Ecosystem C Responses to Nitrogen Additions in Subtropical Grasslands.

Human activities have increased the availability of nutrients, particularly nitrogen (N), across many terrestrial ecosystems worldwide. Given the regulatory impacts of resources and consumers on net primary productivity and subsequent carbon inputs to the soil, disturbances caused by anthropogenic N additions can have major impacts on how C is cycled in grassland ecosystems. The objective of this study was to investigate the impacts of continuous N additions on soil organic C (SOC) stocks and characteristics. Experimental sites consisted of bahiagrass (Paspalum notatum) pastures that have been receiving annual applications of 0, 60, and 120 kg N ha^-1 during the past 6 years. Five composite soil core samples were taken from the 0 to 15 cm and 15 to 30 cm soil depths. Soil C fractionation was performed using the dry sieving method described by Six et al. (1998). Carbon and nitrogen concentrations were determined in the bulk soil and size/density fractions. Nitrogen additions showed no effects on total SOC and N stocks. At the 0 to 10 cm depth, SOC associated with the microaggregate (250 to 53 μm) fraction increased linearly as N level increased (9.6 g C kg soil^-1 for treatments receiving no N vs. 12.1 g kg^-1 soil for treatments receiving 120 kg ha^-1 yr^-1) while in the silt plus clay fraction (< 53 μm) SOC decreased linearly in response to greater N levels (2.2 vs. 1.1 g C kg^-1 soil for 0 and 120 kg N ha^-1 yr^-1 application levels, respectively). Our results suggest that N additions affect the form by which C is stored in coarse-textured soils.