Does grazing of cover crops impact biologically active soil carbon and nitrogen fractions under inversion or no tillage management?

Abstract

Cover crops are a key component of conservation cropping systems. They can also be a key component of integrated crop-livestock systems by offering high-quality forage during short periods between cash crops. The impact of cattle grazing on biologically active soil carbon (C) and nitrogen (N) fractions has not received much attention. We investigated the impacts of tillage (conventional disk and no tillage) and cover crop management (ungrazed and grazed) on biologically active soil C and N fractions from biennial sampling during seven years of continuous management. Soil microbial biomass C was unaffected by cover crop management under conventional tillage, but was enhanced with grazing compared with no grazing under no tillage at a depth of 0 to 6 cm (0 to 2.4 in), as well as at 0 to 30 cm (0 to 12 in). The same effect occurred for the flush of carbon dioxide (CO2) following rewetting of dried soil during 3 days of incubation at a depth of 0 to 6 cm only, while it occurred for cumulative C mineralization during 24 days of incubation at a depth of 0 to 30 cm only. Grazing effects on net N mineralization during 24 days of incubation and residual soil inorganic N were nonexistent. All biologically active fractions of soil C and N were highly stratified with depth under no tillage and less so under conventional tillage. Cumulative stocks of soil C and N fractions to a depth of 0 to 30 cm were generally not significantly different between cover crop management systems, nor between tillage systems, except for (1) lower soil microbial biomass C with than without grazing under conventional tillage, (2) greater soil microbial biomass C with than without grazing under no tillage, and (3) lower cumulative C mineralization during 24 days under no tillage than under conventional tillage. Grazing of cover crops can be recommended as a strategy to promote greater adoption of cover cropping throughout the southeastern United States.