Excess precipitation in Midwest agricultural production areas is often removed artificially via subsurface drainage systems that intercept and divert it to surface waters. Nitrogen (N), either applied as fertilizer or manure or derived from soil organic matter, can be carried as nitrate with the excess water in quantities that may have deleterious effects downstream. A field study was initiated in 1989 in Pocahontas County, Iowa, on 0.05 ha plots of glacially derived clay loams. The objective of this three-phase study was to determine the effect of N application rate on NO3-N concentration and loss in a corn-soybean rotation over a wide range of weather conditions. Nitrogen-rate treatment phases with five seasons each (six for phase II) were imposed on subsurface-drained, continuous-flow-monitored plots over a 16-year period. Phase I N rates ranged from 0 to 168 kg N ha-1 in 56 kg N ha-1 increments. Separate plots were used for each crop in phase I, and significant NO3-N concentration differences were not observed between corn or soybean plots; this led to combining both crops in a split-plot configuration for phases II and III to study system effects. Phase II N rates ranged from 45 to 179 kg N ha-1 in 45 kg N ha-1 increments. Phase III was limited to two rates, 168 and 252 kg N ha-1. Average yearly flow-weighted NO3-N concentrations ranged from 3.

In Iowa and many other Midwestern states, excess water is removed artificially through subsurface drainage systems. While these drainage systems are vital for crop production, nitrogen (N), added as manure or commercial fertilizer, or derived from soil organic matter, can be carried as nitrate-nitrogen (NO3-N) to downstream water bodies. A five-year, five-replication, field study was initiated in the fall of 1999 in Pocahontas County, Iowa, on 0.05 ha plots that are predominantly Nicollet, Webster, and Canisteo clay loams with 3% to 5% organic matter located on glacial till within the Des Moines Lobe. The objective was to determine the influence of seasonal N application as ammonia or liquid swine manure on flow-weighted NO3-N concentrations and losses in subsurface drainage water and crop yields in a corn-soybean rotation. Four aqua-ammonia N treatments (168 or 252 kg N ha-1 applied for corn in late fall or as an early season sidedress) and three manure treatments (218 kg N ha-1 for corn in late fall or spring or 168 kg N ha-1 in the fall for both corn and soybean) were imposed on subsurface-drained, continuous flow-monitored plots. Precipitation during the drainage season (March to November) was slightly below the long-term norm (722 mm) for all four years in the study period and ranged from 615 mm in 2001 (85% of normal) to 707 mm (98% of normal) in 2004. Monthly rainfall was highly variable, and subsurface drainage, or the lack thereof, usually mimicked the precipitation patterns. On average, 69% of subsurface drainage occurred in May and June of each year, with lower amounts in April and July. Four-year average flow-weighted NO3-N concentrations measured in drainage water were ranked: spring aqua-ammonia 252 (23 mg L-1) = fall manure 168 every year (23 mg L-1) > fall aqua-ammonia 252 (19 mg L-1) = spring manure 218 (18 mg L-1) = fall manure 218 (17 mg L-1) > spring aqua-ammonia 168 (15 mg L-1) = fall aqua-ammonia 168 (14 mg L-1). Corn yields were significantly greater (p = 0.05) for the spring and fall manure 218 rates than for non-manure treatments. Soybean yields were significantly greater (p = 0.05) for the treatments with a spring nitrogen application to the previous corn crop. Overall, under the slightly dry to normal precipitation conditions of this study, corn yields and NO3-N concentrations in subsurface drainage were not significantly different (p = 0.05) between fall and spring treatments at the 168 aqua-ammonia or 218 kg ha-1 N manure N rates.

Subsurface drainage in the Upper Midwest is of importance to agricultural production. However, proper management of these systems through in-field management, drainage management, or edge of field practices is needed to limit negative environmental impacts particularly from nitrate-nitrogen leaching losses. One management practice being considered is drainage management where the outflow of subsurface drainage is managed to conserve water and decrease the overall outflow of subsurface drainage. To understand how and when drainage management may be utilized in the upper Midwest it is important to review long-term drainage data to understand the timing, duration, and volumes of subsurface drainage in these climates. An ongoing drainage study from north-central Iowa allows for reviewing fifteen years of subsurface drainage which encompasses a range of climatic conditions. This information has been reviewed with the objective of understanding the timing, duration, and drainage volumes considering temporal drainage flow patterns. In particular, the monthly and seasonal flow patterns have been investigated using this long-term drainage record. On this site with a relatively narrow drain spacing of 7.6 m, drainage volume was approximately 40% of the precipitation. The time period from April through June had approximately 50% of the average annual precipitation and approximately 70% of the average annual drainage. In addition, the percent of annual drainage occurring after August 1 was only approximately 7%. The timing of subsurface flow in these areas specifically during the spring coincides with time of planting, crop germination, and early crop development has implications when considering drainage management practices and the effectiveness of these practices to limit flow and therefore nitrate-nitrogen leaching losses. To minimize outflow of drainage water, these drainage management systems would need to allow for adequate flexibility to ensure crop production while effectively managing subsurface drainage flow to potentially minimize the outflow of water.

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Excess precipitation in Iowa and many other agricultural production areas is removed artificially via subsurface drainage systems that intercept and usually divert it to surface waters. Nitrogen (N), either applied as fertilizer or manure or derived from soil organic matter, can be carried as nitrate (NO3) with the excess water in quantities that can cause deleterious effects downstream. Over a 16-year period, three N-rate treatment phases with five seasons (six for Phase II) each were imposed on conventionally tilled, subsurface drained, continuous-flowmonitored plots. The field study was initiated in the spring of 1989 in Pocahontas County, Iowa on 0.05-ha plots that are predominantly Nicollet, Webster, and Canisteo clay loams with 3-5% organic matter. The objective was to determine the influence of N fertilizer rates on flowweighted NO3-N concentration and loss along with yield in a corn-soybean rotation, over a wide range of weather conditions. Phase I N rates ranged from 0-168 kg N ha-1 in 56 kg N ha-1 increments. Although separate plots were used for each crop in Phase I, significant nitrate N concentration differences were not observed, at comparable rates, between corn or soybean plots; this lead to combining both crops in a split plot configuration for Phases II and III. Phase II N rates ranged from 45-179 kg N ha-1 in 45 kg N ha-1 increments. Phase III data were limited to two N rates, 168 or 252 kg N ha-1. Average yearly flow-weighted NO3-N concentrations (rate) ranged from 3.9 (45 kg N ha-1 in 1995) to 28.7 mg L-1 (252 kg N ha-1, in 2001). Average, flow-weighted NO3-N concentrations ranked in highest to lowest order for all rates (in mg L-1): 252 (23.4a) > 168 (15.5b) >179 (13.2b) > 112 (13.1b) >134 (11.9bc) > 56 (11.1bc) > 0 (9.9cd) >90 (8.1cd) > 45 (5.7cd). Losses were very precipitation dependent and were reflective of individual seasons and treatments imposed. Highest losses (88 kg N ha-1) were recorded in 1991, a high-flow year preceded by below normal precipitation, for the 112 kg N ha-1 rate. Loss was highly variable from year to year depending on drainage patterns. Corn yield ranking for all treatments in highest to lowest order (in kg ha-1): 252 (9313a) > 168 (8657b) > 112 (8211c) > 179 (7964c) >134 (7610c) > 90 (7164c) > 56 (6572d) >45 (6230d) > 0 (5078e). At commonly applied N rates between 168-179 kg N ha-1, average NO3-N concentrations in subsurface drainage were observed to be 13-15 mg L-1 and in an average drainage year (263mm) have approximately 32-38 kg ha-1 NO3-N lost to subsurface drains. Results from this study may have significant implications for fertilizer N management and subsurface drainage NO3-N loss to surface waters in the state, the Mississippi River and the Gulf of Mexico.