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NITROGEN MANAGEMENT

Nitrate Leaching as Influenced by Cover Crops in Large Soil Monoliths

National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011-3120

* Corresponding author (logsdon{at}nstl.gov)

Received for publication April 25, 2001.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Unacceptable levels of NO₃ leaching to ground water and drainage systems can occur under corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotations. Cover crops have the potential to reduce NO₃ leaching, but this process has not been well documented. Lysimeters utilizing large soil monoliths are an excellent approach for studying NO₃ leaching because inputs can be controlled and outputs accurately measured. The objective of this study was to see if fall cover crops could reduce NO₃ leaching from large soil monoliths. We used three (1 by 1 by 1.5 m deep) monoliths in each of two controlled climate chambers with oat (Avena sativa L.) or rye (Secale cereale L.) fall cover crop interplanted into soybean in mid-August. The study was continued for two cover crop cycles in each chamber. In Chamber 1, drainage was significantly reduced due to oat or rye cover crops for the fall through summer of Years 1 and 2 (first cover crop cycle), and NO₃ loss was reduced for most of the same time period. In Chamber 2, NO₃ loss was reduced for the spring-summer season of the second year (first cover crop cycle). Although drainage was less under cover crops for Chamber 1, the soil water content was not consistently lower because of replenishment by watering. The soil monoliths were useful for showing that oat and rye cover crops in a corn–soybean rotation can reduce NO₃ leaching from lysimeters and suggest that the same trend would be true in the field.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
FALL COVER CROPS can reduce N leaching (Owens et al., 1995; Aronsson and Torstensson, 1998; Shepherd and Webb, 1999). They extend the growing season and the uptake of N beyond that for corn and soybean. Small-grain cover crops take up residual N (Ditsch et al., 1993; Kessavalou and Walters, 1999), N released by mineralization during fall and spring, and N released from fall-applied anhydrous NH₃. The cover crops then release this N as their residues decay the next spring or summer.

Concern has been raised over potential harm of cover crops to the next crop (Karlen and Doran, 1991; Johnson et al., 1998). Cover crops could be detrimental if too much N is tied up and not released in a timely manner to the next crop (Vyn et al., 1999), cover crops compete with the cash crop for water in the spring, and potential allelopathic effects of the decomposing cover crop residue reduce main crop growth. Karlen and Doran (1991) showed that cover crops before corn created an early season N deficiency, and even additional N fertilizer did not help make up the difference.

In Iowa, small-grain cover crops were overseeded into soybean in August to allow a longer growing season for the northern climate (Johnson et al., 1998). Soybean does not leave enough residue after harvest and is not anchored well enough to protect the soil from erosion. Cover crops growing after soybean increase surface cover, anchor residue, and reduce rill erosion (Kaspar et al., 2001). Also, there is a longer fall season following soybean than there would be following corn because of earlier soybean maturity and leaf drop, allowing for greater cover crop growth.

Oat often grows well in the fall but is winter-killed; therefore, herbicides are not needed to kill the oat in the spring before planting corn. Rye overwinters and regrows in the spring. For both oat and rye, growth may be reduced in the fall if they are not established early enough or if fall soil moisture is limiting.

Column studies are often considered an inadequate representation of the field, yet field-leaching studies are complicated by difficulty in obtaining effluent loads. Effluent can be collected in tiles, but only if the water table is shallow and tiles are present. Effluent can be collected in pan or wick samplers, but the samplers alter the water flow pattern (Barbee and Brown, 1986; Holder et al., 1991; Boll et al., 1992; Poletika et al., 1992). Concentrations can be measured from soil extractions or from suction-cup samplers, but the relation to effluent concentration is tenuous (Shaffer et al., 1979; Barbee and Brown, 1986; Jaynes et al., 1988). Weather and soil variability further complicate field studies.

Field-scale studies may be conducted in intensively monitored plots, fields, or watersheds. Thurman et al. (1998) evaluated the merits of field studies vs. lysimeter studies. The field has a greater degree of soil variability, which may or may not be captured in undisturbed soil monoliths. The lysimeter or monolith has unrealistic boundaries compared with the field, which influences proper interpretation of surface or subsurface lateral flow, drainage out of the bottom and prevention of evaporation from lower soil layers, and boundary interaction with roots and organisms.

Examining NO₃ leaching in the field is often difficult because some inputs and outputs are difficult to control and measure. Bergström (1987) measured NO₃ leaching from tile-drained plots and from large (9 by 3 by 1 m deep) lysimeters for barley (Hordeum distichum L.), grass (Festuca pratensis L.), and alfalfa (Medicago sativa L.), with and without N fertilizer. Also, some medium lysimeters (1.2 m diam. by 0.75 m deep) and small undisturbed lysimeters (0.295-m-diam. cylinder by 1.18 m deep) were installed. Nitrogen leaching was around 5 kg N ha^-1 yr^-1 for the grass and lucerne. After they were tilled, the N leaching reached 42 kg N ha^-1 in 20 wk, even more than for fertilized barley. The lysimeters gave similar results to the field tile results.

This study augments a field cover crop study, with the emphasis of this study on NO₃ leaching as influenced by fall small-grain cover crops. Our objective is to investigate whether fall cover crops will reduce NO₃ leaching for a corn–soybean rotation grown in undisturbed soil monoliths under a controlled environment.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Extraction of Soil Monoliths
Six undisturbed soil monoliths, 1 by 1 by 1.5 m deep, were taken from Monona silt loam soil (fine-silty, mixed mesic Typic Hapludoll) in southwestern Iowa. Soil characteristics are given in Table 1. Three of the monoliths were taken 2 Sept. 1992 (for Chamber 1), and three were taken 8 Sept. 1994 (for Chamber 2). The monoliths were taken from a ridge-till field that had been in continuous corn from 1964 through 1993 (Karlen et al., 1999). After that, the field was converted to corn–soybean rotation.

Indoor Preparation
Once the monoliths were unloaded from the truck, a specially designed cart was used to transport the 3000-kg monolith during preparation and loading in the rhizotron chamber, which is described in Kaspar et al. (1992). Before and during preparation, the fallow monoliths were watered periodically to keep macrofauna alive and prevent excessive soil drying. They were also inspected to verify that they remained undisturbed. The sand was emptied, and the gap between soil and the sidewall was filled with commercial bentonite clay. Fiberglass wicks (Holder et al., 1991) were attached to the bottom of the soil column and placed through holes in the base. Then the column was placed on its 0.1-m permanent sand-filled base.

The monoliths were then transported to the lower rhizotron chamber where a mounting frame had been constructed. Once the monoliths were in place, they were secured to the mounting frame, which rested on load cells (Fig. 1) . The fiberglass wicks were covered with plastic tubing and directed to the two effluent collectors attached to the frame. The effective wick length below the base of the box was 0.28 m. A floor was built in the upper chamber around the monoliths that isolated the upper and lower chambers thermally (Fig. 1) but still allowed each monolith to move freely to prevent interference with the load cell measurements. Neutron moisture meter (Gardner, 1986) access tubes were installed in the center of each monolith (Fig. 1). The neutron moisture meter was calibrated from the soil removed to install the tubes. Three monoliths were installed in each of two environmental chambers for a total of six monoliths. Each environmental chamber was completely self-contained and independent. The experiment started about 1 yr later in Chamber 2 than in Chamber 1.

View larger version (77K): [in this window] [in a new window]   Fig. 1. Diagram of setup for rhizotron upper and lower chambers.

Planned climate control was based on 30-yr normals for mid-Iowa (Meek and Hatfield, 1994, 2001). Before the climate control was set, the rain had been applied at 25.4 mm wk^-1 (too wet), and the lower chamber temperature was set to a constant 11°C (too cool). Equipment malfunction and water leaks in the chambers caused deviations from the planned environmental control (Table 2). In Year 3 for Chamber 1 and Year 2 for Chamber 2, the rainfall rates were adjusted to 1.25 times normal because chamber evapotranspiration rates were greater than normal field evapotranspiration rates and drainage rates were lower. The winter periods were of variable length (depending on other factors such as overlaid studies or replacement of nonworking monitoring equipment and lights). In the winter, the lights were shut off, and temperature in the upper chamber was kept constant at 2.5°C. Except for the leaching study, no rain was applied in the winter. Although Chamber 1 was barely able to achieve freezing temperatures, Chamber 2 was not; therefore, after the first winter season in Chamber 1, the subsequent winter temperatures were kept just above freezing. Additional simulated rain was often applied in the spring to stimulate drainage after lack of winter precipitation. Also, main crop and cover crop residue was chopped and weighed before being returned to the monoliths.

The monolith comparison complemented field studies, similar to that described by Johnson et al. (1998). Each chamber was completely isolated and independent. Within each chamber, there were three monoliths. Each monolith contained a treatment, either control, oat, or rye cover crop. The cover crops were planted following soybean within a corn–soybean rotation, as was the practice for associated field studies (Johnson et al., 1998; Kaspar et al., 2001). Specifics of the cover crop management in these monoliths are described in Table 2.

Statistics
The data record for this work is in the form of a time series for each measured variable; hence, time series considerations and methods are used (Schumway, 1988). The data for each variable were summarized as 6-mo totals (evaporation, drainage, and NO₃ loss) or as means (water in profile 0.3–1.1 m). The 6-mo totals or means started in the beginning of September after the first planting of cover crops. Because length of the simulated winter varied, the winter season was grouped together and considered to be 3 mo within the 6-mo time. Treatment differences over time for each comparison pair were then determined. The error series, , for each treatment difference on the variables met the usual additive independent and identically distributed error assumptions (Montgomery and Peck, 1982). If each regression model with time as the independent variable is otherwise adequate, the assumptions are as follows:

1. The mean is zero, = 0.
   
2. The error variance is constant, ² = c, where c is a constant.
   
3. The errors are uncorrelated with respect to time, {[t(i)], [t(i - j)]} = 0, where is the Pearson product-moment correlation coefficient, t is the 6-mo time interval, i is the current 6-mo time interval, and j is the time increment index.
   
4. The errors are normally distributed, i.e., N, where N(µ, ) is the normal distribution with mean, µ, and standard deviation, .
   

The 6-mo period was used because the assumption of uncorrelated errors was met. Time was cast as period past the onset of treatments. For each pair of treatment comparisons, the difference was modeled mainly with splined ordinary or rational polynomial (i.e., the ratio of two polynomial functions) segments. When more than one segment was fit, then each segment was developed with a continuity constraint at the joint or transition points (also known as knots). Generally, a single knot was fixed at the midterm point of the series. This methodology is described in Gallant and Fuller (1973) and in Rivlin (1969). A 95% confidence interval was estimated over the entire period of comparison and used to assess the periods of significant difference. If zero was included within the confidence band, then differences were not significant. If the confidence band was completely negative, then the first value was significantly less than the second number. If the confidence band was completely positive, then the second value was significantly less than the first value. The overall methodology is similar to that used in Jaynes et al. (2001).

Measurement Reliability
To estimate measurement reliability and to estimate the closure in the water balance, we compared the residual term (water applied minus drainage minus evaporation) with depth of soil water determined by neutron moisture meter. Totals were used for water applied, drainage, and evaporation to determine the residual soil water storage. The residual measurements represented all depths and positions, but the neutron moisture meter only measured the 0.3- to 1.1-m depth. The soil water depth determined from the neutron moisture meter was adjusted to the soil depth of the monoliths (1.4 m). The weekly neutron moisture meter measurements did not necessarily correspond to the time intervals used for the totals (water applied, drainage, and evaporation), which sometimes resulted in a temporal offset. The sphere of influence for the neutron probe was smaller than for the residual method, which was a spatial resolution difference between the methods. The neutron probe readings were concentrated around the neutron tube in the center of each monolith. The difference between the two determinations of soil water storage was expressed as a fraction of the applied rain and then subtracted from 1.

	RESULTS AND DISCUSSION

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Measurement Reliability
For Chamber 1, the change in soil water depth determined from the neutron moisture meter accounted for 63 to 99% of the variability from the residual determination based on the water balance even though there were spatial and temporal differences (Table 3). For example, the neutron moisture meter did not measure above 0.3 m or below 1.1 m. For the spring-summer season of Chamber 2 in Year 3 (Table 4), there were large discrepancies between the two procedures due to drainage delay following the leaching study. Water deeper than 1.1 m continued to drain during the spring cycle even though simulated rain was delayed several weeks. Some other discrepancies occurred when there were water leaks in the chamber. Unplanned rain due to water leaks overflowed the effluent collectors in a few cases.

For the fall-winter of the third year (cover crop season, Table 5), the control continued to have more drainage than either the oat or rye treatments, and the rye continued to have more drainage than the oat treatment. The oat treatment had more evaporation than the rye treatment. For the spring-summer period of the fourth year, both the oat and rye treatments had less N loss than the control. Compared with the control, the oat treatment had greater soil water storage but less drainage. The rye treatment continued to have less evaporation than the oat treatment. For the fall-winter of the fourth year (off-season for cover crops, Table 5), the oat treatment had more soil water storage than the control or the rye treatment.

The most important information for these Chamber-1 comparisons was the reduced N loss for oat and rye treatments compared with the control during or after the seasons in which cover crops were present (Fig. 2) . The water balance components were less consistent and may have partly expressed natural variation between the monoliths.

View larger version (23K): [in this window] [in a new window]   Fig. 2. Nitrate N load lost from each treatment of Chambers 1 and 2.

For the fall-winter of the second year (off-season for cover crops, Table 6), the oat treatment had more soil water storage than the control or rye treatments and more NO₃–N loss than the rye treatment. The rye treatment had less soil water storage than the control. For spring-summer of Year 3, both the oat and rye treatments had more soil water storage than the rye treatment, and rye had more evaporation than the control.

For the fall-winter of the third year (cover crop year, Table 6), the oat treatment had more soil water storage than the control, and rye had more evaporation than the control. For spring-summer of Year 4, the oat treatment had more soil water storage than the rye treatment. For the fall-winter of the fourth year (off-season for cover crops, Table 6), the oat treatment had more soil water storage than either the rye or control treatments, and both oat and rye treatments had less evaporation than the control.

The significant information for Chamber 2 was that the NO₃–N loss (Fig. 2) was reduced for the oat and rye cover crops (spring-summer of Year 2) even though there was no significant drainage reduction. The NO₃–N loss reduction was due to lower NO₃–N concentrations of the effluent (not shown).

The NO₃ losses were less for the second cover crop cycles than for the first, even for the control treatments (Fig. 2). This may have been influenced by the fallow pretreatment and the imposition of no-till practices on the monoliths during the study.

Comparison with Data of Others
Leaching of NO₃–N from the control was 102 to 165 kg ha^-1 yr^-1 for the first year but only 6 to 24 and 18 to 39 kg ha^-1 yr^-1 for the second and third year (Fig. 2). The first-year leaching was greater than historical base-flow records for the Monona soil under continuous corn, for which annual NO₃–N leaching ranged from 7 to 81 kg ha^-1 yr^-1 (Steinheimer et al., 1998).

Oat cover crop reduced NO₃–N leaching to the ranges of 13 to 60, 9 to 34, and 0 to 13 kg ha^-1 yr^-1 for the first, second, and third years, respectively, of our study. Similarly, rye cover crop reduced NO₃–N leaching to the ranges of 15 to 37, 5 to 32, and 0 to 5 kg ha^-1 yr^-1 for the first, second, and third years, respectively, of our study. The magnitude of these results are similar to published studies.

Field lysimeters and wick-drainage pans have been used by others to examine NO₃–N leaching effects. For rye planted into soybean before leaf drop in a corn–soybean rotation, Owens et al. (1995) measured annual NO₃–N leaching of 15 to 62 kg ha^-1 yr^-1 through field lysimeters. For 3 yr after the first year, Shepherd and Webb (1999) observed field lysimeter NO₃–N loss for the permanent fallow averaging 11 kg ha^-1 for each winter compared with 16 to 22 kg ha^-1 for cover crops [tumbleweed (Artemisia sp.)] and 27 to 31 kg ha^-1 for control without cover crops. Using a passive capillary wick collection system in the field, Brandi-Dohrn et al. (1997) for 1 yr observed significant reductions in NO₃–N leaching for fall rye (7.5–21 kg N ha^-1) compared with fall fallow (14–55 kg N ha^-1) for the period from December through April (precipitation of 702 mm). Differences were not significant in another year when precipitation was 1024 mm for October through May.

Field tile studies have also been used by others for NO₃–N leaching studies. In a field study of perennial ryegrass (Lolium perenne L.) undersown in spring small grains, Aronsson and Torstensson (1998) showed that N tile loss from NO₃–N leaching in the undersown ryegrass treatment was 15, 20, and 48 kg ha^-1 yr^-1 for the first, second, and third years, respectively, and 24, 37, and 41 kg ha^-1 yr^-1 from the control. Johnson et al. (1999) observed only 1 kg N ha^-1 loss for fall rye compared with 15 kg N ha^-1 for fall fallow the first winter. The first-summer N losses were 25 and 33 kg ha^-1 following fall rye and fallow, respectively. The second-fall N losses were 17 and 30 kg ha^-1 for fall rye and fallow, respectively.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Although we did not have complete climate control at all times, we were able to directly measure NO₃ drained from the monoliths to complement the indirect measurements in the field (T.B. Parkin, personal communication, 2001). We were able to simulate growing seasons and grow crops to maturity. A drawback to the monolith study is that we were not able to periodically sample the soil to verify the N cycling as was possible in a complementary field study.

Because of the relatively low measurement error and similarity of our results to published studies, we have confidence that the indoor soil monoliths captured the essence of the cover crop effect for reducing NO₃ leaching. Even though the lysimeters were not field-scale measurements, we would recommend late-summer, interseeded small-grain cover crops as a management tool for reducing NO₃ leaching in corn–soybean rotations.

	ACKNOWLEDGMENTS

 
We acknowledge the contribution of Rich Hartwig in coordinating the preparation and installation of the large soil monoliths and Gavin Simmons, Tim Hart, Nancy Nubel, and Ben Knutson for sampling and measurements.

	NOTES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
¹ Mention of a specific trade name or commercial company is for reader information and does not imply endorsement by the authors or by USDA.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS NOTES RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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