Future climate scenarios indicate more frequent extreme weather events. This includes more severe droughts but also an increase in heavy rain events and flooding. Agricultural systems are of special interest because of their role in food security but also because of their potentially changing role in global carbon and nutrient cycling under these extreme conditions. Plant diversification strategies like more complex crop rotations which support more diverse soil microbial communities with higher functional redundancy might be more resistant to drought and flooding and could help to reduce impacts on microbial carbon and nutrient cycling.
To test how crop diversification affects the response of soil microbial processes to drought and flooding and reoccurring drought and flooding, we manipulated water regimes in lab incubation experiments using soils from four long term rotation experiments across the USA, including a low (one or two crops) vs. high (>3 crops) diversity rotations at each site. The sites range from low precipitation (Colorado), over intermediate precipitation (Michigan and South Dakota) to high precipitation in Maryland. Replicate sets of samples were either allowed to dry out, were gradually flooded or kept at a constant water content (control). We monitored CO2 production during five stress cycles. Additionally, we determined microbial biomass, enzyme activities and N pools during the first and last stress cycle (last harvest in May 2016) in soils from the precipitation extremes.
Results from the first stress cycle show that drought reduced cumulative CO2 production by 25% on average at all sites irrespective of the rotation length. Flooding resulted in no significant changes in most cases except in the long rotation soil from Maryland where the cumulative respiration was significantly lower than the constant water control (86% of control) and the response of the short term rotation soils to flooding which was not significantly different from the control. Soils from the long term rotation in Maryland also produced N2O when flooded, while soils from short rotation lengths did not.
Additional results, which will be obtained during the further course of this experiment in the next few months, on microbial biomass, enzyme activities and especially how microbial processes behave under reoccurring stress conditions will help to assess resilience and resistance of microbial processes in soils from different rotation regimes. These findings could help to evaluate the potential of crop diversification to be a successful mitigation strategy for a future increase in climate extremes.