Soil organic carbon and nitrogen storage estimated with the root-zone enrichment method under conventional and conservation land management across North Carolina

Agriculture is a globally dominating land use, so efforts to restore soil organic carbon (C) and nitrogen (N) lost through historical degradation could have enormous benefits to production and the environment, particularly by storing an organic reserve of nutrients in soil and avoiding the return of a small portion of biologically cycling C to the atmosphere. Estimates of soil organic C and N storage from conservation agricultural management are still limited when considered in proportion to the large diversity of environmental and edaphic conditions. A study was undertaken to determine the total, baseline, and root-zone enrichment stocks of soil organic C and N as affected by land use on 25 research stations distributed throughout North Carolina. Root-zone enrichment of organic matter is that portion influenced by contemporary management, and baseline is that portion dominated by pedogenesis. These fractions were compared with more traditional estimation procedures. Soil organic C and N were strongly negatively associated with sand concentration. Although physiographic region influenced overall soil C and N contents, variations in soil type and research station management within a region were equally influential. Soil organic C and N stocks were strongly affected by land use, which did not interact with the soil textural effect. Across the 25 research station locations, root-zone enrichment of soil organic C followed the order (p < 0.01) conventional-till cropland (11.1 Mg C ha−1) < no-till cropland (21.5 Mg C ha−1) < grassland (29.6 Mg C ha−1) < woodland (38.6 Mg C ha−1). Root-zone enrichment of total soil N followed a similar order, except grassland and woodland effects were reversed. Root-zone enrichment provided an integrated soil-profile assessment and a more targeted response of soil organic C and N change than did more traditional paired land use approaches, primarily due to separation of a variable pedogenic influence among sites. These point-in-time results gave a clear indication that conservation agricultural management approaches will foster surface soil organic C and N restoration across a diversity of soil types in the southeastern United States.