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ASAE Conference Proceeding

This is not a peer-reviewed article.

Nitrate Losses to Subsurface Drains as Affected by Winter Cover Crop, Fertilizer N Rates, and Drain Spacing

E.J. Kladivko, J.R. Frankenberger, B. J. Jenkinson, and N.R. Fausey

Pp. 51-58 Drainage VIII the Proceedings of the Eighth International Drainage Symposium, 21-24 March 2004 (Sacramento, California, USA), ed. Richard Cooke. ,21 March 2004 . ASAE Pub #701P0304

ABSTRACT

Subsurface tile drainage is a common water management practice in agricultural regions with seasonal high water tables. This paper reports the results of a 16-yr subsurface tile drainage research study on a site located in southeastern Indiana, USA. The site includes three tile drain spacings (5, 10, and 20 m) managed for 10 years with chisel tillage in monoculture corn ( Zea mays L. ) and currently managed under a no-till corn-soybean ( Glycine max L. ) rotation. Drainflow volumes are continuously recorded and the water is sampled for nitrate-N on a flow-proportional basis. Additional field measurements have included soil water content and potential, water table, and crop yield. In general, drainflow and nitrate-N losses per unit area increase as drain spacing becomes narrower. Drainflow removed between 8 and 26% of annual rainfall, depending on the year and drain spacing. Nitrate-N concentrations in the drainflow did not vary with spacing, but concentrations have significantly decreased from the beginning to the end of the experiment. Concentrations dropped from a mean of 28 mg L -1 in the 1986-1988 period, to a mean of 8 mg L -1 in the 1997-1999 period. The reduction in concentration was due to both a reduction in fertilizer N rates over the course of the experiment and to the addition of a winter cover crop as a "trap crop" after corn in the corn-soybean rotation. Average annual nitrate-N mass losses dropped from 38 kg ha -1 in the 1986-1988 period, to 15 kg ha -1 in the 1997-1999 period, and losses were greatest for the narrowest spacing. Most of the nitrate-N losses occurred during the dormant season, when most of the drainage occurred. Results of this study underscore the necessity of long-term research on different soil types and in different climatic zones, in order to develop appropriate management strategies for both economic crop production and protection of environmental quality.

KEYWORDS: drainage, tiles, nitrate, water quality, leaching, cover crops

INTRODUCTION

Subsurface tile drainage is a common water management practice in agricultural regions with seasonal high water tables. The practice of subsurface drainage provides many agronomic and environmental benefits, including greater water infiltration, lower surface runoff and erosion, and improved crop growth and yield compared with similar agricultural soils without subsurface drainage [Skaggs and Van Schilfgaarde, 1999]. Subsurface drains have been found to reduce losses of sediment and phosphorus from agricultural fields but to increase losses of nitrate-N through the enhanced leaching of the soil profile [Gilliam et al., 1999]. Research on nitrate leaching into subsurface tile drains has been conducted for many years, but recent concerns about the hypoxic zone in the Gulf of Mexico and similar problems worldwide have caused a renewed interest in tile drain studies. An appropriate balance between increasing drainage intensity (narrower spacing) to improve drainage and decreasing drainage intensity to reduce nitrate-N losses needs to be found for different climatic and soil regions. Nitrate concentrations and mass losses in subsurface tile drains vary with soil organic matter level, yearly weather variations, fertilizer N rates and timing, drain spacing, cover crop growth, and water table control practices [Drury et al., 1996; Kladivko et al., 1999; Jaynes et al., 2001; Randall and Mulla, 2001; Dinnes et al., 2002]. The objectives of our study were 1) to evaluate the effect of three different drain spacings on nitrate leaching into subsurface drains over a 15-yr period, and 2) to measure changes in drainflow and nitrate leaching that would result from converting from conventional monoculture corn with high N fertilizer rates to a no-till corn-soybean rotation with lower fertilizer N rates and a winter "trap crop".

METHODS

A subsurface drainage research facility was established in 1983 at the Southeastern Purdue Agricultural Center (SEPAC) in southeastern Indiana, USA. The site has been described in detail by Kladivko et al. [1991, 1999]. The soil at the site is a Clermont silt loam (fine silty, mixed, mesic Typic Ochraqualf) and is typical of extensive areas of similar soils across southern parts of Ohio, Indiana, and Illinois. The soil was formed in 50 to 120 cm of loess over glacial till. The surface soil at the study site is light gray, low organic matter (1.3%) silt loam containing 66% silt, 22% sand, and 12% clay. The soil is slowly permeable, and has a borderline fragipan at the 120 cm depth that severely restricts further downward drainage. Although subsurface tile drainage had not traditionally been used on these soils due to concerns of siltation in the tiles and the slow permeability of the soil, the past few decades have seen an increase in use of modern, perforated plastic drain tubing in these soils, with good success. The field experimental site has drains (10 cm diameter) installed at spacings of 5, 10, and 20 m at an average depth of 75 cm and a slope of 0.4%. Three drain lines (225 m length) were installed at each spacing, with the outside drain lines on each spacing acting as common drains between treatments. Two replications of each spacing were established in nonrandomized blocks separated by 40 m distance.

The center drains of the 5, 10, and 20 m plots discharge into observation wells at the bottom of the slope. Subsurface drainflow volumes are monitored continuously with tipping bucket flow gauges connected to a datalogger, and flow-proportional samples are collected with automatic water samplers during all time periods in which there is flow. Water samples are frozen until subsequent laboratory analysis. Nitrate-N mass losses were calculated as the product of water flow volumes and concentrations and were expressed on a per hectare basis, assuming that each drainline collects water from midplane to midplane with adjacent drains. A linear interpolation of concentrations was used to estimate concentrations on days between measurement points.

Corn was planted each year from 1984 through 1993, using conventional tillage (chisel plow to a 20 to 25 cm depth in spring, followed by two passes with a disc or field cultivator). In 1994 a no-till, soybean – corn rotation was begun, with the addition of a winter wheat ( Triticum aestivum L .) cover crop after corn as a "trap crop" for N in the soil profile. Fertilizer N rates were gradually reduced during the course of the 16-year experiment, as new knowledge became available and fertilizer rate "philosophy" changed. Fertilizer N rates were 285 kg N ha -1 for the first 5 years of monoculture corn, 228 kg N ha -1 for the last 5 years of monoculture corn, 200 kg N ha -1 in 1995, and 177 kg N ha -1 in 1997 and 1999, all applied as anhydrous ammonia pre-plant. The nitrification inhibitor N-serve was used with the anhydrous ammonia applications through 1995. A small amount (8 to 28 kg N ha -1 ) of "starter" fertilizer N was also applied during the planting operation for corn. Table 1 lists fertilizer N rates and crop yields for the 15-year period.

Table 1. Field Management Practices and Crop Yields. Preplant Total N Winter Crop yields (Mg/ha) Year Crop Tillage Fert. N N-serve? Starter N applied "trap Average both blocks (kg/ha) (kg/ha) (kg/ha) crop"? 5m 10m 20 m

1985

Corn

chisel

285

yes

8

293

no

11.1

11.5

11.3

1986

Corn

chisel

285

yes

21

306

no

8.1

8.3

8.5

1987

Corn

chisel

285

yes

22

307

no

11.6

12.1

12.1

1988

Corn

chisel

285

yes

11

296

no

8.0

8.4

7.7

1989

Corn

chisel

228

yes

20

248

no

9.9

9.6

8.8

1990

Corn

chisel

228

yes

28

256

no

8.3

7.6

7.7

1991

Corn

chisel

228

yes

28

256

no

7.3

7.1

7.9

1992

Corn

chisel

228

yes

28

256

no

10.7

10.6

10.1

1993

Corn

chisel

228

yes

28

256

no

10.2

9.9

10.0

1994

Beans

no-till

0

N/A

0

0

no

4.5

4.2

4.4

1995

Corn

no-till

200

yes

28

228

yes

8.6

8.5

9.2

1996

Beans

no-till

0

N/A

0

0

no

3.3

3.2

3.4

1997

Corn

no-till

177

no

28

205

yes

8.9

8.6

8.8

1998

Beans

no-till

0

N/A

0

0

no

3.3

3.2

3.3

1999

Corn

no-till

177

no

28

205

yes

9.9

10.2

10.2

Hydrology

Annual rainfall over the 15-yr period ranged from a low of 80 cm for 1987 to a high of 137 cm in 1995 (Table 2). Drainflow volumes varied among years as a result of the differences in annual rainfall and the timing of the rainfall within each year. Results from a 3-yr period near the beginning of the experiment (1986 through 1988, the first three full calendar years of nitrate-N measurements) will be compared to results from a 3-yr period near the end of the experiment (1997 through 1999). All results are presented as annual averages, and focusing on these two time periods will serve to highlight the changes that have occurred over the long-term experiment.

Drainflow per unit area decreased as drain spacing became wider, as expected (Table 2). Drainflow as a percent of annual rainfall was 20.2, 13.7 and 10.4% for the 5, 10, and 20m spacings, respectively, during the beginning time period, and 20.4, 18.6, and 14.8%, respectively, during the ending time period. Drainflow amounts varied each year depending on total rainfall and its timing and intensity. Averaged across both blocks, drainflow varied from a low of 6.7 cm (8% of annual rainfall) for the 20m spacing in 1987, to a high of 32.4 cm (26% of annual rainfall) for the 5m spacing in 1985.

Drainflow from Block 2 was greater than from Block 1, as has been the case since the beginning of the experiment. Averaged across the 15-yr period, Block 2 had 60% greater flow than Block 1. Relationships among the three spacings in each of the blocks have been generally consistent, however, with greatest drainflow per area from the 5m spacing and the lowest drainflow per area from the 20 m spacing.

Table 2. Annual Rainfall, Percent of "Normal" Rainfall, and Drainage Efficiencies as Affected by Drain Spacing. Rainfall Drainflow as % of rainfall‡ % of Drain spacing Year (cm) normal† 5 m 10 m 20 m

1985

126.0

111.5

25.8

14.7

12.6

1986

106.1

93.9

20.4

13.9

11.1

1987

80.0

70.8

15.7

11.0

8.3

1988

100.1

88.5

24.4

16.3

11.8

1989

123.1

108.9

24.6

17.1

13.5

1990

125.5

111.0

21.8

14.1

12.6

1991

110.1

97.4

18.0

11.8

10.6

1992

112.0

99.1

14.7

10.4

10.0

1993

132.3

117.0

18.5

14.0

10.6

1994

103.5

91.6

22.8

11.6

10.0

1995

136.6

120.9

16.2

12.2

9.2

1996

115.2

101.9

24.5

19.5

15.8

1997

104.0

92.0

22.6

21.3

16.2

1998

115.4

102.1

19.5

18.2

14.7

1999

87.2

77.1

19.1

16.4

13.4

15-yr av

111.8

99.0

20.6

14.8

12.0

(block 1)

17.6

9.7

8.8

(block 2)

23.5

20.0

15.3

30-yr normal precipitation (1971-2000) from North Vernon, IN

averaged across both blocks

Nitrate-N concentrations

Nitrate-N concentrations in drainflow dropped considerably over the 15-yr period. Concentrations were consistently in the 20 to 30 mg L -1 range in the 1985 to 1988 period and in the 7 to 10 mg L -1 range in the 1996 to 1999 period (Table 3). The large drop in nitrate-N concentrations with time is due to a combination of several factors that have changed over the 15-yr period, and the original design of the drainage experiment did not allow for testing these different factors individually. Probably the two main factors contributing to the lower concentrations are: a) lower fertilizer N rates, and b) growth of a winter cover crop as a "trap crop" for N after corn. The switch from chisel tillage to no-till, and from monoculture corn to a soybean-corn rotation, are not expected to have contributed much to the reduction in nitrate-N concentrations. In addition to management practice changes, the weather and resulting crop yields had an impact on year to year variations in nitrate-N concentrations (and load, as discussed later). During the first 5 yr of the drainage study (beginning in 1984, one year before the nitrate measurements began), preplant fertilizer N rates were 285 kg ha -1 , which was the recommended application rate at that time for a yield goal of 12.5 Mg ha -1 . Several years of poor crop yields (1986, 1988; see Table 1) and resulting high residual soil N, likely contributed to the increasing trend in concentrations over the 1985 to 1989 period. Preplant fertilizer N rates were reduced to 228 kg N ha -1 in the 5-yr period from 1989 to 1993, and concentrations started to show a drop in 1990, in the first "flow season" after the fertilizer application. A rise in nitrate-N concentrations in 1992 probably reflects the poor crop yield in 1991, but concentrations dropped again in 1993 following a high crop yield in 1992. These relatively rapid responses to low crop yield or prolonged dry periods, during the next flow season, have also been observed by others (Randall and Mulla, 2001). The 1994 change to a soybean-corn rotation and lower fertilizer N rates for the corn, did not result in an immediate drop in concentration, but by 1996 the concentrations had dropped again. The lower concentrations are likely a result of both the winter wheat "trap crop" after the corn and the lower fertilizer N rates.

Table 3. Annual Flow-Weighted Mean Nitrate-N Concentrations, as Affected by Drain Spacing Concentration (mg/L)† Year Crop Drain spacing 5 m 10 m 20 m

1985

Corn

19.9

20.0

20.3

1986

Corn

22.4

25.0

23.4

1987

Corn

31.6

33.5

30.5

1988

Corn

26.8

29.0

27.4

1989

Corn

32.6

34.5

30.1

1990

Corn

24.1

22.5

20.7

1991

Corn

13.3

15.6

15.7

1992

Corn

27.7

30.1

28.0

1993

Corn

16.9

17.1

15.3

1994

Beans

19.8

17.4

18.2

1995

Corn

17.8

15.1

15.9

1996

Beans

6.9

7.1

7.1

1997

Corn

9.6

10.7

10.3

1998

Beans

6.5

7.0

7.2

1999

Corn

6.6

6.4

6.9

averaged across both blocks

Reduction of fertilizer N rates and growth of winter cover crops have been shown by other authors to reduce nitrate-N concentrations in drainage, under specific soil, climatic, and crop management scenarios [Jaynes et al., 2001; Dinnes et al., 2002]. The precise fertilizer N rate needed for optimal crop growth and environmental quality is region-specific and varies from year to year, and this remains a major challenge for agriculturalists worldwide. The results from this site in southeastern Indiana suggest that it is possible to decrease average annual nitrate-N concentrations to below 10 mg L -1 (drinking water standard), while still growing corn and soybeans, on soils similar to the Clermont silt loam. The Clermont has a much lower organic matter content (1.3%) than many of the more productive, high organic matter (4-5%) soils of the "Midwest" region of the USA. Research on these higher organic matter soils suggests that it may not be possible to grow corn and soybeans on these soils while consistently maintaining nitrate-N concentrations below 10 mg L -1 , even with the best management practices currently available. The results of all these types of studies underscore the necessity for long-term field experiments in different regions and on different soils, in order to understand the impacts of yearly weather variations, long-term climate, soils, and management on nitrate-N leaching.

Nitrate-N load

Annual nitrate-N loads to drainage water dropped significantly over the 15-yr experimental period (Table 4), due to the large drop in nitrate-N concentrations over the same time period. Annual nitrate-N loads averaged 38 kg N ha -1 in the 1986 to 1988 period and 15 kg N ha -1 in the 1997 to 1999 period. This 60% reduction in load occurred in spite of the fact that drainflow was 29% greater in the 1997 to 1999 period (18.4 cm) than in the 1986 to 1988 period (14.3 cm). The 71% decrease in concentrations, from 28 mg N L -1 in the 1986 to 1988 period, to 8 mg N L -1 in the 1997 to 1999 period, resulted in a large decrease in loads even with a moderate increase in flow in those years.

Table 4. Annual Nitrate-N Loads in Drainflow, as Affected by Drain Spacing. Nitrate-N load (kg/ha)† Year Crop Drain spacing 5 m 10 m 20 m

1985

Corn-9 mo

36.5

25.1

20.2

1986

Corn

47.0

36.6

27.6

1987

Corn

38.3

28.1

19.9

1988

Corn

64.7

46.4

32.0

1989

Corn

96.3

71.1

50.1

1990

Corn

65.6

38.6

33.4

1991

Corn

25.7

19.2

19.1

1992

Corn

45.0

35.3

32.5

1993

Corn

41.2

31.1

21.3

1994

Beans

48.9

21.6

19.0

1995

Corn

39.3

25.3

20.5

1996

Beans

19.8

16.6

13.4

1997

Corn

22.4

23.2

17.3

1998

Beans

15.0

14.8

12.8

1999

Corn

10.9

9.2

8.4

averaged across both blocks

Mass loads calculated only for April - December, 1985, because sampling did not begin until April.

The greatest annual nitrate-N load occurred in 1989, following the poor crop yield in the drought of 1988. A presumably large residual soil nitrate pool, coupled with higher than normal precipitation and drainflow in winter and early spring of 1989, combined to give nitrate-N loads about 50% greater than the highest of all the other years. Randall and Mulla (2001) have also discussed the elevated nitrate-N concentrations and loads that can occur during a wet season which follows a dry season with poor crop growth.

Nitrate-N mass losses per unit area varied similarly to drainflow volumes, with a trend for greater losses from the narrower spacings (Table 4). Nitrate-N losses were 50, 37, and 27 kg ha -1 for the 5, 10, and 20m spacings, respectively, in the 1986-1988 period and 16, 16, and 13 kg ha -1 in the 1997-1999 period. The differences in losses among spacings were not significant in every year, however. As can be expected, differences in loads were larger in years with overall higher loads, and statistically significant differences were more prevalent in those years. All the significant differences were in the 1986 to 1994 period, with 1988 and 1989 having the largest differences.

Seasonal effects on drainflow and nitrate-N loads and concentrations

Nitrate-N loads to the drains exhibit a clear seasonal cycle related to the timing of drainflow in this system. The majority of the drainflow and nitrate-N loads occur during the dormant season, as discussed in previous reports about this site (Kladivko et al., 1991, 1999). Drains typically begin to flow in November or December at this site, although some years there is no flow until January. Drainflow continues throughout the winter during most years, although in some years the drainflow ceased during parts of January or February. This site thus differs from sites further north in the Midwest, where flow ceases most of the winter and occurs primarily in March through May. Flow at this site usually ceases in May or early June, but occasionally there will be some small flow events in July through October. The 14-yr (1985 was omitted because it was a partial year for nitrate analyses) average percent of flow and nitrate-N load occurring in each month is shown in Table 5. The seasonal distributions of flow must be considered when comparing different studies and when designing and evaluating different management strategies for decreasing nitrate loads to drainage water.

Table 5. Average Percent of Annual Drainflow and Nitrate-N load Occurring in each Month, Averaged Across all 6 Drains and 14 Years. Percent of annual:

Flow

N-load

Jan

13.0

13.4

Feb

16.6

16.3

Mar

18.6

17.0

April

16.7

14.8

May

9.7

10.2

June

7.1

8.2

July

1.8

2.6

Aug

0.1

0.1

Sept

0.0

0.1

Oct

0.6

0.9

Nov

5.0

5.5

Dec

10.6

10.8

General observations of the data did not show any obvious seasonal pattern in concentrations that was repeated throughout the years. There were occasionally some lower concentrations in January and some higher concentrations in June, but this did not occur consistently. The tendency for slightly higher concentrations in June for some drains and some years was of little significance when considering overall nitrate-N losses from the system, however. On average only 8.2% of the annual load and 7.1% of the annual flow occurred in June. Both flow and load were minimal in July through October. On average 64% of the annual flow and 63% of the annual nitrate load occurred in the dormant season of November through March. Another 17% of the flow and 15% of the nitrate load occurred in April, much of which time was before any field operations had been done for the next crop. Thus nearly 80% of the annual drainflow and annual nitrate load occurs in the winter and early spring before fertilization for the next crop occurs. Similarly, Drury et al. (1996) found 88 to 95% of the annual nitrate leaching losses occurred from November through April in their drainage study in southwestern Ontario. Management practices to reduce nitrate concentrations and/or drainflow during the dormant season are therefore key to reducing nitrate loads from drained lands.

CONCLUSIONS

Subsurface drainage is an important water management practice in many humid regions of the world, but it also has potential negative effects of increased nitrate leaching through soils. Both drainflow volumes and nitrate-N leaching losses are greater with more intensive drain spacing. When evaluating the "optimal drain spacing" for a given soil and climatic region, an assessment of both crop yield and drainage water quality should be performed. In the 15-yr study reported here, both crop yields and nitrate-N leaching losses were highest for the narrowest spacing (5m) and lowest for the widest spacing (20m). Nitrate-N mass losses were significantly decreased over the 15-yr period of study, by a combination of reductions in N fertilizer rates and growth of a winter cover crop as a "trap crop" after corn. Further research is needed to determine the relative contributions of each of these factors on this soil and to identify an optimal set of practices for both economic and environmental concerns. Additional research comparing the low organic matter soils represented here, and the high organic matter soils of much of the Midwest, USA, is essential for understanding the system and designing appropriate management practices for different regions.

REFERENCES

1. Drury, C.F., C.S. Tan, J.D. Gaynor, T.O. Oloya, and T.W. Welacky. 1996. Influence of controlled drainage-subirrigation on surface and tile drainage nitrate loss. J. Environ. Qual. 25:317-324.

2. Dinnes, D.L., D.L. Karlen, D.B. Jaynes, T.C. Kaspar, J.L. Hatfield, T.S. Colvin, and C.A. Cambardella. 2002. Nitrogen management strategies to reduce nitrate leaching in tile-drained Midwestern soils. Agron. J. 94(1 ) : 153-171.

3. Gilliam, J.W., J.L. Baker, and K.R. Reddy. 1999. Water quality effects of drainage in humid regions, Chap. 24 in R.W. Skaggs and J. Van Schilfgaarde (Eds.), Agricultural Drainage , Agronomy Monograph 38, American Society of Agronomy, Madison, Wisconsin, p.801-830.

4. Jaynes, D.B., T.S. Colvin, D.L. Karlen, C.A. Cambardella, and D.W. Meek. 2001. Nitrate loss in subsurface drainage as affected by nitrogen fertilizer rate, J. Environ. Qual. 30(4): 1305-1314.

5. Kladivko, E.J., J. Grochulska, R.F. Turco, G.E. Van Scoyoc, and J.D. Eigel. 1999. Pesticide and nitrate transport into subsurface tile drains of different spacings, J. Environ. Qual. 28(3): 997-1004.

6. Kladivko, E.J., G.E. Van Scoyoc, E.J. Monke, K.M. Oates, and W. Pask. 1991. Pesticide and nutrient movement into subsurface tile drains on a silt loam soil in Indiana, J. Environ. Qual. , 20(1): 264-270.

7. Randall, G.W., and D.J. Mulla. 2001. Nitrate nitrogen in surface waters as influenced by climatic conditions and agricultural practices, J. Environ. Qual , 30(2): 337-344.

8. Skaggs, R.W., and J. Van Schilfgaarde (Eds.). 1999. Agricultural Drainage , Agronomy Monograph 38, 1328 pp., American Society of Agronomy, Madison, Wisconsin.