Search

We conducted a sequential tracer leaching study on a 24.4 by 42.7 m field plot to investigate the temporal behavior of chemical movement to a 1.2-m deep field drain during irrigation and subsequent rainfall events over a 14-d period. The herbicides atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], and alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] along with the conservative tracer Br were applied to a 1-m wide strip, offset 1.5 m laterally from a subsurface drain pipe, immediately before an 11.3-h long, 4.2-mm [h.sup.-1] irrigation. Three additional conservative tracers, pentafluorobenzoate (PF), o-trifluoromethylbenzoate (TF), and difluorobenzoate (DF) were applied to the strip during the irrigation at 2-h intervals. Breakthrough of Br and the two herbicides occurred within the first 2-h of irrigation, indicating that a fraction of the solute transport was along preferential flow paths. Retardation and attenuation of the herbicides indicated that there was interaction between the chemicals and the soil lining the preferential pathways. The conservative tracers applied during the later stages of irrigation arrived at the subsurface drain much faster than tracers applied earlier. The final tracer, applied 6 h after the start of irrigation (DF), took only 15 min and 1 mm of irrigation water to travel to the subsurface drain. Model simulations using a two-dimensional, convective, and dispersive numerical model without an explicit preferential flow component failed to reproduce Br tracer concentrations in the drain effluent, confirming the importance of preferential flow. This study showed that preferential flow in this soil is not a uniform process during a leaching event.

Drainage water management (DWM) is a potentially valuable management practice for reducing NO3, losses to surface waters in areas of artificial drainage. But the practice is essentially untested in Midwest United States conditions and its water quality and crop yield benefits uncertain. This paper reports results from applying DWM to a 22 ha (54 ac) production field in central Iowa as part of a five-state Conservation Innovative Grant effort to document the impact of DWM across the Midwest. Three of nine plots in an existing tile drainage research site were retrofitted with control structures so that the drainage level could be controlled. Water flow from the tile in each plot, NO3 concentration in the drainage, and crop yield were measured over a four year period from 2006 to 2009.The field was in a two year corn (Zea mays L.)--soybean (Glycine max [L.] Merr.) rotation with nitrogen (N) fertilizer applied before the corn crop only. During four years of monitoring tile flow, there was a significant (p = 0.05) 21% decrease in tile flow, 110 significant decrease in NO3 concentration, and a significant 29% reduction in NO3 load leaching from the DWM treatment compared to conventional drainage. No yield benefits from DWM were observed for the two year average for corn (2006 and 2008),but a significant yield increase of 8% was observed for the two year average for soybean (2007 and 2009). For the four years monitored in this study, it is unclear if the yield increase for soybean versus no increase for corn was due to weather patterns or because corn and soybean responded differently to the raised water table caused by DWM. [ABSTRACT FROM AUTHOR]

Searching for environmentally friendly N management practices in the midwestern United States is an ongoing task in the agricultural community. Many practices have shown promise in reducing N in tile drainage that may contribute to hypox~a in the Gulf of Mexico. In this study, an agricultural system in central Iowa, managed using a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation from 1996 to 2005 was evaluated with the Root Zone Water Quality Model (RZWQM) for crop yield, grain N, annual N loss in tile drainage flow, and residual soil N03-N under high (H, 199 kg N ha ~), medium (M, 138 kg N ha~), and low (L, 69 kg N ha1) N application rates shortly after planting, and a split (S, 69 kg N ha1 shortly after planting and again at midseason) N treatment. The model adequately simulated the responses of yield and N loss to N application rates. Simulated N losses to drainage flow from 1996 to 2005 were 348, 277, and 228 kg N ha~ fdr the H, M, and L treatments, respectively, compared with corresponding measured values ~f 369, 265, and 201 kg N ha~. The S treatment had simulated and measured total N losses in drainage flow of 194 and 172 kg N ha', respectively, from 1999 to 2005. The study al~o demonstrated that RZWQM without soil microbial growth produced very similar simuIation results for crop production and soil N and water balances as RZWQM with dynamic sc~il microbial growth. [ABSTRACT FROM AUTHOR]

Drainage water management (DWM) has been proposed as a solution to reduce losses of nitrate (NO3) from subsurface drainage systems in the midwestern U.S.; however, tests of DWM efficacy have only been performed over short time periods and at a limited number of sites. To fill this gap, the RZWQM-DSSAT hybrid model, previously evaluated for a subsurface-drainage agricultural system in Iowa, was used to simulate both conventional drainage (CVD) and DWM over 25 years of historical weather at 48 locations across the midwestern U.S. Model simulations were used to demonstrate how variability in both climate and management practices across the region affects the ability of DWM to reduce losses o]:1703 in subsurface drainage. The regional average simulated reduction in drain flow was 151 mm yr-1 when using DWM instead of CVD, and the regional percent reduction over the long term was 53%. Reductions in drain flow were offset mainly by increases in surface runoff and evapotranspiration. Similarly for nitrogen (N), the regional average simulated reduction in NO3 losses through subsurface drains was 18.9 kg N ha-1 yr-1, and the regional percent reduction over the long term was 51%. Subsurface drain NO3 loss reductions were counterbalanced mainly by increases in stored soil N, denitrification, and plant N uptake. The simulations suggest that if DWM can be practically implemented throughout the region, particularly in the southern states, then substantial reductions in the amount of NO3 entering surface waters from agricultural systems can be expected. [ABSTRACT FROM AUTHOR]

This article discusses the use of rye cover and gamagrass strips to reduce the nitrate runoff of subsurface tiled drainage in the Midwest U.S. According to the authors, nitrate concentrations and nitrate loads were significantly reduced in subsurface drainage after rye winter cover treatments but cumulative annual drainage was not reduced. However, permanent gamagrass strips did not significantly reduce nitrate concentrations, nitrate loads or annual drainage. They concluded that rye winter cover after soybean and corn crops can reduce nitrates in surface waters impacted by subsurface drainage systems.