Subsurface drainage is an essential water management practice for many poorly drained soils in the U.S. Midwest, but this practice also contributes nitrate-N loads to surface waters. This paper summarizes results from Years 16–31 of a long-term drainage research project in southeastern Indiana and compares results with the first 15 yr of the study. The study compared three drain spacings (5, 10, and 20 m) managed with a no-till corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation, with cover crops in about half of the years. Drainflow and nitrate-N losses per unit area were greatest for the 5-m spacing and lowest for the 20-m spacing. Nitrate-N concentrations did not vary with drain spacing and were generally in the range of 4–9 mg L^–1. Annual nitrate-N loads were linearly correlated with annual flow volumes, reflecting the relatively constant concentrations over the 16-yr period. Whereas nitrate-N concentrations were relatively constant throughout the year, short-term concentration spikes occurred for nitrate-N during June–July of corn years. About 70% of annual drainflow and N loads occurred during the fallow season of November–April. The results underscore the interacting effects of drainage design, crop management, and weather in determining the magnitude of N loss from drained agricultural fields.

中文翻译：

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排水管间距、作物管理和天气对硝酸盐浸出到地下排水管的长期影响

对于美国中西部许多排水不良的土壤而言，地下排水是一项必不可少的水管理措施，但这种做法也会使地表水中的硝酸盐氮负荷增加。本文总结了印第安纳州东南部一个长期排水研究项目 16-31 年的结果，并将结果与​​该研究的前 15 年进行了比较。该研究比较了使用免耕玉米 ( Zea mays L.)-大豆 [ Glycine max (L.) Merr.] 轮作管理的三个排水间距（5、10 和 20 m），其中大约一半的种植覆盖作物年。每单位面积的排水流量和硝酸盐氮损失在5米间距处最大，而在20米间距处最低。硝酸盐-N 浓度不随排水间距变化，一般在 4-9 mg L ^–1的范围内. 年硝酸盐-N 负荷与年流量呈线性相关，反映了 16 年期间相对恒定的浓度。尽管硝酸盐-N 浓度全年相对稳定，但在玉米年的 6 月至 7 月期间，硝酸盐-N 的浓度会出现短期峰值。每年约 70% 的排水流量和氮负荷发生在 11 月至 4 月的休耕季节。结果强调了排水设计、作物管理和天气在确定排水农田氮损失量方面的相互作用。