Soil Aggregation and Organic Matter Mineralization in Forests and Grasslands: Plant Species Effects
Plant-soil feedbacks can alter N cycling rates in terrestrial ecosystems, but the mechanistic relationship between species characteristics, soil properties, and N dynamics is unclear. Plant species may affect patterns of soil aggregation, which can affect soil C and net N mineralization. This mechanism was examined in two common garden experiments: one containing five tree species (European larch [Larix decidua Miller], red oak [Quercus rubra L.], red pine [Pinus resinosa Ait.], white pine [Pinus strobus L.], and Norway spruce [Picea abies (L.) Karst]) and one containing six grass species (big bluestem [Andropogon gerardi Vitm.], indiangrass [Sorghastrum nutans (L.) Nash], prairie sandreed [Calamovilfa longifolia (Hook) Scrib.], switchgrass [Panicum virgatum L.], little bluestem [Schizachyrium scoparium (Michx.) Nash.], and sideoats grama [Bouteloua curtipendula (Michx.) Torr.]). The grass monocultures are burned annually. Soils were wet sieved into four size classes (>2000, 250-2000, 53-250, and <53 µm). Unsieved soil was incubated aerobically for 30 and 387 d to examine C and net N mineralization. For tree species, aggregate weighted mean diameter (WMD) differed between species (P = 0.01), and correlated positively with fungal biomass (r = 0.56). Large macroaggregate (>2000 µm) C concentration ranged from 15 to 26 g kg-1, and was lowest for Norway spruce and red oak (P = 0.07). Aggregate WMD correlated weakly (and negatively) with potentially mineralizable N (r = −0.57) and in situ net N mineralization (r = −0.67), but positively (again weakly) with potentially mineralizable C (r = 0.49). Grass species had no effect on aggregate-size distribution or organic matter concentration in spite of twofold differences in root biomass and threefold differences in N cycling rates. Species-induced changes in soil aggregation explained little of the variation in whole-soil C and N cycling rates, and are therefore unlikely to be an important mechanism explaining species effects on ecosystem processes.
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