Grazing exclusion decreases soil organic C storage at an alpine grassland of the Qinghai-Tibetan Plateau

Grazing exclusion has been proposed as a choice for restoring degraded grasslands on the Qinghai-Tibetan Plateau, but its effects on soil properties are not clear. The present study was designed to investigate whether various soil organic carbon (OC) and nitrogen (N) pools and enzymatic activities were changed through grazing exclusion. A paddock of grassland was fenced in May 2002 for exclusion of livestock grazing, while the surrounding grassland continued conventional grazing by yak (Bos grunniens) and sheep (Ovis aries). Eight years after grazing exclusion, besides a reduction in plant species, the root biomass and soil bulk density in the top 15-cm depth were reduced by 34% and 26%, respectively, compared to the grazed grassland. Grazing exclusion enhanced the C/N ratios of shoots and roots by 18-19%, indicating a quality reduction in the shoot and root litters compared with the non-exclusion. Grazing exclusion also lowered stocks of total soil OC and N, microbial biomass C and N, and acid-extracted carbohydrate C and soil enzymatic activities (per area) of β-glucosidase, urase, and phosphatase in the 0-15 cm soil layer. Under grazing exclusion, less C input from the root-associated sources and possibly greater C output through heterotrophic respiration might have reduced various soil OC storages. However, a significant increase in soil mineral N pool was found under no grazing compared to grazing, possibly due to less plant N demand and uptake and change in N mineralization and/or immobilization. In conclusion, grazing exclusion is not beneficial to soil OC sequestration on the northeastern Qinghai-Tibetan Plateau.