Soil Carbon Dynamics and Aggregate Size Fractions of Legume- or Grass-N-Based Forage-Livestock Systems Defoliated By Grazing or Clipping.

Defoliation management and grassland species composition can influence nutrient cycling and soil C dynamics. We hypothesized that grazing, instead of haying, and inclusion of legumes, versus grass-N systems, would increase soil C stocks. The objective of this study was to determine C and N soil pools in legume- and grass-based, year-round production systems that were defoliated either by grazing or haying during four years. The four treatments were replicated three times. The legume system included the warm-season perennial ‘Florigraze’ rhizoma peanut (RP, Arachis glabrata Benth.), and the grass-N system included ‘Tifton-85’ bermudagrass (BG, Cynodon spp.). Both were overseeded with the cool-season forages rye (Secale cereale) and ryegrass (Lolium multiflorum) during winter, with the legume system also including crimson (Trifolium incarnatum) and red clovers (T. pratense). Grass-N plots received 50 and 30 kg N ha^-1 after each summer and winter defoliation event, respectively. After four complete cycles of the systems, soil was sampled from 0 to 10 cm depth, air dried, and sieved through a 2-mm screen. Subsamples were wet sieved to obtain three aggregate fractions: 1) 250 to 2000 µm, 2) 53 to 250 µm, and 3) < 53µm. Each aggregate size fraction was dried at 65°C and analyzed for C and N. Aggregate Fractions 1 and 2 were separated into light and heavy C fractions. Initial results show greater increase of soil organic carbon (SOC) in grass-N relative to legume-based systems. Grazed treatments also showed greater SOC and N increase compared with haying systems. Results support the conclusion that plant species and defoliation management influence soil C and N dynamics of grasslands.