Winter cover crops as a best management practice for reducing nitrogen leaching

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Abstract

The role of rye as a winter cover crop to reduce nitrate leaching was investigated over a three-year period on a loamy sand soil. A cover crop was planted after corn in the early fall and killed in late March or early April the following spring. No-tillage and conventional tillage systems were compared on large plots with irrigated corn. A replicated randomized block design experiment was conducted on small plots to evaluate a rye cover crop under no-tillage and conventional tillage and with commercial fertilizer, poultry manure and composted poultry manure as nitrogen fertilizer sources. Nitrogen uptake by the cover crop along with nitrate concentrations in groundwater and the soil profile (0–150 cm) were measured on the large plots. Soil nitrate concentrations and nitrogen uptake by the cover crop were measured on the small plots. There was no significant difference in nitrate concentrations in the groundwater or soil profile with and without a cover crop in either no-tillage or conventional tillage. Annual amounts of nitrate–N leached to the water-table varied from 136.0 to 190.1 kg/ha in 1989 and from 82.4 to 116.2 kg/ha in 1991. Nitrate leaching rates were somewhat lower with a cover crop in 1989, but not in 1990. There was no statistically significant difference in corn grain yields between the cover crop and non-cover crop treatments. The planting date and adequate rainfall are very important in maximizing nitrogen uptake in the fall with a rye cover crop. On the Delmarva Peninsula, the cover crop should probably be planted by October 1 to maximize nitrogen uptake rates in the fall. On loamy sand soils, rye winter cover crops cannot be counted on as a best management practice for reducing nitrate leaching in the Mid-Atlantic states.

Introduction

Over the past 10 years, there has been increasing concern about the impact of agricultural chemicals on groundwater resources. This is particularly true in the Chesapeake and Delaware Bay watersheds. Extensive water quality monitoring has determined nutrient loadings, specifically nitrogen and phosphorus, as the major factor responsible for the undesirable changes in Chesapeake Bay (U.S. Environmental Protection Agency, 1982). The states in the Chesapeake Bay drainage area agreed in 1985 to reduce nitrogen and phosphorus inputs into the Bay and its tributaries by 40% by the year 2000. Studies by Staver et al. (1989)indicate that groundwater flow paths play a major role in nonpoint source nitrogen transport into the Chesapeake Bay from Coastal Plain agricultural regions.

Historically, cover crops had been used to reduce soil erosion, fix nitrogen, and as a source of forage in integrated agricultural systems. Since 1945, the development of relatively inexpensive inorganic fertilizers, and the concurrent spatial separation of livestock and grain production, has caused a dramatic reduction in the use of winter cover crops. Much of the recent research on cover crops has focused on the use of legumes to supply nitrogen for future grain crops. Long before nitrogen was recognized as a problem in the environment, scientists had documented the ability of cereal grain cover crops to reduce the leaching of nitrate from the root zone. However, the successful integration of cereal grain cover crops into current cropping systems will require an understanding of the dynamics of cover crop nitrogen uptake and mineralization in order to minimize nitrogen losses to the environment while providing maximum benefit to following grain crops (Rodale Institute, 1993).

Morgan et al. (1942)documented the ability of cereal grain cover crops to reduce the leaching of nitrates and other nutrients from the root zone. Excessive nitrogen leaching during the nongrowing season has been documented by several investigators on Coastal Plain soils. Weil et al. (1987)reported that 80 to 150 kg/ha of nitrogen was lost between October and March from the root zone of irrigated Coastal Plain soils cropped to corn without winter cover crops. Ritter et al. (1991)evaluated the effect of irrigation and nitrogen management on groundwater quality for 4 years. In all but 1 year, the largest mass of nitrate leached was during the fall and winter months when the largest amount of recharge occurs. In another study, Ritter et al. (1993)found the mass of nitrate–N leached from no-tillage irrigated corn ranged from 55.0 to 78.8 kg N/ha/yr and on conventional tillage from 57.1 to 94.0 kg N/ha/yr. Most of the leaching occurred during the late fall and winter.

Much of the research on cover crops has centered on the use of legumes to supply nitrogen for future grain crops (Ebelhar et al., 1984; Hargrove, 1986). Brinsfield et al. (1988)found that a rye cover crop planted after corn harvest assimilated 113.8 kg/ha and 51.5 kg/ha total nitrogen for conventional and no-tillage systems 90 days after planting, respectively. Soil nitrate–N levels declined by more than 10 mg/kg in the top 15 cm of the soil during the study. In a later study, Brinsfield and Staver (1991)found leachate nitrate concentrations were consistently lower when there was a cereal grain cover crop present than those observed in previous years with no cover crops on two Coastal Plain watersheds. The Rodale Institute (1993)reports nonlegumes are about three times more efficient than legumes at reducing nitrogen leaching.

Since the goal of the Chesapeake Bay plan is to reduce nitrogen inputs into the Bay by 40% by the year 2000, it is important to evaluate crop management practices that reduce nitrogen loads to the Bay. A large portion of the nitrogen load to the Bay from the Delmarva Peninsula is in the baseflow of streams. Nearly all of drinking water on the Delmarva Peninsula comes from groundwater. Nitrate concentrations are above 10 mg/l N in groundwater in many parts of the Delmarva Peninsula (Bachman, 1984; Ritter and Chirnside, 1987). Because of these factors a cooperative project was initiated in Pennsylvania, Maryland and Delaware to evaluate the management of a rye (Secale cereale L.) cover crop to reduce the leaching of nitrate to groundwater and to optimize nitrogen recycling on the farm.

Section snippets

Experimental methods

The research was conducted at the University of Delaware Research and Education Center near Georgetown, DE from 1989 to 1991. Two types of studies were initiated to address different aspects of cover crops and nitrogen management. In the first study involving groundwater monitoring, four large plots (0.25 ha) were used to evaluate nitrate leaching in irrigated corn with and without a rye cover crop, under conventional tillage and no-tillage treatments. The layout of the large plots are shown in

Hydrogeology

The water-table aquifer below the plots consists of sands of the Columbia Formation (Pleistocene) and the subcropping of Manokin and Pocomoke sands (Miocene). The thickness of the water-table aquifer varies from 27 to 61 m. The average transmissivity is about 1000 m3/day and the specific yield is 0.14. The annual recharge varies from 300 to 380 mm (Sundstrom and Pickett, 1969). The boring logs indicated a loamy sand topsoil from 0 to 0.3 m and the substratum consisted of medium sand with traces

Conclusions

A rye winter cover crop will not reduce corn yields the following year if irrigation is used.

The planting date and weather conditions are very important in maximizing nitrogen uptake in the fall with a rye cover crop. On the Delmarva Peninsula, the cover crop should probably be planted by October 1 to maximize nitrogen uptake rates in the fall. On sandy soils on the Delmarva Peninsula, it is also important to have adequate rainfall in October and November to obtain good cover crop growth and

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