Soil organic matter dynamics in a North America tallgrass prairie after 9 yr of experimental warming

The influence of global warming on soil organic matter (SOM) dynamics in terrestrial ecosystems remains unclear. In this study, we combined soil fractionation with isotope analyses to examine SOM dynamics after nine years of experimental warming in a North America tallgrass prairie. Soil samples from the control plots and the warmed plots were separated into four aggregate sizes (>2000 mu m, 250-2000 mu m, 53-250 mu m, and <53 mu m), and three density fractions (free light fraction - LF, intra-aggregate particulate organic matter - iPOM, and mineral-associated organic matter - mSOM). All fractions were analyzed for their carbon (C) and nitrogen (N) content, and delta(13)C and delta(15)N values. Warming did not significantly effect soil aggregate distribution and stability but increased C(4)-derived C input into all fractions with the greatest in LF. Warming also stimulated decay rates of C in whole soil and all aggregate sizes. C in LF turned over faster than that in iPOM in the warmed soils. The delta(15)N values of soil fractions were more enriched in the warmed soils than those in the control, indicating that warming accelerated loss of soil N. The delta(15)N values changed from low to high, while C: N ratios changed from high to low in the order LF, iPOM, and mSOM due to increased degree of decomposition and mineral association. Overall, warming increased the input of C(4)-derived C by 11.6 %, which was offset by the accelerated loss of soil C. Our results suggest that global warming simultaneously stimulates C input via shift in species composition and decomposition of SOM, resulting in negligible net change in soil C.