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Grazing exclusion changes soil physical-chemical characteristics, rapidly affects microbial community composition and function, and alters biogeochemical processes, e.g., carbon (C) cycle, over time. However, the temporal dynamics of CO2 emission and CH4 uptake during grassland restoration chronosequences remain poorly understood. We investigated soil CO2 emission and CH4 uptake, the genes related to CO2 and CH4 production and reduction (cbbL, cbbM, chiA, and pmoA), and associated microbial communities under different durations of grazing exclusion (0, 7, 16, 25, and 38 years) to reveal the mechanisms and potential of soil CO2 emission and CH4 uptake in a semi-arid steppe. The results showed that a proper exclusion period could significantly improve soil physical-chemical conditions, vegetation community, and soil C-cycling. The abundance of C-cycling functional genes (cbbL, cbbM, chiA and pmoA), CH4 uptake and CO2 emission rates showed a single-peak pattern with increasing duration of grazing exclusion, peaking at 16 years and then decreasing in the period between 25 and 38 years, indicating that the effect of exclusion weakened when the exclusion period was too long. The changes in C-cycling functional genes and microbial communities are primarily influenced by aboveground net primary productivity (ANPP), and are associated with CO2, CH4, soil water content (SWC), and soil organic carbon (SOC). Structural equation modeling showed that increases in SOC content and pmoA abundance caused by an increase in ANPP accelerated CO2 emission and CH4 uptake rates, respectively. Our results provide valuable insights into the critical role of grazing exclusion in promoting grassland restoration and carbon sequestration, and have potential implications for sustainable land management practices.

As a grassland management strategy in the typical steppe, livestock grazing can affect soil microbes and ecosystem functions via vegetation removal, trampling and manure deposition. However, the effects of grazing and its main consequences (e.g. the shifts in plant functional types and alterations in soil properties) on soil bacterial and fungal community composition remain unresolved. Taking advantage of a 5-year grazing exclosure experiment that tested different grazing intensities [control: no grazing (G0); light: 0.75 sheep ha−1 (G1); moderate: 1.50 sheep ha−1 (G2); heavy: 2.25 sheep ha−1 (G3); and overgrazing: 3.00 sheep ha−1 (G4)], we examined grazing-related changes in plant communities, soil chemistry and soil microbial communities. Grazing intensity affected the diversity and composition of soil bacterial and fungal communities via changes in plant species and soil nutrient contents. Increased grazing intensity reduced soil organic carbon (OC), suppressed the dominant bacterial phyla Proteobacteria and Bacteriodetes, and increased the abundance of oligotrophic bacteria in subordinate phyla (e.g. Verrucomicroba and Chloroflexi). Increased grazing intensity also promoted the dominant fungal phylum Ascomycetes because of the poor-nutrient soil environment. The decreased soil carbon-to-nitrogen (C/N) ratio in the G2 treatment was associated with an effect on the fungal community composition. Structural equation modelling demonstrated that responses of the bacterial community to grazing intensity were related to changes in soil OC and soil pH, whereas the combination of C4 plant community biomass and soil pH drove changes in the fungal community. Our study provides novel evidence that grazing affects plant functional group and soil chemistry through the soil pH and C/N ratio, which in turn affects bacterial and fungal communities. Highlights: Heavy and overgrazing suppressed bacterial phylum Proteobacteria and increased fungal phylum Ascomycete. Soil OC was the

Grazing exclusion is an effective management practice to restore grassland ecosystem functioning. However, little is known about the role of soil microbial communities in regulating grassland ecosystem functioning during long-term ecosystem restorations. We evaluated the recovery of a degraded semiarid grassland ecosystem in northern China by investigating plant and soil characteristics and the role of soil microbial communities in ecosystem functioning after 22 years of grazing exclusion. Grazing exclusion significantly increased the alpha diversity and changed the community structure of bacteria, but did not significantly affect the alpha diversity or community structure of fungi. The higher abundance of copiotrophic Proteobacteria and Bacteroidetes with grazing exclusion was due to the higher carbon and nutrient concentrations in the soil, whereas the high abundance of Acidobacteria in overgrazed soils was likely an adaptation to the poor environmental conditions. Bacteria of the Sphingomonadaceae family were associated with C cycling under grazing exclusion. Bacteria of the Nitrospiraceae family, and especially of the Nitrospira genus, played an important role in changes to the N cycle under long-term exclusion of grazing. Quantitative PCR further revealed that grazing exclusion significantly increased the abundance of nitrogen fixing bacteria (nifH), ammonia oxidizers (AOA and AOB), and denitrifying bacteria (nirK and nosZ1). Denitrifying enzyme activity (DEA) was positively correlated with abundance of denitrifying bacteria. The increase in DEA under grazing exclusion suggests that the dependence of DEA on the availability of NO3– produced is due to the combined activity of ammonia oxidizers and denitrifiers. Our findings indicate that decades-long grazing exclusion can trigger changes in the soil bacterial diversity and composition, thus modulating the restoration of grassland ecosystem functions, carbon sequestration and soil fertility.

Nutrient resorption from senesced leaves is an important mechanism for nutrient conservation in plants. However, little is known about the effect of grazing on plant nutrient resorption from senesced leaves, especially in semiarid ecosystems. Here, we evaluated the effects of grazing on N and P resorption in the three most dominant grass species in a typical steppe in northern China. We identified the key pathways of grazing-induced effects on N and P resorption efficiency. Grazing increased N and P concentrations in the green leaves of Leymus chinensis and Stipa grandis but not in Cleistogenes squarossa. Both L. chinensis and S. grandis exhibited an increasing trend of leaf N resorption, whereas C. squarrosa recorded a decline in both leaf N and P resorption efficiency under grazing. Structural equation models showed that grazing is the primary driver of the changes in N resorption efficiency of the three dominant grass species. For L. chinensis, the P concentration in green and senesced leaves increased the P resorption efficiency, whereas the senesced leaf P concentration played an important role in the P resorption efficiency of C. squarrosa. Grazing directly drove the change in P resorption efficiency of S. grandis. Our results suggest that large variations in nutrient resorption patterns among plant species depend on leaf nutritional status and nutrient- use strategies under overgrazing, and indicate that overgrazing may have indirect effects on plant-mediated nutrient cycling via inducing shifts in the dominance of the three plant species.

Diversity and functioning are the most important topics in the ecology of arbuscular mycorrhizal (AM) fungi. To address what ecological factors determine the species composition of an AM fungal community and what functional consequences different AM fungal communities have on particular plant-soil systems, my dissertation research examines the species composition and functioning of AM fungal communities associated with the same C4 grasses common to Eastern serpentine grasslands and Mid-west tall-grass prairies. In the first chapter, I compare the natural AM fungal community compositions between serpentine grasslands and prairies. Distinct AM fungal communities were revealed in the two types of grasslands, and there was a noticeable correlation between soil properties and fungal communities. Chapter two describes an experiment aiming to test the hypothesis that the local AM fungal communities are better suited to promoting plant growth in that particular soil environment than are AM fungal communities from a different soil environment. In both serpentine and prairie soils, the degree of benefit to Sorghastrum growth provided by the native AM fungal community was always significantly higher than that provided by the foreign community, suggesting that AM fungi and soil are ecologically matched. In Chapter three, I address whether host-specificity exists in community and population levels of arbuscular mycorrhizae through analyzing the spore data collected from the field and greenhouse experiments. Little host-specificity was demonstrated, however, functional difference was observed between AM roots of two different grass species inoculated with the same spore community. Functional difference driven by host identity could have important ecological consequences. My final chapter evaluates the functional difference among populations of the single fungal species associated with various host plant species in the same serpentine grassland. Spores of Gi. gigantea collected on