Grazing and nitrogen addition restructure the spatial heterogeneity of soil microbial community structure and enzymatic activities

Contrasting responses of α- and β-multifunctionality to aboveground plant community in the Qinghai-Tibet Plateau

The aboveground plant communities are crucial in driving ecosystem functioning, particularly being the primary producers in terrestrial ecosystems. Numerous studies have investigated the impacts of aboveground plant communities on multiple ecosystem functions at α-scale. However, such critical effects have been unexplored at β-scale and the comparative assessment of the effects and underlying mechanisms of aboveground plant communities on α- and β-multifunctionality has been lacking. In this study, we examined the effects of aboveground plant communities on soil multifunctionality both at α- and β-scale in the alpine meadow of the Tibetan Plateau. Additionally, we quantified the direct effects of aboveground plant communities, as well as the indirect effects mediated by changes in biotic and abiotic factors, on soil multifunctionality at both scales. Our findings revealed that: 1) Aboveground plant communities had significantly positive effects on α-multifunctionality whereas, β-multifunctionality was not affected significantly. 2) Aboveground plant communities directly influence α- and β-multifunctionality in contrasting ways, with positive and negative effects, respectively. Apart from the direct effects of plant community, we found that soil water content and bacterial β-diversity serving as the primary predictors for the responses of α- and β-multifunctionality to the presence of aboveground plant communities, respectively. And β-soil biodiversity appeared to be a stronger predictor of multifunctionality relative to α-soil biodiversity. Our findings provide novel insights into the drivers of ecosystem multifunctionality at different scales, highlight the importance of maintaining biodiversity at multiple scales and offer valuable knowledge for the maintenance of ecosystem functioning and the restoration of alpine meadow ecosystems.

Grazing-driven shifts in soil bacterial community structure and function in a typical steppe are mediated by additional N inputs

Herbivore grazing and nitrogen (N) fertilization affect soil microbial diversity and community composition both in direct and indirect pathways (e.g., via alterations in soil microenvironment and plant communities); however, their combination effects are still largely unexplored. We carried out a field study to investigate how soil abiotic properties, plant community composition and functional traits altered soil bacterial community structure and function in response to a long-term herbivore grazing (17-year sheep grazing with four stocking rates) and anthropogenic N inputs (6-year N addition with four levels) experiment. We show that a high stocking rate of 8.7 sheep ha-1 (SR8.7) decreased soil bacterial α- and β-diversity, while α- and β-diversity showed hump-shaped and saddle-shaped responses, respectively, with increasing N addition rate, reaching tipping points at the N application rate of 10 g N m-2 year-1 (N10). The synergistic effects of grazing and N addition induced the highest soil bacterial α-diversity at SR2.7 with N10. The contrasting effects of grazing and N addition induced higher soil bacterial β-diversity at SR8.7 with N20. Plant factors (e.g., aboveground biomass of Stipa bungeana and community-weighted mean carbon [CWM_C]), edaphic factors (e.g., soil moisture, pH, NO3--N, and C:nutrients ratios) and their interactions were the most significant factors affecting the diversity and community composition of bacteria. Our structure equation model (SEM) shows that grazing-induced negative effects on soil pH and plant community composition indirectly increased the β-diversity of soil bacteria, while grazing-induced decreased CWM_C had positive effects on bacterial α-diversity and community structure. However, N addition indirectly increased β-diversity of soil bacteria via changes in soil NO3--N and plant community composition, while N addition had negative impacts on bacterial α-diversity and community structure via variations in CWM_C. The interaction of grazing and N addition increased the complexity and stability of the bacterial network. Based on the KEGG database, grazing and N addition could accelerate the soil functional potential of C and N cycling. Our findings suggest that N application at a rate of <10 g N m-2 year-1 with a stocking rate of <5.3 sheep ha-1 could maintain the development of soil bacteria in supporting the most important ecosystem functions and services. Complex responses of soil microbes to grazing and N addition indicate the need for deeper investigations of the impacts of global change on microbial involvement in biogeochemical cycles.

Nitrogen addition regulates the effects of variation in precipitation patterns on plant biomass formation and allocation in a Leymus chinensis grassland of northeast China

Global warming is predicted to change precipitation amount and reduce precipitation frequency, which may alter grassland primary productivity and biomass allocation, especially when interact with other global change factors, such as nitrogen deposition. The interactive effects of changes in precipitation amount and nitrogen addition on productivity and biomass allocation are extensively studied; however, how these effects may be regulated by the predicted reduction in precipitation frequency remain largely unknown. Using a mesocosm experiment, we investigated responses of primary productivity and biomass allocation to the manipulated changes in precipitation amount (PA: 150 mm, 300 mm, 450 mm), precipitation frequency (PF: medium and low), and nitrogen addition (NA: 0 and 10 g N m-2 yr-1) in a Leymus chinensis grassland. We detected significant effects of the PA, PF and NA treatments on both aboveground biomass (AGB) and belowground biomass (BGB); but the interactive effects were only significant between the PA and NA on AGB. Both AGB and BGB increased with an increment in precipitation amount and nitrogen addition; the reduction in PF decreased AGB, but increased BGB. The reduced PF treatment induced an enhancement in the variation of soil moisture, which subsequently affected photosynthesis and biomass formation. Overall, there were mismatches in the above- and belowground biomass responses to changes in precipitation regime. Our results suggest the predicted changes in precipitation regime, including precipitation amount and frequency, is likely to alter primary productivity and biomass allocation, especially when interact with nitrogen deposition. Therefore, predicting the influence of global changes on grassland structure and functions requires the consideration of interactions among multiple global change factors.

Microbial communities in terrestrial surface soils are not widely limited by carbon

Microbial communities in soils are generally considered to be limited by carbon (C), which could be a crucial control for basic soil functions and responses of microbial heterotrophic metabolism to climate change. However, global soil microbial C limitation (MCL) has rarely been estimated and is poorly understood. Here, we predicted MCL, defined as limited availability of substrate C relative to nitrogen and/or phosphorus to meet microbial metabolic requirements, based on the thresholds of extracellular enzyme activity across 847 sites (2476 observations) representing global natural ecosystems. Results showed that only about 22% of global sites in terrestrial surface soils show relative C limitation in microbial community. This finding challenges the conventional hypothesis of ubiquitous C limitation for soil microbial metabolism. The limited geographic extent of C limitation in our study was mainly attributed to plant litter, rather than soil organic matter that has been processed by microbes, serving as the dominant C source for microbial acquisition. We also identified a significant latitudinal pattern of predicted MCL with larger C limitation at mid- to high latitudes, whereas this limitation was generally absent in the tropics. Moreover, MCL significantly constrained the rates of soil heterotrophic respiration, suggesting a potentially larger relative increase in respiration at mid- to high latitudes than low latitudes, if climate change increases primary productivity that alleviates MCL at higher latitudes. Our study provides the first global estimates of MCL, advancing our understanding of terrestrial C cycling and microbial metabolic feedback under global climate change.