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

Authors: MENDOZA-MARTINEZ, VIOLETA - Colorado State University; Acosta Martinez, Veronica; NUNEZ, AGUSTIN - Colorado State University; WRIGHTON, KELLY - Colorado State University; PRENNA, JESSICA - Colorado State University; SCHIPANSKI, MEAGAN - Colorado State University

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2026
Publication Date: 3/9/2026
Citation: Mendoza-Martinez, V., Acosta Martinez, V., Nunez, A., Wrighton, K., Prenna, J., Schipanski, M.E. 2026. Soil heterotrophic respiration after irrigation retirement is differentially influenced by moisture and substrate availability over time. Biogeochemistry. 169:21. https://doi.org/10.1007/s10533-026-01312-4.
DOI: https://doi.org/10.1007/s10533-026-01312-4

Interpretive Summary: Producers forced to stop irrigation practices due to water limitations across the Western U.S. face many challenges to maintain crop productivity. A team of scientists from Colorado State University and USDA-ARS in Lubbock TX conducted an experiment since 2017 in a no-till, maize system in Colorado simulating an irrigation retirement scenario. In 2021 and 2022, the team found reduced available soil water, enzyme activity, soil respiration and biomarkers for bacteria, fungi, protozoa and actinobacteria, and reduced plant C inputs after irrigation retirement. Non-irrigated plots accumulated higher concentrations of dissolved organic carbon (DOC) and, in the absence of new C inputs, soil respiration from older SOC pools did not differ by water treatment. Soil respiration in long-term fallow plots was not correlated with soil moisture, thus suggesting moisture limitation alone did not change microbial activity or SOC turnover rates. However, over shorter time scales, monthly soil moisture measurements had a stronger direct effect on soil respiration than substrate availability as estimated by water-extractable DOC, though both factors only explained 24% of the variability in soil respiration. Given the expected reduction in SOC and soil health parameters with transitions from irrigated to dryland agriculture, practices that optimize plant C inputs and residue retention following irrigation retirement will be required to minimize the potential negative soil health impacts.

Technical Abstract: 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.