Legacy of warming and cover crops on the response of soil microbial function to repeated drying and rewetting cycles

Abstract

The response of soils to extreme weather events will become increasingly important in the future as more frequent and severe floods and droughts are expected to subject soils to drying and rewetting cycles as a result of climate change. These extreme events will be experienced against a backdrop of overall warming. However, farmers are adopting cover cropping as a sustainable management practice to increase soil organic matter, benefit soil health, and to increase the resilience of soils to help mitigate the impacts of climate change. We examined the legacy of warming and cover crops on the response of soil microbial function to repeated drying and rewetting cycles. We introduced open top chambers to warm the soil surface of a field plot experiment in which cover crops (single species monocultures and 4-species polycultures) were grown over the summer after harvest and before planting of autumn sown cash crops in a cereal rotation. Soil samples were collected from warmed and ambient areas of the experimental plots in spring, before harvesting the cereal crop. We quantified respiration (a measure of soil microbial function) with high-frequency CO2flux measurements after 0, 1, 2, 4, or 8 wet/dry cycles imposed in the laboratory and the addition of barley grass powder substrate at a ratio of 10 mg g-1soil. Cover crop mixtures created a negative legacy effect in the soil which resulted in lower cumulative substrate induced respiration than expected from the average of the same species grown in monoculture. Repeated drying and rewetting cycles increased the cumulative substrate induced respiration rate observed, suggesting that repeated perturbations selected for a community adapted to processing the barley shoot powder more quickly. This adaptation may have resulted in a greater osmolyte production or reacquisition by microorganisms exposed to repeated drought events. Osmolytes are rapidly metabolised upon re-wetting and may have primed the decomposition of the barley shoot powder to a greater extent in soils previously exposed to drying and rewetting cycles. When we calculated the cumulative respiration after 8 wet/dry cycles, relative to cumulative respiration after 0 wet/dry cycles (which we infer represents the extent to which microbial communities adapted to repeated drying and rewetting cycles) our data revealed that the legacy of warming significantly reduced, but cover crops significantly increased, soil microbial community adaptation. This adaptation of the soil microbial community was positively correlated with the concentration of water extractable organic carbon in the soils prior to imposing the drying and rewetting cycles and/or adding the substrate. The availability of labile carbon may have mediated the ability of microorganisms to synthesise osmolytes in response to drought. We conclude that cover crops may enhance the ability of the soil microbial community to adapt to drought events and mitigate the impact of warming, possibly due to the provision of labile organic carbon for the synthesis of osmolytes.