Substrate and water availability co-limit soil microbial activity following irrigation retirement in a continuous maize agroecosystem

Background/Question/Methods

Increasing fresh-water scarcity poses a threat to food systems across the globe, but especially those in arid regions. This decline in resources, in conjunction with other effects of climate change, will cause some farmers to transition from irrigated to dryland farming. This shift will reduce crop productivity and will likely have cascading impacts on multiple soil functions. The soil microbiome responds rapidly to environmental disruptions and plays a key role in soil organic carbon (SOC) formation and turnover, thus making them effective predictors of soil health changes. In this study, we focused on the effects irrigation retirement has on soil microbial dynamics. We utilized a field experiment established in 2016 near Fort Collins, CO, that has four replicates of a continuous maize system under two contrasting irrigation practices and three subplots representing a gradient of plant residue inputs. Previous research at this site suggests that the microbial activity is co-limited by soil substrate availability and moisture, rather than soil moisture alone. To evaluate the relative importance of these two factors, we measured surface soil moisture, available dissolved C and N, and their relationship to soil microbial activity—measured as extracellular enzymes activity— four times across the 2021 growing season.

Results/Conclusions

Our preliminary results showed a strong irrigation treatment effect on 3 of 4 microbial extracellular enzyme activities. Enzyme activity was higher in irrigated than non-irrigated plots for β-glucosidase (Bgluc), β-cellobiohydrolase (Bcell), and N-acetylglucosaminidase (NAG) (p< 0.05), but not Leucine aminopeptidase (LAP). Substrate availability also impacted microbial activity as residue input level explained a similar proportion of enzyme activity variation as irrigation treatment for the same 3 enzymes (p< 0.05 for enzymes Bgluc, Bcell, and NAG). We also found a significant interactive effect of irrigation and residue inputs for the N-cycling enzyme (NAG), but not the C-cycling enzymes of Bcell and Bgluc. These results support our hypothesis that soil microbial activity will be impacted by both the reduction in soil moisture as well as the reduction in plant inputs following the transition from irrigated to dryland production systems. This has implications for the net effects of irrigation retirement on soil C stocks and soil overall health.