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.
Contribution of the Rocky Mountain Forest and Range Exp. Stn. and Dep. of Forest Sciences, Colorado State Univ., Fort Collins, CO 80523