Meta-analysis of the effects of grassland degradation on plant and soil properties in the alpine meadows of the Qinghai-Tibetan Plateau

Abstract

Alpine meadows on the Qinghai-Tibetan plateau (QTP) are particularly important for both ecosystem functioning and pastoral livelihoods, but they have been severely degraded in the last several decades. There have been numerous studies on the responses of plants and soils to alpine meadow degradation across the region, but they are mostly focused on single sites and are incapable of determining a general response pattern of plants and soils to degradation on the QTP. A meta-analysis including 61 individual published studies was conducted to examine the biotic and abiotic characteristics in slightly (LDAM), moderately (MDAM), severely (SDAM), and very severely degraded alpine meadows (VDAM). The results showed that the aboveground biomass (AGB) significantly decreased (P < 0.01) by 23.17%, 31.85%, 47.24%, and 66.66% in the LDAM, MDAM, SDAM, and VDAM compared with that in nondegraded alpine meadows (NDAM), respectively. The belowground biomass (BGB) did not change in the LDAM but began to decrease with the MDAM. The AGBs of graminoids significantly decreased in all degraded-level meadows, and sedges significantly decreased from MDAM to VDAM, whereas the AGBs of forbs significantly increased except in the VDAM. The Shannon-wiener index did not change until the VDAM. Soil organic matter (SOM) declined starting with the LDAM, and the magnitude of the decrease increased with degradation severity. The change in total nitrogen (TN) was similar to the change in the SOM. Significant available nitrogen (AN) decrease began with the MDAM and significant (P < 0.05) available potassium (AK) decrease only occurred at depth of 0.0–0.1 m in the SDAM and the VDAM. Soil moisture (SM) showed a significant decrease, whereas soil bulk density (BD) increased in the degraded alpine meadow. The response of AGB was significantly correlated with the changes in SOM and TN only in the SDAM and VDAM, whereas the response of BGB was significantly correlated with the changes in SOM and TN for all degraded stages. In the LDAM and MDAM, SOM and TN mainly came from the dense rooting of the turf layer, and the changes of SOM and TN thus had no correlation with AGB. However, in the SDAM and VDAM, SOM and TN mainly came from AGB and BGB, and the changes of SOM and TN were thus significantly correlated with AGB and BGB. Our results indicate that biological and biochemical processes regulate the plant and soil changes in LDAM and MDAM and that physical processes such as water and wind erosion might be responsible for the changes in plants and soils in SDAM and VDAM. Therefore, restoration strategies should be based on the processes and stages of alpine meadow degradation.