Biogeochemical feedbacks to climate change: Insights into soil moisture controls on soil heterotrophic respiration
Résumé
Soils contain the largest C terrestrial pool and even small changes in soil organic C (SOC) can have a significant impact on the atmospheric CO2 concentration. t he C cycle feedback to climate will largely depend, in magnitude and timing, on the response of soil C to climate changes. However, there is still considerable uncertainty regarding such effects. While the effect of temperature on soil C mineralization has been the subject of much work and considerable debate, much less attention has been paid to the effect of changes in water regime, another predicted component of climate change. However, soil moisture has a key role in regulating soil respiration. Here, we synthesise work regarding the relation between soil moisture and soil C dynamics, focusing on the underlying mechanisms, on the interaction with soil properties and other components of global changes, and discuss how mechanisms and the complexity of soils can be integrated into models. t he effect of soil moisture is complex to predict because, unlike that of temperature, it is strongly dependent on soil characteristics. s oil moisture effect on heterotrophic respiration is represented in most current carbon cycle models by empirical functions, which are often based on limited experimental data. We performed a data-driven analysis of soil moisture respiration relations and showed how these are consistently affected by soil properties such as clay content, organic C content, bulk density. We developed empirical models including the effects of soil texture, soil organic carbon and bulk density which improve the functions currently used in different soil biogeochemical models. p artially responsible for the present state of knowledge may be the idea that, in biogeochemical models, time- and spatially averaged or approximate relationship of microbial activity with moisture is sufficient to reliably predict C fluxes. But soil microorganisms live in a complex 3-D framework of mineral and organic particles defining pores of various sizes, more or less inter-connected, which result in a variety of microhabitats. Most promising perspectives in this area are based on mechanistic approaches, where theoretical linkages between substrate and gas diffusivity in soil pores and heterotrophic respiration are explored in different soil matrices. a new generation of biogeochemical models is based on an explicit representation of soil architecture at a fine scale, which provides in-depth mechanistic understanding, and should help to define relevant descriptors of soil characteristics to improve how larger scale models account for the effects of soil moisture. t he complex interplay between biological, biogeochemical and physical processes is apparent when considering the variability of soil C storage upon a widely used cropping practice: not tillage, where soil moisture also regulates the flows of C between soil litter and mineral layers. The interactions between soil moisture and other components of global change, i.e. land use, cropping practices, temperature, will also result in biogeochemical feedbacks to climate change.