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Journal of Environmental Quality 30:1659-1667 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Plant and Environment Interactions

Ryegrass Cover Crop Effects on Nitrate Leaching in Spring Barley Fertilized with 15NH415NO3

Lars F. Bergström*,a and William E. Jokelab

a Dep. of Soil Science, Swedish Univ. of Agric. Sci., P.O. Box 7072, S-75007 Uppsala, Sweden
b Dep. of Plant and Soil Science, Univ. of Vermont, Burlington, VT 05405-0082

* Corresponding author (lars.bergstrom{at}mv.slu.se)

Received for publication September 8, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Cover crops are a management option to reduce NO3 leaching under cereal grain production. A 2-yr field lysimeter study was established in Uppsala, Sweden, to evaluate the effect of a perennial ryegrass (Lolium perenne L.) cover crop interseeded in barley (Hordeum vulgare L.) on NO3–N leaching and availability of N to the main crop. Barley and ryegrass or barley alone were seeded in mid-May 1992, in lysimeters (0.3-m diam. x 1.2-m depth) of an undisturbed, well-drained, sandy loam soil. Fertilizer N was applied at the same time as labeled 15NH415NO3 (10 atom % 15N) at a rate of 100 kg N ha-1. In 1993, barley was reseeded in May in the lysimeters but with nonlabeled NH4NO3 and no cover crop (previous year's cover crop incorporated just prior to seeding). Barley yields and total and fertilizer N uptake in Year 1 (1992) were unaffected by cover crop. Total aboveground N uptake by the ryegrass was 28 kg ha-1 at the time of incorporation the following spring. Recovery of fertilizer-derived N in May 1993 was about 100%; 53% in soil, 46% in barley, <2% in ryegrass, and negligible amounts in leachate. In May 1994, the corresponding figures were: 32% in soil, <3% in barley, and, again, negligible amounts in leachate. The cover crop reduced concentrations of NO3–N in the leachate considerably (<5 mg L-1, compared with 10 to 18 mg L-1 without cover crop) at most sampling times from November 1992 to April 1994, and reduced the total amount of NO3–N leached (22 compared with 8 kg ha-1).

Abbreviations: FDN, fertilizer-derived nitrogen


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
LEACHING of NO3–N occurs in some soils and climatic conditions at levels of environmental concern, even with N fertilization at economically optimum rates (Baker and Johnson, 1981; Bergström and Johansson, 1991). Consequently, efforts have been made to develop management strategies to further reduce NO3–N leaching potential, including practices such as reduced tillage (Goss et al., 1993; Davies et al., 1996), split fertilizer applications, and use of slow-release N fertilizers (Heathwaite et al., 1993). Cover crops, often referred to as catch crops, are also effective for reducing NO3–N leaching when grown in conjunction with high N requiring crops such as corn (Zea mays L.) and cereal grains (see reviews by Meisinger et al., 1991, and Aronsson, 2000). Cover crops may also benefit soil–crop systems by increasing N supply for subsequent crops, limiting soil erosion, and improving soil structure (Meisinger et al., 1991).

The value of grass cover crops in reducing leachable N has been demonstrated in several experiments. In 3 yr of a 4-yr study on a sandy soil with barley, it was shown that a perennial ryegrass cover crop, interseeded each year and plowed down in the spring, reduced NO3–N leaching in tile-drained field plots by 83% compared with barley without a cover crop stubble-cultivated in early autumn and plowed in the spring (Lewan, 1994). However, during the fourth year, after the ryegrass was plowed under and not replaced by another cover crop, NO3–N leaching was larger from the cover crop treatment due to large inputs of easily decomposable crop residues. Similarly, in a lysimeter study on an irrigated calcareous brown soil, adding a ryegrass cover crop in a wheat (Triticum aestivum L.)–corn rotation reduced NO3–N leaching by 67% (Martinez and Guiraud, 1990). In both these studies, drainage volumes tended to be lower in the cover crop treatment. The influence of cover crop in mitigating NO3–N leaching is usually the result of the combined effects of reduced drainage volume and reduced concentrations of NO3–N in the drainage water. Reduction of NO3–N concentrations in ground water attributed to use of cover crops has also been demonstrated. For example, in a study conducted on the Atlantic Coastal Plain of Maryland, the use of a rye (Secale cereale L.) cover crop following corn reduced the concentrations of NO3–N entering shallow ground water by 29% compared with no-cover crop controls (Meisinger et al., 1990). Various Brassica species, used as cover crops, have also shown ability to significantly reduce high N levels in soil and NO3–N leaching (e.g., Bertilsson, 1988; Muller et al., 1989), while legumes appear to be less efficient for this purpose (Meisinger et al., 1990).

Some interseeded cover crops decrease yields of the main crop. For example, in Scandinavia, perennial ryegrass in spring barley reduced barley yields by <3% (Jensen, 1991; Andersen and Olsen, 1993; Wallgren and Lindén, 1994; Ohlander et al., 1996). However, barley yield reductions were as high as 20% when Italian ryegrass (Lolium multiflorum Lam.) was used (Andersen and Olsen, 1993; Lewan, 1994; Lyngstad and Børresen, 1996).

In the present investigation, a field lysimeter study was established on a sandy loam soil to evaluate the effect of a perennial ryegrass cover crop interseeded in spring barley on (i) nitrate leaching during and after the growing season and (ii) yield and N uptake of the current and subsequent barley crop. This type of information is important to influence future environmental policy. The use of 15N-labeled fertilizer enabled examination of the pathways of added N in the plant and soil, and helped establish a comprehensive N budget for a barley–ryegrass cover crop system.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Procedures
Ten undisturbed soil monoliths of a sandy loam soil (Fluventric Haplumbrept) were collected from a site (Mellby) in southern Sweden (56°29' N, 13°0' E) in June 1991. Some physical and chemical properties of this soil are listed in Table 1. The monoliths were extracted using a coring technique in which a standard polyvinyl chloride (PVC) pipe (0.295-m diam., 1.18-m length) was gently pushed into the soil by a steel cylinder with four cutting teeth, which rotated around the pipe as it penetrated the soil (Persson and Bergström, 1991). After collection, approximately 0.1 m of soil at the base of the soil profiles was removed and replaced with a nylon mesh followed by a 0.05-m-thick drainage gravel (2- to 4-mm diam.) layer, a porous plastic sheet, and a fiberglass lid that was screwed onto the inside of the casing wall. Five holes (0.01-m diam.) were drilled in each bottom lid to provide gravity drainage. In July 1991, the lysimeters were placed in pipes permanently installed (belowground) at a lysimeter station in Uppsala, Sweden (59°49' N, 17°39' E). The pipes were surrounded by grass to avoid advection effects. However, this did not prevent light from penetrating through the sides of the lysimeters. Continuous exposure of the lysimeters to natural field conditions for 10 mo prior to this study and observation of water draining through all columns until the end of April 1992 ensured that the soil profiles were at a moisture content close to field capacity shortly before the start of the experiment. The lysimeter station and the preparation of lysimeters is described in more detail elsewhere (Bergström and Johansson, 1991).


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  		Table 1. Selected soil characteristics of the Mellby soil.

 
All management procedures performed in the lysimeters were done to reproduce field conditions as closely as possible. Just prior to seeding on 14 May 1992, soil in each lysimeter was hand-tilled to simulate a light harrowing. Labeled 15NH415NO3 (10 atom % 15N) was applied at a rate of 100 kg N ha-1, and P and K were applied at rates of 30 kg ha-1 each, and then mixed in the top 5 cm of soil. Simultaneously, spring barley (cv. Golf) was seeded at 200 kg ha-1 and five randomly selected lysimeters were interseeded with perennial ryegrass (cv. Tove) at 8 kg ha-1. The barley was harvested on 25 Aug. 1992 by cutting the aboveground biomass at ground level; the ryegrass was not cut on this occasion. After harvest, the barley plant parts were brought to the laboratory and dried at 40°C. The ryegrass cover crop from two lysimeters was harvested the following spring (12 May 1993) using the same procedure as described above. At the same time those two monoliths plus two from the noncover crop treatment were removed and sectioned into seven depth increments (0–5, 5–10, 10–20, 20–40, 40–60, 60–80, 80–100 cm). All soil in topsoil layers (to a depth of 40 cm) was sieved (2.0 mm) to collect barley and ryegrass roots. However, the finest roots could not be separated with this method.

The remaining six lysimeters (three from each treatment) were reseeded with spring barley and fertilized on 12 May 1993 at the same rates as during the previous season. However, on this occasion unlabeled NH4NO3 was used and no cover crop was seeded. The soil in all lysimeters was hand tilled to a 5-cm depth just before seeding to incorporate fertilizer and, in three lysimeters, the ryegrass cover crop material. The aboveground barley biomass was harvested on 27 Aug. 1993. The experiment was terminated on 3 May 1994 by sectioning the monoliths into the same depth increments as in the previous spring. However, no separation of roots was done at this time because of the limited amount of root mass without the ryegrass cover crop.

In addition to natural precipitation (951 mm), all lysimeters received supplemental irrigation 14 times in summer and autumn during the 2-yr experimental period. This was done to create a leaching potential somewhat greater than average for the Uppsala region, but still within normal range. Irrigation events were 9 to 22 mm each, totaling 169 mm over the 2-yr period. On each occasion, water was added at rates typical of heavy rain events but not in excess of the infiltration capacity of the soil. Combined precipitation and irrigation totaled 1120 mm for the 2-yr period, which is slightly higher than the long-term average precipitation for Uppsala during this period (1054 mm), but considerably less than average for southern Sweden (1546 mm), where the soil cores were collected.

Leachate and Soil Solution Sampling and Analytical Procedures
Leachate from the monoliths was collected and weighed weekly (if available), from May 1992 to April 1994, in glass sampling bottles placed in the lysimeter station. Leachate subsamples were taken for determination of NO3–N and 15N concentrations. Nitrate N and 15N loads were calculated by multiplying the weekly leachate volume by NO3–N and 15N concentrations.

In addition to leachate samples, soil solution was extracted from five depths (10, 20, 40, 60, and 80 cm) within the soil profile of four lysimeters (two from each treatment). This was done by placing 50 kPa suction on 8-mm-diam. ceramic candles installed horizontally through the lysimeter wall. Solution samples were collected on 10 occasions during the experimental period. However, on some occasions, soil moisture was not adequate to obtain a sample in some of the candles. Nitrate N concentrations in all water samples (leachate and soil solution) were determined by flow injection analysis (Tecator AB, Höganäs, Sweden) according to the colorimetric Cd-reduction method (American Public Health Association, 1985).

To determine the inorganic N (NO3–N + NH4–N) content in soil, defrosted field moist soil samples (120 g) were extracted with 2 M KCl solution (300 mL). Concentrations of NO3–N were then determined by the method described above. Concentrations of NH4–N in the soil extracts were determined with a combined flow-injector gas-diffusion method (Tecator, 1984) in which the extract is injected into a carrier stream and mixed with 0.1 M NaOH solution.

In preparation for 15N analysis, subsamples of soil and plant material were ball-milled after grinding to obtain homogeneous samples. Total N and 15N in these samples were determined simultaneously by mass spectrometry. Water samples (approximately 20 mL) were acidified to pH 1 using concentrated sulfuric acid to eliminate NH3 volatilization. The acidified samples were evaporated to dryness and subsequently analyzed for 15N. Microsamples (20–30 mg of soil or 2–6 mg of leachate crystals and plant material) were weighed into aluminum foil capsules. These capsuled samples were then introduced into a mass-spectrometer system (Tracermass 15N/13C; Europa Scientific, Crewe, UK) for 15N determination.

Amounts of fertilizer-derived nitrogen (FDN) in plant parts, soil, and water were calculated as follows (modified from Hauck and Bremner, 1976):

[1]
where N is the amount of N in the plant, soil, or water; a is the atom % 15N in soil, water, or plant not fertilized with labeled N; and s and f are the atom % 15N in the sample (plant, soil, or water) and fertilizer, respectively. The value for a, or natural abundance, was determined from plant, soil, or water samples from nonfertilized areas at the site where the monoliths were collected.

Experimental Design and Statistical Analysis
This experiment uses 10 lysimeters arranged in a completely randomized design, with five replicates of the two treatments (barley with or without a ryegrass cover crop). Two replicates of each treatment were destructively sampled in May 1993 for soil N analysis, leaving only three replicates for the second year. Ceramic samplers for measuring nitrate concentration in soil solution were installed in only two replicates. On some occasions, because only one sample of a treatment could be collected at some depths, statistical analysis was not performed on NO3–N concentrations in soil solution.

Statistical analysis for the plant and associated N variables consisted of T tests comparing cover and no-cover crop treatments. Repeated measures analysis of variance was used to compare treatments with respect to NO3–N concentrations with soil depth as the repeated factor. Separate analyses were performed for the 1993 and 1994 sampling years. If significant year by depth interactions were detected, based on the global F test, simple main effects of cover conditions were tested at each soil depth. Differences in NO3–N concentrations in the leachate were also analyzed using repeated measures with sample date as the repeated factor and cover condition as the grouping factor. Analyses were performed using SAS statistical software (PROC GLM and PROC MIXED; SAS Institute, 1990).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Biomass and Nitrogen Uptake by Plants
Biomass, total N uptake, and fertilizer-derived nitrogen (FDN) uptake of barley and the ryegrass cover crop, and associated P values for 1992 and 1993 are shown in Table 2. Barley biomass production (grain plus aboveground residues) averaged about 8200 kg ha-1 in 1992, which is similar to field production yields for the area. The interseeded ryegrass cover crop had no significant effect on the first year's barley biomass yield or on total N or FDN uptake. The total biomass and N uptake from the combined barley–ryegrass system were not statistically different from barley alone (statistics were limited to only two replicates). Lack of yield reduction has also been shown for corn when rye was used as an interseeded cover crop on a sandy loam soil in Michigan (Rasse et al., 2000). The belowground biomass and total N uptake were significantly greater (P = 0.02) from the treatment with a cover crop because of the substantial amount of root material from the ryegrass. Belowground uptake of FDN was significantly greater (P = 0.09) in the cover crop system, although the difference was <1 kg N ha-1. Combined above- and belowground totals for biomass and N and FDN uptake are not reported in Table 2 because there were only two replicates for the belowground material (vs. five for aboveground). However, statistical analysis for the combined totals from two replicates showed no significant difference. The shoot to root ratio for barley (Table 2) was lower than that found in other studies under similar climatic conditions (Hansson et al., 1987).


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  		Table 2. Biomass and total N and fertilizer N uptake by barley and a ryegrass cover crop and associated P values in 1992 and 1993.

 
In 1993, the residual effects of the previous year's incorporated cover crop could be assessed. Biomass and uptake of FDN were significantly lower (P = 0.07 and 0.01, respectively) in the treatment with a previous cover crop but total N uptake was not (Table 2). Similar yield reductions have been observed by others (Martinez and Guiraud, 1990; Jensen, 1991; Davies et al., 1996). The yield decrease was probably the result of short-term N deficiency and/or unfavorable physical conditions as a result of incorporation of ryegrass residues. Uptake of N by the previous cover crop resulted in extremely low soil NO3–N concentrations (<0.1 mg kg-1, 0–10 cm depth) at the time of seeding, which probably affected early season growth before the added N fertilizer was dissolved and available for barley. Also, mixing of ryegrass residues into the surface 5 cm of soil created a loose soil–residue structure, which may have interfered with good soil–seed contact, slowing germination and early growth. Similarly, Martinez and Guiraud (1990) found that incorporation of ryegrass residue lowered corn grain yields by 13% compared with corn grown following bare fallow. They attributed this yield reduction to extremely high root density of the ryegrass cover crop, which contributed to deterioration of the soil structure and conditions unsuitable for sowing and growth. Other studies have shown that yields were not affected by incorporation of cover-crop residues (Andersen and Olsen, 1993; Breland, 1996) and some have shown positive yield responses (Lewan, 1994; Lyngstad and Børresen, 1996).

One important factor to keep in mind when evaluating the effect of the cover crop on the main crop is the small size (0.07 m2) of the lysimeters used in this study. Although simulated agronomic practices were carried out, the conditions for crop growth were undoubtedly different in the lysimeters compared with a natural field situation.

Drainage and Nitrate Leaching
Drainage volumes collected from the lysimeters averaged 22% of precipitation plus irrigation over the 2-yr study period (Fig. 1). Most drainage occurred in the November to April period, with none occurring during the active growing season when evapotranspiration was highest. This drainage pattern of a net downward movement of water in unfrozen soil during the noncropping period is typical of Scandinavian climatic conditions (Bergström and Jarvis, 1993). Leachate volume averaged 18% lower with a cover crop in the first year and somewhat higher in the second year, but differences were not statistically significant (P > 0.10).



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  		Fig. 1. Effect of spring-seeded (1992) ryegrass cover crop (incorporated in May 1993) on cumulative leachate from the lysimeters; and monthly precipitation (plus irrigation) amounts during the study period.

 
Statistical analysis of leachate NO3–N concentrations using repeated measures analysis of variance indicated that cover crop effects were date-specific (P value for treatment by date interaction <0.01), so we analyzed by individual sampling dates. Concentrations of NO3–N in the leachate were quite variable and showed no consistent treatment effect during the first 6 wk of leachate collection (Fig. 2). However, from the mid-December to early June period concentrations averaged about 3 mg L-1 with the cover crop compared with about 15 mg L-1 without the cover crop, and treatment differences were significant. From December 1993 to March 1994, following the season with no cover crop treatment, leachate from the previously cover-cropped lysimeters continued to show lower NO3–N concentrations, although the differences were not significant at most dates (most likely due to the reduction from five to three replicates). Nitrate concentrations increased during the last weeks of the study, probably a result of increased mineralization with rising spring temperatures, and the cover crop treatment was again significantly lower.



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  		Fig. 2. Concentrations of NO3–N in lysimeter leachate during the period October 1992–April 1994, as affected by a spring-seeded (1992) ryegrass cover crop (incorporated in May 1993). Statistical differences are indicated by ** (0.01), * (0.05), and + (0.10). NS = not significant. Data points without symbols had only one sample for one of the treatments.

 
Due to a combination of decreased NO3–N concentration and somewhat lower volume of drainage water, the ryegrass cover crop reduced the mass of NO3–N leached (Fig. 3). This effect started in mid-November 1992, 6 mo after seeding barley and ryegrass, and continued until the end of the study, resulting in a 63% reduction in mass of NO3–N leached from mid-November 1992 to April 1994 (Table 3). However, the difference was significant only in the November 1992 to June 1993 period. Similar reductions in NO3–N leaching on a sandy soil in a barley–ryegrass system under comparable environmental conditions were recorded by Lewan (1994). Winter cereal rye reduced NO3–N leaching by 32 to 40% compared with winter fallow on a loamy soil in Oregon (Brandi-Dohrn et al., 1997). The large reductions obtained in the present study were mainly a result of the favorable conditions for the ryegrass cover crop (early harvest of the barley and relatively mild winters to allow for good growth and survival of the ryegrass).



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  		Fig. 3. Cumulative NO3–N leached from lysimeters with and without a cover crop during the period October 1992–April 1994.

 

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  		Table 3. Ryegrass cover crop effect on mass of NO3–N leached from May 1992 to April 1994 and associated P values for T tests.

 
Nitrate Concentration in Soil Solution
Concentrations of NO3–N in soil solution extracted from five depths in the soil profile help explain the NO3–N leaching dynamics during the study period (Fig. 4). It should be borne in mind that these data were not statistically analyzed because, as mentioned above, on some occasions only one sample of a treatment could be collected at some depths. At the first sampling time (1 Oct. 1992) large differences were apparent between the two treatments in the entire profile except the lowest (80 cm) depth. Differences first occurred at the 80-cm depth on 28 October, approximately 1 mo before differences began to appear in the leachate. Large differences in concentration were maintained at the lowest depth through January 1994 and throughout most of the profile through October 1993. Treatment differences in the surface layer varied over time but were generally small or nonexistent (except for November 1993 samples) after July 1993, the first sampling after soil incorporation of the ryegrass cover crop. This process progressed downward through the profile, in some cases leading to reversed treatment order, but the cover crop effect did not disappear in the 80-cm depth until the last sampling on 30 Mar. 1994. At that time, NO3–N concentrations were similar throughout the profile.



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  		Fig. 4. Effect of spring-seeded (1992) ryegrass cover crop (incorporated in May 1993) on NO3–N concentrations in soil solution on 10 sampling dates during the study.

 
The delay in cover crop treatment effect on leaching in autumn 1992 and the tendency for a continued effect through the residual year apparently reflect the time required for NO3–N in the soil solution to move from the surface layer, where most of the roots were located and where N uptake and transpiration effects were active, to the 1-m depth, where leachate was collected. Soil solution concentrations in lysimeters without a cover crop showed a pulse of NO3–N that decreased in magnitude as it moved from the 20- to 80-cm depth from October 1993 to January 1994 (Fig. 4). The higher NO3–N concentration from the cover crop treatment in the surface layers in late 1993 may be the result of mineralization of the ryegrass that was incorporated into the soil the previous May. However, this NO3–N did not increase the NO3–N concentrations deeper in the profile during spring 1994, which is in line with the fact that NO3–N concentrations in leachate were significantly lower in the cover crop treatment during spring 1994 (Fig. 2).

Inorganic Nitrogen in the Soil Profile
The concentration of soil NO3–N under the cover crop appeared lower throughout the soil profile the spring following the cover-crop season (May 1993). The difference was especially pronounced in the surface 5-cm layer (Fig. 5). Main effects of cover crop, depth, and interaction were all significant (by repeated measures analysis), although analysis of separate depth increments showed significance in only the upper three layers. One year after the cover crop, NO3–N concentrations were not significantly different. This pattern is similar to that of soil solution NO3–N in the spring sampling times (6 Apr. 1993 and 30 Mar. 1994; Fig. 4).



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  		Fig. 5. Effect of spring-seeded (1992) ryegrass cover crop (incorporated in May 1993) on soil NO3–N concentrations at various depths in May 1993 and 1994, including standard error bars.

 
In May 1993, inorganic N (NO3–N + NH4–N) content in soil with a cover crop (12 kg ha-1) was significantly less (P < 0.10) than in soil without a cover (62 kg ha-1). In the residual year the inorganic N content was 45 kg ha-1 in the treatment with previous year cover crop and 35 kg ha-1 in the treatment without cover (no significant difference). The NH4–N fraction was always less than 10 kg ha-1 and did not vary greatly, so the treatment effects were primarily a function of NO3–N.

Fertilizer-derived N was concentrated in the surface 5 cm of soil both years, although measurable amounts were found throughout the profile in May 1993 (Fig. 6). The total amount of FDN in the soil was not significantly affected by cover crop treatment in either year (Table 4), but the total amount decreased substantially, from an average of 52.9 kg ha-1 in May 1993 to 31.8 kg ha-1 one year later. Only small amounts (<3 kg N ha-1) can be attributed to removal by the barley in the residual year, and even smaller amounts (<0.5 kg N ha-1) were measured in leachate. Most of the loss appears to have been from N that had moved to depths below 20 cm by May 1993 (Fig. 6). This suggests that much of the FDN at those depths was present in inorganic forms and may have been lost to denitrification.



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  		Fig. 6. Total fertilizer-derived N in various depths of the soil profile in May 1993 and 1994, as affected by the presence of a spring-seeded (1992) ryegrass cover crop (incorporated in May 1993). Bars represent standard error. Note that the top three depths represent smaller increments.

 

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  		Table 4. Recovery of fertilizer-derived N in plant, soil, and leachate as affected by the ryegrass cover crop. Note that because N application rate was 100 kg ha-1, % recovery = kg ha-1.

 
Fertilizer Nitrogen Budget
The total recovery of FDN from all sources one year after application (1993) was about 100% (Table 4). Almost the entire amount was recovered in soil and barley, with less than 2% recovered in the ryegrass cover crop and even less in the leachate. The recovery of FDN accounted for after 2 yr (initial year + residual year) in May 1994 totaled only about 80% of that applied (Table 4). The decrease was the result of much lower recovery of FDN in the soil at the end of the second year (only about 30%). Leached FDN accounted for negligible amounts (<0.1 kg ha-1) and second-year recovery by the barley crop was very low. The only known major pathway not accounted for was denitrification, which may be responsible for at least part of the difference.

The low recovery by the cover crop may have been a function of timing and placement of the fertilizer. Although the ryegrass was seeded and fertilized with N at the same time as the barley, growth of the ryegrass was quite limited through much of the growing season because of competition from the barley canopy. Much of the labeled fertilizer N, therefore, was probably taken up by barley or immobilized in soil organic matter before substantial ryegrass root growth occurred.

A similar phenomenon may be responsible for the low recovery of FDN in the leachate. Drainage from the lysimeters did not occur until September, 5 mo after fertilizer N application, because evapotranspiration exceeded precipitation and irrigation. Also, soil moisture was too low to obtain soil solution samples from the ceramic suction samplers until 1 Oct. 1992 (Fig. 4). Consequently, by the beginning of the active leaching period most of the labeled N may have been removed from the available N pool by plant uptake and microbial immobilization, resulting in low amounts of labeled N in the leachate. This phenomenon can be attributed to pool substitution, in which labeled N substitutes for nonlabeled N that otherwise would have been removed from the available N pool by processes such as immobilization. This is part of what has been described as "mineralization–immobilization turnover" (MIT) (Jansson and Persson, 1982) and has been termed "apparent" added nitrogen interaction (ANI) (Jenkinson et al., 1985). A similar phenomenon using 15N-labeled N on corn was observed by Jokela and Randall (1997). In another lysimeter study using 15N-labeled N to evaluate cover crop effects on NO3–N leaching in corn, McCracken et al. (1994) also reported low recovery of labeled N.

Leaching of NO3–N in this study was moderate, averaging 13 and 28 kg ha-1 from treatments with and without a cover crop, respectively (Table 3). It is likely that the measurement of labeled N in the leachate (<0.5 kg ha-1) grossly underestimated the contribution of fertilizer N to leaching because of the phenomenon described above. However, the lack of a control treatment (no fertilizer) prevents comparison with an alternative estimate of fertilizer N contribution (i.e., the difference method). This situation emphasizes the importance of examining both labeled and nonlabeled N pools in studies with 15N.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Despite the fact that a lysimeter study, such as the one presented here, is not fully representative of field conditions (small size of lysimeters, no influence of ground water, etc.), the results obtained in this study clearly showed that a ryegrass cover crop, interseeded in spring barley for one season, substantially reduced NO3–N leaching. In this case, leaching was reduced by two-thirds in the first year and by more than 50% over a 2-yr period. The cover crop reduced NO3–N concentration in the leachate to levels (about 3 mg L-1) well below the U.S. and European drinking water standards, compared with approximately 15 mg L-1 without a cover crop. These large reductions were made possible by a combination of high leaching potential (sandy soils and high amounts of autumn rainfall) and optimum conditions for ryegrass cover crop growth (early-season harvest of the main crop to allow vigorous cover crop growth and mild over-winter climate for continued ryegrass survival). The delay until late autumn, in the cover crop effect on leaching, as well as the continued effect into the second season, resulted from the time required for NO3–N movement through the soil profile, as documented by soil solution sampling.

One potential problem associated with the use of interseeded cover crops is crop yield reduction due to competition for nutrients, water, and light. However, in this study, barley yield was not significantly affected by the presence of the interseeded ryegrass cover crop during the first year, although it was reduced somewhat during the residual year.

The low recovery of FDN in ryegrass and leachate are presumably a result of mineralization–immobilization turnover and point out the limitations of 15N methodology and the importance of examining both labeled and nonlabeled N pools in this type of research.


   ACKNOWLEDGMENTS
 
Financial support for this project was provided by the Swedish Council for Forestry and Agricultural Research. We would like to thank Pär Aronsson for valuable help conducting the experiment, Bert Forsberg for 15N analyses, Jeff Tricou and Faruk Djodjic for graphics and statistical analysis, and Gary Badger for statistical consulting and analysis.


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