Belowground biomass characteristics and soil organic C accumulation of bahiagrass‐based forage systems with or without legumes

Abstract Grasslands are important terrestrial ecosystems that provide feed for livestock, support soil conservation and nutrient cycling, and store soil carbon (C). In many grazed grasslands, restricted nitrogen (N) inputs from fertilizers and minimal legume presence may constrain belowground biomass accumulation and soil C storage because of limited N. Our objective was to compare root–rhizome biomass and composition, root accumulation, and soil organic C (SOC) stocks for three grazed, year‐round forage systems based on bahiagrass ( Paspalum notatum Flügge). Systems included a bahiagrass monoculture during summer, overseeded with cool‐season grasses for winter, and receiving 224 kg N ha −1 year −1 (BG‐N [bahiagrass during the warm season overseeded with ryegrass‐oats mixtures during the cool season, with 224 kg N ha −1 divided equally in cool and warm seasons]); a bahiagrass monoculture during summer, overseeded with a cool‐season grass–legume mixture for winter, and receiving 34 kg N ha −1 year −1 (BG‐CL [bahiagrass (no N‐fertilized) during the warm season overseeded with ryegrass–oats–clovers mixture (34 kg N ha −1 ) during the cool season]); and a bahiagrass–rhizoma peanut ( Arachis glabrata Benth.) mixture during summer, overseeded with a cool‐season grass–legume mixture for winter, and receiving 34 kg N ha −1 year −1 (BG‐RP [bahiagrass and rhizoma peanut (no N‐fertilized) during the warm season overseeded with ryegrass–oats–clovers mixture (34 kg N ha −1 ) during the cool season]). This study assessed belowground responses and estimated decomposition dynamics during 2 years in a long‐term grazing system. During summer, BG‐RP had greater root–rhizome biomass (9770 kg OM ha −1 [where OM stands for organic matter]) than BG‐N (6230 kg OM ha −1 ) and BG‐CL (7320 kg OM ha −1 ). Greater root–rhizome mass in BG‐RP increased N (35%) and C (38%) stocks compared with BG‐N, but root–rhizome accumulation rate was not different among forage systems (14.5–15.9 and 15.6–22.6 kg OM ha −1 day −1 during cool and warm seasons, respectively). Nine years after imposing forage systems, SOC stocks (0–90 cm) had increased and were similar (104 Mg ha −1 ) across forage systems. Integrating legumes with limited N fertilizer application for 9 years resulted in similar SOC as heavily N‐fertilized, grass‐only systems.