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Protecting the EnvironmentTopics:Water qualityGround and surface water pollution from agriculture has farmers, researchers, and policy makers concerned. Upper Midwest farmers are increasingly being told they are at least partly responsible for hypoxia in the Gulf of Mexico, eutrophication of lakes, contaminated wells, and the increasing cost of treating municipal water supplies to make them fit to drink. While there are numerous potential pollutants originating from agricultural fields, nitrates and atrazine in well water and phosphorus in runoff are the most common problems in Wisconsin. NitratesSince 1990 WICST has measured nitrate-N concentrations in the perched water table at Lakeland through a set of 18 shallow (12 feet) tube wells established under the plots. There were also two check well under adjoining grassy areas. Samples have been taken from each of the wells 7 times from 1992-1998. As a second measure, the team also sampled fall nitrates under the plots at both sites to 2 feet in depth between 1990 through 1999. From late fall until early summer, the hydrologic balance is downward, recharging the groundwater. Therefore, soluble nitrates in the soil profile tend to move toward the water table during this period. Major conclusions follow: The average concentrations of nitrate and nitrite-N in the groundwater under the cash grain systems mirrored the levels of purchased inputs used in the systems.Continuous corn, with its relatively heavy applications of inorganic nitrogen, leached the most nitrate on average, and led to well water nitrate concentrations nearly 2 times the safe level for drinking water (safe level set at 10 parts per million, or 45 ppm NO3). The no-till corn-soybean rotation leached less, hovering around the enforcement standard, and the low-purchased-input corn-soybean-wheat/red clover system leached slightly less than that. It appears that even best management practices can result in significant nitrate leaching. All of the systems leach a significant amount of nitrate if shallow wells were to be used for drinking water.Rotational grazing was best, averaging 8.2 ppm nitrate-N in the well water between 1996 and 1998. However, all the systems are close to producing nitrate contamination that exceeds drinking water standards. Presumably, some of this nitrate ends up where it is not wanted. Nutrient budgeting is necessary whatever the source of nitrogen. Although the corn phase in CS4 and CS5 received little to no synthetic Nitrogen, soil nitrate levels were the highest (134 and 117 lb NO3-N /a to 2 ft depth) of any of the other phases in the trials. (As a point of reference, soil samples from grass buffer strips between the plots contained about 70 lb of NO3-N/a to 2 foot depth.) Apparently, the combination of legume nitrogen and manure resulted in this being a “nitrogen-rich” phase. Although following alfalfa and before planting corn is a common window of opportunity in the dairy rotation to apply raw manure, more nitrogen mineralizes than can be used by the corn crop which consequently can be leached to groundwater. Mechanisms of leachingRecent research on the WICST plots focuses on how chemicals leach through soils. The major findings are as follows: Under normal conditions, chemicals can move from the surface to the groundwater in one growing season. In a separate study, a bromide tracer was applied to the Lakeland WICST plots in 1991, and was found in the water table (approximately 3 feet depth) within one season under all cropping systems. Under certain conditions, soils can develop flow-paths that permit rapid leaching of soluble chemicals into tile drains.When soils are relatively dry, water-borne chemicals are transported through small, crooked paths (“pores”) slowly, and may be either degraded or taken up by plants before they leach out of the root zone. However, when the soils become more saturated, other, faster flow paths become hydraulically active. The water bypasses the smaller crooked pathways in favor of larger, straighter and more continuous channels which develop. These flow paths permit very rapid leaching of soluble chemicals into tile drains. For example, our research shows that surface applied nitrate could reach tile drains buried at 1 m within as little as 16 minutes. Once these flow paths are established, even a very gentle rainfall (1/8 inch/hour for 10 hours) can cause around 5% of newly applied nitrate to leach out from the root zone. Energy inputs and outputsThe consumption of non-renewable energy in agriculture is an issue that waxes and wanes in the public interest. When fuel prices are high, input prices go up and the agricultural community flirts with the idea of moving to lower input cropping systems. When thinking about energy use, one can imagine two performance criteria, the actual energy purchased into the system, as well as the ratio of output to input, or an efficiency ratio. As can be seen from the table, inputs are highest in the cash grain systems and within enterprise types, the more diverse systems require less purchased energy. Following in the same path, the ratio of output to input is highest in the forage based systems and the most efficient systems are the more diverse ones. |
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