SOIL AGGREGATION AND ORGANIC-MATTER MINERALIZATION IN FORESTS AND GRASSLANDS - PLANT-SPECIES EFFECTS

Plant-soil feedbacks can alter N cycling rates in terrestrial ecosyste ms, but the mechanistic relationship between species characteristics, soil properties, and N dynamics is unclear. Plant species may affect p atterns of soil aggregation, which can affect soil C and net N mineral ization. This mechanism was examined in two common garden experiments: one containing five tree species (European larch [Larix decidua Mille r], red oak [Quercus rubra L.], red pine [Pinus resinosa Ait.], white pine [Pinus strobus L,], and Norway spruce [Picea abies (L.) Karst]) a nd 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 side oats grama [Bouteloua curtipendula (Michx.) Torr.]), The grass monocul tures are burned annually. Soils were wet sieved into four size classe s (>2000, 250-2000, 53-250, and <53 mu m). Unsieved soil was incubated aerobically for 30 and 387 d to examine C and net N mineralization. F or tree species, aggregate weighted mean diameter (WMD) differed betwe en species (P = 0.01), and correlated positively with fungal biomass ( r = 0.56). Large macro-aggregate (>2000 pm) C concentration ranged fro m 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 potentia lly 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 distributio n or organic matter concentration in spite of twofold differences in r oot biomass and threefold differences in N cycling rates. Species-indu ced changes in soil aggregation explained little of the variation in w hole-soil C and N cycling rates, and are therefore unlikely to be an i mportant mechanism explaining species effects on ecosystem processes.