CONSIDERING NO-TILL AS A NITROGEN-REDUCING BEST MANAGEMENT PRACTICE

 

 

Deanna Osmond¹, Noah Ranells², George Naderman¹, Michael Wagger¹, Greg Hoyt¹, John Havlin¹, and Steve Hodges¹

 

North Carolina State University

Departments of Soil Science¹ and Crop Science²

 

 

 

Regulations in the Neuse River Basin

Current state regulations require that the agricultural community reduce nitrogen loading into the Neuse River by 30% by 2003.  Producers can use several tools or best management practices (BMPs) to keep nitrogen out of groundwater and surface waters.  These include nitrogen management, controlled drainage, riparian buffers (both herbaceous and woody), and a conservation tillage system that includes the use of a cereal cover crop managed to reduce nitrogen loading of the shallow groundwater.

 

Conservation Tillage and Cover Crops

Under the conservation tillage system listed above, the cereal cover crops include rye, wheat, triticale, barley, and oats.  The cover crop must be planted at an adequate seeding rate early in the fall and dessicated late in the spring to allow crops to incorporate as much nitrogen and other nutrients in the biomass as possible.  The summer crop then can be planted by no-till or strip-till methods with a goal of maintaining as much surface residue coverage as possible after planting and at least 30% of the surface must be covered.  This system meets the basic, nationwide definition of “conservation tillage” used since the late 1980s by the Conservation Technology Information Center (CTIC, 2000).

 

Planting and destroying cereal cover crops on specified dates reduces nitrogen movement into shallow groundwater, whether in conservation tillage or conventional tillage cropping systems.   Adding the cereal cover crop not only offers this important nitrogen-sequestering benefit, it also adds to soil surface cover and contributes to soil organic matter.  Obtaining the minimum 30% residue cover through the fall, winter, and even following planting of the spring crop depends on numerous factors.

 

Although a corn crop may offer the 30% residue requirement necessary to qualify as conservation tillage, it does not reduce subsurface nitrogen as well as a cereal cover crop has.  A uniformly high-yielding corn crop, for example, may provide biomass coverage by itself.  However, it is the growth and subsequent absorption of nitrogen during the fall, winter, and spring that make winter cereal cover crops valuable as a nitrogen-reducing BMP.  Likewise, other major summer crops, such as soybeans and cotton are not effective at reducing nitrogen due to their growing season.  Also, they generally do not meet the 30% residue requirement.  Although soybean crops may produce ample residues, a large portion of these residues consists of rapidly decomposing leaves and petioles.  This leaves little residue cover during the winter and spring.   Cotton leaves rapidly decompose, leaving some larger stems, but limited ground cover.

 

In North Carolina, many producers who use no-till or narrow strip-till planting techniques do not meet the conservation tillage standard because they do not have sufficient cover crop or preceding crop residue to meet the minimum 30% residue cover.  This paper reviews the effectiveness of no-till and strip-till planting methods in reducing soluble nitrogen losses to surface waters and groundwater.  These tillage systems are considered distinct practices that are separate from a conservation tillage system in which cereal cover crops are used.

 

 

No-till

 

Nitrogen Loss Pathways

Nitrogen is lost primarily through erosion, leaching, denitrification, and volatilization.  The two that degrade water quality are erosion and leaching.  In humid regions, leaching losses of nitrogen are generally much greater than surface losses (Smith et al., 1990; Drury et al., 1993).  Most nitrogen is in the soluble form and moves through the soil into the shallow groundwater, which subsequently moves to drainage ditches and streams (Mitsch et al., 1999; Gilliam et al., 1985).  Jacobs and Gilliam  (1985) found that in the Coastal Plain of North Carolina, only 6% of the soluble nitrogen was lost as surface runoff, whereas 94% of the nitrogen losses were into the shallow groundwater.  Nitrate-nitrogen levels measured in streams have been found to increase (from 1 milligram per liter to 5 milligrams per liter or more in streams and ditches) in well-drained agricultural watersheds (Mitsch et al., 1999; Gilliam, 1999; Goolsby et al., 1999).  Although no-till is effective in reducing surface losses of nitrogen, the effects of tillage on reducing nitrogen loading to surface waters and groundwater through soluble nitrate-nitrogen losses are not clear. 

 

General Comments about No-Till

A literature review to determine the effectiveness of no-till in reducing leaching losses of nitrogen shows that differences in crop rotations, tillage systems, and other soil and crop management factors complicate the interpretation and comparison of research results.  For example, the duration of no-till practices between relevant studies can vary from several years to several decades.  In some studies, researchers separate the effects of cover crops from tillage type, whereas in other studies these effects are confounded.  For this review, we will consider the cropping practice only by the type of tillage used in establishing a crop stand, exclusive of cover crop.  In general, we will use the term “no-till” since many of the studies reviewed did not meet or recognize the conservation tillage standards defined by the Conservation Technology Information Center.

 

Some of the research demonstrates a reduction in nitrogen loading, while other studies cite an increase in nitrogen loading when no-till is used. Due to the diverse conclusions, it is easy to selectively demonstrate both positive and negative effects of no-till as a nitrogen-reducing BMP for soluble nitrogen leaching losses.  However, several recent reports or review articles of tillage effects on subsurface nitrogen losses have concluded that there is no difference in nitrogen losses due to tillage systems (Mitsch et al., 1999; Smith et al., 1990).   To quote Mitsch, “Tillage methods appear to have little influence on nitrate losses from agricultural fields.” 

 

No-till generally increases the macroporosity of the soil as reduced tillage allows increased aggregation.  Changes in pore size generally allow for enhanced infiltration but can cause an increase in bulk density in high-traffic areas (although no-till often can increase bulk density).  Macropores can increase water and solute movement.  Kamau et al. (1996) studied the effect of tillage and cropping effects on preferential flow through macropores  and solute transport.  Although these researchers were able to determine that macropore pathways played a major role in leaching losses, there was extreme variability in water and solute flow within plots, and they did not find any differences in solute movement due to tillage treatment.  Rasse and Smucker (1999) and Ogden et al. (1999) found that no-till increased flow volume compared to conventional-till.  In both studies, the amount of solute lost, whether nitrate-nitrogen or bromide, was essentially the same, even though flow was greater under no-till.

 

Better soil structure and increased porosity that are associated with no-till systems may increase nitrogen leaching.  Preferential flow of water through larger pores may permit more nitrogen and pesticides to move through the soil profile in no-till than in conventional tillage systems. Conversely, preferential flow actually can reduce the amount of nitrogen lost if the water moves quickly through the pores without equilibrating with the nitrogen in the smaller pores.  Total nitrate-nitrogen loss (mass) is a function of both nitrogen concentration and volume of water flow.  To determine the effects of tillage on nitrate-nitrogen losses into shallow groundwater or tile drains, it is not enough to consider soil nitrogen concentration.  Rather, nitrogen concentration and water movement both must be considered in order to obtain the total mass of nitrogen lost below the root zone.  There is often an increase in the amount of water moving through the soil in no-till systems and, although that may decrease the nitrogen concentration, the total mass of nitrogen lost from the no-till system will be similar to that of conventional-till systems.

 

Rasse and Smucker (1999), like many other researchers, have found that more water was lost under no-till than conventional systems, but that the nitrate concentrations were greater under conventional-tilled.  Total nitrate-nitrogen leached in this study was similar to slightly lower for no-till.  An 11-year study compared no-till to conventional-till (Randall and Iragavarapu, 1995).  Total flow over the years was greater under no-till, but nitrate-nitrogen concentration was greater under conventional-till.  Nitrate-nitrogen losses were greater for no-till in 6 out of the 11 years.  Average flow-weighted nitrate-nitrogen concentrations for the entire study period were 13.4 milligrams per liter for the conventional system and 12.0 milligrams per liter for the no-till system.  Most of the difference in total nitrogen loss between the two systems was due to losses that occurred in a single year.  Grain yields and nitrogen removal were significantly higher 6 out of the 11 years for conventionally tilled corn.

 

Drury et al. (1993)  found higher water losses under no-till and ridge-till but greater nitrate-nitrogen concentrations for the conventional-till treatments.  Total subsurface nitrate losses were 23.5 milligrams per liter, 17 milligrams per liter, 17 milligrams per liter, and 2 milligrams per liter for conventional-till, no-till, ridge-till, and continuous bluegrass pasture, respectively.  Surface nitrate losses, however, were 1.6 lb N/A, 3.3 lb N/A, 2.9 lb N/A and 0.14 lb N/A for conventional-till, no-till, ridge-till, and continuous bluegrass pasture, respectively.  Corn yields and nitrogen removal in plant biomass were greater for the no-till and ridge-till systems in this study than the conventional-tilled system.

 

Izaurralde et al. (1995) found greater nitrogen losses under no-till than conventional-till systems. Other researchers from Canada reported that nitrate leaching losses were greater under no-till than conventional systems, whereas another experiment they conducted showed that the reverse was true: conventional-tilled systems leached more nitrogen (Serem et al., 1997).  Although papers that present data with soil nitrogen concentration are useful, care must be taken in interpreting data that only provides soil nitrogen concentration information because of the possible differences in the amount of water movement.

 

Increased water infiltration can increase available soil moisture.  If water is limiting, extra soil water can increase yields.  Increased crop yields generally absorb fertilizer nitrogen, which can reduce the amount of nitrogen available for leaching.  However, these differences in plant nitrogen uptake are frequently so small that the effects on shallow groundwater nitrate concentrations are negligible.  One reason may be that crops grown under no-till conditions often contain lower nitrogen concentrations in the above-ground biomass than conventionally produced crops (Rasse and Smucker, 1999; Martens, 2000).  In addition, identical rates of nitrogen may not have been applied to both systems.  For instance, no-till burley tobacco generally requires an additional 30 lbs N/A when the cover crop removes 50 to 80 lbs N/A (Hoyt, 1999).  In another example, agronomists recommend in a conservation tillage guide for cotton that an additional 30 lbs of nitrogen be applied to no-till cotton crops that are planted into a desiccated wheat cover crop (Reeves et al., 1993).  This extra nitrogen is needed to offset the affects of immobilization.  This is probably not a general practice, and it is not recommended for cotton in North Carolina.

 

Cropping systems tend to have a greater affect on nitrogen leaching than tillage systems. Kanwar et al. (1997) examined the effects of tillage on tile drainage water quality in the Midwest (Table 1).  Statistically, there were few differences between tillage treatments for subsurface  drainage volume and nitrate loss in drainage water.

 

Table 1.  Effect of various tillage practices and rotation on nitrate-nitrogen losses in drainage water (Kanwar et al., 1997).

 

Rotation	Tillage*
	CP	MBP	RT	NT	Average
………………Nitrate-nitrogen losses, lb/A………………
Corn following corn	58	42	49	57	52
Corn following soybean	32	25	21	21	25
Soybean following soybean	31	29	22	23	26
Average	40	32	30	34

 

*CP = chisel plow

MBP = moldboard plow

RT = ridge-till

NT = no-till

 

In this study the dominance of corn in the rotation was more influential in determining leaching losses of nitrogen than the tillage system.  Typically, there were smaller differences between tillage treatments in the total amount of nitrogen leached.  The maximum of difference in nitrate losses for  tillage treatments was less than 25%.  In contrast, there were much larger differences between the amounts of nitrogen leached due to crop rotation, with a maximum 52% difference due to cropping system.  In general, crop rotation is more significant in determining the amount of nitrogen lost to shallow groundwater than tillage type.

 

Southern Region Studies

Staver and Brinsfield (1998) used a paired agricultural watershed design to study how no-till, conventional-till, and a rye winter cover crop under both forms of tillage affected crop nitrogen uptake and nitrogen subsurface losses in the Coastal Plain of Maryland.  They also examined the effect of planting date on the nitrogen-reducing effectiveness of rye cover crops.  Delaying planting by 30 days reduced cover crop nitrogen accumulation by more than half–from 160 lb N/A to 80 lb N/A showing that the planting date was critical to the nitrogen-reducing effectiveness of cover crops.  Care has to be used in applying Maryland data to North Carolina conditions.  For example, in eastern North Carolina rye cover crops generally do not accumulate more than 50 lb N/A during the entire growing season, even when planted at the correct time, whereas in Maryland the rye cover planted at the correct date accumulated more than 150 lb N/A.

 

Grain and stover yields of no-till corn were slightly higher than that of conventional-till for most years (Staver and Brinsfield, 1998).  A statistical analysis of crop yields was not presented.  Assuming an average corn plant nitrogen concentration of 1%, 6 lb/A of additional nitrogen would be contained in the corn biomass produced under no-till, 4.5 lb N/A in the grain, and 1.5 lb N/A in the stover.  Conversely, there was more nitrogen in the rye cover crop that preceded the conventional-till corn crop.  Nitrogen uptake in the cover crop produced under conventional tillage averaged 19 lb N/A more than the no-till cover crop.  Post-harvest soil nitrate-nitrogen levels were similar throughout the 9-year study when averaged over the sampled soil profile (27.4 lb N/A), despite the more than 12 lb/A advantage in nitrogen uptake for no-till corn and the more than 19 lb/A advantage in nitrogen accumulation in the conventional system with the rye cover crop that preceded the corn.

 

Although crop yield and nitrogen accumulation information are useful and often the only data provided, shallow groundwater data provide more complete depictions of nitrogen dynamics.  Shallow groundwater nitrate concentrations were similar between tillage treatments when there was no cover crop (Staver and Brinsfield, 1998).  Following inclusion of a rye cover crop, a 5-year period of data depicted lower groundwater nitrate concentrations under no-till than under conventional tillage.  After the 5-year period, nitrate concentrations were similar for both tillage treatments.  Because of the changes that occur during the transition to no-till, it is important to monitor long-term experiments on no-till to separate the establishment phase from the semi-equilibrium phase. The shallow groundwater results from Staver and Brinsfield (1998) may represent immobilization of nitrogen during the initial phase of organic matter buildup with no-till.  In addition, experimental fields were not the same.  The groundwater was deeper under conventional tillage treatment than under the no-till treatment, so the lag period of nitrogen movement into the shallow groundwater was longer.  The authors attribute some of the differences in nitrogen loss between treatments to greater nitrogen storage in the profile under the conventional-tilled soil (which was a deeper soil) than the no-till field. Based on their results, they concluded that a winter cover crop is much more effective in reducing soil nitrate leaching than no-till.

 

Sharpley and Smith (1994) compared conventional and no-till wheat systems in a paired-watershed experiment in Oklahoma.  They used conventional tillage in two watersheds and found shallow groundwater concentrations of nitrate-nitrogen averaged 4 milligrams per liter.  Then one field was switched to no-till.  The nitrate-nitrogen concentration of the shallow groundwater under the conventional-tilled watershed varied between 2 and 4 milligrams per liter of nitrate-nitrogen during the next 6 years of the experiment, whereas the nitrate-nitrogen concentration of the shallow groundwater under the no-till system increased immediately and rose as high as 25 milligrams per liter.  The increased nitrate leaching was attributed to poorer wheat yields caused by the inability to incorporate fall fertilizer into the no-till wheat.  Increased competition from rye and cheatgrass also reduced wheat yields. The authors emphasized the importance of target crop productivity in order to use applied fertilizer efficiently.  Various aspects of alternative technologies must be considered if the no-tillage culture is to be acceptable both agronomically and environmentally.

 

Crop yields following a cover crop can be  higher or lower than a non-cover crop system, depending on the environment.  In a 3-year study conducted on a Norfolk sandy loam, Reeves and Touchton (1991) showed that corn yields were lower following a rye cover crop than following fallow at each nitrogen rate (0 to 150 lb N/A), except for the highest nitrogen rate, where yields were similar.  In the Georgia Coastal Plain Neely et al. (1987) compared sorghum yields produced on a Greenville sandy clay loam over 2 years.  Grain sorghum yields were greater after fallow than after a wheat cover crop at nitrogen rates ranging from 0 to160 lb N/A.

 

A long-term study of 10 years in Maryland (Poplar Hill) showed no statistical difference in yield between no-till and conventional-till plots (Coale, 1999).  Early data from these same experiments demonstrated that conventional tillage had a slight (but not statistically significant) yield advantage at 0 and 80 lb N/A.  No-till had a slight, but not statistically significant, yield advantage at 120 and 160 lb N/A (Bandel, 1986).

 

In Kentucky, Frye (1986) developed a nitrogen budget for a Maury silt loam (see below) by measuring nitrogen uptake and losses.  At lower nitrogen fertilizer rates, more nitrogen was translocated to the grain and less was immobilized in the soil in conventional tillage systems than no-till systems.  Approximately the same amount of nitrogen was lost under all tillage treatments and nitrogen rates.

 

 

Another long-term study was conducted in Tennessee, where researchers compared corn yields from different tillage systems for 11 years.  Corn yields were higher in the conventional-tilled plots for 5 of the 11 years, with similar yields in the other 6 (Howard, 2000).  However, in spite of the yield differences, no-till remains an extremely important tool to reduce soil erosion on the highly erodible, sloping silt-loam soils in this area of Tennessee.

 

Researchers in Texas found that nitrogen applied to no-till wheat was more effective in improving grain yield than conventional treatments at all but 100 lb N/A (Hons et al., 1985).  Conversely, grain sorghum yields were significantly higher for conventional tillage at all nitrogen rates, including the no nitrogen treatment.  Cropping sequence had a more pronounced influence on yield, however, than tillage type.

 

Mullins and Mitchell (1989) examined wheat production on a Dothan loam fertilized at 120 and 180 lb N/A.  At both nitrogen rates, wheat yields were greater in conventional tillage than in reduced tillage.  Data from this and other studies that do not include a 0 nitrogen rate or information on how long the site has been in continuous conservation tillage can greatly limit the ability to make accurate comparisons with other research results.

 

Camp et al. (1984) reported no difference in corn or soybean yields between conventional and minimum tillage with subsoiling in a 3-year study on Bonneau and Norfolk soil in the Coastal Plain of South Carolina.  More recently, researchers in South Carolina found no consistent difference in corn, wheat, and cotton yields; plant population; and crop biomass between conventional tillage and no-tillage systems (Hunt et al., 1990; 1997).  They did report a difference in seed cotton yield between the two tillage systems depending on cultivar.  Averaged over 3 years, three of the six cotton cultivars produced greater yields under conservation tillage management compared to only one cultivar with higher yields under conventional tillage.  In a related study, Bauer and Busscher (1996) reported slightly lower lint yield in conservation tillage compared to conventional tillage, but the difference was not significant.  In another study, Torbert and Reeves (1994) evaluated cotton production under several tillage systems produced on three Hapludults: Wickham, Cahaba, and Bassfield.  They found that strip-tillage increased lint yield by 14% over conventional tillage, but there was no difference in total nitrogen uptake.  

 

North Carolina Studies

Studies comparing yield differences between no-till and conventional tillage systems for corn and soybeans have been conducted in North Carolina. A 6-year tillage study was established in the Piedmont and Coastal Plain of North Carolina (Wagger and Denton, 1989).  The treatments included continuous corn and corn-soybean rotations under no-till and conventional-tillage systems.  In general, no-till improved corn yields in 3 out of 5 years in the Piedmont soils but only 1 out of 5 years in Coastal Plain soils.  5-year corn yields increased by 27% under no-till compared to conventional tillage in the Piedmont.  In the Coastal Plain, yields were only 4% higher, which was not statistically different.  Soybean yields were improved by no-till in 2 out of 5 years but decreased 1 year under no-till in the Piedmont.  In the Coastal Plain there were no differences in soybean yield between tillage systems.

 

Conventional and no-till studies also have been conducted on wheat in North Carolina.  Wagger and Denton (1992) demonstrated lowered wheat yields for no-till plots than for conventional tillage plots in 1 of 2 years in the Piedmont.  Although the 2-year yield mean was slightly lower for the no-till treatment, there was no significant difference in yield due to tillage.  In the Coastal Plain, wheat yields were significantly higher 1 year under no-till, but the next year they were significantly lower, and the 2-year mean showed no difference.

 

Another wheat no-till study was conducted in both the Piedmont and Coastal Plain (Weisz and Bowman, 1999).  Twelve wheat varieties were produced for 2 years using both no-till and

conventional-till planting techniques.  Although there was no statistical difference between the yields of no-till and conventional-tilled wheat, yields in the Coastal Plain were 11% less when produced under no-till conditions.  There were no differences between tillage practices in the Piedmont.

 

Data from field trials in Beaufort County, North Carolina, demonstrated conflicting yield responses to tillage (Ambrose, 1993).  In some trials, there were no differences due to tillage, yet other trials demonstrated higher or lower yields for no-till compared to conventional tillage.

 

No-till studies on corn in the Piedmont suggest a yield advantage over conventional tillage at similar nitrogen rates (Wagger and Denton, 1989).  Since more nitrogen is utilized with higher yields, less nitrogen would be available for leaching.  These findings suggest that crediting no-till as a nitrogen-reducing BMP for corn is appropriate in the Piedmont.  Accordingly, we estimate a 40% fertilizer nitrogen use efficiency for conventional corn produced in the Piedmont, and a 55% fertilizer use efficiency for no-till corn.  These data only support the use of no-till as an nitrogen-reducing BMP for corn produced in the Piedmont.

 

Dr. Rich McLaughlin (1999) monitored shallow groundwater at the Center for Environmental Farming Systems in Goldsboro, NC in conventional and no-till systems.  For approximately 18 months, nitrate-nitrogen concentrations were measured in water from shallow groundwater wells.  Five of the wells were under no-till and 6 of the wells under conventional-till cropping systems.  Average nitrate-nitrogen concentration was 8.3 mg N/l under no-till and 6.1 mg N/l under conventional-till.  Since soil types were not the same, there may be some confounding of the data due to the interaction of soil type and tillage.

 

Additionally shallow groundwater data was collected over 2 years in Pamlico County.  The 2-year crop rotation was corn, wheat, and soybeans.  There were 12 plots under continuous no-till and 4 plots under conventional tillage.  No statistical differences in shallow groundwater nitrogen concentrations were detected between the two tillage systems (Hardy, 1999).

 

Osmond and Schultheis (1999) conducted a tillage by nitrogen fertility experiment in which cucumbers were planted after wheat harvest.  Tillage treatment included conventional, no-till, and strip till.  Nitrogen rates were 0, 40, 80, 120, and 160 lb N/A.  Yield, grade, nitrogen content of the cucumbers and stover were measured, and fertilizer nitrogen use efficiency was calculated. In the first year of the study, tillage had no effect on yield, nitrogen content, or nitrogen use efficiency (which was very poor).  However, nitrogen rates had a significant impact on yield, nitrogen content, and nitrogen use efficiency. In year 2 of the study, there were statistical differences in yields due to tillage and nitrogen rates.  Conventional tillage, at the same nitrogen rates, significantly increased cucumber yields over either strip-till or no-till.

 

Several field studies in the North Carolina Coastal Plain have compared conventional tillage with conservation tillage for cotton using variations of no-till and strip-till with wheat as a cover crop. The same nitrogen rate was used for all treatments.   Some of these studies included sloping, crust-prone soils.  The major finding was that strip tillage with under-row subsoiling increased nitrogen use efficiency compared to no-tillage.  Generally one or more versions of conservation tillage was statistically similar to conventional tillage, but seldom superior.  Since yields are

similar between tillage systems, a conservation tillage system would be recommended to prevent soil erosion on the more sloping and crust-prone areas (Naderman, 2000). 

 

Summary

During a national conference on no-till, it was concluded that no-till had little effect on increasing or decreasing nitrogen movement into shallow groundwater (Logan, 1987).  Under some unique conditions, no-till may reduce subsurface losses of nitrate-nitrogen with certain crops.

 

The available data from the Piedmont of North Carolina demonstrate that at the same nitrogen fertilization rates, yields of no-tilled corn are much greater.  Studies with the sprinkling infiltrometer of North Carolina State University Soil Science Department have indicated that the no-tillage treatment at Upper Piedmont Research Station at Reidsville can allow twice as much water to infiltrate as conventional-till.  There is ample evidence that increased water capture will occur, even with short-term conservation tillage, where significant residue cover exists with crust-prone surface soils and a significant slope gradient (Naderman, 2000).  The very large and sustained corn yield increases in the Piedmont region of North Carolina suggest that nitrogen is used more efficiently under no-till systems and subsurface nitrogen losses may be reduced.  This information supports greater fertilizer nitrogen use efficiency with no-till corn than conventionally grown corn in the Piedmont.

 

Most of the data reviewed from the United States, the South, and North Carolina supports the conclusion that the method of tillage, considered alone and separately from cover crop practices, has little effect on nitrogen movement into the shallow groundwater.  We have reviewed the available data and submitted our findings to outside reviewers.  It is our best professional judgment that no-till systems without a cereal winter cover crop do not represent a nitrogen-reducing BMP in the Coastal Plain of North Carolina for any crop.  This does not preclude the use of no-till or strip-till planting techniques to reduce production costs and to reduce sediment losses.  Agricultural systems of best management practices should include practices that reduce all pollutants. 

 

 

References

Ambrose, Gaylon.  1993.  Beaufort County On-farm Test Report.  North Carolina Cooperative Extension.  Washington, NC.

 

Bandel, V.A. 1986.  “Nitrogen management for no-tillage corn” In Proceedings Southern Region No-Till Conference.  R.E. Phillips (ed).  University of Kentucky.  Southern Region Series Bulletin 319.  Lexington, KY.

 

Bauer, P.J. and W.J. Busscher.  1996. Winter cover and tillage influences on Coastal Plain cotton production.  J.  Prod. Agric.  9:50-54.

 

Camp C.R., G.D. Christenbury, and C.W. Doty. 1984.  Tillage effects on crop yield in Coastal Plain soils.  Trans. Am. Soc. Ag. Eng. 27:1729-1733.

 

Coale, F.  1999.  University of Maryland.  Personal communication on yields and tillage type in the Coastal Plain of Maryland.

CTIC.  2000. Conservation Technology Information Center.  http://www.ctic.purdue.edu.

 

Drury, C.F., D.J. McKenney, W.I. Findlay, and J.D. Gaynor.  1993.  Influence of tillage on nitrate loss in surface runoff and tile drainage.  Soil Sci. Soc. Am. J. 57:797-802.

 

Frye, W.W.  1986.  “Kentucky no-tillage update” In Proceedings of the Southern Region No-Till Conference.  R.E. Phillips (ed).  University of Kentucky.  Southern Region Series Bulletin 319.  Lexington, KY.

 

Gilliam, J.W., T.J. Logan, and F.E. Broadbent. 1985. Fertilizer use in relation to the environment.  In: O.P. Engelstad (ed.) Fertilizer and Use and Technology.  Am. Soc. Agron., Madison, WI.

 

Gilliam, J.W. 1999. North Carolina State University, Dept. of Soil Science. Personal communication on stream and ditch nitrate levels under different land uses.

 

Goolsby, D.A., W.A. Battaglin, G.B. Lawrence, R.S. Artz, B.T. Aulenbach, R.P. Hooper, D.R. Keeney, and G.J. Stensland. 1999.  Fluxes and sources of nitrogen in the Mississippi-Atchafalaya River Basin:  Topic 3 report for the integrated assessment on hypoxia in the Gulf of Mexico.  NOAA Coastal Ocean Program Decision Analysis Series No. 17.  NOAA Coastal Oceans Program, Silver Spring, MD. 130pp.

 

Hons, F.M., R.G. Lemon, and V.A. Saladino. 1985. Tillage and cropping sequence effects on yields and nitrogen use efficiency.  In The Rising Hope of Our Land: Proceedings of the Southern Region No-Till Conference Proceedings.  W.L Hargrove, F.C. Boswell, and G.W. Langdale. The University of Georgia. Griffin, GA.

 

Howard, D.  2000.  The University of Tennessee Agricultural Experiment Station, Jackson, TN.  Personal communication on yields and tillage treatments in Western Tennessee.

 

Hoyt, G. 1999.  North Carolina State University, Dept. of Soil Science.  Personal communication on no-till tobacco and nitrogen rates.

 

Hunt, P.G., T.A. Matheny, D.L. Karlen, and S.H. Roach.  1990.  Performance of corn, wheat, and cotton in a two-year rotation on a Norfolk loamy sand soil after 10 years of conservation or conventional tillage. Southern Conservation Tillage Conference, Raleigh, NC.

 

Hunt, P.G., P.J. Bauer, and T.A. Matheny.  1997.  Crop production in wheat-cotton doublecrop rotation with conservation tillage.  J. Prod. Agric. 10:462-465.

 

Izaurralde, R.C., Y. Feng, J.A. Robertson, W.B. McGill, N.G. Juma, and B.M. Olson. 1995.  Long-term influence of cropping system, tillage method, and nitrogen source on nitrate leaching.  Can. J. Soil Sci. 75:497-505.

 

Jacobs, T.J. and J.W. Gilliam.  1985.  Riparian losses of nitrate from agricultural drainage waters.  J. Environ. Qual. 14:472-478.

 

Kamau, P.A., T.R. Ellsworth, C.W. Boast, and F.W. Simmons.  1996.  Tillage and cropping effects on preferential flow and solute transport.  Soil Sci. 161:549-561.

 

Kanwar, R.S., T.S. Colvin, and D.L. Karlen. 1997.  Ridge, moldboard, chisel, and no-till effects on tile water quality beneath two cropping systems.  J. Prod. Agric. 10:277-234.

 

Logan T.J. (ed.).  1987  Effects of conservation tillage on groundwater quality. Lewis Publishers, Chelsea, MI.

 

Martens, D.A.  2000.  Nitrogen Cycling Under Different Soil Management Systems.  Adv. Agron. 80:In press.

 

McLaughlin, R.A.  1999.  North Carolina State University, Dept. of Soil Science.  Raw data for monitoring wells at Cherry Farms.

 

Mitsch, W.J.,  J.W. Day, J.W. Gilliam, P.M. Groffman, D.L. Hey, G.W. Randall, and nitrogen. Wang.  1999.  Reducing nitrogen loads, especially nitrate-nitrogen, to surface water, groundwater, and the Gulf of Mexico:  Topic 5 report for the integrated assessment on hypoxia in the Gulf of Mexico.  NOAA Coastal Ocean Program Decision Analysis Series No. 19.  NOAA Coastal Oceans Program, Silver Spring, MD. 111pp.

 

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