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Purpose: Grasslands are facing the threat of climate change and intensive land use. Soil respiration (Rs) in grassland ecosystems can be potentially altered by changes in precipitation and land use. We aimed to quantify the impact of changes in precipitation and common land use practices in an Inner Mongolia grassland, i.e., mowing and grazing, on soil respiration. Methods: We performed an in situ experiment with altered precipitation (+ 50%, ambient, and -50%) and land use (control or fencing, mowing, and grazing) to explore their impacts on soil respiration and its autotrophic (Ra) and heterotrophic (Rh) components. Results: Altered precipitation had stronger impacts on abiotic and biotic drivers than land use, leading to stronger impacts on Rs and its components. Over the 3-year experiment, Rs, Ra and Rh decreased by 36%, 42% and 33% with reduced precipitation and increased by 29%, 36% and 25% with increased precipitation, respectively. Grazing and mowing caused relatively small decreases in Rs compared to fencing (generally < 10%). However, precipitation and land use interactively impacted abiotic and biotic drivers and thus Rs. The decrease in Rs with reduced precipitation was greater with grazing (38%) and mowing (37%) than with fencing (32%). Conclusions: Rs and its components may decrease under the projected decrease in precipitation and may further decrease with grazing and mowing compared to fencing. Therefore, land use should be considered when predicting grassland carbon cycling in response to future precipitation changes. [ABSTRACT FROM AUTHOR]

Soil microbes assemble in highly complex and diverse microbial communities, and microbial diversity patterns and their drivers have been studied extensively. However, diversity correlations and co-occurrence patterns between bacterial, fungal, and archaeal domains and between microbial functional groups in arid regions remain poorly understood. Here we assessed the relationships between the diversity and abundance of bacteria, fungi, and archaea and explored how environmental factors influence these relationships. We sampled soil along a 1500-km-long aridity gradient in temperate grasslands of Inner Mongolia (China) and sequenced the 16S rRNA gene of bacteria and archaea and the ITS2 gene of fungi. The diversity correlations and co-occurrence patterns between bacterial, fungal, and archaeal domains and between different microbial functional groups were evaluated using α-diversity and co-occurrence networks based on microbial abundance. Our results indicate insignificant correlations among the diversity patterns of bacterial, fungal, and archaeal domains using α-diversity but mostly positive correlations among diversity patterns of microbial functional groups based on α-diversity and co-occurrence networks along the aridity gradient. These results suggest that studying microbial diversity patterns from the perspective of functional groups and co-occurrence networks can provide additional insights on patterns that cannot be accessed using only overall microbial α-diversity. Increase in aridity weakens the diversity correlations between bacteria and fungi and between bacterial and archaeal functional groups, but strengthens the positive diversity correlations between bacterial functional groups and between fungal functional groups and the negative diversity correlations between bacterial and fungal functional groups. These variations of the diversity correlations are associated with the different responses of microbes to environmental factors, especially aridity. Our findings demonstrate the complex responses of microbial community structure to environmental conditions (especially aridity) and suggest that understanding diversity correlations and co-occurrence patterns between soil microbial groups is essential for predicting changes in microbial communities under future climate change in arid regions. [ABSTRACT FROM AUTHOR]

Aims Functional group composition of a plant community is mainly driven by environmental factors and is one of the main determinants of grassland biodiversity and productivity. Therefore, it is important to understand the role of plant functional groups (PFGs) in mediating the impact of environmental conditions on ecosystem functions and biodiversity. Methods We measured plant biomass and species richness (SR) of grasslands in 65 sites on the Mongolian Plateau and classified 157 perennial herbaceous plants into two main PFGs (namely grasses and forbs). Using the random forest model and ordinary least squares regression, we identified that environmental factors (i.e. aridity index, soil total nitrogen [STN] and pH) were significantly related to the SR and aboveground biomass (AGB) of PFGs. We then used structural equation modeling to explore the relationship between the identified environmental factors and community SR and biomass, and the role of PFGs in driving this relationship. Important Findings We found that aridity index had unimodal relationships with both AGB and SR of the PFGs and the whole community. All SR and biomass metrics were significantly related to STN and pH. The relationship between aridity index and community biomass was mediated by an increase in the AGB of grasses. The influence of STN and pH on community SR was mainly due to their regulation in the SR of forbs. Our results indicate that community composition and the identity of the PFGs play a key role in linking environmental factors to ecosystem functioning. [ABSTRACT FROM AUTHOR]

Over the last century, anthropogenic activities have increased nitrogen (N) deposition considerably, which significantly affects ecosystem processes and has the potential to induce N saturation in the future. The continuous increase in N deposition may cause a non-linear response in soil respiration (Rs), an important component of carbon (C) cycling. However, little is known about N saturation threshold of soil respiration. In this study, we conducted coordinated experiments in four grassland types across northern China with four N addition levels to explore patterns in the Rs saturation threshold. Our results showed that an Rs saturation threshold generally exists in grassland ecosystems in response to N addition gradients. The N saturation threshold of Rs occurred at an average rate of 50 kg N ha−1 yr−1, but varied widely with grassland type; the N saturation threshold occurred at rates of 100, 50, 50, and 25 kg N ha−1 yr−1 in the alpine meadow, meadow steppe, typical steppe, and desert steppe, respectively. Autotrophic respiration (Ra) and heterotrophic respiration (Rh) responded to N addition gradients differently. Ra increased initially and became saturated at a rate of 50 kg N ha−1 yr−1 and declined thereafter. In contrast, Rh decreased monotonically after N addition. Structural equation models further confirmed that the effects of N addition gradients on Rs were primarily determined by the non-linear response of belowground biomass. Interestingly, the compiled global dataset showed that the N saturation threshold of Rs increased with precipitation and soil moisture. These findings indicate that the stimulating effect of N deposition on Rs and Ra might diminish with increasing N deposition in the future, especially in dry grassland ecosystems. [ABSTRACT FROM AUTHOR]

Background and aims: Human activities have significantly increased nitrogen (N) and phosphorous (P) inputs to terrestrial ecosystems. However, the impact of N and P enrichment on soil microbial community structure and functioning in temperate and alpine grassland ecosystems remains unclear. Methods: In this study, we investigated the responses of soil microbial communities to nutrient (N and P) additions in two temperate and one alpine grassland ecosystems in China. We measured soil chemical properties, microbial community composition (indicated by the phospholipid fatty acids, PLFA) and potential enzyme activities related to carbon (C), N, and P cycling in the peak growing season after 4 years of nutrient addition. Results: We found that N addition reduced soil pH and increased soil total N content at two meadow sites, P addition increased soil total P content at all three sites, but both N and P additions had minimal effects on soil organic C content. Bacteria and total microbial abundances did not change after N and P additions, while fungi and arbuscular mycorrhizal fungi (AMF) abundances were suppressed by N addition. Moreover, the activity of soil extracellular enzymes involved in C, N and P cycling and their stoichiometric ratios were not responsive to N and P additions, except for inhibition of acid phosphatase by P addition at the temperate meadow site. Conclusions: Despite significant changes in soil chemistry (e.g., pH and available nutrients), soil microbial biomass (except fungi and AMF abundances), community structure, and enzyme activities (except phosphatase) were generally resistant to 4 years of N and P addition in the three temperate and alpine grassland ecosystems in China. [ABSTRACT FROM AUTHOR]

Changes in species composition across communities, i.e., β-diversity, is a central focus of ecology. Compared to macroorganisms, the β-diversity of soil microbes and its drivers are less studied. Whether the determinants of soil microbial β-diversity are consistent between soil depths and between abundant and rare microorganisms remains controversial. Here, using the 16S-rRNA of soil bacteria and archaea sampled at different soil depths (0–10 and 30–50 cm) from 32 sites along an aridity gradient of 1500 km in the temperate grasslands in northern China, we compared the effects of deterministic and stochastic processes on the taxonomic and phylogenetic β-diversity of soil microbes. Using variation partitioning and null models, we found that the taxonomic β-diversity of the overall bacterial communities was more strongly determined by deterministic processes in both soil layers (the explanatory power of environmental distance in topsoil: 25.4%; subsoil: 47.4%), while their phylogenetic counterpart was more strongly determined by stochastic processes (the explanatory power of spatial distance in topsoil: 42.1; subsoil 24.7%). However, in terms of abundance, both the taxonomic and phylogenetic β-diversity of the abundant bacteria in both soil layers was more strongly determined by deterministic processes, while those of rare bacteria were more strongly determined by stochastic processes. In comparison with bacteria, both the taxonomic and phylogenetic β-diversity of the overall abundant and rare archaea were strongly determined by deterministic processes. Among the variables representing deterministic processes, contemporary and historical climate and aboveground vegetation dominated the microbial β-diversity of the overall and abundant microbes of both domains in topsoils, but soil geochemistry dominated in subsoils. This study presents a comprehensive understanding on the β-diversity of soil microbial communities in the temperate grasslands in northern China. Our findings highlight the importance of soil depth, phylogenetic turnover, and species abundance in the assembly processes of soil microbial communities. [ABSTRACT FROM AUTHOR]

Nematodes are a crucial component of the soil food web, and play a key role in maintaining biodiversity and regulating soil nutrient cycling. Many studies have been done on the effects of N addition on the soil biota, but the effects of N addition on nematodes in plant litter versus in soil are not well understood, especially in semi-arid steppe ecosystems. Here we investigated the composition of nematode communities in plant litter versus in surface soil in two climatically contrasting years in a simulated N enrichment experiment with four N levels (0, 25, 50 and 100 kg N ha−1 yr−1) in a temperate semi-arid grassland. We found that nematodes in plant litter were more sensitive to N addition than that in surface soil, that is, the abundance and generic richness of nematodes in plant litter, but not in surface soil, manifest a significant difference among N addition rates at both community and guild levels. The abundance and generic richness of nematodes were significantly higher in 2015 (a year with relatively wet weather) than 2016 (a year with relatively dry weather) in soil, but the opposite was true on the abundance of nematodes in litter. The plant parasites and omnivores-predators were more abundant, whereas the bacterivores and fungivores were less abundant, in 2015 than 2016. Our results also indicate that litter C/N ratio and soil moisture have greater impact than litter biomass and plant species richness on the abundance of nematodes in litter layer, while soil moisture and resource availability have greater impact than soil N availability and pH on the abundance of nematodes in the top soil. These results provide new insights into the dynamics of nematode communities, and suggest that the litter layer is very important to maintain a stable nematode community in the steppe ecosystem under the fluctuating climate. Unlabelled Image • Nematodes were less sensitive to N addition in surface soil than in plant litter. • Plant parasites and omnivores-predators were more abundant in wet year. • Bacterivores and fungivores were more abundant in dry year. • Litter C/N ratio is negatively related to the abundance of nematodes in litter. • Soil moisture is positively associated with the abundance of nematodes in soil. [ABSTRACT FROM AUTHOR]

• Bacterial communities and brGDGTs are compared along an aridity transect. • Cyclic brGDGTs do not respond to changes in arid soils of pH > 7.5. • 5- and 6-methyl brGDGTs show opposite trends along the aridity transect. • Acidobacteria abundances show no relationships with brGDGTs, pH or aridity. • Actinobacteria and Verrucomicrobia possibly produce brGDGTs in arid soils. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids of certain soil bacteria, and their relative distributions are used as a proxy for air temperature and soil pH. While temperature is recorded by the degree of methylation, soil pH is reflected by the amount of internal cyclization and the relative abundance of 6-methyl isomers. Since the exact producers of brGDGTs remain enigmatic, the mechanisms underlying their empirical relationships with temperature and soil pH, and thus the reliability of brGDGT-based paleorecords, are not well understood, especially in arid regions where mean annual precipitation (MAP) is less than 500 mm. Here, we evaluate the influence of soil pH and aridity on brGDGT distributions in grassland soils along an aridity transect (MAP = 173–415 mm) in Inner Mongolia. While the absolute and fractional abundance of 6-methyl brGDGTs increases with increasing soil pH and aridity, following the trend in the global surface soil calibration dataset, the degree of cyclization does not. This indicates that in arid regions, soil pH reconstructions based on the relative contribution of 6-methyl brGDGTs are likely more reliable than those based on the degree of cyclization. Furthermore, 5- and 6-methyl brGDGTs respond differently to aridity, supporting prior suggestions that the distribution of brGDGTs could be the result of changes in bacterial community composition instead of the direct physiological alteration of molecular structures by the source organisms. Analysis of the bacterial community composition in the same soil transect indicates that the relative abundance of Acidobacteria, the phylum hosting potential brGDGT source-organisms, shows a poor relationship with aridity. Instead, Verrucomicrobia (r2 = 0.70, p < 0.01) , and its subclass Spartobacteria (r2 = 0.70, p < 0.01) in particular, show a significant negative correlation with aridity, resembling that of 5-methyl brGDGTs. Similarly, Actinobacteria are positively correlated with aridity (r2 = 0.59, p < 0.01), following the same trend as that of 6-methyl brGDGTs. The ability of certain cultures of Verrucomicrobia and Actinobacteria to produce iso- C 15:0 fatty acids that could serve as building blocks for brGDGTs hints that Verrucomicrobia and Actinobacteria could possibly produce brGDGTs in arid soils. [ABSTRACT FROM AUTHOR]

• Microbial necromass abundance is regulated by community production in subsoil. • Microbial necromass abundance is more related to its preservation in topsoil. • Climate is the primary predictor for microbial necromass distribution in topsoil. • Soil properties best explain microbial necromass spatial distribution in subsoil. • Subsoil shows elevated preservation of microbial necromass relative to biomass. Microbial carbon has recently been highlighted to play a key role in the formation and persistence of soil organic carbon, bearing significant implications for regulating ecosystem carbon stocks under global changes. However, microbial carbon distribution and the link between biomass and necromass components are poorly understood in natural soils, especially at depth. Here, we employ various microbial biomarkers, including phospholipid fatty acids (PLFAs), amino sugars and glycerol dialkyl glycerol tetraethers (GDGTs), to investigate the spatial distribution patterns of microbial biomass and necromass components in the top- (0–10 cm) versus subsoils (30–50 cm) across Chinese temperate grasslands along an aridity gradient. We find that bacterial necromass components are better preserved relative to bacterial biomass in the sub- than topsoil, possibly due to a stronger association of microbial necromass with calcium and/or lower nitrogen competition between plants and microbes at depth in these neutral-to-alkaline soils. As a result, there is a stronger link between bacterial necromass components (especially for core lipid branched GDGTs and muramic acid) and their producers (reflected by intact polar lipid-derived branched GDGTs) in the sub- than topsoil, while such a trend is not observed for fungi- or archaea-derived components. Furthermore, using linear mixed effect model analyses, we find that aridity index best explains the concentration variance of most microbial biomarkers in the topsoil, whereas edaphic properties (i.e., pH and macronutrients) also contribute significantly to their variance in the subsoil. These findings highlight different links between microbial necromass and biomass components and distinct preservation mechanisms for microbial carbon at different soil depths, which is crucial for improved understanding of microbial carbon sequestration potentials at different depths in a changing environment. [ABSTRACT FROM AUTHOR]