| Livestock grazing is one of main using patterns in grassland ecosystem. Irrational grazing not only threatens the biodiversity and stability of grasslands, but also alters ecosystem structure and functioning, leading to increased C and N losses. However, recently, the extent in which grazing, affects C and N cycling, especially belowground process in global grasslands remains unclear, although numerous individual studies have been conducted. In this meta-analysis study, after reviewed more than 2500 published paper about the effects of grazing on C and N cycling in grassland ecosystem, we extracted data from 105 independently papers that focused this issue and then established the analysis database. The compiled database included 21 variables which contained aboveground C and N pools, belowground C and N pools [i.e., soil C and N, litter C and N, root C and N, microbial C and N], C:N ratio[i.e., soil C:N(SCN), root C:N(RCN), litter C:N(LCN), microbial C:N ratio(MCN)], fluxes [i.e., soil respiration(Rs), soil net N mineralization(SNNM), and soil net N nitrification(SNNN)], and related soil parameters such as soil bulk density(BD), soil moisture(SM), soil temperature(ST), soil pH, and soil depth. Based on the database and this analysis method, we further analyzed and revealed the general patterns of C and N responses to livestock grazing from field experiments.On average, grazing significantly decreased C pools in bulk soils, belowground plant mass, soil microbes, and litter by 10.28, 13.72, 21.62 and 8.93%, respectively, and N pools by 13.38, 4.40, 24.40 and 10.39%, respectively. Among these pools, microbial C and N pools showed the largest decreased responses to grazing compared to other variables. In contrast, soil and root C:N ratios increased under grazing whereas microbial and litter C:N ratios decreased under grazing compared to control. Grazing also significantly increased soil respiration(Rs), soil net N mineralization, and soil net N nitrification by 4.25, 30.63 and 12.88%, respectively. In addition, grazing increased soil bulk density(BD), soil pH and temperature but decreased soil moisture.Grazing intensity affected belowground C and N cycles with different magnitudes and even directions. Light grazing increased soil C pools(SCP) and soil N pools(SNP) by 0.78% and 3.24%, respectively. However, moderate and heavy grazing significantly decreased SCP by 3.45% and 9.92% and SNP by 8.41% and 13.04%, respectively. Similarly, light grazing increased belowground plant C pool(BPCP) and litter C:N ratio(LCN) by 2.99% and 10.14%, respectively, while moderate and heavy grazing decrease BPCP by 3.17% and 24.1% and L CN by 22.61% and 30.18%, respectively. However, microbial biomass N(MBN) and litter N pool(LNP) showed the larger decreases in response to light grazing compared with those to moderate and heavy grazing. For belowground C and N fluxes, Rs increased by 11.53% under light intensity, whereas moderate and heavy intensities decreased Rs by 12.7% and 32.6%, respectively. The weighted response ratios of SNNM decreased by 48.87% to 10.85% from light to heavy grazing intensities. However, light grazing did not affect the response ratios of SNNN, but moderate and heavy grazing intensities significantly increased by 13.43% and 103.06%, respectively.Our meta-analysis also showed that both biotic and abiotic factors significantly influence the carbon and nitrogen cycling in response to grazing. Grazing disturbance in semi-humid/humid regions had decreased SCP, BPCP, SNP and BPNP greater than those of arid/semi-arid regions. Similarly, the weighted response ratio Rs [RR++(Rs)] in semi-humid/humid regions was 0.099±0.023(P < 0.01), which was slightly higher than these in arid/semi-arid regions. However, MBC and LCP exhibited the larger negative effects in response to grazing under arid/semi-arid regions compared to those under semi-humid/humid regions. Different livestock type showed different magnitudes of changes(even direction) for many of the considered variables. Grazing duration and MAT displayed significant correlations with RR(SCP) and RR(SNP). The MAP exhibited a significant correlation with the response of SCP, but it was not correlated with response of SNP to grazing. In addition, the responses of SCP exhibited a significant positive correlation with RR(SNP).In conclusion, grazing activist significantly influenced the C and N cycling in grassland ecosystem, as well as altered the distribution pattern of C and N between above and belowground at global level. Grazing intensity affected C and N pools with different magnitudes and even directions in response to livestock grazing. Future land-surface models may need to differentially treat with grazing intensity and climate factors in order to develop more precise process-based mechanism for forecasting the feedback of grassland ecosystems to climate change. |
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