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Published online 25 August 2005
Published in Soil Sci Soc Am J 69:1644-1651 (2005)
DOI: 10.2136/sssaj2004.0365
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Nutrient Management & Soil & Plant Analysis

Separation of Pig Slurry and Plant Utilization and Loss of Nitrogen-15-labeled Slurry Nitrogen

Peter Sørensen* and Ingrid K. Thomsen

Danish Institute of Agricultural Sciences, Dep. of Agroecology, Research Centre Foulum, PO Box 50, 8830 Tjele, Denmark

* Corresponding author (peter.sorensen{at}agrsci.dk)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Separation of slurry by centrifugation concentrates nutrients and facilitates the transport of surplus nutrients away from livestock farms. Nitrogen-15 labeled pig slurries containing urine 15N or fecal 15N were produced to compare the utilization and fate of N in separated and unseparated pig slurry. There was good agreement between the mineral fertilizer equivalence (MFE) of manure fractions estimated from 15N uptake and by a traditional non-isotopic method, but the 15N method was more precise. The weighted utilization of N in separated slurry was similar to the N utilization in unseparated slurry. The MFE of slurry total N was 75 to 79% after incorporation before sowing spring barley (Hordeum vulgare L.) and 59 to 64% after surface application in winter wheat (Tritium aestivum L.). Both the origin of the slurry N and the fractionation influenced the N availability. The uptake of urinary 15N in the first barley crop was 35 to 53% and the uptake of fecal 15N 21 to 44% with the lowest availability of 15N in the dry-matter-rich fraction (DMR). The uptake of 15N in the following cover crop and barley crop was low (1–4.5%). The residual N effect of the manures in the year after application (MFE) was equivalent to 1% for the liquid fraction, 3% for the slurry, and 5% for the DMR. The amount of 15N remaining in soil 15 mo after application was 30 to 53% for urinary N and 44 to 61% for fecal N. It is concluded that the overall utilization of N is unaffected by slurry separation, but the separation facilitates a better distribution of nutrients.

Abbreviations: ANI, added-nitrogen interaction • DMR, dry-matter-rich fraction • LIQ, liquid fraction • MFE, mineral fertilizer equivalence


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN MANY EUROPEAN COUNTRIES pig production concentrates on fewer and larger farms. To avoid some areas consequently receiving heavy loads of manure, Denmark has introduced restrictions on the amount of manure N that can be applied per hectare. Even with these restrictions many areas continuously receive more P in animal manure than is removed with crops. Phosphorus is strongly adsorbed in soil, but there is increasing concern that a large accumulation of P will result in increasing losses of P to the aquatic environment (Sims et al., 1998).

The increased land area needed for spreading of manure and increased transport costs has accentuated the need for alternative use of manure. Physical separation of animal slurry has been suggested as a tool for improving the distribution of nutrients and avoiding overfertilization on farms with a high livestock density. Slurry can be divided into a dry-matter-rich fraction (DMR) and a liquid fraction (LIQ) by centrifugation. The DMR contains most of the slurry P in a rather concentrated form (Møller et al., 2002) and enables transport of surplus nutrient to other areas. The DMR has a higher C/N ratio than unseparated slurry whereas the LIQ has a low C/N ratio and provides better soil infiltration, reducing the risk of ammonia volatilization. Thus, slurry separation may influence the turnover of slurry N in soil as well as the loss of volatile N after application.

A high first-year utilization of N in animal manure is a prerequisite for keeping losses of N to the environment at a minimum. Improved understanding of the cycling of animal manure N is important for optimizing the utilization of N in agriculture. Isotopic labeling has been used to study the fate of manure N in agroecosystems (Kirchmann, 1990; Sørensen et al., 1994; Thomsen et al., 1997; Muñoz et al., 2003; Chantigny et al., 2004a). In principle, the labeling of manure components should be homogeneous, but a homogeneous labeling of manure can be difficult to obtain (Sørensen et al., 1994). Chantigny et al. (2004b) found some heterogeneity in the 15N enrichment of pig slurry (mixture of feces and urine) after feeding with a diet containing ingredients of variable 15N enrichment. Sørensen and Thomsen (2005) found a sufficient uniformity of feces N after feeding pigs on a 15N-labeled diet and recommended that labeled feces and urine N should be investigated separately in N cycling studies.

The aim of this study was to compare the N utilization and N losses from separated and unseparated pig slurry. The recovery and loss of labeled N in pig slurry fractions were measured after incorporation before sowing spring barley and after surface application in winter wheat. Residual N effects in the year after application of the slurry fractions were also quantified.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Separation of Pig Slurry
Two growing pigs were fed on a 15N-labeled diet for 10.5 d as described in detail by Sørensen and Thomsen (2005), and two pigs from the same brood were fed on a similar unlabeled diet. The major ingredients of the diet were barley and peas. The uniformity of labeling of fecal N was evaluated by incubation in N-free quartz sand and in soil, and the uniformity was found to be sufficient after a few weeks of decomposition (Sørensen and Thomsen, 2005).

Two similar pig slurries were established by mixing 15N-labeled urine and feces with their unlabeled counterparts, aiming at a feces N/urine N ratio of 0.35:0.65, which was the average excretion ratio observed and within the normal range for growing pigs. After 1 wk of anaerobic storage at 20°C, the slurries were separated into a DMR and a LIQ by four repeated centrifugations (10 min at 4500 x g). The average composition of the slurries before and after the separation is shown in Table 1. After separation the fractions were stored in closed containers at 15°C for 8 (Exp. 1) or 2 wk (Exp. 2).


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  		Table 1. Composition of pig slurry fractions used in the two field experiments. The average composition of the two paired slurry fractions containing either labeled fecal or labeled urinary N is shown.

 
Field Experiments
Spring Barley 2001–2002 (Exp. 1)
In April 2001 microplots confined in polyvinyl chloride cylinders (30-cm i.d., 0.0707 m2, 30 cm length) were established by pushing the cylinders to the 25-cm soil depth in an arable field at Research Centre Foulum, Denmark (56°29' N lat., 9°34' E long.). The field had been cropped with spring barley in 2000. The soil was a loamy sand (Typic Hapludult, mixed, mesic) with 9% clay (<2 µm), 12% silt (2–20 µm), 76% sand (20µm–2 mm), 0.165% N, 1.72% C, and pH (H2O) was 6.8. Temperature and precipitation data for the experimental years are shown in Fig. 1 .



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  		Fig. 1. Monthly averages of air temperature and precipitation in the two experimental years of the field experiments.

 
The slurry fractions used in this experiment were derived from urine and feces sampled 5 d after the start of 15N feeding (Sørensen and Thomsen, 2005). The slurry fractions were applied by removing the upper 15-cm of soil, placing the manure in a layer and then returning the soil to the plot (simulated plowing). On 2 May 2001, the unseparated slurry and the LIQ were applied to the plots to bring 75 kg ammonium N ha–1 (Table 2). The DMR was applied at a rate equivalent to 100 kg total N ha–1 either alone or supplemented with unlabeled ammonium nitrate to bring the mineral N application rate to 75 kg N ha–1. The DMR is rich in P and normally the application rate should be restricted (based on the P content) and then supplemented with mineral N. The amounts of total N and inorganic N applied with each manure type are shown in Table 2. To establish a mineral fertilizer response curve, separate plots were supplied with 0, 50, 75, 100, 125, 150, and 175 kg N ha–1 in unlabeled ammonium nitrate. A linear response between N application and total N uptake in grain plus straw was observed. There were four replicates of each treatment, and the experiment was designed in four randomized blocks with plots placed in four rows with 2.5 m between each row and 50 cm between each plot.


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  		Table 2. Total N and inorganic N applied as pig slurry fractions and mineral fertilizer and 15N enrichment of the manures used in the two field experiments.

 
The plots and the surrounding soil were sown with spring barley and ryegrass (Lolium perenne L. cv. Lasso), a late-developing cultivar suited for over wintering. Essential plant nutrients except N were applied to all plots at a level sufficient for optimal growth. The soil surrounding the plots was fertilized with 75 kg N ha–1 in an NPK fertilizer. During the growing season the field including the plots was treated with herbicides and fungicides according to normal practice in Denmark. On 20 Aug. 2001, the mature barley crop including the undersown grass was harvested 1 cm above soil surface. The grass regrowth was cut 1 cm above the soil surface on November 20th and removed. In March 2002, the 0- to 20-cm soil layer was removed, mixed and returned to the same plot to simulate plowing. The plots and surrounding soil were sown to spring barley on 15 Apr. 2002 and essential nutrients including 75 kg N ha–1 in ammonium nitrate were applied to all the plots. The plots applied with 75 kg N ha–1 in unlabeled fertilizer in 2001 were applied with labeled ammonium nitrate in 2002 (Table 2). The barley was harvested on August 14th by cutting 5 cm above the soil surface and the stubble including the major root residues was removed, washed, dried, and weighed before analysis of total N and 15N content.

Winter Wheat 2001–2002 (Exp. 2)
Confined microplots were installed in September 2001 after plowing a field near Exp. 1. The slurry fractions used in this experiment derived from urine and feces sampled Day 10 to 11 after the start of 15N feeding (Sørensen and Thomsen, 2005). The N application rate and time of the different manures are shown in Table 2. The DMR (101 kg N ha–1) were applied in a layer at the 15-cm depth before sowing on 26 Sept. 2001 (simulated plowing). All plots including the surrounding area were sown to winter wheat (Triticum aestivum L. cv. Bill) on 26 Sept. 2001. In March 2002, all plots received 45 kg N ha–1 in unlabeled ammonium nitrate. A treatment with spring application of DMR (101 kg N ha–1) was included by placing the manure on the soil surface between the wheat rows. To ensure that the total application of plant available N was similar in all the manure treatments, all treatments receiving DMR were supplemented with 50 kg N ha–1 in unlabeled ammonium nitrate on 30 Apr. 2002 (50% of the DMR-N was assumed to be available). The LIQ (100 kg ammonium N ha–1) was applied to the crop by surface-banding on the same date in spring. The unseparated pig slurry (100 kg ammonium N ha–1) and 15N-labeled ammonium nitrate (100 kg N ha–1) were likewise applied by surface-banding. A mineral fertilizer response curve was prepared by including treatments with 0, 50, 125, and 150 kg N ha–1 in unlabeled ammonium nitrate in separate plots. There were four replicates of each treatment randomized in four blocks. The weather on 30 April (manure application date) was cloudy and windy (8 m s–1), and the air temperature was about 8°C. The winter wheat was harvested by cutting 5 cm above the soil surface on 16 Aug. 2002 and the stubble including the major root residues was removed, washed, dried, and weighed before analysis.

Analytical Methods
In both field experiments, the soil was sampled from the 0- to 20- and 20- to 40-cm depth for 15N and mineral N analysis after harvest in 2002. The 0- to 20-cm soil in the whole plot was removed and weighed, and the soil carefully mixed before sampling and pooling eight soil cores from each mixed plot. Four soil cores were taken from the 20- to 40-cm depth in each plot and pooled to one sample.

Total N in slurry samples was determined using a Kjeldahl method (Tecator Kjeltec Auto 1030, Tecator, Höganäs, Sweden). Inorganic N in soil and manure was extracted by 2 M KCl for 1 h (2.5:100 for manure; 1:4 for soil), followed by centrifugation and filtration through glass filters. Ammonium and nitrite + nitrate N in extracts were measured by flow colorimetry (Autoanalyzer II, Bran + Luebbe GmbH, D-22803 Norderstedt, Germany). Ammonium N in Kjeldahl extracts was concentrated by diffusion as described by Jensen (1991). Total N in oven-dried soil (40°C) and plant material (80°C) was determined by elemental analysis and 15N enrichments were determined at the Stable Isotope Facility at University of California-Davis using a mass spectrometer coupled to the elemental analyzer (Europa Scientific Integra, Crewe, UK). Soil 15N determinations were corrected for the 15N natural abundance of soil N (0.369 atom% 15N).

The experiment was performed with four replicates giving a total of 116 plots for the two experiments including N response curves. Analysis of experimental variance was performed using the SAS procedure GLM (SAS, 1989) and least significant differences (LSD) were calculated if the main effects or interactions were significant (P < 0.05). The MFE of the labeled manure was calculated by: MFE = (% uptake of 15N from manure) x 100/(% uptake of 15N from mineral fertilizer) (Christensen, 2004). The MFE of the manures was likewise calculated by the traditional non-isotopic method (Muñoz et al., 2004).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Slurry Separation
The separation in the present study was made by centrifugation in the laboratory on a small scale due to the limited amount of 15N-labeled slurry that was available. Nevertheless the composition of the slurry fractions was similar to fractions obtained by full-scale centrifugation by Møller et al. (2002) as regards dry matter content and distribution of total P. The N concentrations were slightly higher than observed by Møller et al. (2002), probably because the unseparated slurry also had a higher N content in this study. After the centrifugation most of the dry matter and P was present in the DMR as also observed by Møller et al. (2002). About 77% of the total P, 26% of the total N, 13% of total K, and 16% of the weight was found in the DMR (data not shown). The 15N cross-labeling allowed an estimation of the fate of feces N and urine N during the separation and revealed that most of the fecal N ended up in the DMR and most of the urine N in the LIQ. Still, 40 to 45% of the N contained in DMR derived from urine and 26 to 30% of the N in the LIQ came from feces (data not shown).

The Fate of 15N Applied to Spring Barley
The recovery of 15N in the first barley crop increased in the order DMR < pig slurry < LIQ < mineral fertilizer from 21 to 56% (Table 3). As expected urinary N was generally more available than fecal N. However when comparing urine N and fecal N in different fractions, it turned out that fecal N in the LIQ was more available than urinary N in the DMR. Most urinary N is normally assumed to be plant available, but the manure components interact due to microbial processes (Jensen et al., 1999). Thus, urinary N may be immobilized into more stable components when feces are decomposed. This can take place both during manure storage and after application to soil and can explain the lower availability of urine N in the DMR. The fecal N in the LIQ was probably mainly in soluble compounds present at excretion (about 10% of the fecal N was ammonium) or mineralized during storage.


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  		Table 3. Recovery of labeled N from separated and unseparated pig slurry in crops and soil (Aug. 2002) after application to spring barley in spring 2001. Percentage of applied labeled N (n = 4).

 
The uptake of 15N in the ryegrass cover crop grown in the autumn after barley harvest was 1 to 3% of the input and highest for the DMR (Table 3). This was similar to the 15N uptake found after application of 15N-labeled ammonium N in pig slurry (Sørensen and Amato, 2002). The uptake of 15N in barley in the second year after application was 1.7 to 4.5% with the highest uptake from urinary N in DMR and fecal N in pig slurry (Table 3). In the second year the uptake of 15N was slightly higher from the DMR and unseparated pig slurry than from the mineral fertilizer and LIQ. These rather small uptakes of 15N in the second year after application are comparable with other experiments with 15N-labeled pig, ruminant, and poultry manures (Chantigny et al., 2004a; Christensen, 2004; Thomsen, 2004).

The recovery of 15N in the 0- to 20-cm soil layer varied from 19% for mineral fertilizer N applied in 2001 to 51% for fecal N in the DMR (Table 3). Within manure types the recovery of fecal N was higher than urinary N, but more urinary N in DMR remained in soil than fecal N applied in the LIQ. Thus, not only the origin of the N but also the fractionation had a significant influence on the retention in soil. The recovery of 15N in the 20- to 40-cm soil layer was highest for N applied in DMR and fecal N in the unseparated slurry. Thus, the slurry material containing most particulate material resulted in the highest downward transport of 15N. A possible explanation is that earthworms were more active in soil with applied particulate material and that the earthworms selected the particulate material for ingestion and thereby increased the transport of 15N included in particulate material to deeper soil layers. Another explanation could be that the slowly mineralized N was more prone to leaching, especially during the intercropping period either as mineral or dissolved organic N.

Residual mineral N in soil after harvest of the barley in 2002 was not significantly influenced by the different manure types applied in the previous year. After harvest the soil contained 24 to 29 kg inorganic N ha–1 at the 0- to 40-cm depth (data not shown).

The total recovery in crops and soil of 15N applied with the manure types was 86 to 88% and did not differ significantly from the mineral fertilizer N recovery (85%) (Fig. 2) . Thus 12 to 15% of the applied 15N was unaccounted for after the two growing seasons. The total recovery was rather high compared with other studies with 15N-labeled fertilizer N applied to spring barley under comparable conditions (Glendining et al., 2001). The N unaccounted for was probably lost by leaching, denitrification, and ammonia volatilization and part of it could have been transported to soil layers deeper than 40 cm (Macdonald et al., 2002; Muñoz et al., 2003). The recovery of mineral fertilizer 15N in the soil–plant system differed for the two application years with the lowest recovery in the last application year (Table 3). This difference occurred despite the fertilizer applied in 2002 having resided 1 yr less and indicates that 15N losses varied mainly during the first months after application.



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  		Fig. 2. Nitrogen-15 balance based on the weighted mean recovery of fecal and urinary N after incorporation of labeled dry-matter-rich (DMR) and liquid fractions (LIQ), unseparated pig slurry, and labeled mineral fertilizer N to spring barley in spring 2001. Labeled N in soil was measured after harvest of the second barley crop in August 2002.

 
The Fate of 15N Applied to Winter Wheat
The uptake of 15N of different origin ranged from 9 to 78% and was more variable in the wheat crop than in spring barley (Table 4). The wheat had a higher uptake of 15N from mineral fertilizer, LIQ, and from unseparated slurry than barley because these were applied to the established crop and most of the 15N assimilated by the wheat was translocated to aerial parts of the crop. The uptake of 15N from the DMR was lower than in barley due to higher losses of N. The crop uptake of manure 15N was twice as high after the application of DMR in spring compared with the autumn application (22 vs. 9% uptake). The weighted uptake of total 15N from unseparated pig slurry was 46% and comparable with results by Chantigny et al. (2004a) who found 50% uptake of 15N-labeled pig slurry N in the aboveground plant parts after slurry application to growing maize on a sandy loam soil.


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  		Table 4. Recovery of labeled N from separated and unseparated pig slurry in crops and soil (Aug. 2002) after application to winter wheat. Percentage of applied labeled N (n = 4).

 
A significant proportion of the N in the DMR (38%) was lost after application to winter wheat irrespective of whether it was applied in the autumn or in spring (Fig. 3) . Presumably the loss occurred mainly by leaching and denitrification when the fraction was incorporated in the autumn and mainly by ammonia volatilization when the fraction was applied to the soil surface with the growing wheat in spring. A later lysimeter study on the same soil type has confirmed that the loss after autumn application mainly occurred by nitrate leaching (P. Sørensen, unpublished data, 2004). Jensen et al. (2000) found that a significant proportion of cattle slurry N applied in the autumn could be used by a winter wheat crop under specific circumstances with a low winter percolation and a high rooting depth. However, under the present conditions most of the plant available N applied to winter wheat in the autumn was lost.



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  		Fig. 3. Nitrogen-15 balance based on the weighted mean recovery of fecal and urinary N after application of labeled dry-matter-rich (DMR) and liquid fractions (LIQ), unseparated pig slurry, and labeled mineral fertilizer N to winter wheat. The DMR was either incorporated in the autumn before sowing or surface-applied in spring 2002. The other fertilizers were surface-banded in the growing crop in spring. Labeled N in soil was measured after harvest of the winter wheat in August 2002.

 
If part of the organic manure N is released late in the growing season, there may be more mineral N in soil after harvest that can be lost during the autumn. However, the different manure types did not significantly influence residual mineral N levels in soil after harvest of the wheat. The soil contained 23 to 28 kg mineral N ha–1 at the 0- to 40-cm depth (data not shown).

The highest uptake of 15N in the wheat crop and the lowest amount of unaccounted for 15N was observed after mineral N application where only 5% of the 15N was unaccounted for. This was a high 15N recovery compared with other studies with winter wheat (Macdonald et al., 2002). The winter wheat had a higher uptake of 15N from the LIQ than from the unseparated slurry whereas the recovery of 15N in soil was nearly similar for these two manure types. Fifteen percent of the LIQ N was unaccounted for, whereas 25% of the unseparated slurry N was unaccounted for (Fig. 3). This difference in loss may relate to a better infiltration of the LIQ resulting in less ammonia volatilization (Frost et al., 1990; Sommer and Hutchings, 2001). Assuming that the extra loss from slurry compared with mineral N was due to volatilization then 10% of the total N in the LIQ and 20% of the unseparated slurry N was lost by volatilization.

The recovery of 15N in soil was similar for DMR incorporated before sowing wheat in autumn and barley in spring (Tables 3 and 4). The weighted recovery of the DMR-15N in soil (0–40 cm) was 52% after both crops. As less labeled N was recovered in soil after surface application of DMR to the wheat in spring (39%, Table 4), the lower uptake of autumn-applied DMR-15N was due to more 15N being immobilized in soil. The autumn-applied DMR was added just before sowing the crop and labeled N was available for crop uptake at a plant development stage where the initial root system was being established. Therefore more 15N probably remained in roots whereas the root system was already established at the spring application and most of the labeled N taken up was transported to the aboveground parts. A greater contact between the manure and the soil matrix with autumn application of DMR could likewise result in increased microbial immobilization of 15N (Sørensen and Jensen, 1998).

Mineral Fertilizer Nitrogen Equivalence of Manure Fractions
The MFE of slurry fractions following incorporation before sowing spring barley and after surface banding in winter wheat is shown in Table 5. There was similarity between MFE calculated by the 15N method and MFE calculated by the traditional method where the crop N uptake after manure application is related to a mineral N response curve, but the 15N method showed less variability (Table 5). If significant added-N interaction (ANI) takes place due to mineralization–immobilization turnover (Jenkinson et al., 1985), the 15N method will not give a true measure of the manure N effect (Kirchmann, 1990), but ANI did not seem to take place in the present study. When labeled manure is well mixed with soil, ANI may occur (Sørensen and Jensen, 1998). In this study, the manure was placed in a discrete layer either at the 15-cm depth or at the soil surface like in standard farm conditions and ANI was insignificant or at least similar for mineral fertilizer and manures.


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  		Table 5. Plant availability of N in separated and unseparated pig slurry compared with mineral fertilizer N (mineral fertilizer equivalence, MFE) calculated on basis of total N uptake and 15N uptake in spring barley and winter wheat crops (grain + straw), and grain yields obtained after the manure/fertilizer applications.

 
The MFE was expected to equal the mineral N content of the manures and the same amount of mineral N was applied to all the barley plots, except where DMR was applied alone. However, the grain yield was higher after application of LIQ and DMR plus mineral N than after mineral fertilizer alone, and the MFE was also higher than the mineral N content applied with these manures (Table 5). In the DMR 30 to 40% of the total N was ammonium N, but the MFE was 42 to 50% when it was incorporated before sowing spring barley, indicating that net mineralization of N occurred from the DMR.

The plant availability of fecal N in unseparated pig slurry expressed as MFE was 59% (calculated from Table 4), and this was about twice that reported for ruminant fecal N in slurry (Thomsen et al., 1997; Sørensen and Jensen, 1998). The MFE of urinary N in the pig slurry was 84% and comparable with the reported availability for urinary N in ruminant slurry (Thomsen et al., 1997; Sørensen and Jensen, 1998). The high plant availability of 59% of fecal N to barley was apparently not in accordance with an incubation study where the mineralization of the same feces was measured in soil (Sørensen and Thomsen, 2005). Only about 30% of the fecal N was in mineral form after 32-wk incubation in soil. A reason for the discrepancy could be that unlabeled urinary N instead of fecal N was immobilized after the slurry application in the field due to decomposition of organic material in feces (Jensen et al., 1999), whereas immobilized N mainly derived from the fecal 15N in the incubation study of Sørensen and Thomsen (2005). However, the availability of urinary N in slurry was also high in the field, and all the discrepancy could not be explained by immobilization of urinary N. A second reason could be that the mixing with soil in the incubation experiment resulted in less net mineralization than when the slurry was placed in a discrete layer in the field experiment (Sørensen and Jensen, 1998).

After incorporation to barley in spring, losses of 15N were similar from all sources. The MFE of slurry was generally higher after incorporation to spring barley than after surface-banding in winter wheat (Table 5), probably because the ammonia loss was reduced by incorporation. In winter wheat, the LIQ infiltrated the soil better than untreated slurry, probably resulting in a lower ammonia loss, whereas the DMR had higher losses. Thus, the overall plant availability of slurry N was not affected by separation, either under conditions with minimal N loss (barley with cover crop) or under conditions with ammonia volatilization (winter wheat). The overall utilization was lowest when the DMR was applied in the autumn, but since only about 25% of the slurry N is in the DMR, losses from this fraction has minor influence on the overall N utilization after separation. Nitrogen may also be lost by ammonia volatilization during storage of the DMR if the solid manure is not covered, resulting in a reduced overall N utilization, but such storage losses were avoided in the present study.

Despite the low MFE of the DMR in wheat, the grain yield was not significantly lower after application of 101 kg DMR-N ha–1 plus 50 kg mineral N ha–1 compared with application of 100 kg N ha–1 in mineral fertilizer (Table 5). This indicates that the DMR had a beneficial effect on wheat growth not related to the N availability, especially after the autumn application. The wheat grain yield was significantly higher after application of mineral fertilizer than after application of the unseparated slurry due to the ammonia loss from the surface-applied slurry.

Many farmers grow autumn-sown crops like winter wheat and they have to apply some of the slurry to a growing crop in spring. Under such circumstances the nitrogen use efficiency can be improved after separation by using the LIQ in the winter crop and using the DMR on other areas where it can be incorporated in spring before sowing a crop.

Residual Nitrogen Effect of Manure Fractions
The treatment with application of labeled mineral fertilizer to spring barley in 2002 enabled calculations of the residual MFE also in the second year after application of separated and unseparated pig slurry. The extra residual N effect compared with mineral N application was 4.5% for the DMR, 1.2% for the LIQ and 2.7% for unseparated slurry (Table 6). Thus, the overall residual N effect of slurry fractions was similar to that of the unseparated slurry.


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  		Table 6. Residual N effect of separated and unseparated pig slurry in the year after application of 15N-labeled manure, calculated as mineral fertilizer equivalence (MFE) from the weighted uptake of fecal and urinary labeled N in spring barley in relation to the uptake of labeled fertilizer N applied in the following year (2002).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The overall crop uptake and loss of N were similar for unseparated pig slurry and for corresponding separated DMR + LIQ when the fractions were applied in spring to the same crop (spring barley or winter wheat). It is concluded that the overall utilization of N is unaffected by slurry separation when the manure fractions are applied to the same crop as the unseparated slurry, but the separation facilitates a better distribution of nutrients. Thus the nitrogen use efficiency can be improved after separation by using the LIQ in the winter crop and using the DMR on other areas where it can be incorporated in spring before sowing a crop. A significant amount of manure N was lost after application of the DMR to winter wheat irrespective of whether it was incorporated in the autumn or surface-applied in spring, but the plant availability was highest after surface application. The overall utilization of pig slurry N was higher in spring barley (MFE: 75–79%) where gaseous N losses were prevented by slurry incorporation than after surface application in winter wheat (MFE: 59–64%). There was a good agreement between the MFE of manure fractions measured by 15N uptake and measurement by a traditional non-isotopic method. Extra residual N effects in the year after application compared with mineral N application were equivalent to 1% for total N in the LIQ, 2.7% for unseparated pig slurry ,and 4.5% for the DMR.


   ACKNOWLEDGMENTS
 
The work was supported by the Ministry of Food, Agriculture and Fisheries (projects: HAR98-DJF-1 and VMPIII aktstykke, slurry technology). We thank the staff in "Research Unit Organic Matter and Microbial Ecology" for skilled technical assistance.

Received for publication November 24, 2004.


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




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P. Sorensen and I. K. Thomsen
Production of Nitrogen-15-Labeled Pig Manure for Nitrogen Cycling Studies
Soil Sci. Soc. Am. J., August 25, 2005; 69(5): 1639 - 1643.
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