Soil heterotrophic respiration after irrigation retirement is differentially influenced by moisture and substrate availability over time

Water limitations are forcing producers to transition large areas of currently irrigated farmland into dryland agriculture across the Western U.S. with unclear effects on global soil carbon (C) dynamics. An experiment established in 2017 in a no-till, maize system in Colorado suggested that soil heterotrophic respiration (Rh) following irrigation retirement was co-regulated by water and available C. We continued Rh measurements in 2021–2022 along with monthly soil samplings to explore the interactive effects of soil moisture and available C on microbial community composition and activity. Plant C inputs, available soil water, bacteria, fungi, and protozoa fatty acid methyl ester (FAME) biomarkers, enzyme activity, and Rh decreased after irrigation retirement, while actinobacteria abundance was not affected. Non-irrigated plots accumulated higher concentrations of dissolved organic carbon (DOC) and, in the absence of new C inputs, Rh from older SOC pools did not differ by irrigation treatment, suggesting limited microbial access to available C under low moisture. Short-term Rh variation was primarily moisture-driven, whereas cumulative residue inputs explained longer-term differences. Overall, microbial activity under irrigation retirement was co-limited by water and substrate availability. Management strategies that enhance soil moisture retention and maintain residue inputs are essential to sustain soil C cycling and resilience.