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Nitrogen enrichment impacts ecosystems productivity by altering different hierarchical community processes, ranging from responses of individual species traits to species reordering and ultimately species turnover. Most previous studies have focussed on examining the responses of these processes separately, but little has been done to examine how hierarchical community responses are linked and contribute to the changes in productivity.To fill this knowledge gap, we conducted a 4‐year nitrogen addition field experiment to investigate the hierarchical community responses of above‐ground net primary productivity (ANPP) in an alpine meadow ecosystem.We found that the individual species response contributed more to ANPP changes under nitrogen addition, while species reordering and species turnover contributed little over the 4‐year experiment. In particular, plant height was a key trait to ultimately explain and bridge nitrogen addition effects on ANPP across different community levels. Nitrogen addition promoted plant height and growth of individual species, and reordered functional groups by increasing taller grasses and sedges and decreasing shorter forbs and legumes. Species turnover had a weak effect on ANPP changes due to low biomass of both lost and gained species. Finally, taller persistent plants promoted ANPP more with nitrogen addition.Synthesis. This study provides a new perspective on how ecosystem productivity responds to nitrogen enrichment in a hierarchical manner linked to plant height, which is helpful for elucidating complex community processes and establishing direct links between plant traits and ecosystem function. [ABSTRACT FROM AUTHOR]

Global hydroclimatic variability is increasing with more frequent extreme dry and wet years, severely destabilizing terrestrial ecosystem productivity. However, what regulates the consequence of precipitation extremes on productivity remains unclear. Based on a 9‐year field manipulation experiment on the Qinghai–Tibetan Plateau, we found that the responses of gross primary productivity (GPP) to extreme drought and wetness were differentially regulated by nitrogen (N) deposition. Over increasing N deposition, extreme dry events reduced GPP more. Among the 12 biotic and abiotic factors examined, this was mostly explained by the increased plant canopy height and proportion of drought‐sensitive species under N deposition, making photosynthesis more sensitive to hydraulic stress. While extreme wet events increased GPP, their effect did not shift over N deposition. These site observations were complemented by a global synthesis derived from the GOSIF GPP dataset, which showed that GPP sensitivity to extreme drought was larger in ecosystems with higher N deposition, but GPP sensitivity to extreme wetness did not change with N deposition. Our findings indicate that intensified hydroclimatic variability would lead to a greater loss of land carbon sinks in the context of increasing N deposition, due to that GPP losses during extreme dry years are more pronounced, yet without a synchronous increase in GPP gains during extreme wet years. The study implies that the conservation and management against climate extremes merit particular attention in ecosystems subject to N deposition. [ABSTRACT FROM AUTHOR]

Determination of tipping points in nitrogen (N) isotope (δ15N) natural abundance, especially soil δ15N, with increasing aridity, is critical for estimating N‐cycling dynamics and N limitation in terrestrial ecosystems. However, whether there are linear or nonlinear responses of soil δ15N to increases in aridity and if these responses correspond well with soil N cycling remains largely unknown. In this study, we investigated soil δ15N and soil N‐cycling characteristics in both topsoil and subsoil layers along a drought gradient across a 3000‐km transect of drylands on the Qinghai–Tibetan Plateau. We found that the effect of increasing aridity on soil δ15N values shifted from negative to positive with thresholds at aridity index (AI) = 0.27 and 0.29 for the topsoil and subsoil, respectively, although soil N pools and N transformation rates linearly decreased with increasing aridity in both soil layers. Furthermore, we identified markedly different correlations between soil δ15N and soil N‐cycling traits above and below the AI thresholds (0.27 and 0.29 for topsoil and subsoil, respectively). Specifically, in wetter regions, soil δ15N positively correlated with most soil N‐cycling traits, suggesting that high soil δ15N may result from the "openness" of soil N cycling. Conversely, in drier regions, soil δ15N showed insignificant relationships with soil N‐cycling traits and correlated well with factors, such as soil‐available phosphorus and foliage δ15N, demonstrating that pathways other than typical soil N cycling may dominate soil δ15N under drier conditions. Overall, these results highlight that different ecosystem N‐cycling processes may drive soil δ15N along the aridity gradient, broadening our understanding of N cycling as indicated by soil δ15N under changing drought regimes. The aridity threshold of soil δ15N should be considered in terrestrial N‐cycling models when incorporating 15N isotope signals to predict N cycling and availability under climatic dryness. [ABSTRACT FROM AUTHOR]

Climate warming and extreme climatic events are threatening ecosystem processes and functions. However, it remains unclear how climate warming changes ecosystem stability in facing extreme rainfall events.Here, we investigated the temporal stability of above‐ground net primary productivity (ANPP) in an alpine meadow via a 5‐year warming experiment, during which two flooding events occurred, in an alpine meadow.We first found that warming significantly increased the temporal stability of ANPP in facing flooding events by increasing resistance to and decreasing recovery from flooding events. Second, we found that warming shifted the plant community structure by increasing the dominance of grasses and reducing species richness and asynchrony. Last, we detected the higher temporal stability of ANPP under warming, which was mainly ascribed to the warming‐induced increase in dominant species stability (Deschampsia caespitosa).Synthesis. These findings indicate that climate warming may mitigate the shock of flooding events on the temporal stability of community productivity via altering plant community structure in alpine grasslands. Our study highlights that to buffer ecosystems from climatic extremes, we should focus on promoting the maintenance or selection of dominant species rather than only focusing on increasing species richness. [ABSTRACT FROM AUTHOR]

Plant functional traits play important roles in determining plant responses to environmental change and further shaping community composition, but the role of plant stoichiometry remains poorly understood in regulating community responses to continually increased nitrogen (N) deposition.In this paper, we used a 5‐year manipulative experiment with six N addition rates (Nrate) to explore how the N response of different plant functional groups including forbs, grasses, sedges and legumes links to above‐ground plant carbon‐to‐N ratio (C:N, related to N use strategies) in an alpine meadow.We found that plant C:N explained 45.8% and 42.6% of N response variation among functional groups in plant above‐ground net primary productivity (ANPP) and abundance. Over increasing Nrate, functional groups with higher C:N (grasses and sedges) tended to grow better and have greater abundance, yet those with lower C:N (forbs and legumes) were at risk of loss, subsequently mitigating the N‐caused decline in community‐level plant C:N. This was mainly due to that functional groups with higher C:N not only possessed higher N use efficiency but also had greater competitiveness for above‐ground light due to higher plant height. In addition, we also found that plant C:N differences among functional groups remained relatively constant over Nrate, suggesting that the ability of this trait to indicate functional group‐specific N responses was stable.Synthesis. Our results show that the N responses of different herbaceous plants can be well explained by plant C:N. Thus, we suggest the prediction of plant community composition changes under N deposition would be greatly improved by considering this easily measured stoichiometric trait. [ABSTRACT FROM AUTHOR]