Soil Organic Carbon and Nitrogen Dynamics of Year-Round Legume- or Nitrogen-Fertilized Grass-Based Forage Systems Defoliated By Grazing or Clipping.

Conversion from annual row cropping to pastureland enhances soil C and N accumulation due to greater belowground mass, tissue turnover, and nutrient cycling, but defoliation management and species composition can influence their dynamics. We hypothesized that soil C and N accumulation increase under grazing and legume inclusion relative to haying or N-fertilized grass systems. The objective was to determine total soil organic C (SOC) and N (N) dynamics of year-round legume- and N-fertilized grass-based forage systems, defoliated by grazing or haying for 5 yr following conversion from row cropping. Treatments were all combinations of two forage systems and two defoliation methods replicated three times. The warm-season component was ‘Florigraze’ rhizoma peanut (Arachis glabrata) for the legume system and ‘Tifton-85’ bermudagrass (Cynodon spp.) for the grass system. Both systems were overseeded in fall with rye (Secale cereale) and ryegrass (Lolium multiflorum), and the legume system also was overseeded with 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. Prior to establishment and after five complete cycles of the systems, soil was sampled in 0-10, 10-20, 20-40, 40-70, and 70-100 cm layers. Samples were dried and ground in a ball mill before analysis. Aboveground plant residual biomass and litter and root-rhizome biomass were measured. All plant tissue samples were air-dried, weighed, and ground to pass a 1-mm screen. Root-rhizome and soil subsamples were analyzed by dry combustion for C and N. Root-rhizome and residual biomass were greater on grass-N systems and under grazing than on legume-based systems or for hay defoliation. These results support the conclusion that defoliation management and species influence SOC and N dynamics after land conversion.