Potential Impacts of Changing Precipitation Patterns on Biological Nitrogen Fixation in Soybean (Glycine Max L.) as Mediated by Landscape Position and Tillage

PurposeExpected changes in rainfall patterns will affect the timing of N-mineralization and other N transformations, potentially promoting or suppressing biological nitrogen fixation (BNF). We test the hypotheses that BNF is more sensitive to changing rainfall patterns in summit vs. toeslope positions and in till vs. no-till consistent with patterns of soil texture and organic matter.MethodsAt a site in the upper Midwest USA, we measured soybean BNF 15N natural abundance at different landscape positions with and without supplemental rainfall and in till vs. no-till rainfall exclusion shelters to lengthen the dry periods between rainfall events. ResultsSoybean BNF was 41% higher at summit than toeslope positions, consistent with lower soil OM and coarser texture at summits. When precipitation was increased by 20%, BNF decreased at summit positions and was unaffected at toeslope positions. In a separate tillage experiment, with 3-week (but not 2-week) rainfall intervals, %BNF decreased 15% under conventional tillage and increased 14% under no-till. ConclusionsChanging rainfall patterns affected BNF differentially depending on landscape position and tillage in well-drained Alfisols. BNF was greater in summit than in toeslope positions and decreased with added rainfall. BNF under conventional tillage was more sensitive to longer rainfall intervals than was BNF under no-till. Models that incorporate these interactions will be better able to characterize legume crop performance and N use across landscapes and improve global estimates for BNF.