Effect of grazing intensity on carbon and nitrogen in soil and vegetation in a meadow steppe in Inner Mongolia

The effects of grazing on grassland ecosystems must be understood to develop effective soil conservation measures and sustain livestock production. The objective of this research was to characterize the impact of dairy cow grazing intensity on C and N in soil and vegetation on a meadow steppe. The meadow steppe site, located near Hailaer, Inner Mongolia, China, has been grazed by dairy cattle for more than 30 years, with the last 23 years managed by the same herders. The study site never received any form of mineral fertilizer. Grazing intensity was classified as light (LG), moderate (MG) and heavy (HG) according to the vegetation utilization rate. Grazing affected biomass, total N (TN) and 15N levels, but not 13C, of above-ground plants. The above-ground biomass and δ15N decreased but TN content increased with the grazing intensity for all nine non-legume species while five legume species were not affected. Coarse root biomass also decreased with grazing intensity while TN or δ15N responses to grazing showed no consistent patterns. Soil OC and total N content decreased significantly with grazing intensity for all three sampling depths (0–10, 10–20 and 20–30 cm) while nitrate, ammonium and 13C levels were not affected by grazing intensity in all sampling depths. The decreases in soil OC suggest carbon storage in the meadow steppe declined as grazing intensity increased. Combined with the decreases in soil TN content, this also means heavy grazing could lead to decreases in soil quality and fertility. Uneven grazing intensity is an inherent feature of a season-long grazing system and there are risks to soil quality and environmental health associated with HG. We need to find the proper balance between light to heavy grazing intensity through proper grazing management and in some cases using judicious herding to distribute livestock. Some combination of HG, MG and LG could be desirable in sustainable rangeland grazing production.