Impacts of water regime and land-use on soil CO2 efflux in a small temperate agricultural catchment

Abstract

Within the context of climate change, it is necessary to improve our mechanistic understanding of processes that release CO2 to the atmosphere. This study was designed to investigate spatial variation of soil CO2 efflux (FCO2) at the landscape (catchment) scale, which has received less attention so far in comparison to more homogeneous, smaller areas. More specifically, this study investigated the impacts of water regime, land-use and crop rotation on FCO2 in a small temperate agricultural catchment. FCO2 was measured manually with closed dynamic chambers over a 1 year period (36 weekly to bi-weekly measurement dates) at 22 sites that were selected to represent most of the diversity of drainage classes, soil types and land-uses in a small catchment in North-Western France. FCO2 was modelled empirically at each site as a function of soil temperature and soil water content. As a secondary step, impacts of water regime, land-use and crop rotation on heterotrophic respiration (HR) were also sought, with HR being estimated empirically as a fraction of FCO2 on the basis of partitioning coefficients taken from the literature. Results showed that water regime, land-use and crop rotation all had significant impacts on FCO2 on an annual scale. Poorly-drained sites emitted less CO2 than well-drained and moderately-well drained sites, which could be explained by reduced soil CO2 production and/or limitations in soil transport in more saturated environments. In addition, larger annual FCO2 were measured on grasslands compared to croplands (wheat and maize crops), which is likely due to the presence of larger amounts of available soil carbon and nutrients and to the presence or denser rooting systems. Due to the literature-based determination of partitioning coefficients, the results related to HR estimates were affected by more uncertainties than those related to FCO2 but they showed that the water regime tended to impact them similarly as FCO2, while HR was likely less affected by land-use. Altogether, the results of this study, performed at scale of a small agricultural temperate catchment, confirm that specific patterns of FCO2 result from varying soil CO2 production and transport processes, resulting from complex interactions between soil environment characteristics (soil carbon content, water regime,…) and land-use/crop rotations.