Throughout Earth's history, drought has been a common crisis in terrestrial ecosystems; in human societies, it can cause famine, one of the Four Horsemen of the apocalypse. As the global hydrological ...cycle intensifies with global warming, deeper droughts and rewetting will alter, and possibly transform, ecosystems. Soil communities, however, seem more tolerant than plants or animals are to water stress-the main effects, in fact, on soil processes appear to be limited diffusion and the limited supply of resources to soil organisms. Thus, the rains that end a drought not only release soil microbes from stress but also create a resource pulse that fuels soil microbial activity. It remains unclear whether the effects of drought on soil processes result from drying or rewetting. It is also unclear whether the flush of activity on rewetting is driven by microbial growth or by the physical chemical processes that mobilize organic matter. In this review, I discuss how soil water, and the lack of it, regulates microbial life and biogeochemical processes. I first focus on organismal-level responses and then consider how these influence whole-soil organic matter dynamics. A final focus is on how to incorporate these effects into Earth System models that can effectively capture dry-wet cycling.
Soil heterotrophic respiration and nutrient mineralization are strongly affected by environmental conditions, in particular by moisture fluctuations triggered by rainfall events. When soil moisture ...decreases, so does decomposers' activity, with microfauna generally undergoing stress sooner than bacteria and fungi. Despite differences in the responses of individual decomposer groups to moisture availability (e.g., bacteria are typically more sensitive than fungi to water stress), we show that responses of decomposers at the community level are different in soils and surface litter, but similar across biomes and climates. This results in a nearly constant soil-moisture threshold corresponding to the point when biological activity ceases, at a water potential of about −14 MPa in mineral soils and −36 MPa in surface litter. This threshold is shown to be comparable to the soil moisture value where solute diffusion becomes strongly inhibited in soil, while in litter it is dehydration rather than diffusion that likely limits biological activity around the stress point. Because of these intrinsic constraints and lack of adaptation to different hydro-climatic regimes, changes in rainfall patterns (primary drivers of the soil moisture balance) may have dramatic impacts on soil carbon and nutrient cycling.
The dominant pools of C and N in the terrestrial biosphere are in soils, and understanding what factors control the rates at which these pools cycle is essential in understanding soil CO2 production ...and N availability. Many previous studies have examined large scale patterns in decomposition of C and N in plant litter and organic soils, but few have done so in mineral soils, and fewer have looked beyond ecosystem specific, regional, or gradient-specific drivers. In this study, we examined the rates of microbial respiration and net N mineralization in 84 distinct mineral soils in static laboratory incubations. We examined patterns in C and N pool sizes, microbial biomass, and process rates by vegetation type (grassland, shrubland, coniferous forest, and deciduous/broadleaf forest). We also modeled microbial respiration and net N mineralization in relation to soil and site characteristics using structural equation modeling to identify potential process drivers across soils. While we did not explicitly investigate the influence of soil organic matter quality, microbial community composition, or clay mineralogy on microbial process rates in this study, our models allow us to put boundaries on the unique explanatory power these characteristics could potentially provide in predicting respiration and net N mineralization. Mean annual temperature and precipitation, soil C concentration, microbial biomass, and clay content predicted 78% of the variance in microbial respiration, with 61% explained by microbial biomass alone. For net N mineralization, only 33% of the variance was explained, with mean annual precipitation, soil C and N concentration, and clay content as the potential drivers. We suggest that the high R2 for respiration suggests that soil organic matter quality, microbial community composition, and clay mineralogy explain at most 22% of the variance in respiration, while they could explain up to 67% of the variance in net N mineralization.
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► We measured microbial respiration and net N mineralization in 84 soils. ► Surface mineral soils were collected from across biomes and vegetation types. ► We built structural equation models to describe these processes. ► Models explained 79% & 33% variance in respiration & N mineralization, respectively. ► Puts bounds on influence of microbial community, OM quality, clay mineralogy.
Ecosystems across the biosphere are subject to rapid changes in elemental balance and climatic regimes. A major force structuring ecological responses to these perturbations lies in the ...stoichiometric flexibility of systems – the ability to adjust their elemental balance whilst maintaining function. The potential for stoichiometric flexibility underscores the utility of the application of a framework highlighting the constraints and consequences of elemental mass balance and energy cycling in biological systems to address global change phenomena. Improvement in the modeling of ecological responses to disturbance requires the consideration of the stoichiometric flexibility of systems within and across relevant scales. Although a multitude of global change studies over various spatial and temporal scales exist, the explicit consideration of the role played by stoichiometric flexibility in linking micro-scale to macro-scale biogeochemical processes in terrestrial ecosystems remains relatively unexplored. Focusing on terrestrial systems under change, we discuss the mechanisms by which stoichiometric flexibility might be expressed and connected from organisms to ecosystems. We suggest that the transition from the expression of stoichiometric flexibility within individuals to the community and ecosystem scales is a key mechanism regulating the extent to which environmental perturbation may alter ecosystem carbon and nutrient cycling dynamics.
A mechanistic understanding of microbial assimilation of soil organic carbon is important to improve Earth system models’ ability to simulate carbon‐climate feedbacks. A simple modelling framework ...was developed to investigate how substrate quality and environmental controls over microbial activity regulate microbial assimilation of soil organic carbon and on the size of the microbial biomass. Substrate quality has a positive effect on microbial assimilation of soil organic carbon: higher substrate quality leads to higher ratio of microbial carbon to soil organic carbon. Microbial biomass carbon peaks and then declines as cumulative activity increases. The simulated ratios of soil microbial biomass to soil organic carbon are reasonably consistent with a recently compiled global data set at the biome level. The modelling framework developed in this study offers a simple approach to incorporate microbial contributions to the carbon cycling into Earth system models to simulate carbon‐climate feedbacks and explain global patterns of microbial biomass.
A major thrust of terrestrial microbial ecology is focused on understanding when and how the composition of the microbial community affects the functioning of biogeochemical processes at the ...ecosystem scale (meters-to-kilometers and days-to-years). While research has demonstrated these linkages for physiologically and phylogenetically "narrow" processes such as trace gas emissions and nitrification, there is less conclusive evidence that microbial community composition influences the "broad" processes of decomposition and organic matter (OM) turnover in soil. In this paper, we consider how soil microbial community structure influences C cycling. We consider the phylogenetic level at which microbes form meaningful guilds, based on overall life history strategies, and suggest that these are associated with deep evolutionary divergences, while much of the species-level diversity probably reflects functional redundancy. We then consider under what conditions it is possible for differences among microbes to affect process dynamics, and argue that while microbial community structure may be important in the rate of OM breakdown in the rhizosphere and in detritus, it is likely not important in the mineral soil. In mineral soil, physical access to occluded or sorbed substrates is the rate-limiting process. Microbial community influences on OM turnover in mineral soils are based on how organisms allocate the C they take up - not only do the fates of the molecules differ, but they can affect the soil system differently as well. For example, extracellular enzymes and extracellular polysaccharides can be key controls on soil structure and function. How microbes allocate C may also be particularly important for understanding the long-term fate of C in soil - is it sequestered or not?
Extracellular enzymes in soil are central to the decomposition of plant and microbial detritus and they are increasingly incorporated into soil biogeochemical models as drivers of detritus breakdown. ...In enzyme-driven models, a critical parameter is the functional lifespan of the enzymes, yet this is poorly constrained by experimental data. We evaluated how long soil enzymes remain active in five soils spanning from arctic tussock tundra to a tropical forest/grassland soil. We incubated soils under continuous fumigation with CHCl3 vapor to kill microbes and prevent the synthesis of new enzymes. We monitored the activities of six hydrolytic and two oxidative enzymes over a 12-week incubation. Initial activities of the various soil enzymes varied substantially across the ecosystems; they were generally highest (per gram soil) in the tussock tundra, followed by temperate hardwood forest, grassland, tropical forest/grassland, and then chaparral. In tussock tundra organic soil, activities of all enzymes decreased rapidly following first-order decay curves; the half-lives of enzyme activity were typically several weeks. In the mineral soils, the time course of loss of hydrolytic enzyme activities could always be described by a first-order decay curve, but in many cases, activity could equally be described by a zero-order, linear, decay function because the rate of loss was slow. Although α-glucosidase lost activity rapidly, for other enzymes substantial activity remained even after 12 weeks of incubation. This likely resulted from stabilization by mineral surfaces, stabilization that might constrain activity against native polymeric substrates. Measured turnover rate constants fell within the broad range that model have used, but that range remains exceedingly broad.
•Extracellular enzyme potential activities in decay over time in soils incubated under CHCl3.•In mineral soils, activity loss was generally slow, with most enzymes persisting for 12 weeks.•In a tundra organic soil, activities declined rapidly, with half-lives of roughly 2–3 weeks.
A major goal of microbial ecology is to identify links between microbial community structure and microbial processes. Although this objective seems straightforward, there are conceptual and ...methodological challenges to designing studies that explicitly evaluate this link. Here, we analyzed literature documenting structure and process responses to manipulations to determine the frequency of structure-process links and whether experimental approaches and techniques influence link detection. We examined nine journals (published 2009–13) and retained 148 experimental studies measuring microbial community structure and processes. Many qualifying papers (112 of 148) documented structure and process responses, but few (38 of 112 papers) reported statistically testing for a link. Of these tested links, 75% were significant and typically used Spearman or Pearson's correlation analysis (68%). No particular approach for characterizing structure or processes was more likely to produce significant links. Process responses were detected earlier on average than responses in structure or both structure and process. Together, our findings suggest that few publications report statistically testing structure-process links. However, when links are tested for they often occur but share few commonalities in the processes or structures that were linked and the techniques used for measuring them.
Few publications reported statistically testing microbial community structure-process links; 75% of tested links were significant, though had few commonalities in which processes or structures were measured and the techniques used.
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