Abstract
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 20m) 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 4-9mg 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.
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