Agriculture has been identified frequently as a major contributor of nitrate-N to surface water. The transport of nitrate-N to surface waters can occur through subsurface drainage systems or base flow. The amount of drainage water leaving the landscape largely depends on climate and soil properties (i.e., rainfall, soil texture, soil infiltration rate).
Nitrogen is a naturally occurring element that is essential to plant growth and crop production. In a soil system nitrate-N is continually supplied through mineralization of soil organic matter. Other sources of N include fertilizers, animal manures, municipal sewage wastes, agricultural and industrial wastes, atmospheric deposition, and nitrogen fixation, all of which either occur as nitrate-N or have been converted to nitrate-N through mineralization and nitrification.
In Minnesota, research on the effects of agricultural drainage on water quality has centered on the following factors: climatic conditions, rate and time of nitrogen application, cropping systems, source of nitrogen, and tillage. These research efforts primarily focus on the transport and movement of nitrate-N to surface waters because it is highly soluble, and thus susceptible to movement through the drainage system.
Long-term research in Minnesota has demonstrated a strong relationship between precipitation and subsurface drainage volume. When wet years follow very dry years, a substantial amount of nitrate-N from soils high in organic matter is susceptible to loss in subsurface drainage. For instance, flow-weighted nitrate-N concentrations in subsurface drainage were measured for four years (1973-1976) from continuous corn that received 18 lbs N/acre per year near Lamberton, MN. Mean annual concentrations were 13, 19, 19 and 0 mg/L during the four year period and no drainage occurred in 1976. In 1977, nitrate concentrations averaged 28 mg/L with slightly above average rainfall.
In a study at Waseca, four plots were fallowed (no crop grown and no N applied) from 1987 through 1993. The plots were tilled periodically to control weeds. Following three dry years, the 1990 nitrate-N concentrations in subsurface drainage water averaged 57 mg/L. Concentrations decreased in 1991, 1992, and 1993 to 38, 25, and 23 mg/L, respectively.
Based on data from these studies it appears that high concentrations of nitrate-N can easily be lost to subsurface drainage from high organic matter soils even if no N or very small amounts of N are applied, especially when dry weather limits crop production. These losses occur regardless of soil or nutrient management practices. Results like these suggest agricultural drainage may contribute to water resource quality problems downstream, especially from nitrate loadings that occur after flood events like those of 1990-1993 following the widespread drought in 1988 and 1989.
Additional research related to climate was undertaken to find out whether or not there is scientific evidence for long-term trends toward increased stream flow in the Minnesota River. The study involved an investigation of the impact of climatic variations on stream flow. Daily stream flow records from the U.S. Geological Survey for the Minnesota River at Mankato were analyzed from 1904-1994. Modelling results show the direct effect of climate change accounts for a significant increase in maximum monthly stream flow of 40 cfs annually since 1904. The climatic model explained about 70% of the yearly variation in the log of monthly maximum stream flow.
After compensating for the effects of climate, the remaining increase in stream flow was also significant. The most likely sources for the remaining stream flow increase include drainage of wetlands and installation of subsurface drainage, channelization of streams and loss of floodplains, changes in vegetation and cropping systems, expansion of urban areas, and climatic effects not accounted for by the climate model. These results offer strong scientific evidence for increases in maximum monthly stream flow (and hence flooding) that are largely driven by an increasingly wetter climate. The effects of non-climatic influences (i.e., changing land use and drainage of wetlands) on increased stream flow is less important than climatic changes, but not negligible.
Studies in Iowa and Minnesota have shown that cropping system type (crop type and rotation) has more impact on nitrate-N concentrations found in drainage water than differences in tillage system. At Lamberton, MN, a comparison was made of drainage water nitrate-N concentrations between row crop systems (continuous corn and a corn-soybean rotation) and perennial crops (alfalfa and a Conservation Reserve Program grass-alfalfa mix). When N was applied based on a soil nitrate-N test, nitrate-N concentrations beneath the row crops averaged between 14 and 40 mg/L. Nitrate-N concentrations beneath the perennial crops ranged from 0.3 to 4 mg/L. The smaller concentrations beneath perennial crops are due to a longer period of greater root activity where cycling of N is optimized. In addition, nitrate-N losses from the row crop fields were greater because of higher flow volumes from the plots. Nitrate-N losses from the row crops were 30 to 50 times higher than from the perennial crops.
An 11-year study of the effects of tillage at Waseca showed that a switch from conventional tillage to no tillage had a minimal impact on nitrate-N losses to subsurface drain flow. The results show that nitrate-N losses from no tillage plots were slightly lower than from the conventional tillage plots, however slightly more water and thus more nitrate-N drained from the no-till plots making the difference in nitrate-N losses negligible.
Higher concentrations of nitrate-N are found in soil water beneath row
crops like corn (top) than beneath perennial crops like alfalfa
(bottom). In addition, row crop production results in higher nitrate-N
runoff losses because of higher flow volumes leaving the field.
Cropping systems that can maintain profitability while including a
perennial crop are one possible method of limiting nitrate-N losses to
surface waters.
Nitrogen losses to surface waters through drainage systems has been documented in a number of research studies including some conducted at Lamberton and Waseca. These studies primarily showed that N losses depend upon the N application rate and amount of precipitation. Time of application (see next page) and the type of crop grown have also been shown to influence nitrate-N loss to drainage systems. However, little information is available on N losses to drainage systems when different sources of N are applied.
At Waseca, a study was begun in 1994 to study the movement of N from two different sources through the soil and into drainage systems. The two N fertilizer methods included the application of liquid dairy manure and urea. Similar application rates were calculated for each source of N. The results show that N source had no effect on nitrate-N concentration in drainage water and nitrate-N content in the top five feet of the soil profile in the fall.