Alpine meadow degradation, usually involving decreased soil N and patchy landscapes, is challenging for natural restoration. However, the mechanism underlying plant species coexistence during degradation is unclear. In this study, we evaluated plant N niche complementarity in degraded alpine meadows by a 15N-labeling (15NO3-, 15NH4+ and 15N-glycine) experiment. At the community level, the degraded alpine meadow showed larger root and all plant 15N concentrations and preferred glycine over NO3- compared with the undegraded alpine meadow. At the species level, dominant species in the undegraded alpine meadow consistently preferred NO3-. For the degraded alpine meadow, generalist species, common to both meadows, showed diverse preferences, while unique species generally preferred glycine, among which the uneven distribution could reduce glycine competition. We observed that differentiation in N sources and the uneven distribution of unique species may explain the stability of degraded alpine meadows. Our results suggested that plant spatial distribution could be powerful for community stability and emphasized the importance of considering fine-scale perspectives in studies of niche theory. This study have important implication for restoration of degraded alpine meadows.

The impacts of human-driven environmental changes on the stability of natural grasslands have been assessed by comparing differences between manipulative warming and grazing plots and reference plots. However, little is known about whether or how ambient climate regulates the effects of manipulative treatments. A 36-year observational dataset shows that there is a nonlinear response of community stability to ambient climate. Manipulative warming and grazing decrease community stability with experiment duration through an increase in legume coverage and/or decrease in species asynchrony, due to exceeding the threshold of background annual mean air temperature with decreasing background annual mean air temperature through time during the 10-year experiment period. Moreover, the temperature sensitivity of community stability is more sensitive under the ambient treatment than under the manipulative treatments. Therefore, our study emphasizes the importance of the context dependency of the response of community stability to human-driven environmental changes.

The cultivation of artificial grassland, which requires regular sowing and harvesting, is an important and common practice in grassland management and restoration on the Qinghai-Tibetan Plateau (QTP). However, the key factors and successional processes that determine its interannual variability of net ecosystem exchange (NEE) remains unclear. In this study, we analyzed 6 years of eddy covariance observations, quantified seasonal and annual NEE, net carbon uptake period length (CUP), two largest carbon emission periods length (CEPs), and daily minimum and maximum NEE (NEEmin, NEEmax) responses to management measures in an alley silvopasture system with planted Elymus nutans on the QTP. We examined NEE variations before and after sowing and harvesting and investigated how sowing and harvesting managements affect NEE. The five-year life cycle study revealed that this artificial perennial grassland was a weak net carbon sink with an uptake of -180 g C m–2 per plant cycle (2012–2016). Inter-annual variations in NEE was determined by CUP and the first largest carbon emission periods (CEP1). Sowing delayed the beginning of CUP (BCUP) by about 42 days and caused spring C uptake to decline by -36 g C m–2. In contrast, harvesting caused the ending of CUP (ECUP) to advance about 25 days, and caused a decease of autumn C uptake about -33 g C m–2. Sowing and harvesting shortened the CUP and extended the CEP1, which reduces the carbon sequestration potential of the artificial grassland. Based on our results, we recommend that policy makers and authorities consider the effect of timing of sowing and harvesting on C sequestration. Our results provide a new insight for grassland management and the inter-annual variability of NEE on climate mitigation and regional sustainability.