Temporal Dynamics of Total and Particulate Organic Carbon and Nitrogen in Cover Crop Grazed Cropping Systems

Soil organic C and N are important indicators of agricultural sustainability, yet numerous field studies have revealed a multitude of responses in the extent and rate of change imposed by conservation management and, therefore, a lack of clarity on responses. We conducted an evaluation of total and particulate organic C and N in the surface 30 cm on a Typic Kanhapludult in northern Georgia during 7 yr of tillage (conventional disk and no tillage) and cover crop utilization (ungrazed and grazed by cattle). Soil organic C and total soil N were greater under no tillage (NT) than under conventional tillage (CT) at depths of 0 to 3 and 3 to 6 cm but were lower under NT than CT at depths of 12 to 20 and 20 to 30 cm. Total soil N accumulated with time at a depth of 0 to 6 cm under both tillage systems and the rate tended to be greater under NT than under CT (0.039 vs. 0.021 Mg N ha(-1) yr(-1), p = 0.10). Soil organic C accumulated with time at a depth of 0 to 6 cm under all management systems, but there was a significant tillage x cover crop interaction (0.68 and 1.09 Mg C ha(-1) yr(-1) with ungrazed cover crop management under CT and NT, respectively, and 0.84 and 0.66 Mg C ha(-1) yr(-1) with grazed cover crop management under CT and NT, respectively). At a depth of 0 to 30 cm, there was only a tillage trend (1.00 and 1.59 Mg C ha(-1) yr(-1) under CT and NT, respectively; p = 0.09). Particulate organic C was more dramatically different than soil organic C between tillage regimes at a depth of 0 to 30 cm (-0.49 and 0.35 Mg C ha(-1) yr(-1) under CT and NT, respectively; p < 0.001). Grazing of cover crops had little negative impact on soil C and N fractions, suggesting that NT and grazing of cover crops could provide a broader-spectrum conservation cropping approach in the southeastern United States.