Soil biological activity and nitrogen mineralization in grass and grass–legume forage systems

Abstract Pasture systems can significantly impact soil carbon (C) and nitrogen (N) mineralization by influencing organic inputs, root turnover, and microbial processes. This study evaluated soil biological activity (SBA), soil C, and N mineralization in long‐term N‐fertilized grass or grass–legume grazing systems. Soil samples (0‐ to 10‐cm depth) were collected in 2022 from 10‐year grazing systems at the University of Florida's North Florida Research and Education Center in Marianna, FL. Three grazing systems were evaluated: N‐fertilized bahiagrass ( Paspalum notatum Flüggé) during summer and overseeded with N‐fertilized ryegrass ( Lolium multiflorum Lam ) and oat ( Avena sativa L.) during winter (BGN); unfertilized bahiagrass overseeded with ryegrass, oat, and a mixture of clovers ( Trifolium spp.; BG); and bahiagrass‐rhizoma peanut (RP; Arachis glabrata Benth.) mixture overseeded with the same ryegrass–oat–clover mixture as for BG (BGRP). A 24‐day soil incubation was conducted to evaluate SBA, C mineralization, inorganic N, microbial biomass carbon (MBC), C and N stocks, and δ 13 C from CO 2 . On average, BGN and BG had 12% greater SBA than BGRP. Carbon and N stocks were 32% greater in BGN compared with BGRP, although the proportion of total C mineralized did not differ among treatments. Nitrification was 34% greater in BGN than in BGRP. These results indicate a tradeoff between reduced synthetic N inputs and lower soil C and N accumulation in grass–legume systems. However, despite lower C and N stocks, the BGRP system maintained comparable relative C mineralization and exhibited lower inorganic N availability, suggesting a lower nitrogen loss risk.