Soil carbon and nitrogen stocks in nitrogen-fertilized grass and legume-grass forage systems

Soils under grasslands contribute significantly to terrestrial carbon (C) and nitrogen (N) pools, and grassland management practices directly impact pool size through effects on plant biomass. Our objective was to quantify above and belowground plant biomass and soil C and N accumulation for year-round forage systems based on N-fertilized grasses (GN; 290 kg N ha(-1) yr(-1)) or legume-grass mixtures (LG; 30 kg N ha(-1) yr(-1)) when grazed or harvested for hay. Soil C and N accumulation (0-0.4 m) were evaluated before and 6 yr after imposing treatments. Average root-rhizome mass did not differ for GN and LG systems (3690 and 3020 kg OM ha(-1), respectively). Aboveground residual biomass was 55% greater for GN than LG (4890 vs. 3150 kg OM ha(-1), respectively) and 46% greater for grazed than hayed swards (4770 vs. 3270 kg OM ha(-1), respectively) at the end of the warm season. Aboveground C and N pools were 70 and 58% greater for GN than LG, respectively. After 6 yr, soil C and N concentration (0-0.1 m) increased 6.7 and 0.8 g C kg(-1) and 0.6 and 0.2 g N kg(-1), for GN and LG, respectively. Annual C accumulation (0-0.1 m) was 1.3 and 0.2 Mg C ha(-1) for GN and LG systems, respectively. Thus, a year-round forage system based on N-fertilized grasses accumulated more plant biomass and surface soil C and N than a legume-grass mixture, with the grass system advantaged by presence of a perennial C4 species and application of moderate to high N fertilizer rates.