The impacts of deficit irrigation on crop production and sustainable soil management
Flynn, Nora E., author
Fonte, Steven, advisor
Comas, Louise, committee member
Stewart, Catherine, committee member
von Fischer, Joseph, committee member
Growing issues of water scarcity around the planet highlight a need for more efficient use of agricultural water. Deficit irrigation (DI) offers a promising option to reduce water use with relatively small impacts on crop yield, when properly managed. However, the impacts of DI management on above and belowground crop growth and the interactions between plants and soil are complex and need further study. There are concerns that DI, because it often reduces crop biomass, could reduce soil carbon (C) stocks, and negatively impact soil processes related to soil health. Additionally, DI alters soil moisture conditions with significant implications for soil C turnover and for the movement, transformation, and fate of soil nitrogen (N). At the same time soil N could buffer crops from water stress. Therefore, the goal of this research was to examine the potential impacts of DI on crop production and water stress and implications for soil C and N dynamics. Chapter 2 explores the effect of DI on maize above and belowground growth, soil microbial community composition, soil aggregation as well as soil C concentrations in surface soils (0-20 cm) and at depth (40-60 cm). Deficit irrigation increased root length density in deep soils (40-60 cm), with a trend towards higher soil C in treatments with the most root growth. Deficit irrigation also reduced total microbial biomass in the surface layer and led to shifts in microbial community composition. While aggregation and soil C were not strongly impacted by DI here, increased root growth under DI could eventually increase soil C and benefit a range of soil health related parameters, which are advantageous for crop production in water-limited systems. Chapter 3 quantifies greenhouse gas emissions from DI compared to full irrigation and suggests that DI can reduce both N2O and CO2. While this is a promising result, we also found that yields were reduced under DI, such that yield-scaled emissions were higher under DI compared to FI. The tradeoff between reducing emissions at the cost of reducing yield is important to recognize in the development of more sustainable agricultural practices. An additional important observation in this study was that emissions from this drip-irrigated maize system appeared to be much lower than from sprinkler or furrow irrigated maize systems reported elsewhere in the Great Plains. Chapter 4 sought to elucidate the impact of DI on the fate of N and the interactions between water and N in a drip-irrigated maize system. Yield and the amount of N at the end of the growing season in the harvested material vs. N lost via N2O emissions or remaining in the soil. Deficit irrigation reduced grain yield compared to full irrigation was quantified. Less N was taken up by maize under DI, leaving more residual nitrate in the soil at the end of the growing season, which is vulnerable to subsequent loss via leaching or emissions. While DI reduced consumptive water use in this experiment, yields were also reduced, thereby reducing water use efficiency. Overall, the findings of this study suggest that farmers should apply less fertilizer when utilizing DI. Chapter 5 examines the impact of DI and N level on above and belowground growth of five different sorghum genotypes in a greenhouse experiment. We found that DI led to an increase in root biomass allocation for all the sorghum genotypes, and that a low N treatment further increased root biomass allocation and specific root length (SRL) compared to a high N treatment under DI. Importantly, increasing root biomass allocation did not decrease aboveground biomass which is a common tradeoff in drought-stressed agriculture. In summary, this research indicates that DI alters crop growth in important ways beyond just grain yield. Deficit irrigation can increase maize and sorghum root growth, which has important implications for water and nutrient acquisition and for building soil C. This finding is especially significant in semi-arid systems, where maintaining and building soil C presents a significant challenge for long term soil health. We also showed that DI can be used to reduce greenhouse gas emissions, but it is important to note that such management can also reduce yield. Overall, this research will help inform farmers and policymakers in making decisions around the adoption of DI practices. Most importantly, this work suggests that proper implementation of DI offers promise to maintain crop growth with less water and that doing so could maintain or increase soil C stocks and would require less N fertilizer application compared to full irrigation.
Includes bibliographical references.
Includes bibliographical references.