Whole-soil warming shifts species composition without affecting diversity, biomass and productivity of the plant community in an alpine meadow

Soil Bacterial β-Diversity as a Key Determinant of Belowground Productivity in Warming Alpine Ecosystems

Climate warming has profound effects on terrestrial ecosystems, with biodiversity playing a crucial role in modulating ecosystem productivity responses. While extensive studies have investigated how plant species richness (α-diversity) influences aboveground productivity under warming conditions, the contributions of plant and soil microbial β-diversity to belowground net primary productivity (BNPP) remain poorly understood. In this study, we conducted a 6-year warming experiment in an alpine meadow to investigate the response patterns and drivers of BNPP, as well as the α- and β-diversity of plant and soil microbial communities. Our results showed that warming increased BNPP by 41.41%-90.3%, with biodiversity metrics collectively accounting for about 86% of the variation in BNPP. Notably, while climate warming significantly reduced the α-diversity of both plant (p = 0.067) and soil bacterial communities (p < 0.05), soil bacterial β-diversity showed a marked increase. The enhancement in soil bacterial β-diversity was closely linked to increased gene abundance associated with ammonification and nitrification processes, identified as key drivers of BNPP under warming conditions. These findings underscore the pivotal role of soil microbial β-diversity in supporting BNPP under warming conditions. Our study highlights the need to preserve belowground microbial heterogeneity to maintain ecosystem functioning and enhance carbon sequestration efforts in the face of global climate change.

Patterns and Mechanisms of Legume Responses to Nitrogen Enrichment: A Global Meta-Analysis

Nitrogen (N), while the most abundant element in the atmosphere, is an essential soil nutrient that limits plant growth. Leguminous plants naturally possess the ability to fix atmospheric nitrogen through symbiotic relationships with rhizobia in their root nodules. However, the widespread use of synthetic N fertilizers in modern agriculture has led to N enrichment in soils, causing complex and profound effects on legumes. Amid ongoing debates about how leguminous plants respond to N enrichment, the present study compiles 2174 data points from 162 peer-reviewed articles to analyze the impacts and underlying mechanisms of N enrichment on legumes. The findings reveal that N enrichment significantly increases total legume biomass by 30.9% and N content in plant tissues by 13.2% globally. However, N enrichment also leads to notable reductions, including a 5.8% decrease in root-to-shoot ratio, a 21.2% decline in nodule number, a 29.3% reduction in nodule weight, and a 27.1% decrease in the percentage of plant N derived from N2 fixation (%Ndfa). Legume growth traits and N2-fixing capability in response to N enrichment are primarily regulated by climatic factors, such as mean annual temperature (MAT) and mean annual precipitation (MAP), as well as the aridity index (AI) and N fertilizer application rates. Correlation analyses show that plant biomass is positively correlated with MAT, and tissue N content also exhibits a positive correlation with MAT. In contrast, nodule numbers and tissue N content are negatively correlated with N fertilizer application rates, whereas %Ndfa shows a positive correlation with AI and MAP. Under low N addition, the increase in total biomass in response to N enrichment is twice as large as that observed under high N addition. Furthermore, regions at lower elevations with abundant hydrothermal resources are especially favorable for total biomass accumulation, indicating that the responses of legumes to N enrichment are habitat-specific. These results provide scientific evidence for the mechanisms underlying legume responses to N enrichment and offer valuable insights and theoretical references for the conservation and management of legumes in the context of global climate change.

Warming suppresses grassland recovery in biomass but not in community composition after grazing exclusion in a Mongolian grassland

We conducted a 4-year temperature manipulation experiment in a Mongolian grassland to examine the effect of daytime and nighttime warming on grassland recovery after grazing exclusion. After constructing a livestock exclusion fence in the grassland, we established daytime and daytime-and-nighttime warming treatments within the fenced area by a combination of open-top chambers (OTC) and electric heaters. We measured the numbers of plants and aboveground biomass by species after recording percentage vegetation cover every summer for three warming treatments inside the fence-non-warming, daytime warming, and daytime-and-nighttime warming-and for the grassland outside of the fence. OTCs increased daytime temperature by about 2.0 °C, and heaters increased nighttime temperature by 0.9 °C during the growing period. Grazing exclusion had little effect on grassland biomass but reduced the abundance of poorly palatable species and modified plant community composition. Daytime warming decreased soil moisture and lowered aboveground biomass within the fenced grassland but had little effect on plant community composition. Nighttime warming lowered soil moisture further but its effects on grassland biomass and community composition were undetectable. We concluded that recovery of plant biomass in grasslands degraded by grazing would be lowered by future climate warming through soil drying. Because warming had little effect on the recovery of community composition, adverse effects of warming on grassland recovery might be offset by improving plant productivity through mitigation of soil drying by watering. Soil drying due to nighttime warming might have detectable effects on vegetation when warming persists for a long time.

Warming differentially affects above- and belowground ecosystem functioning of the semi-arid alpine grasslands

Rapid climate warming is altering multiple ecosystem functions of alpine grasslands. However, the responses of the above- and belowground ecosystem multifunctionality (EMF) to climate warming might exhibit difference in semi-arid alpine grasslands. Based on manipulative field experiments at an alpine meadow and an alpine steppe, we explored warming effects on the functioning of alpine grassland ecosystems on the Tibetan Plateau. Warming significantly decreased plant diversity and aboveground biomass, but tended to increase belowground biomass, soil carbon, and soil nutrient contents. Experimental warming generally had neutral effects on the EMF of both alpine grasslands. Nevertheless, warming differentially affects the above- and belowground ecosystem functioning of Tibetan semi-arid alpine grasslands, with the aboveground EMF (AEMF) deceased but the belowground EMF (BEMF) increased under warmer conditions. Our results further showed that the negative effect of experimental warming on AEMF was mainly regulated by the changes of plant and soil biodiversity. However, plant productivity had a pivotal role in propelling the positive effect of warming on BEMF. Our results emphasized the potential impacts of plant and soil biodiversity, productivity, and soil nutrients in maintaining the EMF of alpine grasslands, which could offer novel views for sustainable management of Tibetan semi-arid alpine ecosystems.

Elevation Influences Belowground Biomass Proportion in Forests by Affecting Climatic Factors, Soil Nutrients and Key Leaf Traits

Forest biomass allocation is a direct manifestation of biological adaptation to environmental changes. Studying the distribution patterns of forest biomass along elevational gradients is ecologically significant for understanding the specific impacts of global change on plant resource allocation strategies. While aboveground biomass has been extensively studied, research on belowground biomass remains relatively limited. Furthermore, the patterns and driving factors of the belowground biomass proportion (BGBP) along elevational gradients are still unclear. In this study, we investigated the specific influences of climatic factors, soil nutrients, and key leaf traits on the elevational pattern of BGBP using data from 926 forests at 94 sites across China. In this study, BGBP data were calculated from the root biomass to the depth of 50 cm. Our findings indicate considerable variability in forest BGBP at a macro scale, showing a significant increasing trend along elevational gradients (p < 0.01). BGBP significantly decreases with increasing temperature and precipitation and increases with annual mean evapotranspiration (MAE) (p < 0.01). It decreases significantly with increasing soil phosphorus content and increases with soil pH (p < 0.01). Key leaf traits (leaf nitrogen (LN) and leaf phosphorus (LP)) are positively correlated with BGBP. Climatic factors (R2 = 0.46) have the strongest explanatory power for the variation in BGBP along elevations, while soil factors (R2 = 0.10) and key leaf traits (R2 = 0.08) also play significant roles. Elevation impacts BGBP directly and also indirectly through influencing such as climate conditions, soil nutrient availability, and key leaf traits, with direct effects being more pronounced than indirect effects. This study reveals the patterns and controlling factors of forests' BGBP along elevational gradients, providing vital ecological insights into the impact of global change on plant resource allocation strategies and offering scientific guidance for ecosystem management and conservation.

Nitrogen and phosphorus addition mediate soil priming effects via affecting microbial stoichiometric balance in an alpine meadow

Priming effect (PE) plays a crucial role in regulating the decomposition of soil organic matter (SOM). Multiple empirical results have shown that nitrogen (N) and phosphorus (P) addition can significantly alter the direction and intensity of PE, which may significantly affect carbon turnover in grasslands, especially in alpine meadows that are sensitive to N and P enrichment. To evaluate the PE responses to N and/or P addition, we conducted an incubation experiment by adding 13C-labeled glucose and nutrient additions (+N, +P, and +NP) in soils collected from an alpine meadow. The soils were incubated for 30 days and soil/microbial properties and enzyme activities were measured. Partial correlation and linear regression analyses were then performed to investigate their correlations with PE. The results showed that mean PE intensity among all treatments was 0.61 mg C g-1 soil or 1.35 (ratio). Nitrogen addition increased PE intensity, which was attributed to the better match between soil resources and microbial demands and enhanced enzyme activities. However, the PE intensity in P-addition soils was lower than that in control soils. This discrepancy may be related to the P-induced decrease of N availability and stronger microbial C/N imbalance. No significant response of PE intensity to NP addition was detected, and this could be explained by the offset of positive N effects and negative P effects on microbial decomposition. In this experiment, N or P addition altered the PE intensity by mediating the match between soil C:N:P ratio and microbial demands, which supported the stoichiometric decomposition hypothesis. Overall, our study highlights the importance of considering the C, N and P coupling in regulating PE, and underscores the need for further investigation into the effects of soil P on microbial activity and SOM decomposition.

Effects of whole-soil warming on CH4 and N2 O fluxes in an alpine grassland

Global climate warming could affect the methane (CH4 ) and nitrous oxide (N2 O) fluxes between soils and the atmosphere, but how CH4 and N2 O fluxes respond to whole-soil warming is unclear. Here, we for the first time investigated the effects of whole-soil warming on CH4 and N2 O fluxes in an alpine grassland ecosystem on the Tibetan Plateau, and also studied the effects of experimental warming on CH4 and N2 O fluxes across terrestrial ecosystems through a global-scale meta-analysis. The whole-soil warming (0-100 cm, +4°C) significantly elevated soil N2 O emission by 101%, but had a minor effect on soil CH4 uptake. However, the meta-analysis revealed that experimental warming did not significantly alter CH4 and N2 O fluxes, and it may be that most field warming experiments could only heat the surface soils. Moreover, the warming-induced higher plant litter and available N in soils may be the main reason for the higher N2 O emission under whole-soil warming in the alpine grassland. We need to pay more attention to the long-term response of greenhouse gases (including CH4 and N2 O fluxes) from different soil depths to whole-soil warming over year-round, which could help us more accurately assess and predict the ecosystem-climate feedback under realistic warming scenarios in the future.

Trade-off in the partitioning of recent photosynthate carbon under global change

There may be trade-offs in the allocation patterns of recent photosynthetic carbon (RPC) allocation in response to environmental changes, with a greater proportion of RPC being directed towards compartments experiencing limited resource availability. Alternatively, the allocation of RPC could shift from sources to sinks as plants processing excess photosynthates. It prompts the question: Does the pattern of RPC allocation vary under global changes? If so, is this variation driven by optimal or by residual C allocation strategies? We conducted a meta-analysis by complicating 273 pairwise observations from 55 articles with 13 C or 14 C pulse or continuous labeling to assess the partitioning of RPC in biomass (leaf, stem, shoot, and root), soil pools (soil organic C, rhizosphere, and microbial biomass C) and CO2 fluxes under elevated CO2 (eCO2 ), warming, drought and nitrogen (N) addition. We propose that the increased allocation of RPC to belowground under sufficient CO2 results from the excretion of excess photosynthates. Warming led to a significant reduction in the percentage of RPC allocated to shoots, alongside an increase in roots allocation, although this was not statistically significant. This pattern is due to the reduced water availability resulting from warming. In conditions of drought, there was a notable increase in the partitioning of RPC to stems (+7.25%) and roots (+36.38%), indicative of a greater investment of RPC in roots for accessing water from deeper soil. Additionally, N addition led to a heightened allocation of RPC in leaves (+10.18%) and shoots (+5.78%), while reducing its partitioning in soil organic C (-8.92%). Contrary to the residual C partitioning observed under eCO2 , the alterations in RPC partitioning in response to warming, drought, and N supplementation are more comprehensively explained through the lens of optimal partitioning theory, showing a trade-off in the partitioning of RPC under global change.