When the wells run dry: soil organic carbon dynamics during the transition from irrigated to dryland cropping systems

Núñez, Agustín, author
Schipanski, Meagan, advisor
Cotrufo, M. Francesca, committee member
Davis, Jessica, committee member
Paustian, Keith, committee member
Journal Title
Journal ISSN
Volume Title
Colorado State University. Libraries
In many parts of the world, irrigation must decrease due to declining water availability and increased demand from other water users. The Ogallala Aquifer, one of the biggest aquifers in the world, is one example where declining groundwater levels threaten agricultural productivity and social communities across large parts of the semiarid High Plains. In this semiarid region, irrigation is not only fundamental for crop productivity, but it also has positive effects on soil organic carbon (SOC). However, little is known about the changes in SOC dynamics during the transition from irrigated to dryland cropping systems, which has important potential implications for the long-term productivity of these agricultural systems as well as the potential for the soils of the region to be a net sink or source of CO2. The general objective of my dissertation was to study how irrigation retirement affects SOC dynamics in semiarid agricultural systems of the Ogallala Aquifer Region. I used field experiments to quantify the early changes in crop productivity and C inputs, soil microbial communities, C outputs and SOC formation and turnover during the transition from irrigated to dryland cropping systems. Irrigation retirement had a stronger influence on C inputs than on C outputs because plants responded faster and to a greater magnitude than soil microorganisms to water limitations. Given intrinsic differences in growing season and water requirements, crops vary in their sensitivity to water stress, and wheat agroecosystems were less affected by irrigation retirement than maize agroecosystems. After three growing seasons, there was lower microbial activity and SOC formation in dryland (retired) than irrigated maize, but we did not find changes in the decomposition rate of old SOC. In winter wheat, low differences in soil moisture and crop productivity resulted in almost no changes in microbial activity and SOC dynamics after irrigation retirement. These short-term study results suggest that large losses of crop productivity and C inputs without changes in C outputs will decrease the formation of new SOC, thus affecting SOC storage on the longer term. I confirmed this outcome with on-farm observations of the longer-term effect of irrigation retirement on SOC stocks under different management options. After 7-10 years, sites that used to be irrigated and transitioned back to dryland systems had lower SOC than long-term irrigated sites and had the same SOC stocks as long-term dryland fields, confirming the relatively short legacy effect of irrigation. An exception to this was the transition from irrigated agriculture to perennial, ungrazed grasslands enrolled in the Conservation Reserve Program (CRP). Fields that transitioned into CRP were able to maintain intermediate SOC levels that did not differ from the currently irrigated controls. Taken together, the results of my dissertation indicate that there will be rapid and significant losses of SOC during the transition from irrigated to dryland cropping systems in the Ogallala Aquifer Region. These losses will occur mainly in response to changes in C inputs. Therefore, comparison of biomass and residue production could be used to rapidly identify crop and vegetation management strategies with higher potential to minimize the negative impact of irrigation retirement on SOC.
2021 Fall.
Includes bibliographical references.
isotopes, soil organic matter, wheat, maize, irrigation, soil respiration