Global methane (CH₄) emissions have reached approximately 600 Tg per year, 20%–40% of which are from wetlands. Of the primary factors affecting these emissions, the water table level is among the most uncertain. Here we conduct a global meta-analysis of chamber and flux-tower observations of CH₄ emissions and employ a novel mechanistic model to show that wetlands have maximum emissions at a critical level of inundation and discuss its origin. This maximum arises from an interplay between methanogenesis, methanotrophy, and transport, whose rates vary differently with the inundation level. The specific location of the critical water level above the soil surface may differ depending on wetland characteristics, for example temperature or the presence of macrophytes with aerenchyma. However, data suggest that globally a water level of about 50 cm is the most favorable to CH₄ emissions. Keeping the water level away from this critical value could reduce methane emissions in human-made wetlands, which comprise at least one fifth of the global wetland area.

Microbial growth is a clear example of organization and structure arising in nonequilibrium conditions. Due to the complexity of the microbial metabolic network, elucidating the fundamental principles governing microbial growth remains a challenge. Here, we present a systematic analysis of microbial growth thermodynamics, leveraging an extensive dataset on energy-limited monoculture growth. A consistent thermodynamic framework based on reaction stoichiometry allows us to quantify how much of the available energy microbes can efficiently convert into new biomass while dissipating the remaining energy into the environment and producing entropy. We show that dissipation mechanisms can be linked to the electron donor uptake rate, a fact leading to the central result that the thermodynamic efficiency is related to the electron donor uptake rate by the scaling law $\eta \propto {\mu }_{ED}^{-1/2}$ and to the growth yield by $\eta \propto Y^{4/5}$. These findings allow us to rederive the Pirt equation from a thermodynamic perspective, providing a means to compute its coefficients, as well as a deeper understanding of the relationship between growth rate and yield. Our results provide rather general insights into the relation between mass and energy conversion in microbial growth with potentially wide application, especially in ecology and biotechnology. Significance Understanding the principles underlying microbial growth is paramount to the cycle of carbon and nutrients in the biosphere, bioremediation technologies, and biochemical engineering, as well as to natural selection and evolution. Yet, fundamental questions remain on the links between mass and energy balances in microbial metabolism and growth. Guided by a nonequilibrium thermodynamics framework, we interpret extensive literature data on microbial growth. The analysis reveals how mass and energy conversion are tightly coupled by scaling laws relating the thermodynamic efficiency to the electron donor uptake rate and the growth yield. Most importantly, these results appear to be universal, in that they apply across microbial species and metabolic pathways, and pave the way for a general thermodynamic theory of microbiological systems.

Soil heterotrophic respiration (R_h) represents an important component of the terrestrial carbon cycle that affects whether ecosystems function as carbon sources or sinks. Due to the complex interactions between biological and physical factors controlling microbial growth, R_h is uncertain and difficult to predict, limiting our ability to anticipate future climate trajectories. Here we analyze the global FLUXNET 2015 database aided by a probabilistic model of microbial growth to examine the ecosystem-scale dynamics of R_h and identify primary predictors of its variability. We find that the temporal variability in R_h is consistently distributed according to a Gamma distribution, with shape and scale parameters controlled only by rainfall characteristics and vegetation productivity. This distribution originates from the propagation of fast hydrologic fluctuations on the slower biological dynamics of microbial growth and is independent of biome, soil type, and microbial physiology. This finding allows us to readily provide accurate estimates of the mean R_h and its variance, as confirmed by a comparison with an independent global dataset. Our results suggest that future changes in rainfall regime and net primary productivity will significantly alter the dynamics of R_h and the global carbon budget. In regions that are becoming wetter, R_h may increase faster than net primary productivity, thereby reducing the carbon storage capacity of terrestrial ecosystems. Significance The variability of the different ecosystem carbon fluxes directly influences the global carbon budget and future climate trajectory. Heterotrophic respiration is particularly uncertain and the extent to which its dynamics are driven by climate, soil, and vegetation properties remains poorly understood. Here we combine a global carbon flux dataset with a probabilistic model of microbial growth to show that despite its complexity the ecosystem-scale heterotrophic respiration can be described as a function of rainfall characteristics and vegetation primary productivity, regardless of ecosystem type. This emergent simplicity in ecosystem-scale heterotrophic respiration may help close the ecosystem carbon budgets and provide a quantitative framework to forecast the possible impacts of climate change on the soil carbon balance.