Hess, L. 2017. Climate change and agriculture: The effects of extreme rainfall patterns on nitrogen leaching from field crop systems in the U.S. Midwest. Dissertation, Stanford University, Stanford, California.

As global surface temperatures rise due to human-induced greenhouse gas emissions, increases in atmospheric moisture and changes in atmospheric circulation result in altered precipitation patterns. The proportion of total precipitation that falls in extreme events is increasing in many places around the world, including the United States (U.S.) Midwest, a globally important region of agricultural production. Agriculture is a major source of reactive nitrogen (N) inputs to the environment, where N can have a range of detrimental consequences. In particular, N leached from agriculture contributes to widespread eutrophication of estuaries and coastal ecosystems, and contamination of groundwater where it poses threats to human health. The implications of more extreme rainfall patterns for N cycling and losses in agricultural systems are uncertain. In this dissertation, I evaluate the effects of more extreme rainfall patterns on N leaching from field crop systems in the U.S. Midwest, and the extent to which the N leaching response depends on cropping system management.
In Chapter 2, I use a short-term laboratory incubation to evaluate the effects of larger, less frequent water additions on N cycling in and potential N leaching from agricultural soils under conventional, no-till, and biologically-based management. I found that larger, less frequent water additions increased total N leaching compared to smaller, less frequent water additions, and that differences in N leaching were mainly explained by differences in drainage following water additions. Soils from biologically-based cropping systems exhibited greater potential N leaching than soils from conventional and no-till cropping systems, likely due to higher concentrations of labile organic matter and rates of N turnover. Chapters 3 and 4 are based on a 234-day manipulative rainfall experiment conducted at the Kellogg Biological Station Long Term Ecological Research site in southwest Michigan, in which extreme rainfall patterns were imposed in conventional and no-till cropping systems. Chapter 3 evaluates the effects of extreme rainfall patterns on deep drainage and soil water content. In contrast to predictions by other researchers, I found no evidence that extreme rainfall patterns increase surface soil dryness or variability in surface soil water content. I also found that extreme rainfall patterns increased water storage in deep soil layers as well as water flux beyond the root zone. Chapter 4 builds on Chapter 3, combining simulations of water flux with observations of ecosystem N dynamics to evaluate the effects of extreme rainfall patterns on nitrate leaching. In conventional cropping systems, extreme rainfall patterns increased nitrate leaching, while in no-till cropping systems, extreme rainfall patterns had no effect on nitrate leaching. Extreme rainfall patterns also increased net N mineralization and inorganic N concentrations in surface soils in both cropping systems, consistent with increased nitrate leaching under extreme rainfall patterns in the conventional cropping system only. I suggest that the difference in the response of nitrate leaching to extreme rainfall patterns observed between the conventional and no-till cropping systems may be related to hydrological flow paths, with greater macropore or bypass flow in no-till soils. Together, these findings suggest that altered rainfall patterns driven by climate change may exacerbate nitrate leaching from agricultural systems. Moreover, cropping system management (e.g. conventional vs. no-till vs. biologically-based) appears to play a complex but important role in determining the extent of the effect.

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