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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. |
