Long-term irrigation reduces soil carbon sequestration by affecting soil microbial communities in agricultural ecosystems of northern China

Irrigation has become one of the main approaches to improve agricultural production in an arid area. The variation of soil moisture after irrigation has the potential to affect soil microbial community composition and soil organic carbon (SOC) storage, and thus, the imbalances in the terrestrial ecosystem carbon cycle. However, the impact of long-term irrigation on the relationships between soil microbial community and SOC sequestration in semiarid agroecosystems is still poorly understood. We took advantage of a 7-year irrigation experiment in a winter wheat-maize rotation system in northern China, whereby the non-irrigation was subject to rain-fed conditions. We aimed to investigate the effects of long-term irrigation on soil microbial communities and their linkages with soil carbon sequestration. Seven years of irrigation significantly increased soil moisture content by 39% but decreased SOC concentration of topsoil (0-20 cm) by 4.2% on average across all sampling times. The responses of soil microbial communities to irrigation were highly taxa dependent. Irrigation significantly decreased fungal biomass, fungal-to-bacterial ratio and Gram-positive-to-Gram-negative bacterial ratio, and did not affect the bacterial community biomass. The decreased SOC concentration under the long-term irrigation was mainly caused by the changes in the ratio of fungi-to-bacteria. Our findings highlight the important role of soil fungal-to-bacterial ratio in mediating the response of SOC dynamics to a future drier climate in semiarid agricultural ecosystems. Highlights Irrigation influences the soil microbial community and carbon stock in semiarid agroecosystems. Highlights the role of fungi:bacteria ratio in mediating the response of SOC dynamics to irrigation Long-term irrigation decreased soil carbon content by changing the microbial community. Reduced SOC storage after irrigation was due to the decreased ratio between fungi and bacteria.