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Soil Science Society of America Journal - Soil Biology & Biochemistry

Intercropping with Switchgrass Improves Net Greenhouse Gas Balance in Hybrid Poplar Plantations on a Sand Soil

 

This article in SSSAJ

  1. Vol. 81 No. 4, p. 781-795
    unlockOPEN ACCESS
     
    Received: Sept 20, 2016
    Accepted: Apr 17, 2017
    Published: August 31, 2017


    * Corresponding author(s): hal.collins@ars.usda.gov
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doi:10.2136/sssaj2016.09.0294
  1. H.P. Collins *a,
  2. P.A. Faya,
  3. E. Kimurab,
  4. S. Fransenc and
  5. A. Himesd
  1. a USDA-ARS Grassland, Soil and Water Research Lab. 808 E. Blackland Road Temple, TX 76502
    b Texas AgriLife Extension Service Dep. of Soil and Crop Sciences Vernon, TX 76384
    c Washington State Univ. Irrigated Agricultural Research and Extension Center 24106 N. Bunn Rd Prosser, WA 99350
    d Greenwood Resources Inc. Portland, OR 97201
Core Ideas:
  • A critical issue in the production of biofuels has been the competition with food crops
  • Intercropping switchgrass and hybrid poplar was found to be a viable bioenergy production strategy
  • Intercropped biomass production offset increases in GHGs and GWP
  • N2O emissions factors averaged 1% of the applied N over the study

Abstract

In the Pacific Northwest, commercial hybrid poplar (Populus generosa Henry × Populus canadensis Moench.) is managed at low stocking densities under irrigation for high-value timber production. The objectives of this study were to measure greenhouse gas emissions (CH4, CO2, and N2O) during intercropping of switchgrass (Panicum virgatum L.) with hybrid poplar; estimate losses of fertilizer-N as N2O, and estimate global warming potentials (GWP) of the intercrop. Cumulative above-ground biomass-C of the poplar monoculture (PM) closely matched the four year growing season (GS) soil CO2–C emissions, where aboveground biomass of the switchgrass monoculture (SM) and intercrop (IC) exceeded GS CO2–C emissions by 14.1 Mg C ha-1. Soil CH4–C uptake was not significantly different between treatments, while GS N2O-N emissions for PM were ∼80% lower than both IC and SM. N2O emissions factors averaged 0.7% of the applied N-fertilizer. Cumulative contributions of CO2 emissions to GWP were offset by biomass-C resulting in a near zero balance (−5.1 Mg CO2eq ha -1) for the PM, where, IC and SM sequestered significantly more CO2 resulting in a net GWP of −42.5 and –32.2 Mg CO2eq ha-1, respectively. Intercropping with switchgrass can improve the net greenhouse gas balance of hybrid poplar. Continued research is needed on the effects of irrigated bioenergy production on GHG emissions in intercropped systems as they will become increasingly important as agricultural water use, water availability and quality are challenged by climate change.


Abbreviations

    DM, dry matter; GHG, greenhouse gas; GS, growing season; GWP, global warming potential; NEF, nitrogen emissions factor; PM, monoculture poplar; IC, poplar/switchgrass intercrop; SM, switchgrass monoculture

Major shifts in crop production will occur as farmers prepare to supply the demand for biomass feedstocks for production of renewable biofuels. These shifts will affect agroecosystem services related to water use, carbon storage, nutrient cycling and greenhouse gas (GHG) emissions that have direct effects on air, water, and soil quality (Popp et al., 2014). Perennial grasses are an important source of feed and fiber and are considered sustainable bioenergy crops because they have the capacity to produce large quantities of biomass, can be grown on marginal lands, improve soil quality and protect soils from erosion (Lemus and Lal, 2005; Sartori et al., 2006; Casler et al., 2009; Gelfand et al., 2013). Switchgrass (Panicum virgatum L.) is a prominent biomass crop for the US biofuel industry (Sanderson et al., 2007; Casler et al., 2009) because it yields well on a variety of soil types, is drought-tolerant, and has low fertility requirements (McLaughlin and Kszos, 2005; Lemus and Lal, 2005; Kimura et al., 2015). The root system of switchgrass has been shown to offset C losses and promotes C sequestration by adding a significant amount of organic matter to the soil (Ma et al., 2000a, 2000b; Liebig et al., 2005, 2008; Collins et al., 2010).

A concern in the production of biofuels has been the potential for crop lands to be converted to production of dedicated biofuel crops (Tenenbaum, 2008; HLPE, 2013; Searchinger and Heimlich, 2015). A potential solution to offset this concern is to grow dedicated energy crops on marginal lands (Kang et al., 2013; Gelfand et al., 2013) or maximize land use by intercropping or alley cropping with other perennial crops (Blasier et al., 2012; Cacho et al., 2015; Tian et al., 2016). Intercropping is the growing of two or more crops of different species or varieties in close proximity to maximize the capture of photosynthetically active radiation and enhance the yield of both species compared with their growth in monoculture. This biological synergism results in improved ecosystem services through the sharing of space, soil and water resources, mutual protection from pests, greater nutrient availability, and enhanced biodiversity of soil microbes, insects and animals (Power, 2010).

There are many examples of successful intercropping systems. These systems commonly include tree or other woody species often from tropical or semi-tropical environments within agroforestry production systems (Suresh and Rao, 1999; Nissen et al., 2001; Prasad et al., 2010), combined with annual food crops grown using low inputs (Gliessman, 2007; Lithourgidis et al., 2011; Gebru, 2015). Few studies of intercropping woody species with annual food crops in North America have been conducted (Thevathasan and Gordon, 2004; Rivest et al., 2009; Evers et al., 2010). Thevathasan and Gordon (2004) summarized 15 yr of intercropping research with ten tree species and four annual grain crops. They found younger tree stands did not reduce grain yields when intercropped. However, competition increased with older stands of hybrid poplars with all annual grain crops (Evers et al., 2010). Intercropping of trees with annuals was shown to improve N and C cycling (Allen et al., 2004; Fang et al., 2010), enhance wildlife habitat (Stainback and Alavalapati, 2004), and reduce erosion and ground and surface water contamination (Zamora et al., 2009; Bergeron et al., 2011).

Intercropping with perennial species is an additional option for forest landowners across the United States due to its potential for promoting environmental benefits and increase economic returns. Gamble et al. (2014) found that intercropping among rows of poplar and willows provided suitable conditions for establishment of several perennial biomass crops including switchgrass, prairie cordgrass (Spartina pectinate L.), alfalfa (Medicago sativa L.) and wheatgrass (Thinopyrum intermedium L.), but the potential for resource competition increased with time. Switchgrass and loblolly pine (Pinus taeda L.) interactions have been investigated in several intercropping studies established on the coastal plain of the southeastern United States (Cacho et al., 2015; Tian et al., 2015; 2016).

Several studies have shown the benefits of intercropping woody species and switchgrass on yield and soil properties (Blasier et al., 2012; Gamble et al., 2014; Cacho et al., 2015; Muwamba et al., 2015; Tian et al., 2016). These benefits result from a diversification of outputs and a shortened time to reach economic production (Holzmueller and Jose, 2012; Susaeta et al., 2012). Evers et al. (2010) reported that tree-based intercropping systems stored more C than conventional cropping systems by increasing C storage in the biomass of planted trees and increased soil organic matter storage through C inputs to the soil. Further, they suggested that tree based intercropping in temperate regions not only sequestered atmospheric CO2, but also reduced N2O emissions compared with conventional monocultures.

Soils are an important source and sink of anthropogenic CO2, CH4 and N2O in terrestrial ecosystems. Changes in land use or cropping strategy can significantly affect the release of these gases and contribute to the greenhouse effect of warming global temperatures (Snyder et al., 2009; Signor and Cerri, 2013; IPCC, 2013). Nitrous oxide and CH4 produced in the soil are small compared with CO2 but have radiative forcing 298 and 25 times greater than CO2 respectively, over a 100 yr period (IPCC, 2013).

Emissions of CH4 from soil depends on the source and sink of C resources of soils under different land uses and is ubiquitous in temperate, tropical, boreal, grasslands and forests (Powlson et al., 1997; Le Mer and Roger 2001; Palm et al., 2002; USAFGGI, 2008). Nitrous oxide is emitted from ecosystems through microbially mediated pathways of nitrification and denitrification (Snyder et al., 2009; Signor and Cerri, 2013; IPCC, 2013). Agriculture is responsible for greater than 67% of N2O emissions, and result primarily from animal waste management and crop fertilization (IPCC, 2013; Gelfand et al., 2013). The magnitude of these emissions depends on management, climatic conditions, N fertilization, and availability of N and site soil properties (Smith et al., 2003; Signor and Cerri, 2013).

The Intergovernmental Panel on Climate Change (IPCC) estimates that fertilized soils have an average N2O emission factor of 1% (range 0.3 to 3%) of the fertilizer N applied (IPCC, 2013). Emissions of N2O from N-fertilized agricultural fields has been found to range between 0.001 and 6.8% (Bouwman, 1996; Snyder et al., 2009; Signor and Cerri, 2013; IPCC, 2013). Adler et al. (2007) reported soil N2O emissions are the largest source of greenhouse gas emissions associated with bioenergy crop production. Oates et al. (2016) found N2O emissions from annual crops (corn, Zea mays L.; soybean, Glycine max L. Merr.; canola, Brassica napus L.) were 142% higher than from perennials [switchgrass; miscanthus (Miscanthus × giganteus); poplar (Populus spp.), and restored prairie], with fertilized perennials 190% higher than unfertilized perennials. Emissions ranged from 3.1 to 19.1 kg N2O-N ha-1 yr-1 for annuals and 1.1 to 6.3 kg N2O-N ha-1 yr-1 for perennials with N2O peak fluxes associated with precipitation and fertilization. Schmer et al. (2012) found CH4 emissions had a range of -3.8 to 2.4 g ha-1 d-1 for fertilized monoculture switchgrass and −6.8 to -3.8 g ha-1 d-1 for unfertilized switchgrass. Nitrous oxide flux was affected by N treatment, soil temperature and water filled pore space. The flux of N2O ranged from 0.24 to 8.6 g ha-1 d-1 for fertilized switchgrass. Few studies have evaluated greenhouse gas emissions from the intercropping of woody and perennial grasses such as switchgrass. Evers et al. (2010) reported mean N2O emission from monoculture and tree-based intercropping were 10.7 and 7.5 g ha-1 d-1, respectively.

In the Pacific Northwest, commercial hybrid poplar (Populus generosa × P. canadensis) have been successfully managed at low stocking densities under irrigation for high-value timber production (Stanton et al., 2002). These plantations have been managed at a density of 1536 stems ha-1 for 6- to 8-yr pulpwood rotations or at a density of 358 stems ha-1 for 12- to 15-yr saw log rotations. The open understory created by low stocking rates can be used for the production of energy feedstocks; intercropped with perennial grasses such as switchgrass prior to canopy closure. We hypothesize that the intercropping of switchgrass with hybrid poplar significantly increases the flux of GHG but reduces the GWP under irrigated conditions due to increased biomass production. This study is the first in the Pacific Northwest on GHG emissions from the intercropping of switchgrass with hybrid poplar and provides early information on the effects of intercropping on GHG’s as the bioenergy industry develops. The objectives of this research were to (i) describe seasonal patterns of GHG fluxes (CH4, CO2, N2O) during intercropping of switchgrass with hybrid poplar, (ii) estimate growing season losses of N2O, and amount of N-fertilizer losses as N2O, and (iii) estimate global warming potentials produced by intercropping vs monocultures of poplar or switchgrass.


METHODS AND MATERIALS

The hybrid poplar-switchgrass intercropping study was established on the GreenWood Resources Inc., Boardman Tree Farm, Boardman, OR (45°46˝ N, 119°32˝ W; elevation 192 m) in 2011. The site has a mean annual temperature of 11.7°C and during the winter months (November–March) receives an average of 170 mm precipitation as rain and snow (Fig. 1). Crops grown in the region require irrigation. Soil at the field site is Quincy fine sand (mixed, mesic Xeric Torripsamments). Selected soil characteristics are presented in Table 1. Soil bulk density was determined using a 7.6-cm diameter impact core sampler according to Blake and Hartage (1986). Particle size was determined by the hydrometer method (Gee and Bauder, 1986), and pH and EC determined using the method of Robertson et al. (1999). Soil elemental concentrations were determined by the Mehlich 3 extraction method (Mehlich, 1984).

Fig. 1.
Fig. 1.

Monthly precipitation, average air and soil temperatures at the study site, Boardman, OR, for 2011 to 2014 crop years. Monthly minimum and maximum temperatures are shown for air temperature.

 

View Full Table | Close Full ViewTable 1.

Physical and chemical properties of the Quincy fine sand (Xeric Torripsamments) soil at the GreenWood Resources Inc., Boardman Tree Farm.

 
Depth BD† Sand Silt Clay pH EC Organic C‡ Total N P
cm Mg m-3 g kg-1 dS m-1 g kg-1 mg kg-1
0–15 1.63 917 56 27 7.6 0.133 6.3 0.50 45
15–30 1.54 927 52 21 7.7 0.114 1.7 0.11 34
Elemental composition
K S B Zn Mn Cu Fe Ca Mg
cm mg kg-1 cmol kg-1
0–15 250 9 0.5 2.7 9.3 0.8 38 7.4 1.9
15–30 195 8 0.5 2.3 9.0 0.8 34 7.7 1.9
BD, soil bulk density.
Carbonates removed.

The experimental design was a randomized complete block comprised of two hybrid poplar clones—P. generosa (PC4) and P. canadensis (OP367)—planted within four experimental blocks. Main plots comprise three intercropping patterns and switchgrass cultivars assigned to subplots. The three levels of the intercrop were: (1) grass cultivars inter-planted with poplar trees (IC), (2) monoculture grass cultivars (SM), and (3) monoculture trees (PM) (Fig. 2). Field plots comprised an area occupied by 60 poplar trees configured as 5 rows × 12 trees within each row that formed four intercropped areas. Each plot was planted in 2011 with 6 m × 3 cm diameter poplar poles obtained from the GreenWood Resources Inc., nursery, Boardman Tree Farm, Boardman, OR. Three switchgrass cultivars, ‘Kanlow’ (USDA-NRCS, 2011a), ‘Blackwell’ (USDA-NRCS, 2011b), and ‘Trailblazer’ (Vogel et al., 1991) were seeded at a rate of 11.2 kg pure live seed ha-1 on 10 June to 13 June 2011 using a 3 m (width) Tye Drill (Great Plains Manufacturing, Salina, KS) with double disk openers. Grass monocultures were randomized and planted adjacent to the intercropped plots with four replications.

Fig. 2.
Fig. 2.

(a) Experimental design and (b) example of a replicate of the intercropping switchgrass and hybrid poplar set up, showing location of gas flux chambers among trees and monocultures. The blue dots represent locations of gas flux chambers. The photos show the area planted between poplar trees after harvest. Switchgrass monocultures were planted in replicated blocks (foreground).

 

All monoculture and intercropped switchgrass plots, received a blended dry granular fertilizer of 112 kg N ha-1 as urea (45–0–0), 28 kg P ha-1 as mono-ammonium phosphate (11–52–0), 112 kg K ha-1 as potash (0–0–60), and 56 kg S ha-1 as ammonium sulfate (21–0–24) in April and 112 kg N ha-1 as urea after the first harvest in July each year with a 3 m (width) Barber spreader (Barber Engineering Company, Spokane, WA).

Switchgrass was harvested annually in early July (1–8 July) and October (1–5 October) each year with a Hesston discbine swather (AGCO Corporation, Duluth, GA). Following cutting and before windrowing the switchgrass hay was aerated to speed drying with up to five passes of a New Holland tedder (New Holland Agriculture). The hay was then bailed with a Case International Harvester 8555 baler (Racine, WI) and removed from the field. Aboveground biomass after wind rowing and tedding was collected from the center of each plot (5.6 m2) and weighed to determine yield. Biomass dry matter production was determined for each plot using 0.5- to 1.0-kg subsamples dried at 50°C for up to 5 d until a constant weight was reached and biomass yields converted to a dry matter basis.

Annual hybrid poplar tree biomass yield was estimated using regression algorithms to obtain tree weight from measurements of tree age and clone from standing tree diameter and height. These algorithms were developed from previous destructive harvest sampling in the development of the poplar clones used in this study (personal communication, Brian Stanton, Greenwood Resources Inc.). Poplar leaf and branch biomass was also collected after leaf fall in November each year and were included in the estimates of aboveground biomass. Dried subsamples of switchgrass, poplar leaves and branches were ground with a Wiley mill (Thomas Scientific, Swedesboro, NJ) equipped with a 1-mm screen. Total C and N samples (0.2 g) were determined by dry combustion on an Elementar model Vario EL III CNS Analyzer (Elementar, Hanua, Germany). The C content of the leaves and branches varied from year to year so estimates of the aboveground biomass were based on the yearly analyses. Poplar C ranged from 0.45 to 0.51 g g-1 in branches and 0.38 to 0.42 g g-1 in leaves.

Weeds were controlled annually with glyphosate [N-(phosphonomethyl) glycine] at 13.0 g a.i. ha-1 in late March during switchgrass dormancy. Irrigation was applied by solid set sprinklers positioned within the tree rows. Rain gauges were placed in each plot to quantify the amount of water applied over the growing season. The cumulative amount and duration of irrigation applied during the season was managed by Greenwood Resources Inc. with an average of 225 cm ha-1 from April to October in the establishment year and increased to 300 cm ha-1 by year four of the study determined from the rain gauge data.

Greenhouse Gas Flux Measurements: CO2, CH4, N2O

In situ GHG fluxes were measured using the closed static chamber method (Hutchinson and Mosier, 1981; Hutchinson and Livingston, 1993). Duplicate chambers were installed in each replicate of the P. generosa (PC4) monoculture, Kanlow switchgrass/PC4 poplar intercropped treatment, and Kanlow switchgrass monoculture, and sampled according to the USDA-ARS GRACEnet (Greenhouse gas Reduction through Agricultural Carbon Enhancement network) protocols (Parkin and Venterea, 2010). Two base frames were inserted within each treatment replicate at the beginning of the growing season and remained in the field until removal prior to harvests and replaced afterward (Fig. 2). Chambers were placed in the center of each plot with replicates within 6 m of each other. Each base frame was a 30.5-cm diameter × 15-cm height PVC cylinder driven into the soil surface to a depth of 10 cm. Greenhouse gas fluxes were measured by fitting the chamber base frames with a vented PVC cap (30.5-cm inner diameter by 7.5-cm height) that contained a sampling port. The caps had a 2.5-cm diameter hole allowing air to exit and minimize air turbulence when caps were placed. The hole was sealed with a rubber stopper during the measurement period. Vegetation was removed from the chamber area and soil respiration measured.

Fluxes in the establishment year (2011) were measured weekly following irrigation events from June through September. In subsequent years soil fluxes of CO2, CH4 and N2O were measured weekly throughout the growing season following irrigation events from April through September. Depending on the time of season, an average of 25 mm of water was applied at each irrigation event. The amount of irrigation water applied was calculated from a localized hybrid poplar crop model developed by Greenwood Resources Inc. (Gochis and Cuenca, 2000) and 25 yr of historical local AgriMet weather station data (Oregon State University, Hermiston Research and Education Center, Hermiston, OR; https://www.usbr.gov/pn/agrimet/) hourly weather data that used evapotranspiration and growing degree days plus multiple independent local extended weather forecasts. In addition, soil moisture was monitored with real-time soil moisture sensors. Irrigation was applied once a week. Gas sample collection took place 2 h after irrigation ended and within the 1000 and 1200 h window depending on irrigation scheduling (Parkin and Venterea, 2010). The change in concentration of gases within each chamber was determined by withdrawing 35 mL of air from the headspace every 20 min over an hour period after the cap was placed on the chamber base using 60-mL polypropylene syringes. Gas samples were immediately transferred to an evacuated 12 mL Labco Exetainer (Labco Limited, High Wycombe, Buckinghamshire, UK) vial and taken to the laboratory for determination of CO2, CH4 and N2O by gas chromatography. At the time of gas sampling, chamber temperature and air temperature before and after measurements were recorded using a digital differential thermocouple thermometer (Omega HHM290 Supermeter; Omega Engineering Inc., Stamford, CT). Samples were stored in an incubator at 25°C and analyzed immediately. A Varian CP-3800 GC (Varian, Palo Alto, CA) equipped with a thermal conductivity, flame ionization and electron capture detector was used to measure CO2, CH4 and N2O concentrations.

Fluxes of CO2, CH4 and N2O were calculated from the slope of gas concentration over time, based on chamber temperature, and volume and surface area of the chamber (Parkin and Venterea, 2010). Each sampling was checked for nonlinearity of fluxes according to the protocols outlined by the USDA-ARS GRACEnet. Cumulative fluxes were determined by summing individual flux measurements over the growing season. Time-integrated seasonal fluxes of CO2, CH4 and N2O from each treatment were calculated by averaging the flux between sampling times and multiplying by the interval between sampling dates (Collins et al., 2011). The GHG emission factor of each treatment was expressed as the percentage of the N applied as commercial fertilizer and calculated: N2O emissions factor = [(N2O-Nfertilized – N2O-NTree only)/Napplied] × 100; where N2O-Nfertilized is the total N2O emission of intercrop or monoculture treatments, N2O-NTree only is the total N2O emission from the monoculture tree treatment, and Napplied is the kg N ha-1 applied to the intercrop and monoculture treatments.

Soil Sampling Analyses

Soil samples (0–15 cm depth) were collected at each gas sampling interval within 1 m of each sampling chamber and sieved to pass a 2-mm screen with plant fragments removed. Gravimetric soil moisture content, NH4+–N and NO3–N concentrations, and pH were determined for each sample. Gravimetric soil moisture of each sample was determined by oven drying at 105°C for 24 h. An estimate of the average field capacity was determined using a volumetric soil–water method described by Hook and Burke (2000). Briefly, air-dried sieved (2 mm) soil was packed lightly into 50-cm3 graduated cylinders and 5 mL of distilled water was slowly added. The cylinder was covered with perforated parafilm (American National Can, Greenwich, CT) and allowed to equilibrate. After 24 h, soil volume and water content of the wetted front was determined. Mineral N was obtained by extracting 10-g soil subsamples with 1 mol L–1 KCL and analyzed on a QuikChem AE flow-injection analyzer (Lachat Zellweger, Loveland, CO). Soil pH was measured using a 1:2 soil/deionized water solution on a Corning 445 pH Meter (Corning Incorporated, Corning, NY) (Robertson et al., 1999). Soil carbonates were removed from each sample with 0.33 mol L–1 H3PO4 before C analyses (Follett and Pruessner, 2001). Total C concentration of soil samples (25 mg) were determined by dry combustion on an Elementar model Vario EL III CNS Analyzer (Elementar, Hanua, Germany).

Statistical Analysis

Averages and standard errors of the mean for soil moisture, NO3–N, NH4–N, and CO2–C, CH4–C, N2O-N were determined for each sampling interval. Data from paired chambers within each replicate was averaged. Analysis of Variance was conducted using PROC MIXED GLM of (SAS Institute, 2011). Years and replicates were considered random, while the treatments were considered fixed effects. Means among treatments for soil NO3–N, NH4–N, and CO2–C, CH4–C, N2O-N cumulative gas emissions and global warming potentials (GWP) were compared using the Tukey-Kramer significant difference test. All tests for significance were conducted at the P ≤ 0.05 levels (Table 2).


View Full Table | Close Full ViewTable 2.

ANOVA table from MIXED GLM analysis of data from the monoculture poplar, poplar switchgrass intercrop and monoculture switchgrass treatments for the 2011 to 2014 crop years.

 
Seasonal flux N emission factor Biomass C Global warming potential†
CO2–C CH4–C N2O-N CO2 CH4 N2O NetGWP
Year (Y) *** NS‡ *** *** *** *** NS *** ***
Trt (T)§ ** NS *** NS *** NS NS *** ***
Time (I)¶ *** * *** ***
Y×T * NS *** NS *** NS NS ** ***
Y×I NS NS *** ***
T×I NS *** *** ***
Y×T×I NS NS NS NS
*Significant at P < 0.05; ** Significant at P < 0.01; *** Significant at P < 0.001.
Global warming potential (GWP) was calculated for the total flux over the 167 d growing season, so no analysis of time by year or treatment by time. Analyses for biomass C and global warming potential were conducted for cumulative production after 4 yr of cropping; Net GWP = (Soil CO2eq + N2OCO2eq + CH4CO2eq) – (aboveground biomassCO2eq).
NS, not significant.
§Trt, treatments were monoculture poplar, poplar switchgrass intercrop and monoculture switchgrass.
Time, sampling times 1 (April to June) and time 2 (July to September).


RESULTS AND DISCUSSION

Air and Flux Chamber Temperature, and Soil Water Content

Average growing season air temperatures were similar among the 2011 through 2014 growing seasons ranging from a low of 10.5°C to a high of 30.4°C with an average annual precipitation of 163 mm (Fig. 1). Soil temperatures averaged 19.2°C for the first half (April–June) of the growing season and 22.9°C for the second half (July–September). The gas flux chamber temperatures were 1 to 2°C higher than the ambient air temperature at the time of sampling (data not shown).

Gravimetric soil moisture contents were above field capacity (FC = 0.11 g g-1) during much of the growing seasons averaging 0.124, 0.146, and 0.164 g g-1 for the PM, IC, and SM, respectively. Monthly soil moisture of the surface 0- to 15-cm soil layer averaged 0.12 g g-1 from April to May and increased above field capacity from July to September for the PM and IC and significantly greater for the SM (Fig. 3). Increasing summer temperatures and higher evapotranspiration from the hybrid poplar trees and switchgrass resulted in greater irrigation applications in the later years of the study. Soil moisture was greater in the IC plots than PM or SM which may have resulted from cooler soil temperatures due to tree shading and the soil cover of the intercropped switchgrass that reduced evaporation and/or water demand by the switchgrass. Soil density in 2011 was 1.63 Mg m-3 and increased to 1.93 Mg m-3 in 2014. The increase in soil density resulted from the wheeled equipment used during fertilizing, harvesting, tedding and baling operations.

Fig. 3.
Fig. 3.

Mean monthly soil moisture of the Quincy soil for the 0- to 15-cm soil layer of the monoculture poplar (PM), poplar/switchgrass intercrop (IC) and monoculture switchgrass (SM) averaged over the 2011 to 2014 growing seasons. Error bars are standard errors of each treatment for April to May, n = 48; for June to September, n = 64.

 

Soil NH4–N and NO3–N

Nitrate was the dominant form of inorganic N in the 0- to 15-cm soil layer of each sampling date over the 2011–2014 growing seasons (Fig. 4), representing greater than 90% of the mineral N. Soil mineral N (NH4–N and NO3–N) of the non-fertilized PM averaged 4 mg N kg-1 soil during each growing season (Fig. 4). Soil mineral N was significantly higher following fertilization in the spring and summer each year for the IC and SM reaching 80 to 90 mg N kg-1 soil following the July applications in 2013 and 2014. Mineral N of the IC and SM declined to PM levels after 2 wk of application, except in 2013 where mineral N remained elevated for the following 6 wk.

Fig. 4.
Fig. 4.

Average monthly ammonium N (NH4–N) plus nitrate N (NO3–N) concentrations for the 0- to 15-cm soil layer of the monoculture poplar (PM), poplar/switchgrass intercrop (IC) and monoculture switchgrass (SM) treatments for the 2011 to 2014 growing seasons. Error bars for each month by treatment are standard errors, n = 16.

 

Greenhouse Gas Emissions: CO2, CH4, and N2O

CO2–C Flux rates, Seasonal Emissions and Biomass Production

Seasonal (167 d) CO2–C flux patterns from the PM, IC and SM were similar among years, increasing as soil temperature increased and switchgrass and poplar matured during the growing season (Fig. 5). In 2011, CO2–C soil respiration was similar among the PM, IC, and SM with a range of 40 to 70 kg CO2–C ha-1 d-1 from July to September, which resulted from the decomposition of the hybrid poplar harvest debris in the establishment year. In subsequent years (2012–2014) average daily CO2–C emissions of the IC and SM were higher than the PM. The higher emissions likely resulted from greater microbial activity and root respiration from the switchgrass and hybrid poplar. Although CO2–C respiration fluxes were lower for the PM the pattern of emissions over the growing season were similar to the IC and SM. Peak CO2–C fluxes occurred during June and July.

Fig. 5.
Fig. 5.

Average monthly carbon dioxide (CO2–C) flux rates from the monoculture poplar (PM), poplar/switchgrass intercrop (IC) and monoculture switchgrass (SM) treatments for the 2011 to 2014 growing seasons. Error bars for each month by treatment are standard errors, n = 16.

 

Nikiema et al. (2011) and others (Lee et al., 2007; Schmer et al., 2012) reported CO2 flux rates under fertilized and non-fertilized switchgrass ranged from 15 to 60 kg CO2–C ha-1 d-1 and reported N fertilization had no effect on soil respiration rates. Peichl et al. (2006) measured CO2 flux rates from a poplar (Populus eltoides × Populus nigra clone DN-177) intercropped with barley (Hordeum vulgare L.) and a barley monoculture in southern Ontario, Canada. They found soil respiration rates ranged from 72 to 120 kg CO2 ha-1 d-1 in the barley monoculture system and were slightly higher in the poplar intercropping system at rates between 72 to 192 kg CO2 ha-1 d-1. Peichl et al. (2006) also reported annual soil respiration was 3.7 and 2.8 Mg C ha-1 y-1 measured during the growing season (July–October) from the poplar intercropping and barley monoculture cropping systems, respectively. Growing season soil respiration in our study averaged 3.2, 3.9, and 4.0 Mg C ha-1 for the PM, IC, and SM, respectively and was similar to that reported by Peichl et al. (2006).

Annual biomass C of the perennial switchgrass and hybrid poplar increased each year of the study (Table 3). Biomass C of the PM on average tripled each production year; IC biomass doubled after the second production year, where SM production peaked in 2013 then declined. The decline in SM production was not significantly different among years two through four. Cumulative biomass C produced over the 4 yr of cropping was 27.6, 25.5, and 14.4 Mg C ha-1 (65.7, 60.7, and 34.3 Mg DM ha-1) for the IC, SM, and PM, respectively. The four year cumulative values of the PM and IC biomass produced was not the sum of tree biomass measured each year but was derived from the total tree biomass produced by 2014; where the IC includes the cumulative switchgrass biomass harvested each year. The IC and SM produced significantly more biomass than the PM. Collins et al. (2010) and Kimura et al. (2015) reported similar cumulative dry matter and C yields of irrigated switchgrass monocultures over 3 yr on a site located close to the current study on a Quincy soil. Cumulative three-year yields ranged from 34 to 47 Mg DM ha-1 (15–20 Mg C ha-1) depending on the cultivar. Collins et al. (2010) reported biomass yields of the switchgrass cultivar Kanlow was 3.3 Mg DM ha-1 in the establishment year and 47 Mg DM ha-1 (20 Mg C ha-1) over the three year study which exhibited a similar production track as found in the current study. They also reported profile root biomass of Kanlow produced after three seasons represented 3.3 Mg C ha-1 to 1-m depth and that soil C increased 20% in the 0- to 15-cm soil layer. After 4 yr of cropping in the current study soil C increased 0.2, 2.6, and 3.9 Mg C ha-1 in the 0- to 15-cm of the PM, IC, and SM, respectively (Table 3).


View Full Table | Close Full ViewTable 3.

Aboveground biomass-C, soil C and time-integrated growing season CO2–C emissions from the monoculture poplar, poplar/switchgrass intercrop and monoculture switchgrass treatments for the 2011 to 2014 growing seasons.

 
Year Biomass production
Monoculture
poplar
Poplar +
switchgrass
Monoculture
switchgrass
Mg C ha-1
2011 0.5Da 0.7Da 0.6Ba
2012 1.7Cb 5.9Ca 7.6Aa
2013 5.8Bb 10.3Ba 9.3Aa
2014 14.4Ab 19.7Aa 8.0Ac
Cumulative‡ 14.4b 27.6a 25.5a
Soil C sequestered (2011–2014)
Mg C ha-1
Cumulative‡ 0.2c 2.6a 3.2a
Growing season soil CO2–C emissions§
Mg CO2–C ha-1
2011 4.2Aa 3.6Ba 3.9Aa
2012 3.2Bb 5.0Aa 4.3Aa
2013 2.6Bb 3.3Ba 3.8Aa
2014 2.9Bb 3.6Ba 4.0Aa
Cumulative 12.9b 15.5a 16.0a
Change in C = (dry matter biomass C + Soil C) – (CO2–C)
C ha-1 season-1
2011 -3.7Da -2.9Da -3.3Ca
2012 -1.5Cc 0.9Cb 3.3Ba
2013 3.2Bb 7.0Ba 5.5Aa
2014 11.5Aa 16.1Aa 4.0Ab
Cumulative 1.7b 14.7a 12.7a
Means followed by the same uppercase letter within a treatment among years are not significantly different at P < 0.05; Means followed by the same lowercase letters among treatments within a year are not significantly different at P < 0.05.
Four year cumulative values of the monoculture poplar and poplar/switchgrass intercrop biomass produced are not the sum of biomass measured each year but the total tree biomass produced in 2014 and cumulative switchgrass biomass harvested each year for the intercrop.
§CO2–C emissions measured over the growing season (167 d, April–September); Negative values indicate CO2 soil respiration exceeded biomass C production.

Tree based intercropping systems are considered to be C sinks because the incorporation of trees within the intercrop allows for greater CO2 sequestration from the atmosphere and subsequently higher C storage (Thevathasan and Gordon, 2004; Evers et al., 2010). Peichl et al. (2006) found that after 13 yr of poplar intercropping the total C pool of above- and belowground components yielded 68.5 and 96.5 Mg C ha-1 in a barley monoculture and poplar intercropped system, respectively. They also reported that the total C pool of the poplar intercrop was 41% greater than the monoculture. Gamble et al. (2014) in a two year poplar-switchgrass non-irrigated intercropping study reported switchgrass yields ranged 2.8 to 5.2 Mg DM ha-1 (1.2–2.2 Mg C ha-1) at two sites in Northern Minnesota. Switchgrass intercropped with pine in the southeastern United States produced 4.1 to 10.3 Mg DM ha-1 after 2 yr of cropping (Albaugh et al., 2012; Krapfl et al., 2015; Tian et al., 2015) compared with yields of established monoculture switchgrass stands of 16 to 36 Mg DM ha-1 (McLaughlin and Kszos, 2005; Albaugh et al., 2012). Compared with other temperate agroforestry systems, the intercropped switchgrass biomass C found in the present study after 2 yr was 4.2 Mg C ha-1 (10 Mg DM ha-1) and after 4 yr was 5.3 Mg C ha-1 (13 Mg DM ha-1) and were 33% less than the switchgrass monoculture (Table 3). The reduction in switchgrass production in the IC was attributed to shading by the hybrid poplar trees (Table 3). The greater yield in IC compared with other intercropping studies was attributed to irrigation and fertilization management.

Total seasonal CO2–C emissions were similar among treatments and exceeded biomass production in 2011 (Table 3). These emissions were attributed to decomposition of hybrid poplar harvest debris since switchgrass and poplar development was minimal. Seasonal CO2–C emissions from the IC, SM, and PM peaked in 2012 then declined and were significantly greater in the IC and SM than PM (Table 3). Negative values indicate CO2–C emissions exceeded biomass C production, where positive values were the sequestering of C. The greatest sequestration of C into biomass occurred in 2014. By the fourth year (2014) biomass C production was 5, 5.5, and 2 times greater than soil CO2–C emissions for the PM, IC, and SM, respectively. Cumulative four year biomass C produced including the C sequestered into the soil of the PM was slightly greater than the four year seasonal CO2–C emissions, where, biomass production for the SM and IC exceeded CO2–C emissions by an average of 14.1 Mg C ha-1. The greater C assimilated within the intercrop compensated for higher C losses via soil respiration and other C losses from the system which resulted in a net accumulation of carbon, compared with the PM (Peichl et al., 2006: Nikiema et al., 2011; Schmer et al., 2012). There are several things to consider from this interpretation: first, this approach was not an attempt to short circuit accepted methods of determining net ecosystem production (NEP), but to show that the biomass produced over a growing season and the increase in soil organic C significantly offset growing season soil CO2–C fluxes. The definition of NEP is the difference between the amount of organic C fixed by photosynthesis in an ecosystem and total ecosystem respiration (Reco) and represents the organic C available for storage within the system or loss from it by some export process (Woodwell and Whittaker, 1968). Soil respiration (SR) has been reported to represent 30 to 60% of total Reco with an average of 50% depending on the type of ecosystem (Bloemen et al., 2010; Reichstein et al., 2012; Delogu et al., 2013). Randerson et al. (2002) suggested that NEP could equate to C accumulation in the ecosystem, where Lovett et al. (2006) contended it was only valid if inputs and outputs of organic C are negligible. Yuste et al. (2005) measured soil respiration in a mixed temperate forest (Pinus sylvestris L. and Quercus robur L.) and determined that the ratio of SR:Reco ranged from 0.58 to 0.76. Further, they reported that the contribution of SR to Reco varied seasonally with minimum contributions during summer (<50% of Reco) and maximum contributions during winter (>94% of Reco). A significant portion of the heterotrophic soil respiration was influenced by the location in the soil profile and by the total organic C content of soils. If the assumption is made that SR is 50% of Reco and that switchgrass root production was similar to that Collins et al. (2010) reported for switchgrass the estimate of C accumulation would be lower but still positive for the intercropped and switchgrass treatments after 4 yr of production.

Another consideration was that soil CO2–C flux was only measured during the growing season. However, sampling during this period represented the major production period of plant biomass, organic C accumulation and the maximum period of soil respiration. The Reco would be significantly reduced in the fall and winter months since both switchgrass and poplar enter into dormancy in October and soil respiration rates decrease significantly as soil temperatures decline through fall and winter months. Poplar and switchgrass typically break dormancy in April-May and the sampling covered this period. It is noted that additional measurements made during the winter and early spring would improve the estimates of the effect of intercropping on CO2–C emissions.

CH4–C Flux rates and Seasonal Emissions

Methane (CH4–C) fluxes were similar among treatments throughout the growing season (Fig. 6). Daily soil methane uptake rates averaged -0.72, -0.76, and -0.42 g CH4–C ha-1 d-1 for the PM, IC, and SM, respectively. Methane uptake varied during the growing season (2011–2014) with highest uptake occurring in the spring (April–June) then declined from July to September. Increasing soil moisture later in the season likely inhibited microbial activity and reduced diffusion (Khalil and Baggs, 2005; Le Mer and Roger, 2001). Following N fertilization after the July harvest methane uptake from the IC and SM soil increased while no effect was observed for the PM. Soil methanotrophic activity (CH4 consumption) generally increases to a soil’s field capacity then decreases when soil water content exceeds field capacity and gaseous transfer is reduced. At low water contents (22–60%), methanotrophy is dependent on the level of soil fertility (Khalil and Baggs, 2005; Le Mer and Roger, 2001). After fertilization and increasing soil moisture CH4–C exhibited several spikes in emissions from the SM ranging from 0.5 to 1.3 g CH4–C ha-1 d-1 over a few weeks after fertilization that reduced CH4–C uptake. Methane oxidation has been shown to decline with application of N fertilizers. Le Mer and Roger (2001) reported that urea and NH4–based fertilizers inhibit CH4 oxidation where NO3–based fertilizers do not. The inhibition of CH4 oxidation in soils by NH4 originates from the competition at the level of the methane monooxygenase, leading to a reallocation of the CH4 oxidizing activity toward nitrification and the toxicity of NO2 that can be produced inhibiting oxidation (Le Mer and Roger, 2001).

Fig. 6.
Fig. 6.

Average monthly methane (CH4–C) flux rates from the monoculture poplar (PM), poplar/switchgrass intercrop (IC) and monoculture switchgrass (SM) treatments for the 2011 to 2014 crop growing seasons. Error bars for each month by treatment are standard errors, n = 16.

 

Seasonal methane uptake for the PM, IC, and SM was lower than a number of studies have reported in the literature. Methane fluxes reported in the literature from switchgrass production range from -3.8 to 2.4 g CH4–C ha-1 d-1 for fertilized monoculture switchgrass and −6.8 to -3.8 g CH4–C ha-1 d-1 for unfertilized switchgrass and are predominately sinks for CH4–C (Nikiema et al., 2011; Schmer et al., 2012; Wile et al., 2014; Raun et al., 2016). Total growing season soil CH4–C uptake was not significantly different among the PM, IC and SM treatments (Table 4).


View Full Table | Close Full ViewTable 4.

Seasonal CH4–C emissions from the monoculture poplar, poplar/switchgrass intercrop and monoculture switchgrass treatments for the 2011- 2014 crop years.

 
Cropping year Monoculture poplar Poplar/switchgrass intercrop Monoculture switchgrass
April-June July- September Growing season† April-June July- September Growing season April-June July- September Growing season
g CH4–C ha-1
2011 −59Aa −59Aa −59ABa −59Aa −52Aa −52Aa
2012 −48Aa‡ −63Aa -111Aa −47Aa −44ABa −78ABa −70Aa§ -2Aa −72Aa
2013 -31Aa -38Aa −69Aa -28Aa -35Aa −59Aa −86Ab -11Aa −75Aa
2014 −75Aa −54Aab -129Aa −47Aa§ −81Bb -129Ba −48Aa -10Aa −57Aa
Cumulative -154a -214a -368a -122a§ -219a -324a -203a§ −76a -256a
Average −51a −53b −92a −41a −55b −81a −68a§ -25a −64a
CH4–C emissions measured over the growing season (167 d, April–September) each year were interpolated linearly between measurements to estimate cumulative flux. Negative values indicate that CO2 emissions exceeded biomass production.
Means followed by the same uppercase letters within a sampling time among years are not significantly different at P < 0.05; Means followed by the same lowercase letters among treatment within a sampling time and year among treatments are not significantly different at P < 0.05.
§Compares the values between sampling time 1 (April to June) and time 2 (July to Sep) at P < 0.05.

N2O-N Flux rates and Seasonal Emissions

Nitrous oxide fluxes from the PM were similar among the 2011- 2014 growing seasons averaging 3.7, 1.3, 1.0, and 2.5 g N2O-N ha-1 d-1, respectively (Fig. 7). Data are presented as the daily N2O-N flux rates from the PM, IC and SM treatments. N2O-N fluxes for the IC and SM showed minimal increases following the spring fertilizer applications with daily fluxes increasing each year after fertilization, averaging 5.0, 8.2 and 14.2 g N2O-N ha-1 d-1 for 2012, 2013, and 2014. Major N2O-N increases occurred after the second fertilizer application in July with the greatest average monthly fluxes occurring during the 2013 growing season that persisted for up to 5 wk after application. Highest emissions recorded were 240 g N2O-N ha-1 d-1 occurring within 3 wk after application in 2013 and 200 g N2O-N ha-1 d-1 in 2014. N2O-N emissions from the IC and SM declined to background levels comparable to that of the PM by mid-August each year. We attributed the lower N2O-N fluxes to greater aeration of the soil relative to low soil water contents below field capacity. Soil water content is related to water-filled pore space and soil NO3 concentrations are key factors affecting N2O-N emissions (Liu et al., 2007; Snyder et al., 2009; Signor and Cerri, 2013). The soil water contents among the treatments increased later in the growing season and in later years as compaction increased from 1.63 to 1.93 Mg m-3 suggesting N2O emissions were generated from denitrification rather than nitrification as water applications increased (Ruser et al., 2006; Buchkina et al., 2013). Evers et al. (2010) found that high soil water contents correlated with N2O-N emission from summer to spring in both monoculture and hybrid poplar-barley intercropped fields. In the IC soil water was higher, due to a reduction in evaporation from soil as a result of shading, which could enhance denitrification. Evers et al. (2010) reported N2O-N emissions from monoculture and intercropped systems were 3.9 and 2.7 kg ha-1, respectively. Schmer et al. (2012) found N2O-N fluxes ranged from 0.24 to 8.6 g N2O-N ha-1 d-1 for fertilized switchgrass. Raun et al. (2016) in a study in the upper mid-west reported that average N2O emissions ranged from 1.28 g N ha-1 d-1 in unfertilized switchgrass to 25.8 g N ha-1 d-1 in fertilized switchgrass. They also reported that the maximum daily N2O emission was 270 g N ha-1 d-1 in fertilized switchgrass which was similar to that found in the present study.

Fig. 7.
Fig. 7.

Nitrous oxide (N2O-N) flux rates from the monoculture poplar (PM), poplar/switchgrass intercrop (IC) and monoculture switchgrass (SM) for the 2011 to 2014 crop growing seasons. Error bars for each date by treatment are standard errors, n = 8.

 

Cumulative growing season N2O-N emissions from the PM averaged 241 g N2O-N ha-1 over 2011 to 2014 with a low of 113 g N2O-N ha-1 in 2013 (Table 5). Seasonal emissions from the fertilized IC and SM increased significantly from establishment in 2011 (∼257 g N2O-N ha-1), peaking in 2013 for the fertilized IC and SM averaging 1936 and 2854 g N2O-N ha-1, respectively, then declined by 15 and 50% in 2014 (Table 5). Total fluxes of N2O-N from IC and SM were 3, 21, and 5 times greater than PM for the 2012, 2013, and 2014 growing seasons, respectively. Total N2O-N emissions for the four year study were 963, 4485, and 5263 g N2O-N ha-1, for the PM, IC and SM, respectively, with the highest emissions (75%) occurring from July through September.


View Full Table | Close Full ViewTable 5.

Seasonal N2O-N flux and emissions factors for the monoculture poplar, poplar/switchgrass intercrop and monoculture switchgrass treatments for the 2011 to 2014 crop years.

 
Cropping year Monoculture poplar Poplar/switchgrass intercrop Monoculture switchgrass
April-June July-September Growing
season†
July-September Growing season April-June July-September Growing season
g N2O-N ha-1 season-1
2011 280Aa 280ABa 249Ba 249Ca 264Ba 264Ca
2012 141ABa‡ 99Bb 240ABb 165Ba§ 449Ba 615BCa 142Ba§ 657Ba 799BCa
2013 51Ba 62Bb 113Bb 111Ba§ 1825Aa 1936Aa 123Ba§ 2731Aa 2854Aa
2014 168Ab 163ABb 331Ab 875Aa 809Ba 1685ABa 522Aab 878Ba 1400Ba
Cumulative 360b 603b 963b 1151a§ 3333a 4485a 787ab§ 4532a 5263a
Average 120b 151b 241b 384a§ 833a 1121a 262ab§ 1133a 1316a
Emission factor = (kg N2O-N ha-1)/(kg fertilizer N ha-1) x 100
2011
2012 0.31Ba§ 0.41Bb 0.35Aa 0.26Ab§ 0.59Ba 0.43Ba
2013 0.48Ba§ 1.81Ab 0.87Aa 0.48Aa§ 3.01Aa 1.74Aa
2014 0.79Aa 0.72Bb 0.75Aa 0.47Aa§ 1.10Ba 0.78Ba
Average 0.53a§ 0.98a 0.66a 0.40a§ 1.56a 0.77a
N2O-N emissions measured over the growing season (167 d, April–September) each year were interpolated linearly between measurements to estimate cumulative flux.
Means followed by the same uppercase letters within a sampling time between years are not significantly different at P < 0.05; Means followed by the same lowercase letters within a sampling time and year among treatments are not significantly different at P < 0.05.
§Compares the values between sampling time 1 (April to June) and time 2 (July to September) within a year and treatment at P < 0.05.

Growing season emission factors (NEF) were similar for IC and SM which is consistent with the findings of others (Evers et al., 2010; Nikiema et al., 2011; Schmer et al., 2012; Wile et al., 2014; Oates et al., 2016). In 2012 when fertilization began NEF for the IC and SM averaged 0.4% (0.90 kg fertilizer N ha-1), then increased significantly to 0.87 and 1.74% (2.0 and 3.9 kg fertilizer N ha-1) in 2013 then declined to 0.75% (1.7 kg fertilizer N ha-1) in 2014 for the growing season. There were significant seasonal differences between spring and summer periods when fertilizer was applied. Spring NEF had a range of 0.32 to 0.80% with the highest fertilizer N loss in 2013. The summer period (July–September) showed a significantly greater loss of N ranging from 0.72 to 3.0% being highest in 2013. Growing season emissions factors (NEF) of N averaged over the 4 yr accounted for 0.9% (6.0 kg N ha-1) of the applied fertilizer for IC and 1.0% (6.7 kg N ha-1) for SM (Table 5). During the growing season the unfertilized PM had an average loss of 0.27 kg N ha-1. From N2O-N flux data presented by Schmer et al. (2012) we estimated the NEF from a non-irrigated monoculture switchgrass study in the Northern Great Plains as 0.25%. Wile et al. (2014) reported that cumulative seasonal (May–November) N2O emissions from monoculture switchgrass stands were <1 kg N2O-N ha-1 an NEF of 0.8% with highest emissions released from switchgrass fertilized with 120 kg N ha-1. McGowan (2015) found fertilizer induced emission factors of switchgrass increased from 0.7% at 50 kg N ha-1 to 2.6% at 150 kg N ha-1, demonstrating a nonlinear increase in N2O emissions from fertilized switchgrass.

Global Warming Potentials (GWP)

Greenhouse gas emissions were estimated over the growing season each year and interpolated linearly between measurements to estimate cumulative greenhouse gas losses and GWP of greenhouse gasses, relative to CO2. Seasonal N2O GWP of the PM was similar among years averaging 114 kg CO2eq ha-1 compared with significant increases each year for the IC and SM due to switchgrass fertilization (Table 6). The Quincy soil was a sink for CH4 with an average uptake of 3.3 kg CO2eq ha-1 among the PM, IC, and SM. The cumulative four year seasonal GWP based on CH4 and N2O for the PM averaged 444 kg CO2eq ha-1 compared with 2094 and 2610 kg CO2eq ha-1 for the IC and SM where N fertilization of the IC and SM increased the CO2eq ha-1 fivefold. The combined average GWP of CO2, CH4 and N2O for the 2011 to 2014 growing seasons was 12.0, 14.7, and 15.3 Mg CO2eq ha-1 for the PM, IC, and SM, respectively with CO2 representing >95% GWP.


View Full Table | Close Full ViewTable 6.

Net global warming potentials (GWP) for the 2011 to 2014 growing seasons of the monoculture poplar, poplar/switchgrass intercrop and monoculture switchgrass treatments.

 
Biomass C production Carbon dioxide Nitrous oxide Methane Net GWP†
Mg CO2eq ha-1 season-1
Monoculture poplar
2011 1.8Da‡ 15.5Aa 0.136Aa -0.0027Aa +13.8Aa
2012 6.1Cb 11.8Bb 0.113ABb -0.0037Aa +5.8Aa
2013 21.3Bb 9.5Bb 0.053Bb -0.0023Aa -11.8Ba
2014 52.9Ab 10.7Bb 0.155Ab -0.0043Aa −42.1Cb
Average 11.9b 0.114b -0.0033a
Cumulative§ 52.9b 47.4b 0.457b -0.0130a −5.1a
Poplar/switchgrass intercrop
2011 2.4Da 12.7Ba 0.122Ca -0.0027Aa +10.4Aa
2012 21.5Ca 18.4Aa 0.288BCa -0.0026Aa -2.8Bab
2013 37.7Ba 12.1Bab 0.907Aa -0.0020Aa -24.7Cc
2014 72.1Aa 13.4Ba 0.789ABa -0.0043Aa −57.9Dc
Average 14.2a 0.527a -0.0029a
Cumulative§ 101.2a 56.6a 2.106a -0.0116a −42.5c
Monoculture switchgrass
2011 2.1Ba 14.2Aa 0.129Ca -0.0023Aa +12.2Aa
2012 27.8Aa 15.7Aa 0.374BCa -0.0033Aa -11.7Bb
2013 34.2Aa 14.0Aa 1.261Aa -0.0028Aa -18.9Bb
2014 29.2Ac 14.6Aa 0.855ABa -0.0020Aa -13.7Ba
Average 14.6a 0.655a -0.0026a
Cumulative 93.3a 58.5a 2.619a -0.0104a -32.2b
Net GWP = (Soil CO2eq + N2OCO2eq + CH4CO2eq) – (aboveground biomassCO2eq); Positive values indicate the system is emitting CO2eq to the atmosphere, negative values sequestering CO2eq from the atmosphere.
Means followed by the same uppercase letters within a treatment among years are not significantly different at P < 0.05; Means followed by the same lowercase letters among treatments within a year, the average or cumulative values are not significantly different at P < 0.05.
§Four year cumulative value of the monoculture poplar and poplar/switchgrass intercrop biomass produced is not the sum of biomass measured each year but uses the total tree biomass produced in 2014 and cumulative switchgrass biomass harvested each year for the intercrop.

The net GWP (GWPNet) was calculated as (Soil CO2eq + N2OCO2eq + CH4CO2eq) – (aboveground biomassCO2eq). Positive values indicated the system was emitting CO2eq to the atmosphere, negative values sequestering CO2eq from the atmosphere. GWPNet in 2011 among all systems averaged +12.1 Mg CO2eq ha-1. As discussed earlier CO2 emissions exceeded biomass C in the first 2 yr of the PM and first year of the IC and SM treatments resulting from decomposition of previous poplar harvest debris. The contributions of CO2eq emissions from soil to GWP were offset each successive year by the C fixed in the crop biomass with the hybrid poplar sequestering more atmospheric CO2eq than switchgrass biomass. By year four of the study, the GWPNet of PM (−42.1 Mg CO2eq ha-1) and IC (−57.9 Mg CO2eq ha-1) showed a significantly greater sink for atmospheric CO2 sequestered by the poplar biomass than the SM (-13.7 Mg CO2eq ha-1).

The four year cumulative net GWP values of the PM and IC biomass produced was the total tree biomass produced by 2014 and sum of the switchgrass biomass harvested each year. Cumulative 4 yr CO2eq emissions to net GWP were offset by the C fixed in the crop biomass resulting in a net balance of GWP for the PM (−5.1 Mg CO2eq ha-1), where, IC and SM sequestered significantly more CO2eq to yield a net GWP of −43 and -32 Mg CO2eq ha-1, respectively.

SUMMARY

Highly productive, commercial hybrid poplar plantations are being managed in the Pacific Northwest for high-value timber production at relatively low stocking densities under irrigation. The open understory was used to produce switchgrass biomass prior to canopy closure in an intercropping system. Intercropping hybrid poplar with switchgrass was found to be a viable alternative to biomass production for bioenergy production than solely in monoculture. The resulting increase in biomass production was found to offset increases in GHGs and GWP resulting from irrigation and fertilization.

Cumulative 4 yr biomass C produced in the PM closely matched the 4 yr CO2–C emissions (1.7 Mg C ha-1), where biomass production for the SM and IC exceeded CO2–C emissions averaging 14.1 Mg C ha-1. CH4–C uptake was not significantly different between treatments, while GS N2O-N emissions for PM were ∼80% lower than both IC and SM. N2O emissions factors averaged 0.7% of the applied N-fertilizer. Cumulative contributions of CO2eq emissions to GWP were offset by the C fixed in the aboveground biomass resulting in a net GWP zero balance for the PM after 4 yr, where, IC and SM sequestered significantly more atmospheric CO2 to yield a net GWP of −42 and -32 Mg CO2eq ha-1. However, later production years (2013– 2014) of the PM showed significant net GWP yearly offsets by the monoculture PM. This study only evaluated the GHG emissions during the growing season, additional measurements made during the late fall; winter and early spring are needed to develop a complete understanding of the effects of intercropping on GHG emissions. However as found intercropping significantly reduced the GWP to a greater extent than when switchgrass was grown in monoculture.

Continued research on the effects of irrigated bioenergy production and the effects of irrigation water applied on GHG emissions in intercropped systems is needed. A suggestion from our observations for future irrigated intercropping hybrid poplar-switchgrass production is to closely match the irrigation needs of each crop. We estimated that the switchgrass within intercropping received 127% more water than needed for production. The over application of irrigation water resulted in increasing soil compaction from management operations and increased soil moisture leading to higher N2O-N fluxes and reduced CH4–C oxidation. We suggest a separate irrigation strategy using drip irrigation for hybrid poplar and sprinkler for switchgrass with conversion of switchgrass to drip in later years. Knowledge of the processes regulating GHG emissions from irrigated biomass production will become increasingly important as agricultural water use, water availability and quality are challenged by climate change.

Acknowledgments

This work was supported by the USDA Agricultural Research Service and in part from funding from the USDA Agriculture and Food Research Initiative, contract #2011-6700930001 and supports the USDA-ARS GRACEnet. The authors thank J. Mieirs (Washington State University, IAREC, Prosser, WA) for field assistance, R. Cochran (USDA-ARS) and M. Silva (Washington State University) for sample processing and laboratory analyses. USDA is an equal opportunity provider and employer.

 

References

Footnotes


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