Nitrogen (N) losses from agricultural lands in the Midwest United States are contributing to the expansion of the hypoxic zone in the Gulf of Mexico. This study evaluated the importance of inter‐annual variability in precipitation, land cover, and N fertilizer use on NO₃ + NO₂‐N loads in seven United States Midwestern Rivers using the backward stepwise regression analysis. At the annual scale, fluctuations in the current and previous years’ precipitations explained much of the variation in streamflow, baseflow, and N‐load. Previous years precipitation effects were associated with fillable soil porosity. In some years, higher residual soil N from previous dry years also contributed to an increase in N‐load. Area under soybean production (SOY), a surrogate for replacement of prairies and small grains was generally not a significant explanatory variable. Fertilizer use from 1987 to 2012 was also not a significant explanatory variable in the annual analysis. Precipitation in both the current and previous months and previous year were important in explaining variation in monthly streamflow, baseflow, and N‐load. SOY was significant in one or two months from June to August, but had a higher p‐value than precipitation. We conclude recent increases in river N‐loads are primarily due to wet climate and minimally due to the changes in land cover or N fertilizer use. Under current cropping systems and agronomic N application rates, tile water remediation will be necessary to reduce river N‐loads.

中文翻译：

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降水是美国中西部河流氮负荷变化的主要驱动因素

美国中西部农业土地上的氮损失正在加剧墨西哥湾的缺氧区。这项研究评估了年降水量，土地覆盖率和氮肥用量对NO ₃  + NO ₂的年际变化的重要性。使用向后逐步回归分析得出美国中西部七条河流的N负荷。在年尺度上，当前和前几年降水的波动解释了水流，基流和氮负荷的大部分变化。前几年的降水影响与可充填土壤孔隙度有关。在某些年份，以前干旱年份较高的残留土壤氮也导致了氮负荷的增加。大豆生产面积（SOY），代替大草原和小谷物的替代物通常不是重要的解释变量。在年度分析中，1987年至2012年的肥料使用量也不是重要的解释变量。当前和前几个月以及前一年的降水对于解释月流量，基流量和氮负荷的变化都很重要。p值大于降水。我们得出的结论是，近期河流氮负荷的增加主要是由于潮湿的气候，而最小程度是由于土地覆盖或氮肥使用的变化。在当前的耕作制度和农艺氮肥施用量下，为减少河流氮负荷，必须对地砖进行水修复。