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TECHNICAL REPORTS

Plant and Environment Interactions

Leaching and Crop Uptake of Nitrogen from Nitrogen-15-Labeled Green Manures and Ammonium Nitrate

Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7072, SE-75007 Uppsala, Sweden

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

Received for publication December 4, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Green manures can be used as an N source for agricultural crops as a substitute for inorganic N fertilizers. The effects of using green manures on leaching and uptake of N by spring barley (Hordeum vulgare L.) were evaluated in a 2-yr lysimeter study. Ryegrass (Lolium perenne L.) and red clover (Trifolium pratense L.) labeled with ¹⁵N were applied in May of the first year at 160 kg total N ha^–1. Simultaneously, ¹⁵NH₄¹⁵NO₃ was applied at 80 kg N ha^–1 to additional lysimeters and others were left without N additions (control). During the second year, all lysimeters, except the control, received 80 kg N ha^–1 as unlabeled NH₄NO₃. The cumulative, average loads of total N leached during the two years were: 37 (control), 62 (NH₄NO₃), 50 (ryegrass manure), and 73 (red clover manure) kg ha^–1. The differences among the treatments were not significant (P > 0.05), but the control had significantly smaller (P < 0.05) leaching loads than the treatments. About 24% of ryegrass- and red clover–derived N and 43% of NH₄NO₃ were removed through spring barley grain and stover during the two growing seasons. Thus, the N use efficiency in barley was substantially larger when grown with inorganic N fertilizer than when grown with green manure. Viewed in combination with the tendency for larger N leaching loads under red clover manure, claims about water quality benefits of legume-based green manures should be evaluated with regard to the timing of N release and demand for N by the plant.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
NEGATIVE EFFECTS of agricultural activities on surface and ground water quality have been a topic of concern in many parts of the world for several decades, especially in cold and humid regions where the net downward movement of water may cause large leaching losses of nitrogen (N) during the noncropping period (Kladivko et al., 1991; Davies and Sylvester-Bradley, 1995). This has led to an increasing interest and growing recognition of new methods for production of crops in an environmentally safe manner (Kirchmann et al., 2002). In the last few years, organic farming has been proposed as a possible way to reduce leaching of N from agricultural soils. In such farming systems, synthetic N fertilizers are not allowed, and the N inputs mainly originate from animal and green manures. This has led to an increasing interest in inclusion of legume- or grass-based green manures to agricultural cropping systems (Dou et al., 1995).

To be considered as an effective N source for most commercial agricultural crops, green manures must supply the crop with sufficient N, which requires a synchrony between N release from green manures and crop demand. In cold, humid regions, the green manures have to release a large amount of mineral N in spring and early summer when N uptake by the crop is rapid. In a study in the U.S. Upper Midwest, legume cover crops, used as green manures, released about 50% of their N within 4 wk of incorporation in spring (Stute and Posner, 1995), which coincided with the period of most rapid N uptake by corn (Zea mays L.). However, if the N in green manure is released too late in the season, or after the growing season, it may leach through soil and contaminate surface and ground waters. This has been clearly demonstrated when other organic N sources have been used. For example, in a study in which equal amounts of poultry manure N and inorganic fertilizer N were applied in spring to barley, leaching of manure-derived N was about one order of magnitude higher than fertilizer N during a 3-yr period (i.e., 28 and 3.5 kg N ha^–1), primarily due to leaching during autumn and winter (Bergström and Kirchmann, 1999). Long-term use of cover crops, and thereby large inputs of crop residues, have also demonstrated that N leaching may increase compared with cropping systems without cover crops (Hansen et al., 2000), even though cover crops commonly reduce N leaching over the short term (Bergström and Jokela, 2001). Hansen et al. (2000) showed that NO₃ leaching was 29% higher in plots with 24 yr of cover crops than in plots without cover crops. Both these examples suggest that to avoid the risk of increasing N leaching, precaution is called for when using organic N additions to agricultural soils.

The objectives of this study were to: (i) compare annual leaching and crop uptake of N in arable cropping systems that include addition of green manures with those in which only inorganic N fertilizer have been used; (ii) quantify how much of the total N leaching load and crop N uptake measured over a 2-yr period derive from a single spring application of green manure; and (iii) determine the influence of different types of green manure (in this study represented by red clover and perennial ryegrass) on leaching and crop uptake of N.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Experimental Design and Field Procedures
Twelve undisturbed soil monoliths of a sandy loam soil (Fluventic Haplumbrept) were collected from a field site in southern Sweden (56°29'N, 13°0'E) in June 1991 and April 1994. This soil has been cultivated for more than 150 yr, with spring-sown cereals as the most common crops grown. Some physical and chemical properties of this soil are listed in Table 1. The monoliths were collected with a coring machine in which a polyvinyl chloride (PVC) pipe (0.295-m diameter, 1.18-m length) was mounted in a steel drill cylinder with four cutting teeth at the lower end (Persson and Bergström, 1991). With the PVC casing mounted, the unit was placed on the ground and the drill cylinder was put into rotation to carve out a soil monolith that was gently pushed into the fixed casing. 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 diameter) layer, a porous plastic sheet, and a fiberglass lid that was screwed onto the inside of the casing wall. Five holes were drilled in each bottom lid to provide gravity drainage. After the lysimeters had been prepared for leaching measurements, they were placed in pipes permanently installed (belowground) in a lysimeter station in Uppsala, Sweden (59°49'N, 17°39'E), in which they were arranged in a completely randomized design. Continuous exposure of the soil columns to natural field conditions for several years before this study and observation of water eluting at the bottom of all columns a couple of weeks before this study was started ensured that the soil profiles had a moisture content close to field capacity from the start. Between collection and the start of this experiment (1 May 2000), cereal crops were grown in the lysimeters each year. The lysimeter station and the preparation of lysimeters for leaching studies are described in more detail elsewhere (Bergström, 1992).

In addition to natural precipitation, all lysimeters received supplemental irrigation from spray bottles on six occasions during the 2-yr period. On each occasion, between 7.5 and 15 mm of water was added at rates typical of heavy rainfall in the area but not in excess of the infiltration capacity of the soil. In total, 75 mm was added during the experimental period.

Results presented below as yearly amounts refer to 12-mo periods from 1 May to 30 April the following year.

Preparation of Nitrogen-15-Labeled Green Manures
Nitrogen-15-labeled red clover and ryegrass were obtained by growing plants in pots with pure sand fertilized with ¹⁵N-labeled (NH₄)₂SO₄ (about 50 atom % ¹⁵N) plus addition of all other plant nutrients. At an early stage of development (i.e., after 8 to 10 wk from emergence), the aboveground biomass was harvested, weighed, and dried at 50°C. Roots were isolated from the sand by washing and root biomass was determined and dried at 50°C. Mixtures of shoots and roots were applied to the lysimeters in proportion to the biomass yield of the pots. All materials were analyzed for dry matter, total C and N, and ¹⁵N enrichment after milling and passing through a 2-mm mesh (see Table 2). Concentrations of total N were similar in ryegrass and clover for both shoots and roots. The proportion of roots was higher with red clover than with ryegrass.

The materials added to the lysimeters were only slightly crushed by hand, not milled, and varied in size from a few up to 30 mm. The water holding capacity of the mixtures of shoots and roots amounted to about 6.0 and 5.5 mL g^–1 dry matter of ryegrass and red clover, respectively. In terms of the dry matter addition per lysimeter, ryegrass could bind 228 mL and red clover 209 mL of water.

Leachate Sampling and Analytical Procedures
Leachate draining through the soil columns was collected in glass sampling bottles placed in the lysimeter station. The bottles were weighed and emptied weekly, from May 2000 to the end of April 2002, to determine leachate volumes. Subsamples were taken for determination of total N and ¹⁵N concentrations if sufficient amounts of leachate were available.

To determine the total N concentrations in leachate samples, inorganic and organic N constituents were oxidized by K₂(SO₄)₂ + NaOH to NO₃. Nitrate concentrations were then determined by flow injection analysis (Model 5012 analyzer; Tecator AB, Höganäs, Sweden) according to the colorimetric Cd reduction method (American Public Health Association, 1985).

In preparation for ¹⁵N analysis, a portion of about 150 mL of each leachate sample was acidified with one or two drops of 5 M citric acid to a pH of 1 to 2 before evaporation. The remaining paste having a sufficient concentration of total N for ¹⁵N determination was then analyzed on a mass spectrometer (MS; Thermo Finnigan Mat, Delta Plus, Bremen, Germany).

After harvest, crop samples were dried in an oven at 40°C, and straw and grain were separated, weighed, and milled. Subsamples of each fraction were then finely ground and the contents of ¹⁵N were determined using the same MS system as described above for leachate samples.

Calculations and Statistical Analysis
Due to the fact that the natural abundance of ¹⁵N in natural systems can vary significantly (Hauck, 1982), ¹⁵N atom percentages were determined in all samples, including the unlabeled controls. The amounts of ¹⁵N in samples were, therefore, obtained by multiplication of total amount of N with the actual atom % ¹⁵N. To obtain the amounts of N derived from the respective N source added during the first year by use of the isotope method, the amounts of ¹⁵N in crop and leachate samples of the control were subtracted from the corresponding ¹⁵N amounts in the fertilizer and green manure treatments. The obtained values were then divided by the amounts of ¹⁵N in the applied fertilizer and green manure. The amounts of N in crop and leachate derived from fertilizer and green manure were also estimated by use of the difference method (i.e., a direct comparison with an unfertilized control) (Jansson and Persson, 1982).

Statistical treatment of data on leaching, and yields and N uptake by crops was done by one-way analysis of variance using the SAS procedure ANOVA (SAS Institute, 1985). Mean value comparisons between the different treatments were done by Duncan's multiple range test and Tukey's studentized range test.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Precipitation and Drainage Conditions
During the 2-yr period (1 May 2000–30 Apr. 2002), combined precipitation and irrigation totaled 1344 mm, which is about 21% higher than the long-term average in the Uppsala region. The wettest period occurred during summer and autumn 2000, with 560 mm of total water inputs from 1 May to 31 December, which is about as much as the average annual precipitation (554 mm) in the area. The highest monthly rainfall (153 mm) occurred in July 2000.

The amounts of leachate for the different treatments are shown in Fig. 1. The largest amounts of leachate during the 2-yr period were measured in the control lysimeters (575 mm), followed by the NH₄NO₃ treatment (516 mm). These two had significantly larger (P < 0.05) accumulated leachate volumes than the treatments in which ryegrass (420 mm) and red clover (400 mm) manures were used during the initial year. Considering the whole period, leachate volumes collected from the different lysimeters averaged 27% of total water inputs (precipitation plus irrigation), which is the same as that found in a similar lysimeter study with animal manures (Bergström and Kirchmann, 1999).

View larger version (13K): [in this window] [in a new window]   Fig. 1. Annual amounts of leachate during the two years. Bars represent standard deviation (n = 3).

Total Nitrogen and Nitrogen-15 in Leachate
Concentrations of total N in leachate showed a similar pattern during the experimental period in all treatments (Fig. 2). Peak concentrations occurred in late autumn and early winter during the initial year (2000–2001), reaching average levels of 10 (control), 22 (NH₄NO₃), 23 (ryegrass manure), and 33 (red clover manure) mg N L^–1 in the respective treatments. A less pronounced peak also occurred in the spring of 2002. During the period between these two peaks the concentrations tended to be less variable, although quite different between treatments. When no N was applied, the average total N concentration during 2001 was around 5 mg L^–1. In the lysimeters receiving only NH₄NO₃ the corresponding level was 5 to 10 mg L^–1, whereas in the lysimeters receiving green manures during the initial year it was typically within the 10 to 15 mg L^–1 range. Both concentration peaks coincided with intensive drain-flow periods, although the first peak was preceded by a long period with no drainage outflow in the lysimeters.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Mean total N concentrations in leachate on each sampling occasion.

Crop Yields and Nitrogen in Crops
Grain dry matter yields during the first year were highest for the NH₄NO₃ (3310 ± 600 kg ha^–1) and red clover treatments (3600 ± 760 kg ha^–1), which were significantly higher (P < 0.05) than the control and ryegrass treatment (Table 4). The grain yields of the ryegrass treatment and the control were not significantly different (P > 0.05) and averaged about 55% of the other treatments. However, the straw dry matter yield was highest in the lysimeters with ryegrass (6110 ± 1440 kg ha^–1). During the residual year (2001), all treatments except the control were fertilized with 80 kg N ha^–1, and there was no difference in yields between the treatments to which N was added (P > 0.05) (Table 4).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
How Could Fertilizer and Green Manure Additions Affect Drainage Volumes?
As expected, the results of this study revealed that lysimeters that received NH₄NO₃ had somewhat smaller drainage volumes than unfertilized control lysimeters (Fig. 1) due to higher crop yields. Transpiration of water was simply higher from lysimeters with a more vigorous crop. Accordingly, one could therefore also expect similar drainage from lysimeters treated with the inorganic N fertilizer as compared with green-manured lysimeters, since they had similar crop yields. However, green manure–treated lysimeters had significantly lower drainage volumes. We examined whether an increase in the water storage capacity in soil due to the green manure addition could explain the reduced drainage.

The green manure materials had a water holding capacity of, on average, 650 mL per 100 g dry weight. We applied 39.2 g ryegrass and 37.7 g red clover residues, which is equivalent to an increase in the water storage capacity of about 3.7 mm per lysimeter. Data showed that the amounts of drainage from the lysimeters were reduced by 40 to 50 mm per year when green manure materials were added during Year 1 (Fig. 1). This reduction can be explained if the green manure materials were able to withhold 3.7 mm of rain water during at least 11 to 13 major rain events, assuming complete dryness of the organic manures between events or, more realistically, roughly 2 mm of water during 20 to 30 rain events, assuming the green manures to be partially moist between rain events. Daily rainfall records showed that there were 21 events exceeding 10 mm during Year 1 and 11 during Year 2. The number of major drain-flow events each year was 10 to 15. Thus, the reduced amounts of drainage from manure-amended lysimeters can largely be explained by the increase in soil water storage, although 3.7 mm of water only account for 1.5% of the total water storage capacity of the 1-m soil profile. It can, therefore, be concluded that the presence of green manure material incorporated into soil can reduce drainage volumes significantly.

Fluctuations of Nitrogen in Leachate
The short-term changes of N in leachate are well described by fluctuations in concentrations, whereas the long-term effect on natural water bodies is largely determined by the total loadings over extended periods of time. Therefore, we chose to focus the discussion on N dynamics in leachate on concentrations rather than loads.

The increases in total N concentrations in leachate after the first growing season were more pronounced in lysimeters that received green manures than those receiving NH₄NO₃ (Fig. 2). In the ryegrass and red clover treatments, the concentrations increased by 15 to 17 mg total N L^–1 from early November to the middle of December. The corresponding increase in NH₄NO₃–fertilized lysimeters was only 3 mg total N L^–1. The larger increases in the manured lysimeters was probably caused by increased mineralization, which is presumed less in the NH₄NO₃–fertilized lysimeters to which no organic N was added. All lysimeters had the same initial organic matter content; the only difference among treatments was the source of N added.

As a result of the much higher total N concentrations in the red clover–manured lysimeters than in those that received NH₄NO₃, the leaching loads were on average about 18% smaller in the NH₄NO₃–fertilized lysimeters during the 1.5-mo period from early November onward, despite the fact that they had about 30% larger drainage volumes during this period. Larger leaching loads of N (14 kg N ha^–1 yr^–1) in plots with long-term previous use of cover crops than in plots without cover crops were reported by Hansen et al. (2000). They also attributed this difference to increased N mineralization as a result of inputs of cover crop residues and subsequent release of N during the noncropping season. Based on ¹⁵N data in our study, leaching of fertilizer- or manure-derived N during the 1.5-mo period was 1.2 and 4.1 kg N ha^–1 when NH₄NO₃ and red clover manure were applied, respectively. These amounts correspond to 1.5 and 2.6% of applied N in the respective treatment. Thus, the relative difference in leaching of N derived from the two N sources, as estimated by the isotope method, was larger than of total N, although the absolute amounts of N leached were much smaller. A similar trend, with smaller percentages of N leached during the first year after N application expressed with the isotope method than in a direct comparison with an unfertilized control, was shown by Bergström and Kirchmann (1999) in a study comparing N leaching from NH₄NO₃ and poultry manure.

As mentioned above, the increase in total N concentration was about 15 mg L^–1 during autumn 2000 in leachate of both the ryegrass and the red clover treatments. However, the leaching loads were much smaller when ryegrass manure was used (17 kg total N ha^–1) compared with red clover manure (32 kg total N ha^–1). The main reason for this was the fact that the total N concentration peak started at a much lower level (about 8 mg L^–1) in the lysimeters receiving ryegrass manure than in those receiving red clover manure (about 16 mg L^–1) (Fig. 2). In a study in which the residual N effects of different green manure crops were investigated, the average mineral N content in soil to a 90-cm depth was about 12 kg ha^–1 less after Italian ryegrass (Lolium multiflorum Lam.) had been used than after barley without green manure additions (Wallgren and Lindén, 1991). In contrast, on average between 12 and 32 kg ha^–1 more mineral N was found in late autumn after incorporation of legume green manures than after barley alone. These examples show that the risk for leaching of N associated with the use of green manures is much higher for legumes than ryegrass. In this context, it is worth noting that the main reason for using green manures is to capture N for subsequent crops, and N₂–fixing legumes are, therefore, the only alternative. In fact, in organic farming legumes are, by far, the dominant crops used for green manuring.

Effects of Green Manures on Mineralization–Immobilization Turnover in Soil
Comparing soil N uptake in crops reveals the turnover of green manure N and fertilizer N after incorporation into soil (Table 6).

Crop Utilization and Leaching of Nitrogen Related to Green Manure Application
The results of this study on crop uptake and leaching refer to one set of conditions over two years. Therefore, only short-term effects caused by the different N additions on crop utilization and leaching are possible to assess based on the present data. Since the main reason for using green manures is, as mentioned above, to capture N for subsequent crops in a crop rotation, only the red clover manure will be considered here and compared with NH₄NO₃.

During the first year (May 2000–April 2001), total N leaching was 46 kg N ha^–1 and harvested barley grain yield of N was 37 kg N ha^–1, when red clover manure was used. The corresponding figures for NH₄NO₃ were 35 and 38 kg N ha^–1, respectively. Expressed in relation to total removal of N (excluding gaseous losses), leaching of N was 56% when red clover manure was used and 48% in NH₄NO₃–fertilized lysimeters. Consequently, the corresponding figures for N in harvested grain were 44% (red clover manure) and 52% (NH₄NO₃). These results indicate that the first-year efficiency of NH₄NO₃ was higher than that of red clover green manure, which was probably due to poor synchronicity of N release from the green manure and demand for N by barley. However, we have to keep in mind that in this study the green manures were applied in spring, which is only possible on light-textured soils in Sweden or in cold climatic regions elsewhere. In other cases, green manures are usually incorporated into soil during autumn, which typically cause larger leaching losses. Bergström (1986) showed that an increase of as much as 117 kg inorganic N ha^–1 in a 1-m profile of an agricultural soil occurred during a 3-mo period after incorporation of alfalfa (Medicago sativa L.) in late summer. Such N is very vulnerable to leaching when there is no crop cover and a net downward movement of water in soil. A low first-year N efficiency of any applied fertilizer or manure is probably the key reason for large leaching loads over the long term, although this could not be verified by the results of this study.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
During the first autumn period, leaching loads of total N were about 20% less when NH₄NO₃ was used compared with red clover manure. Leaching loads of N derived from the respective N source during the same period was about 70% less when NH₄NO₃ was used. Therefore, claims about water quality benefits of legume-based green manures should be carefully examined. In light of this, together with the fact that plant uptake of fertilizer-derived N during the first season was considerably higher than of N derived from red clover manure, we can conclude that the first-year efficiency of NH₄NO₃ was better than that of red clover manure.

In addition to affects on N leaching and N use efficiency, incorporation of green manures into soil also had a significant impact on the soil water balance. Drainage volumes were about 20% lower when green manures were used compared with only NH₄NO₃, despite similar crop yields in the different treatments. This could be explained by increased soil water storage as a result of the green manure addition.

	ACKNOWLEDGMENTS

 
This work was funded by a grant from the Oscar and Lily Lamm Foundation, to which we express our sincere thanks. We would also like to thank Stefan Ekberg, Inger Juremalm, and Annelie Mejbert for help with analytical work.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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