Responses of soil heterotrophic respiration to moisture availability: An exploration of processes and models

Soil moisture strongly affects the dynamics of soil organic matter and is an important environmental variable in all models predicting changes in soil carbon stocks from site to global scales. Despite this, the mechanisms determining the response of heterotrophic soil respiration to soil moisture remain poorly quantified, being represented in most current carbon cycle models as simple empirical functions. With the aim of providing an overview and new insights into the mechanisms involved, here we review the observations and theory behind the respiration-moisture relationship. We start by calculating best estimates of average empirical relationships using published data and comparing the results for contrasting soil types. The theoretical linkages between substrate and gas diffusivity in soil pores and heterotrophic respiration are then explored as a function of temperature and textural properties. Based on this theoretical model we interpret the variability of moisture effects observed in previous empirical studies. Transient CO2 efflux–moisture relationships are discussed next, reviewing the theory and models developed in the last years with particular emphasis on the ‘Birch effect’. We continue by giving an overview of recent pore-scale models and consider how these can be used to gain a more mechanistic understanding of carbon storage and stabilization in variably saturated soils. From this review we conclude that current empirical models are useful but limited approximations, with questionable predictive capacity. Significant efforts are still necessary to represent the full range of soil moisture responses in a unifying model with a sound theoretical basis that can help identify common underlying processes. Equations present here, combining diffusion, texture and substrate to model respiration, are a step forward in this direction.