Glanville, K. 2020. Impacts of changing precipitation on nitrogen cycling in different landscape positions and cropping systems. Dissertation, Michigan State University, East Lansing, MI.
Soil nitrogen (N) influences crop yields and can interact with climate change. Soil N has many transformations and transfers that are hard to quantify and control. These N transformations and transfers are mediated by many factors, including temperature, water, and carbon. Thus, impending climate change may strongly affect N cycling across cropping systems.
To minimize N losses and increase crop production, we must maximize N use efficiency (NUE). Past research shows precipitation and soil moisture act as the primary physical drivers of terrestrial N cycling and losses. To improve NUE with changing precipitation patterns, controls on N cycling in terrestrial systems must be identified. Thus, experiments to elucidate the linkage between hydrological and biogeochemical controls are valuable (Chapter 1). Many aspects of the N cycle are influenced by a changing climate – two are especially important: nitrous oxide fluxes (N2O) and biological nitrogen fixation (BNF).
N2O is a powerful greenhouse gas with over 250 times the radiative forcing of CO2. In Chapter 2, I test the hypothesis that changing rainfall patterns strongly alter N2O fluxes in agricultural soils as modulated by cropping system. I use rainfall manipulation shelters to expose soils to the same amount of rainfall delivered at different intervals (3-days, 14-days, and 28-days). Results from the 2016 and 2017 field seasons show cumulative N2O fluxes were 1.4 to 2 times higher when rainfall occurred in 28-day rather than shorter intervals in corn systems. Fluxes were related to changes in denitrifier enzyme activity for both years. In switchgrass systems N2O emissions were not significantly affected by rainfall intervals.
In Chapter 3, I test the hypothesis that changing rainfall patterns that alter N2O fluxes will be modulated by landscape position as landscape position affects soil texture and carbon. Over two field seasons cumulative N2O fluxes were higher in toeslope positions than in summit positions, and longer rainfall intervals had higher fluxes in summits only, consistent with higher soil carbon and finer soil texture in toeslope positions. Knowledge of these landscape patterns deserve inclusion in models of current and future climate change effects in order to better quantify and mitigate agricultural N2O fluxes.
In Chapter 4, I test the hypothesis that BNF is particularly vulnerable to changing rainfall patterns in till vs. no-till and in summit vs. toeslope positions due to differences in texture and organic matter. Results reinforce the importance of topographic position for predicting soybean BNF and show that summit positions are more sensitive to additional rainfall. Results also show changes in rainfall intensity affect BNF in tilled differently than in no-till soils. Models that incorporate these interactions will be better able to characterize legume crop performance and N fixation across landscapes and improve global estimates for BNF.
Understanding these interactions in the agricultural US Midwest may help us improve sustainability of N use in cropping systems with a changing climate.
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