Tile spacing impacts on Zea mays L. yield and drainage water nitrate load

The nitrogen (N) load to surface water introduced by subsurface tile drains is a function of soluble N concentration and drainflow volume. A 6-year study characterized the impact of tile spacing on the inter-related factors of monoculture corn yield, N concentration in drainage water, and total drainage water mass N loss for a poorly drained Mollisol. Tile drains were installed at spacings of 10, 20, and 30m. Yields and grain N were quantified as a function of lateral distance from the tile. Drainflow was monitored continuously with nitrate (NO3-N) measured in daily, flow proportional aliquots. A 20 m tile spacing (20 m TS) maximized crop yield (7933 kg ha(-1)) and N accumulation in grain (109 kg ha(-1)) with slightly lower yields (< 2%) and grain N (7%) observed in both 10 and 30 m TS. Six-year mean NO3-N concentrations in drainage water were 22.2, 17.7, and 24.3 mg L-1 for 10, 20, and 30 m TS, respectively, and these values were significantly different (P <= 0.05). For all treatments, year-to-year variation in mean concentration was pronounced (6.8-34.9 mg L-1 for 20 m TS), and, in general, annual mean NO3-N concentration decreased 11 mg L-1 for every additional 1 Mg ha(-1) yield recorded in the previous year. Annual drainflows were greatest for 20 m TS (1327 m(3) ha(-1) year(-1)), 19 and 30% greater than for 10 and 30 m TS, respectively. Mass loss was inversely related to tile spacing (6-year means of 29.9, 26.6, and 22.6 kg ha(-1) for 10, 20, and 30 m TS, respectively). Analysis of daily NO3-N concentrations as a function of flow volumes found a slight tendency for higher concentrations to occur in conjunction with the higher flow volumes (e.g. < 7 mg L-1 at < 5 m 3 ha(-1) day(-1) versus > 15 mg L-1 for > 40 m(3) ha(-1) day-1). For 20 m TS, the slope of the log-log relationship between daily NO3-N load and flow volume was > 1 in 4 of 6 years, and results from all treatments suggest that significant dilutions of drainage water concentrations with large or prolonged flushing events does not occur. The magnitude of daily and seasonal flow variations far exceeded that of NO3-N concentration making discharge the more dominant factor in determining temporal variation in N export. (c) 2004 Elsevier B.V. All rights reserved.