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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.

Nonpoint-source pollution has been linked to agricultural practices;however, there is a need for quantitative information describing theeffect of specific farming practices on ground and surface water quality. Lack of information at the watershed scale limits our ability to make decisions about the effect of potential changes in either farming practices or landscape management that would enhance water quality. A multidisciplinary study was designed to evaluate the effect of farming practices on subsurface drainage, surface runoff, stream discharge, groundwater, volatilization, and soil processes that influence water quality. Walnut Creek watershed is a 5130-ha intensively cropped area in central Iowa on the Des Moines Lobe landform region. Soils within the watershed are in the Clarion-Nicollet-Webster (Typic Hapludoll- Aquic Hapludoll-Typic Haplaquoll) soil association, and the underlying surficial material is glacial tiff. Land use is predominantlycorn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. Fertilizer use, herbicide application, tillage practices, and crop selection were obtained through surveys of each field operator. Atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazin e-2,4-diamine], cyanazine [2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]am inol-2- methylpropanenitrile], EPTC [S-ethyl dipropyl carbamothioate], and metolachlor [2-chloro-N-(2-ethyl-6--methylphenyl)-N- (2-methoxy-1-methylethyl)acetamide] are the primary herbicides used within the watershed at rates similar to those for the state. Nitrogen fertilizer was applied as anhydrous annnonia on 60% of the corn fields at an average rate of153 kg ha-1 for the 1991--1994 period, but the frequency of corn fields receiving <112 kg ha- 1 has increased. [ABSTRACT FROM AUTHOR]

Nonpoint source contamination of surface and groundwater resources with nitrate-N (NO3-N) has been linked to agriculture across the midwestern USA. A 4-yr study was conducted to assess the extentof NO3-N leaching in a central Iowa field. Water flow rate was monitored continuously and data were stored on an internal datalogger. Water samples for chemical analysis were collected weekly provided there was sufficient flow. Twelve soil cores were collected in spring, early summer, mid-summer, and after harvest for each of the 4yr. Nitrate-N concentrations in shallow groundwater exhibited temporal trends and were higher under Clarion soil than under Okoboji or Canisteo soil. Denitrification rates were two times higher in Okoboji surface soil than in Clarion surface soil and the highest denitrification potential among subsurface sediments was observed for deep unoxidized loess. Sod profile NO3-N concentrations decreased with depth and were the same below 30 cm for fertilized corn (Zea mays L.) and soybean (Glycine max L. Merr.). Nitrate-N concentrations in subsurface drainage water exceeded 10 mg L-1 for 12 mo and were between 6 and 9 mg L-1 for 32 mo during the 4-yr study. The temporal pattern of NO3-N concentrations in subsurface drainage water was not related to the timing of fertilizer N application or the amount of fertilizer N applied. Total NO3-N losses to subsurface drains were greatest in 1993 (51.3 kg ha-1) and least in 1994 (4.9 kg ha-1). Most of the subsurface drainage water NO3-N was lost when crop plants were not present (November--May), except in 1993. Our results indicate thatNO3-N losses to subsurface drainage water occur primarilyas a result of asynchronous production and uptake of NO3-N in the soil and the presence of large quantities of potentially mineralizable N in the soil organic matter. [ABSTRACT FROM AUTHOR]

Herbicide transport in subsurface drainage can result in unacceptable levels of contamination in surface waters. This study assessed the extent of attrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3-,5-triazi ne-2,4-diamine] and metribuzin [4- amino-6-(1,1-dimethyethyl)-3-(methylthio)-1,2,4-tr iazin-5(4H)-one] transport to subsurface drainage and shallow groundwater. A corn (Zea mays L) and soybean [Glycine max(L.) Merr.] rotation was used with corn receiving banded atrazine applications of 459 g ha- 1 in 1992 and 561 g ha-1 in 1994. Soybean were treated with metribuzin at 420 g ha-1 in 1993 and 1995. Monthly flow-weighted average concentrations ofatrazine in drainage water did not exceed 3 mu g L-1 and annual losses ranged from 0.02 to 2.16 g ha-1 during the 4-yr study. Less than 3% of the groundwater samples contained atrazineconcentrations exceeding the 3 mu g L-1 maximum contaminant level for drinking water (MCL). Atrazine was detected more frequently in groundwater beneath the lowest parts of the field, despite greater than average sorption to soils in that area. Metribuzin was alsofound in groundwater, but only half as frequently as atrazine. The patterns observed in subsurface drainage and groundwater reflected thepersistence of atrazine and metribuzin in soil. Atrazine was detected in >90% of surface soil samples up to 23 mo after application, whereas metribuzin was rarely detected during the second year following application. Atrazine was found far more commonly than metribuzin in soil below 30 cm depth. [ABSTRACT FROM AUTHOR]

There is a lack of quantitative information describing the impact offarming on water quallity at the watershed scale. This study documents the surface water quality of Walnut Creek a 5130-ha watershed with about 86% of the land used for crop production. Starting in 1990, flow and concentrations of NO3-N and four herbicides atrazine [6-chloro-N-ethyl-N'-(1- methylethyl)-1,3,5-triazine-2,4-di-amine], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)a cetamide], metribuzin [4-amino-6- (1,1-dimethylethyl)-3-(methyl-thio)-1,24-triazin-5 (4H )-one], and metolachlor [2-chloro-N-(2-ethyl-6- methylphenyl) - N - (2-methoxy -1-methylethyl)acetamide] were measured at eight locations. Nitrate-N concentrations often exceeded 10 mg L-1 during May, June, and July. Total losses from the watershed ranged from 4 to 66 kg ha-1 yr-1 and represented 6 to 115% of the N applied as fertilizer in any year. Atrazine and metolachlor were detected at concentrations >0.2 mu g L- 1 in about half of all the water samples, while alachlor and metribuzin were seldom detected. Median concentrations for atrazine and metolachlor were below 1 mu g L-1 for all locations within thewatershed. During runoff events, herbicide concentrations in the stream increased while NO3-N concentrations decreased. Yearlylosses from the watershed ranged from 0.2 to 7.5 g ha-1 for atrazine and from 0.3 to 6.7 g ha-1 for metolachlor. These losses represent 0.18 to 5.6% of the atrazine and 0.047 to 1.6% of the metolachlor applied in any year. [ABSTRACT FROM AUTHOR]

A significant portion of the NO3 from agricultural fields that contaminates surface waters in the Midwest Corn Belt is transported to streams or rivers by subsurface drainage systems or "tiles." Previous research has shown that N fertilizer management alone is not sufficient for reducing NO3 concentrations in subsurface drainage to acceptable levels; therefore, additional approaches need to be devised. We compared two cropping system modifications for NO3 concentration and load in subsurface drainage water for a no-till corn (Zea mays L.)-soybean (Glycine max [L.] Merr.) management system. In one treatment, eastern gamagrass (Tripsacum dactyloides L.) was grown in permanent 3.05-m-wide strips above the tiles. For the second treatment, a rye (Secale cereale L.) winter cover crop was seeded over the entire plot area each year near harvest and chemically killed before planting the following spring. Twelve 30.5x42.7-m subsurface-drained field plots were established in 1999 with an automated system for measuring tile flow and collecting flow-weighted samples. Both treatments and a control were initiated in 2000 and replicated four times. Full establishment of both treatments did not occur until fall 2001 because of dry conditions. Treatment comparisons were conducted from 2002 through 2005. The rye cover crop treatment significantly reduced subsurface drainage water flow-weighted NO3 concentrations and NO3 loads in all 4 yr. The rye cover crop treatment did not significantly reduce cumulative annual drainage. Averaged over 4 yr, the rye cover crop reduced flow-weighted NO3 concentrations by 59% and loads by 61%. The gamagrass strips did not significantly reduce cumulative drainage, the average annual flow-weighted NO3 concentrations, or cumulative NO3 loads averaged over the 4 yr. Rye winter cover crops grown after corn and soybean have the potential to reduce the NO3 concentrations and loads delivered to surface waters by subsurface drainage systems.

The relationships between N fertilizer rate, yield, and NO3 leaching need to be quantified to develop soil and crop management practices that are economically and environmentally sustainable. From 1996 through 1999, we measured yield and NO3 loss from a subsurface drained field in central Iowa at three N fertilizer rates: a low (L) rate of 67 kg ha(-1) in 1996 and 57 kg ha(-1) in 1998, a medium (M) rate of 135 kg ha(-1) in 1996 and 114 kg ha(-1) in 1998, and a high (H) rate of 202 kg ha(-1) in 1996 and 172 kg ha(-1) in 1998. Corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] were grown in rotation with N fertilizer applied in the spring to corn only. For the L treatment, NO3 concentrations in the drainage water exceeded the 10 mg N L(-1) maximum contaminant level (MCL) established by the USEPA for drinking water only during the years that corn was grown. For the M and H treatments, NO3 concentrations exceeded the MCL in all years, regardless of crop grown. For all years, the NO3 mass loss in tile drainage water from the H treatment (48 kg N ha(-1)) was significantly greater than the mass losses from the M (35 kg N ha(-1)) and L (29 kg N ha(-1)) treatments, which were not significantly different. The economically optimum N fertilizer rate for corn was between 67 and 135 kg ha(-1) in 1996 and 114 and 172 kg ha(-1) in 1998, but the net N mass balance indicated that N was being mined from the soil at these N fertilizer levels and that the system would not be sustainable.