Subsurface drainage is a beneficial water management practice in poorly drained soils but may also contribute substantial nitrate N loads to surface waters. This paper summarizes results from a 15-yr drainage study in Indiana that includes three drain spacings (5, 10, and 20 m) managed for 10 yr with chisel tillage in monoculture corn (Zea mays L.) and currently managed under a no-till corn-soybean [Glycine max (L.) Merr.] rotation. In general, drainflow and nitrate N losses per unit area were greater for narrower drain spacings. Drainflow removed between 8 and 26% of annual rainfall, depending on year and drain spacing. Nitrate N concentrations in drainflow did not vary with spacing, but concentrations have significantly decreased from the beginning to the end of the experiment. Flow-weighted mean concentrations decreased from 28 mg L(-1) in the 1986-1988 period to 8 mg L(-1) in the 1997-1999 period. The reduction in concentration was due to both a reduction in fertilizer N rates over the study period and to the addition of a winter cover crop as a "trap crop" after corn in the corn-soybean rotation. Annual nitrate N loads decreased from 38 kg ha(-1) in the 1986-1988 period to 15 kg ha(-1) in the 1997-1999 period. Most of the nitrate N losses occurred during the fallow season, when most of the drainage occurred. Results of this study underscore the necessity of long-term research on different soil types and in different climatic zones, to develop appropriate management strategies for both economic crop production and protection of environmental quality.
Soil drainage conditions are important to land use decisions. Traditionally, anaerobic conditions induced by poor drainage have been evaluated by observing soil color related to Fe and Mn oxides, using α, α‐dipyridyl dye, measuring dissolved O2, and measuring EH We believe that there is further need for a device that is scientifically sound and easy to use. Therefore, our goals were to develop and test a device that mimics natural soil processes, visually indicates soil reduction, and is robust. Our concept was to coat a rod or tube with a colored soil mineral that dissolves on reduction, insert the device into a soil, remove it after a few weeks or longer, and observe if some of the coating had been lost. If the coating was not dissolved, no reduction occurred, but if it was dissolved, reducing conditions must have prevailed. After trying several kinds of coatings and tubes, we chose ferrihydrite (FH) coating on polyvinyl chloride (PVC) pipe. We call the device an Indicator of Reduction in Soils (IRIS). As the study progressed we added semi‐quantitative interpretations by measuring depleted areas using a digital camera and image analysis. We tested IRIS tubes in the lab and in soils in Indiana, Minnesota, and North Dakota, and concluded they performed as expected. Reduction rates increased between February and April and were related to increasing soil temperature, turnover (flux) of soil OC, and content (inventory) of OC. Reduction rates decreased after April, presumably because the nutrient supply for microbes decreased.
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