Comparison of soil carbon and nitrogen stocks at grazing-excluded and yak grazed alpine meadow sites in Qinghai–Tibetan Plateau, China

Abstract

The effects of yak grazing on C and N below-ground pools were evaluated by comparing five grazing-excluded sample site pairs (5 years of grazing exclusion) at the Eastern edge of the Qinghai–Tibetan Plateau. Soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), bulk density, pH and soil particle size fractions were analyzed in these samples taken at a depth of 10cm. Our results showed TK stock and bulk density had increasing trends with grazing activity. pH presented a slight increase, but the differences were not significant. On the contrary, SOC, TSN and TP stocks declined in grazing plots (P<0.05) and respectively decreased by 32.9%, 27.4%, and 10.5%, while TK stocks increased by 6.8%. P may become limited elements over time owing to an imbalance of inputs and outputs. Elemental stocks calculated by the equivalent mass method indicated the reported SOC losses based on the conventional method might be overestimated and that sampling depth of 7.64cm rather than same with sampling depth for grazing-excluded site (10cm) should be applied for grazing site if conventional method is used to calculate carbon stock. The soil silt content in grazing-excluded sites was greater than that in grazed zones (P<0.05), whilst greater soil specific surface areas were also found in the grazing-excluded sites, indicating that manual fencing led to heterogeneous distribution of soil particles and those silts may play a primary role in nutrient stocks in this region.