Root-zone enrichment of soil organic carbon and nitrogen under grazing and other land uses in a humid-temperate region

Grazing lands can be considered a conservation land use providing value to farmers and society. An underappreciated value of these lands may be in C and N storage and cycling. However, soil organic C (SOC) and total soil N (TSN) storage under humid-temperate zone pastures have not been well characterized. A survey of 31 farms in Virginia USA characterized SOC and TSN depth distributions under a diversity of grassland management scenarios, as well as in comparison with long-term no-till cropland and woodland uses on the same farms. Root-zone enrichment calculations separated management-controlled SOC and TSN stocks from a baseline condition characterized at 30-cm depth. Total stock of SOC at 0–30-cm depth varied from 46 to 88 Mg C ha⁻¹ (5%–95% range from 304 soil profiles) across all land uses. Root-zone enrichment of SOC was maximized under mature pastures (≥20 years old) at 38.3 ± 1.6 Mg C ha⁻¹, which was not different from that under woodland (39.8 ± 1.2 Mg C ha⁻¹), but was greater than under no-till cropland (28.3 ± 1.3 Mg C ha⁻¹) and conventional-till cropland (15.1 ± 5.1 Mg C ha⁻¹). Root-zone enrichment of TSN was optimized at stocking rate of ~1 Mg live weight ha⁻¹, but was not affected by stocking method, N fertilization history, or low levels of hay feeding. These results suggest that grazed pastures in Virginia are storing significant amounts of SOC and TSN, with at least half due to management-induced compared with pedogenic-controlled accumulation.