Warming and grazing affect soil labile carbon and nitrogen pools differently in an alpine meadow of the Qinghai-Tibet Plateau in China

Purpose Small but highly bioactive labile carbon (C) and nitrogen (N) pools are of great importance in controlling terrestrial C and N fluxes, whilst long-term C and N storage is determined by less labile but relatively large sizes of C and N pools. Little information is available about the effects of global warming and grazing on different forms of C and N pools in the Qinghai-Tibet Plateau of China. The aim of this study was to investigate the effects of warming and grazing on the sizes of different soil labile C and N pools and N transformation in this region. Materials and methods A free-air temperature enhancement system in a controlled warming-grazing experiment had been implemented since May 2006. Infrared heaters were used to manipulate temperature, and a moderate grazing intensity was simulated by Tibetan sheep. After 3 years' warming, soil samples were taken from the four treatment plots no warming with no grazing; no warming with grazing; warming with no grazing; and warming with grazing. Concentrations of inorganic N in the 40-cm soil profiles were measured by a flow injection analyser. Microbial biomass C (MBC) and microbial biomass N (MBN) were measured by the fumigation-extraction method, and soluble organic C (SOC) and soluble organic N (SON) were determined by high-temperature catalytic oxidation. Total N (TN), C isotope composition (δ¹³C) and N isotope composition (δ¹⁵N) were determined using an isotope ratio mass spectrometer. Net N transformation under low temperature was studied in a laboratory incubation experiment. Results and discussion Warming and grazing treatments affected soil C and N pools differently, and these effects varied with soil depth. Warming significantly increased TN, MBC, MBN, and SON and decreased δ¹³C at the 10-20 and 20-30 cm soil depths, whilst grazing generally decreased SON at the 10-20 and 20-30 cm, and MBC at 20-30 cm. At the 0-10 cm depth, neither warming nor grazing alone affects these soil parameters significantly, indicating that there could be considerable perturbation on the soil surface. However, grazing alone increased NO₃^--N, total inorganic N, SOC and δ¹⁵N at the 0-10 cm depth. Incubated at 4°C, warming (particularly with grazing) led to net immobilization of N, but no-warming treatments led to net N mineralization, whilst nitrification was strong across all these treatments. Correlations between MBC and SOC, and TN and MBN or SON were positive. However, SON was less well correlated with TN and MBN compared with the highly positive correlations between SOC and MBC. Conclusions It is clearly demonstrated that warming and grazing affected labile C and N pools significantly, but differently after 3 years' treatments Warming tended to enlarge labile C and N pools through increased litter inputs, whilst grazing tended to increase inorganic N pools, decrease SON and accelerate N cycling. Grazing might modify the mode that warming affected soil C and N pools through its strong impacts on microbial processes and N cycling. These results suggested that interactive effects of warming and grazing on C and N pools might have significant implications for the long-term C and N storage and productivity of alpine meadow ecosystem in the Qinghai-Tibet Plateau of China.