Managing Global Resources for a Secure Future

Quantifying soil organic carbon (SOC) and microbial biomass in soil profile is critical for accurately predicting SOC loss or accumulation trends under emerging agronomic practices. This study was conducted in perennial pasture and annual sorghum grazing systems to evaluate SOC in soil profile layers (0-24”) under different tillage, cover crop rotations and sequences for nutrient cycling. Microbial biomass was estimated in these soil profile layers to evaluate the potential for SOC mineralization. Grazing pressure (cattle stocking rate) and nitrogen fertilization are major drivers of SOC dynamics in low fertile South Eastern soils. SOC in soil profile was significantly lower under high grazing pressure compared to low grazing pressure. Perennial and cool season grass sequence retained more SOC in soil profile compared to perennial grass and a cool season legume sequence. Interestingly, perennial grass and legume sequence retained more SOC at 12-24” depth. In annual sorghum grazing systems, cover crop rotations were most effective than no-till in conserving SOC, as sorghum rotation with grass pea plots accumulated almost 9.7 MgC/ha more SOC compared to monoculture sorghum plots. More than 60% of this accumulation was in subsurface profile at a depth of 12-24”. Fungal and bacterial biomass carbon significantly decreased by soil depth, which indicates lower mineralization potential in subsurface soil. Deep-rooted cover crop rotations have potential to rapidly increase subsurface SOC, and are an effective strategy to promote SOC addition and storage in these cropping systems.