Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/1997
Publication Date: N/A
Citation: Interpretive Summary: There are large amounts of agricultural land in the USA and other countries where wet soil conditions have been alleviated with tile drains. These drainage systems provide a way for excess water in the soil to drain away to nearby ditches or streams. Removing the excess water allows the soil to warm up faster in spring and to provide more oxygen to plant roots, and both of these conditions improve crop yields. However, the water carried in these tile drains also contains nutrients, soil sediment, and sometimes pesticides, which can impair water quality in the streams and rivers into which this water flows. Also, in years when there is too much snow melt or rainfall in early spring, the rapid flow of water through these drainage systems may increase the chance of flooding and property damage. In this research we asked the question, 'Are there ways to reduce the loss of both water and nutrients in tile drainage systems by altering the cropping system?' We showed that perennial crops are very effective in reducing loss of both water and nitrate (a potentially toxic form of nitrogen). Both alfalfa, which produces a high value feed crop, and a mixture of grasses and legumes, like those used in the soil protection effort called the Conservation Reserve Program, protected tile drainage water quality better than annual crops, like corn or soybean. These research results may be used by farmers to alter both the flow of water to streams and rivers and the quality of that agricultural runoff by including more perennial crops in their cropping systems.
Technical Abstract: Subsurface drainage of gravitational water from the soil profile through tiles is a common practice used to improve crop production on poorly drained soils. Previous research has often shown significant concentrations of nitrate-N (NO3-N) in drainage water from row-crop systems, but little drainage research has been conducted under perennial crops such as those used in the Conservation Reserve Program (CRP). Four cropping systems (continuous corn, a corn-soybean rotation, alfalfa, and CRP) were established in 1988 to determine above ground biomass yields, N uptake, residual soil N (RSN), soil water content, and nitrate losses to tile drainage water as influenced by cropping system. Hydrologic year rainfall during the 6 yr study ranged from 23% below normal to 66% above normal. In dry years, yields were limited, RSN accumulated at elevated levels in all crop systems but especially in the row-crop systems, soil water reserves and RSN were reduced to as deep as 2.7 m in the alfalfa and CRP systems, and tile drainage did not occur. Drainage occurred only in the corn and soybean systems in the year of normal rainfall. In years of excess precipitation, drainage from the row-crop systems exceeded that from the perennial crops by 1.1 to 5.3X. Flow-weighted average NO3-N concentrations in the water during the flow period of this study were continuous corn = 32, corn-soybean rotation = 24, alfalfa = 3, and CRP = 2 mg/L. Nitrate losses in the drainage water from the continuous corn and corn-soybean systems were about 37X and 35X higher, respectively, than from the alfalfa and CRP systems due primarily to greater season-long evapotranspiration resulting in less drainage and uptake and/or immobilization of N by the perennial crops.