Short-term impacts of high levels of nitrogen fertilization on soil carbon dynamics in a tropical pasture

Bermudagrass (Cynodon spp.) is widely recognized by its ability to respond positively to nitrogen (N) fertilization. Nitrogen often promotes shoot and root growth and, consequently, increase carbon (C) inputs into the soil via above- and below-ground biomass. The objective of this 2-yr field study was to assess the short-term effects of increased N levels on soil C dynamics of a Tifton 85 bermudagrass pasture grown in a tropical soil. Treatments consisted of five levels of N fertilization (0, 60, 120, 180 and 240 kg ha(-1) N) applied at each harvest for a total of four harvest events per year. Soil samples were collected at the 0 to 0.1 and 0.1 to 0.2 m depths for determination of soil C content. The latter was also fractionated into free-light (FLF), particulate organic carbon (POC), and mineral-associated organic carbon (C-min) fractions. Results indicated that N levels showed no effect on total C content and distribution among the various size-density fractions. The top layer (0-0.1 m) was more sensitive to changes in soil C for more recalcitrant fractions (POC and C-min) relative to 0.1-0.2m layer. Our findings suggest that acceleration of the decomposition rate induced by N fertilization may have offset the potential accumulation of soil C associated with high inputs of above- and below-ground biomass, but additional research is warranted to better elucidate long-term impacts of nitrogen fertilization on soil C dynamics in grassland of tropical regions.