Effects of grazing on carbon and nitrogen in plants and soils in a semiarid desert grassland, China

Grazing can modulate the feedback between vegetation and soil nutrient dynamics (carbon and nitrogen), altering the cycles of these elements in grassland ecosystems. For clarifying the impact of grazing on the C and N in plants and soils in the desert grassland of Ningxia, China, we examined the plant biomass, SOC (soil organic carbon), total soil N and stable isotope signatures of plants and soils from both the grazed and ungrazed sites. Significantly lower aboveground biomass, root biomass, litter biomass and vegetation coverage were found in the grazed site compared to the ungrazed site, with decreases of 42.0%, 16.2%, 59.4% and 30.0%, respectively. The effects of grazing on plant carbon, nitrogen, δ15N and δ13C values were uniform among species. The levels of plant carbon and nitrogen in grasses were greater than those in the forbs (except for the carbon of Cynanchum komarovii and Euphorbia esula). Root 15N and 13C values increased with grazing, while the responses of root carbon and nitrogen to grazing showed no consistent patterns. Root 15N and 13C were increased by 79.0% and 22.4% in the grazed site compared to the ungrazed site, respectively. The values of SOC and total N were significantly lower in the grazed than in the ungrazed sites for all sampling depths (0–10 and 10–20 cm), and values of SOC and total N at the surface (0–10 cm) were lower than those in the deeper soils (10–20 cm). Soil δ15N values were not affected by grazing at any sampling depth, whereas soil δ13C values were significantly affected by grazing and increased by 19.3% and 8.6% in the soils at 0–10 and 10–20 cm, respectively. The soil δ13C values (−8.3‰ to −6.7‰) were higher than those for roots (−20.2‰ to −15.6‰) and plant tissues (−27.9‰ to −13.3‰). Our study suggests that grazing could greatly affect soil organic carbon and nitrogen in contrast to ungrazed grassland and that grazing appears to exert a negative effect on soil carbon and nitrogen in desert grassland.