Since fungi and bacteria are the dominant decomposers in soil, their distinct physiologies are likely to differentially influence rates of ecosystem carbon (C) and nitrogen (N) cycling. We used ...meta‐analysis and an enzyme‐driven biogeochemical model to explore the drivers and biogeochemical consequences of changes in the fungal‐to‐bacterial ratio (F : B). In our meta‐analysis data set, F : B increased with soil C : N ratio (R2 = 0.224, P < 0.001), a relationship predicted by our model. We found that differences in biomass turnover rates influenced F : B under conditions of C limitation, while differences in biomass stoichiometry set the upper bounds on F : B once a nutrient limitation threshold was reached. Ecological interactions between the two groups shifted along a gradient of resource stoichiometry. At intermediate substrate C : N, fungal N mineralisation fuelled bacterial growth, increasing total microbial biomass and decreasing net N mineralisation. Therefore, we conclude that differences in bacterial and fungal physiology may have large consequences for ecosystem‐scale C and N cycling.
Censuses of tropical forest plots reveal large variation in biomass and plant composition. This paper evaluates whether such variation can emerge solely from realistic variation in a set of commonly ...measured soil chemical and physical properties.
Controlled simulations were performed using a mechanistic model that includes forest dynamics, microbe-mediated biogeochemistry, and competition for nitrogen and phosphorus. Observations from 18 forest inventory plots in Guanacaste, Costa Rica were used to determine realistic variation in soil properties.
In simulations of secondary succession, the across-plot range in plant biomass reached 30% of the mean and was attributable primarily to nutrient limitation and secondarily to soil texture differences that affected water availability. The contributions of different plant functional types to total biomass varied widely across plots and depended on soil nutrient status. In Central America, soil-induced variation in plant biomass increased with mean annual precipitation because of changes in nutrient limitation.
In Central America, large variation in plant biomass and ecosystem composition arises mechanistically from realistic variation in soil properties. The degree of biomass and compositional variation is climate sensitive. In general, model predictions can be improved through better representation of soil nutrient processes, including their spatial variation.
The theory of ecological stoichiometry predicts that the microbial biomass should regulate production of extracellular enzymes to target the resource in shortest supply. Therefore, microbial ...communities on decomposing leaf litter should optimize allocation to C-, N-, and P-degrading enzymes according to the stoichiometry of the foliar substrate. Because extracellular enzymes are the proximate agents of leaf litter decay, shifts in microbial enzyme allocation may influence overall rates of litter mass loss. To test these hypotheses, I measured fungal growth and the activities of acid phosphatase (AP), beta-glucosidase (BG), cellobiohydrolase (CB) and glycine aminopeptidase (GAP) on decaying leaf litter of five plant species over the course of a 394-day decomposition experiment. I used regression and correlation analyses to link to interspecific variation in mass loss rates with enzyme activities and foliar nutrient content. Enzymes explained 35% of the variance in foliar decay rates across plant species, yet fungal abundance and enzyme activities were unrelated to foliar concentrations of N, P, K, or 9 other nutrients. Furthermore, relative activities of C-, N-, and P-acquiring enzymes did not vary across litter types despite wide variance in foliar C:N and C:P ratios. This weak relationship between litter stoichiometry and decomposition rates suggests that nutrients are not the primary control on microbial growth or enzyme allocation in this tropical forest. However, substantial interspecific differences in fungal abundance and enzyme activities imply that differences in litter composition strongly influence microbial communities and the ecosystem processes they mediate.
•Fungal abundance and enzyme activities were measured on decomposing leaves.•Enzyme activities predicted 35% of the variance in foliar decay rates.•Enzyme activity and stoichiometry were unrelated to foliar nutrient content.•Nutrients are not the primary control on microbial growth or enzyme allocation in this ecosystem.
Rates of ecosystem nitrogen (N) cycling may be mediated by the presence of ectomycorrhizal fungi, which compete directly with free‐living microbes for N. In the regenerating tropical dry forests of ...Central America, the distribution of ectomycorrhizal trees is affected by succession and soil parent material, both of which may exert independent influence over soil N fluxes. In order to quantify these interacting controls, we used a scale‐explicit sampling strategy to examine soil N cycling at scales ranging from the microsite to ecosystem level. We measured fungal community composition, total and inorganic N pools, gross proteolytic rate, net N mineralization and microbial extracellular enzyme activity at multiple locations within 18 permanent plots that span dramatic gradients of soil N concentration, stand age and forest composition. The ratio of inorganic to organic N cycling was correlated with variation in fungal community structure, consistent with a strong influence of ectomycorrhiza on ecosystem‐scale N cycling. However, on average, > 61% of the variation in soil biogeochemistry occurred within plots, and the effects of forest composition were mediated by this local‐scale heterogeneity in total soil N concentrations. These cross‐scale interactions demonstrate the importance of a spatially explicit approach towards an understanding of controls on element cycling.
Seasonally dry tropical forests (SDTF) are located in regions with alternating wet and dry seasons, with dry seasons that last several months or more. By the end of the 21st century, climate models ...predict substantial changes in rainfall regimes across these regions, but little is known about how individuals, species, and communities in SDTF will cope with the hotter, drier conditions predicted by climate models. In this review, we explore different rainfall scenarios that may result in ecological drought in SDTF through the lens of two alternative hypotheses: 1) these forests will be sensitive to drought because they are already limited by water and close to climatic thresholds, or 2) they will be resistant/resilient to intra- and inter-annual changes in rainfall because they are adapted to predictable, seasonal drought. In our review of literature that spans microbial to ecosystem processes, a majority of the available studies suggests that increasing frequency and intensity of droughts in SDTF will likely alter species distributions and ecosystem processes. Though we conclude that SDTF will be sensitive to altered rainfall regimes, many gaps in the literature remain. Future research should focus on geographically comparative studies and well-replicated drought experiments that can provide empirical evidence to improve simulation models used to forecast SDTF responses to future climate change at coarser spatial and temporal scales.
Although tropical forests occupy a small fraction of the earth's total land area, they play a disproportionately large role in regulating the global carbon cycle. Yet controls on both primary ...productivity and decomposition in tropical forests are not well-studied in comparison with temperate forests and grasslands, despite their extreme biogeochemical heterogeneity. To evaluate the relative importance of climate and foliar chemical variables in driving decomposition in tropical forests, I performed a meta-analysis of reported leaf litter decay rates throughout tropical forest ecosystems. Using a model selection procedure based on Akaike's Information Criterion, I found that temperature and precipitation played little direct role in regulating decomposition rates, except in montane forests where cool temperatures slowed decay. Foliar concentrations of calcium, magnesium, nitrogen, phosphorus, and potassium were important predictors of mass loss rates, although each of these factors explained a very small amount of variance when considered in isolation. The large amount of unexplained variation in decomposition rates observed both within and across tropical forest sites may be due to other factors not explored here, such as soil biota or complex plant secondary chemistry. Carbon cycling in tropical forests seems to be modulated by the availability of multiple nutrients, underscoring the need for additional manipulative experiments to explore patterns of belowground nutrient limitation across the biome. Because models of decomposition developed in temperate ecosystems do not appear to be generalizable to wet tropical forests, new biogeochemical paradigms should be developed to accommodate their unique combination of climatic, edaphic, and biotic factors.
Conversely, restoration efforts might employ ‘bottom-up’ approaches that focus on the soil microbes which release and transport plant-available nutrients, secrete growth-promoting hormones, and ...mediate plant community succession through plant-soil feedbacks. ...recently, most restoration efforts failed to consider whether and how to re-introduce beneficial microbes to a site–and without intervention, recovery of the soil microbiome in revegetated sites is often incomplete (Watson et al., 2022). ...developments might include the invention of new equipment/infrastructure to help native species to spread and re-establish (Temmink et al., 2020); the deployment of sensors and satellites for monitoring restoration projects at scale; and the data infrastructure necessary to share, analyse, and synthesise information about restoration outcomes (Perring et al., 2015). Yet it is very challenging to understand the factors that limit project success, as many landscape restoration efforts do not solicit sufficient involvement of local stakeholders in planning, data collection or monitoring (Evans et al., 2023). ...large-scale rehabilitation of degraded lands will also involve careful attention to the socioeconomic factors that drive land use change–which itself comprises another major challenge to twenty-first century restoration.
Soil moisture constrains the activity of decomposer soil microorganisms, and in turn the rate at which soil carbon returns to the atmosphere. While increases in soil moisture are generally associated ...with increased microbial activity, historical climate may constrain current microbial responses to moisture. However, it is not known if variation in the shape and magnitude of microbial functional responses to soil moisture can be predicted from historical climate at regional scales. To address this problem, we measured soil enzyme activity at 12 sites across a broad climate gradient spanning 442–887 mm mean annual precipitation. Measurements were made eight times over 21 months to maximize sampling during different moisture conditions. We then fit saturating functions of enzyme activity to soil moisture and extracted half saturation and maximum activity parameter values from model fits. We found that 50% of the variation in maximum activity parameters across sites could be predicted by 30‐year mean annual precipitation, an indicator of historical climate, and that the effect is independent of variation in temperature, soil texture, or soil carbon concentration. Based on this finding, we suggest that variation in the shape and magnitude of soil microbial response to soil moisture due to historical climate may be remarkably predictable at regional scales, and this approach may extend to other systems. If historical contingencies on microbial activities prove to be persistent in the face of environmental change, this approach also provides a framework for incorporating historical climate effects into biogeochemical models simulating future global change scenarios.
Ecosystem carbon losses from soil microbial respiration are a key component of global carbon cycling, resulting in the transfer of 40–70 Pg carbon from soil to the atmosphere each year. Because these ...microbial processes can feed back to climate change, understanding respiration responses to environmental factors is necessary for improved projections. We focus on respiration responses to soil moisture, which remain unresolved in ecosystem models. A common assumption of large-scale models is that soil microorganisms respond to moisture in the same way, regardless of location or climate. Here, we show that soil respiration is constrained by historical climate. We find that historical rainfall controls both the moisture dependence and sensitivity of respiration. Moisture sensitivity, defined as the slope of respiration vs. moisture, increased fourfold across a 480-mm rainfall gradient, resulting in twofold greater carbon loss on average in historically wetter soils compared with historically drier soils. The respiration–moisture relationship was resistant to environmental change in field common gardens and field rainfall manipulations, supporting a persistent effect of historical climate on microbial respiration. Based on these results, predicting future carbon cycling with climate change will require an understanding of the spatial variation and temporal lags in microbial responses created by historical rainfall.
Tropical dry forests are already undergoing changes in the quantity and timing of rainfall, but there is great uncertainty over how these shifts will affect belowground carbon (C) cycling. While it ...has long been known that dry soils quickly release carbon dioxide (CO2) upon rewetting, the mechanisms underlying the so-called 'Birch effect' are still debated. Here, we quantified soil respiration pulses and their biotic predictors in response to simulated precipitation events in a regenerating tropical dry forest in Costa Rica. We also simulated the observed rewetting CO2 pulses with two soil carbon models: a conventional model assuming first-order decay rates of soil organic matter, and an enzyme-catalyzed model with Michaelis-Menten kinetics. We found that rewetting of dry soils produced an immediate and dramatic pulse of CO2, accompanied by rapid immobilization of nitrogen into the microbial biomass. However, the magnitude of the rewetting CO2 pulse was highly variable at fine spatial scales, and was well correlated with the size of the dissolved organic C pool prior to rewetting. Both the enzyme-catalyzed and conventional models were able to reproduce the Birch effect when respiration was coupled directly to microbial C uptake, although models differed in their ability to yield realistic estimates of SOC and microbial biomass pool sizes and dynamics. Our results suggest that changes in the timing and intensity of rainfall events in tropical dry forests will exert strong influence on ecosystem C balance by affecting the dynamics of microbial biomass growth.