Divergent responses of CO2 and CH4 fluxes to changes in the precipitation regime on the Tibetan Plateau: Evidence from soil enzyme activities and microbial communities

Biodiversity in mountain soils above the treeline

Biological diversity in mountain ecosystems has been increasingly studied over the last decade. This is also the case for mountain soils, but no study to date has provided an overall synthesis of the current state of knowledge. Here we fill this gap with a first global analysis of published research on cryptogams, microorganisms, and fauna in mountain soils above the treeline, and a structured synthesis of current knowledge. Based on a corpus of almost 1400 publications and the expertise of 37 mountain soil scientists worldwide, we summarise what is known about the diversity and distribution patterns of each of these organismal groups, specifically along elevation, and provide an overview of available knowledge on the drivers explaining these patterns and their changes. In particular, we document an elevation-dependent decrease in faunal diversity above the treeline, while for cryptogams there is an initial increase above the treeline, followed by a decrease towards the nival belt. Thus, our data confirm the key role that elevation plays in shaping the biodiversity and distribution of these organisms in mountain soils. The response of prokaryote diversity to elevation, in turn, was more diverse, whereas fungal diversity appeared to be substantially influenced by plants. As far as available, we describe key characteristics, adaptations, and functions of mountain soil species, and despite a lack of ecological information about the uncultivated majority of prokaryotes, fungi, and protists, we illustrate the remarkable and unique diversity of life forms and life histories encountered in alpine mountain soils. By applying rule- as well as pattern-based literature-mining approaches and semi-quantitative analyses, we identified hotspots of mountain soil research in the European Alps and Central Asia and revealed significant gaps in taxonomic coverage, particularly among biocrusts, soil protists, and soil fauna. We further report thematic priorities for research on mountain soil biodiversity above the treeline and identify unanswered research questions. Building upon the outcomes of this synthesis, we conclude with a set of research opportunities for mountain soil biodiversity research worldwide. Soils in mountain ecosystems above the treeline fulfil critical functions and make essential contributions to life on land. Accordingly, seizing these opportunities and closing knowledge gaps appears crucial to enable science-based decision making in mountain regions and formulating laws and guidelines in support of mountain soil biodiversity conservation targets.

Microbiota-induced asymmetry in coastal methane emission potential under experimental precipitation gradients

Climate models predict that the frequency and intensity of extreme precipitation events will increase globally. Despite carbon budget in coastal wetlands is known to be sensitive to precipitation variability, in where CH4 productions and potential mechanisms remain poorly understood. We investigated CH4 emission potential and its drivers after 7-year of field experiments with five precipitation gradients (-60%, -40%, ambient condition, +40%, +60%) in Yellow River Delta, China. The response of CH4 emission potential to precipitation gradients exhibited significant asymmetry, with the highest emission potential occurring under +40% precipitation. 13C-isotope tracing experiment discovered the primary contribution of acetoclastic methanogenic pathway. +40% precipitation significantly improved the accumulation of aboveground biomass, soil organic carbon and total nitrogen. Microbial community abundance, but not composition, referring to metagenome-assembled genomes also actively responded to precipitation changes. For example, +40% precipitation increased the relative abundance of Methanosarcinia and Methanobacteria. Furthermore, CH4 emission potential was also promoted by higher microbial enzyme activity. Collectively, CH4 emission potential in response to 7-year experimental precipitations was regulated by microbiota-driven, showing obvious asymmetry.

Soil Moisture Threshold of Methane Uptake in Alpine Grassland Ecosystems

Methane (CH4) uptake in alpine ecosystems is an important component of the global CH4 sink. However, large uncertainties remain regarding the magnitude and spatial patterns of CH4 uptake, owing to its extensive spatial variability, diverse controlling factors, and limited regional-scale observations. Here, we investigated field ecosystem CH4 uptake along a 3200-km transect across various alpine grasslands on the Qinghai-Tibetan Plateau (QTP). We found a substantial spatial variation in in situ CH4 uptake among alpine grasslands, with the highest rates in drier regions of the mid-western QTP. Soil moisture was the most important factor controlling CH4 uptake, exhibiting a remarkably low threshold of 6.2 ± 0.1 v/v %. Below this threshold, CH4 uptake was constrained by soil moisture, moisture-induced nitrogen limitation, and high temperatures. Above this threshold, CH4 uptake was mainly limited by gas diffusion and low temperatures. By integrating grid predictors with a random forest model trained on 1851 field measurements encompassing both our observations and a regional synthesis across the QTP, we estimated a regional CH4 uptake of 0.88 ± 0.020 Tg CH4 year-1 from all alpine grasslands on the QTP. This higher estimate, primarily driven by alpine steppes, was significantly greater than current regional estimates from global CH4 models. Our findings highlight the importance of CH4 sink in dry alpine ecosystems characterized by low soil moisture, suggesting that the contribution of CH4 sink in drylands may have been substantially underestimated in the current global CH4 budget.

The Response Mechanism of the cbbM Carbon Sequestration Microbial Community in the Alpine Wetlands of Qinghai Lake to Changes in Precipitation

The dramatic changes in precipitation patterns on the Tibetan Plateau affected the carbon-sequestering microbial communities within wetland ecosystems, which were closely related to the responses and adaptation mechanisms of alpine wetland ecosystems to climate change. This study focused on wetland soils subjected to different precipitation gradient treatments and employed high-throughput sequencing technology to analyze the soil cbbM carbon-sequestering microbial communities. The results indicated that Proteobacteria were the dominant microbial community responsible for carbon sequestration in the Wayan Mountain wetland. A 50% increase in precipitation significantly raised the soil moisture content, while a 50% reduction and a 25% increase in precipitation notably enhanced the total soil carbon content. The 25% reduction in precipitation increased the differences in microbial community composition, whereas both the 50% increase and the 50% reduction in precipitation decreased these differences. The soil pH and temperature had the most significant impact on the carbon-sequestering microbial communities. In conclusion, changes in precipitation affect the cbbM carbon sequestration characteristics of soil microbial communities, and a moderate reduction in water input benefited carbon sequestration in wetlands.

Soil temperature and fungal diversity jointly modulate soil heterotrophic respiration under short-term warming in the Zoige alpine peatland

Global warming has changed carbon cycling in terrestrial ecosystems, but it remains unclear how climate warming affects soil heterotrophic respiration (Rh). We conducted a field experiment in the Zoige alpine peatland to investigate the mechanism of how short-term warming affects Rh by examining the relationships between plant biomass, soil properties, soil microbial diversity, and functional groups and Rh. Our results showed that warming increased Rh after one growing season of warming. However, warming barely changed the bacterial functional groups involved in the carbon cycle predicted by the functional annotation analysis. According to the Mantel test, NO3- was found to be the primary determinant for bacterial and fungal communities. The results of the Structural Equation Model (SEM) indicate that soil temperature and fungal diversity jointly modulate Rh, suggesting that short-term warming may not affect Rh by altering the structural and functional composition of microorganisms, which provides new insight into the mechanisms of the effects of warming on Rh in terrestrial ecosystems.

Response Characteristics and Community Assembly Mechanisms of nirS-Type Denitrifiers in the Alpine Wetland under Simulated Precipitation Conditions

The nitrogen cycling process in alpine wetlands is profoundly affected by precipitation changes, yet the dynamic response mechanism of denitrifiers to long-term precipitation shifts in the alpine wetland of the Qinghai-Tibet Plateau remains enigmatic. Utilizing high-throughput sequencing analysis of nirS-type functional genes, this study delved into the dynamic response mechanism of nirS-type denitrifiers to precipitation changes in the alpine wetland of Qinghai Lake. The findings revealed that nirS-type denitrifiers in the alpine wetland of Qinghai Lake were primarily Proteobacteria, and Alpha diversity exhibited a negative correlation with the precipitation gradient, with deterministic processes predominating in the community assembly of denitrifying microbes. A 50% increase in rainfall shifted the community assembly process of denitrifiers from deterministic to stochastic. Dominant microflora at the genus level responded significantly to precipitation changes, with aerobic bacteria comprising the majority of differentially abundant taxa (55.56%). As precipitation increased, the complexity of the microbial interaction network decreased, and a 25% reduction in precipitation notably elevated the relative abundance of three key functional groups: chemoheterotrophic, aerobic chemoheterotrophic, and nitrogen fixation. Precipitation notably emerged as the primary regulator of nirS-type denitrifiers in the alpine wetland of Qinghai Lake, accounting for 51% of the variation in community composition. In summary, this study offers a fresh perspective for investigating the ecological processes of nitrogen cycling in alpine ecosystems by examining the diversity and community composition of nirS-type denitrifiers in response to precipitation changes.

Tidal organic input restricts CO2 sequestration capacity of estuarine wetlands

The inland and estuary wetlands that characterized by different natural environment perform distinctly in soil carbon (C) sink. It was deemed that estuary wetland has a higher organic C accumulation rate than inland wetland, due to its higher primary production and tidal organics input, thus having higher organic C sink capacity. While from CO₂ budge in view, whether does the large organic input from tide restrict CO₂ sequestration capacity of estuary wetland has not been discussed comparing with inland wetland. In this study, inland and estuary wetlands were selected to study the potential of CO₂ sequestration capacity. It was found that inland wetland had most of soil organic carbon (SOC) derived from plant C, which brought remarkable organic C content and nourished higher microbial biomass, dehydrogenase, and β_glucosidase than estuary wetland. The estuary wetland instead accumulated less SOC, a considerable proportion of which came from tidal waters, therefore supporting lower microbial biomass and enzyme activities than that in inland wetland. However, estuary wetland was evaluated having higher capability in SOC mineralization than inland wetland in consideration of soil respiration (SR) and SR quotient. It was concluded that tidal organic C accelerated the SOC mineralization in estuarine wetland, thus weakening the CO₂ sequestration. These results implied the importance of pollution control for reservation CO₂ sink function in estuarine wetland.

Soil biogeochemical responses to multiple co-occurring forms of human-induced environmental change

Human activities cause a multitude of environmental issues, including increased temperatures and altered precipitation patterns associated with climate change, air pollution, and other impacts of urbanization. One area highly affected by these issues is the Sonoran Desert, specifically the Phoenix metropolitan area where urbanization is among the most rapid in the United States. Most studies investigate these multiple environmental change factors independently or sometimes in pairs, but rarely all together as co-occurring forms of change. We examined how the simultaneous manipulation of increasing temperatures, altered precipitation patterns, nitrogen deposition, and urbanization influenced soil respiration and mineral N pools in the Sonoran Desert. Soil was collected from urban and exurban sites, from both nitrogen-fertilized and control plots. To simulate projected climate change, the soils were incubated in microcosm at the annual average Phoenix temperature as well a 2 ℃ increase under a factorial precipitation treatment of decreased frequency and increased pulse size. Our results show that C and N dynamics were altered by all four forms of environmental change. However, the dominance of significant 3- and 4-way interactions among the four environmental factors for both respiration and mineral N pools demonstrates that the impact of any given form of environmental change will depend on the levels of the other environmental factors. In other words, the cumulative effect of altered precipitation, fertilization, temperature, and urbanization on soil biogeochemical processes is not necessarily predictable from their individual impact.

Interactive effects of global change drivers as determinants of the link between soil biodiversity and ecosystem functioning

Biodiversity, both aboveground and belowground, is negatively affected by global changes such as drought or warming. This loss of biodiversity impacts Earth's ecosystems, as there is a positive relationship between biodiversity and ecosystem functioning (BEF). Even though soils host a large fraction of biodiversity that underlies major ecosystem functions, studies exploring the relationship between soil biodiversity and ecosystem functioning (sBEF) as influenced by global change drivers (GCDs) remain scarce. Here we highlight the need to decipher sBEF relationships under the effect of interactive GCDs that are intimately connected in a changing world. We first state that sBEF relationships depend on the type of function (e.g., C cycling or decomposition) and biodiversity facet (e.g., abundance, species richness, or biomass) considered. Then, we shed light on the impact of single and interactive GCDs on soil biodiversity and sBEF and show that results from scarce studies studying interactive effects range from antagonistic to additive to synergistic when two individual GCDs cooccur. This indicates the need for studies quantitatively accounting for the impacts of interactive GCDs on sBEF relationships. Finally, we provide guidelines for optimized methodological and experimental approaches to study sBEF in a changing world that will provide more valuable information on the real impact of (interactive) GCDs on sBEF. Together, we highlight the need to decipher the sBEF relationship in soils to better understand soil functioning under ongoing global changes, as changes in sBEF are of immediate importance for ecosystem functioning.