Critical transition of multifunctional stability induced by nitrogen enrichment in grasslands differing in degradation severity

Bacterial community assembly processes mediate soil functioning under cadmium stress in the agroecosystem

Elucidating the effects of community assembly processes on soil functioning represents a crucial challenge in theoretical ecology, particularly under cadmium (Cd) stress, where our understanding remains limited. In this study, we therefore used amplicon sequencing and a quantitative-PCR-based chip to analyze the changes in bacterial community characteristics, soil functioning and their interrelationships in agroecosystems under different levels of Cd stress. The results indicated that Cd stress led to a decline in community diversity (Z-score), network complexity and stability, an increase in species turnover, and a regulation of community structure. Cd stress significantly increased the relative importance of dispersal limitation and homogeneous selection, reducing community drift and rendering the community more deterministic. Finally, Cd stress significantly reduced soil functional potential (Z-score) and soil functional stability (Z-score), impairing soil carbon, nitrogen, phosphorus, and sulfur cycling. It is noteworthy that correlation and random forest analyses revealed significant effects of specific community assembly processes, including dispersal limitation, homogeneous selection, drift (and others), on changes in soil functional potential (Z-score). The results emphasize the pivotal role of community assembly processes in dictating soil functioning under Cd stress, thereby offering novel insights into the comprehension of microbial-driven mechanisms governing soil functioning.

Ecosystem multifunctionality enhancement by short-term nitrogen addition in semi-arid saline-alkaline grassland of northern China

The vast area of saline-alkaline grasslands in the agro-pastoral ecotone of northern China has important production and ecological functions. Nitrogen (N) deposition changes the function and structure of vulnerable grasslands. However, the impacts of N deposition on ecosystem multifunctionality (EMF) remains unknown. To address this issue, a three-year in-situ study was carried out between 2018 and 2020 to assess the direct impacts of N addition on grassland ecosystem function. Eight N addition levels were applied: 0, 1, 2, 4, 8, 16, 24, and 32 g·N·m-2·yr-1. Plant-soil-microbe equilibrium properties, productivity, and plant community structure were monitored, and the impacts of N addition rate (NAR) and year (NAY) on grassland EMF were analyzed. Short-term N addition enhanced multiple individual ecosystem functions such as dominant species, aboveground biomass, soil stoichiometry, and litter, and remarkably decreased the structure of the plant community and soil physical and chemical performance. Furthermore, short-term N addition enhanced grassland aboveground multifunctionality (AGMF) and overall EMF, and had a neutral effect on belowground multifunctionality (BGMF). The primary effect of N addition was the enhancement of AGMF by increasing aboveground biomass, thereby enhancing grassland EMF; however, grassland EMF showed relatively minor fluctuations at N addition rates of >16 g·N·m-2·yr-1. The results of this study show that short-term N addition indirectly regulates grassland EMF by increasing aboveground biomass, and provide novel insights into the asynchronous response of AGMF and BGMF of grassland ecosystems to short-term N addition. The addition of an appropriate amount of N is essential to enhance grass yield and maintain grassland EMF in order to manage saline-alkaline grasslands within the agro-pastoral ecotone.

Temporal asynchrony of plant and soil biota determines ecosystem multifunctional stability

The role of plant biodiversity in stabilizing ecosystem multifunctionality has been extensively studied; however, the impact of soil biota biodiversity on ecosystem multifunctional stability, particularly under multiple environmental changes, remains unexplored. By conducting an experiment with environmental changes (adding water and nitrogen to a long-term grazing experiment) and an experiment without environmental changes (an undisturbed site) in semi-arid grasslands, our research revealed that environmental changes-induced changes in temporal stability of both above- and belowground multifunctionality were mainly impacted by plant and soil biota asynchrony, rather than by species diversity. Furthermore, changes in temporal stability of above- and belowground multifunctionality, under both experiments with and without environmental changes, were mainly associated with plant and soil biota asynchrony, respectively, suggesting that the temporal asynchrony of plant and soil biota has independent and non-substitutable effects on multifunctional stability. Our findings emphasize the importance of considering both above- and belowground biodiversity or functions when evaluating the stabilizing effects of biodiversity on ecosystem functions.

Grassland degradation amplifies the negative effect of nitrogen enrichment on soil microbial community stability

Although nitrogen (N) enrichment is known to threaten the temporal stability of aboveground net primary productivity, it remains unclear how it alters that of belowground microbial abundance and whether its impact can be regulated by grassland degradation. Using data from N enrichment experiments at temperate grasslands with no, moderate, severe, and extreme degradation degrees, we quantified the temporal stability of soil microbial abundance (hereafter 'microbial community stability') using the ratio of the mean quantitative PCR to its standard deviation over 4 years. Both bacterial and fungal community stability sharply decreased when N input exceeded 30 g N m-2  year-1 in non-degraded grasslands, whereas a reduction in this threshold occurred in degraded grasslands. Microbial species diversity, species asynchrony, and species associations jointly altered microbial community stability. Interestingly, the linkages between plant and microbial community stability were strengthened in degraded grasslands, suggesting that plants and soil microbes might depend on each other to keep stable communities in harsh environments. Our findings highlighted the importance of grassland degradation in regulating the responses of microbial community stability to N enrichment and provided experimental evidence for understanding the relationships between plant and microbial community stability.

Microbial regulation of feedbacks to ecosystem change

Microbes are key biodiversity components of all ecosystems and control vital ecosystem functions. Although we have just begun to unravel the scales and factors that regulate microbial communities, their role in mediating ecosystem stability in response to disturbances remains underexplored. Here, we review evidence of how, when, and where microbes regulate or drive disturbance feedbacks. Negative feedbacks dampen the impacts of disturbance, which maintain ecosystem stability, whereas positive feedbacks instead erode stability by amplifying the disturbance. Here we describe the processes underlying the responses to disturbance using a hierarchy of functional traits, and we exemplify how these may drive biogeochemical feedbacks. We suggest that the feedback potential of functional traits at different hierarchical levels is contingent on the complexity and heterogeneity of the environment.

Soil fertility shifts the relative importance of saprotrophic and mycorrhizal fungi for maintaining ecosystem stability

Soil microbial communities are essential for regulating the dynamics of plant productivity. However, how soil microbes mediate temporal stability of plant productivity at large scales across various soil fertility conditions remains unclear. Here, we combined a regional survey of 51 sites in the temperate grasslands of northern China with a global grassland survey of 120 sites to assess the potential roles of soil microbial diversity in regulating ecosystem stability. The temporal stability of plant productivity was quantified as the ratio of the mean normalized difference vegetation index to its standard deviation. Soil fungal diversity, but not bacterial diversity, was positively associated with ecosystem stability, and particular fungal functional groups determined ecosystem stability under contrasting conditions of soil fertility. The richness of soil fungal saprobes was positively correlated with ecosystem stability under high-fertility conditions, while a positive relationship was observed with the richness of mycorrhizal fungi under low-fertility conditions. These relationships were maintained after accounting for plant diversity and environmental factors. Our findings highlight the essential role of fungal diversity in maintaining stable grassland productivity, and suggest that future studies incorporating fungal functional groups into biodiversity-stability relationships will advance our understanding of their linkages under different fertility conditions.