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Journal of Environmental Quality 32:1405-1413 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Plant and Environment Interactions

Fate of Urine Nitrogen in Three Soils throughout a Grazing Season

M. L. Decau*, J. C. Simon and A. Jacquet

UMR-INRA950, Université de Caen, esplanade de la paix, 14032 Caen, France

* Corresponding author (cliquet{at}ibba.unicaen.fr)

Received for publication March 5, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The fate of 15N-labeled cattle (Bos taurus) urine (52 g N m-2), applied to a 0.4-m2 surface area on three dates between May and October to three different pasture soils, was studied using 2-m2 lysimeters. Over a period of two years, the sward recovered most of the 15N, but the amount recovered decreased with application date (62% in spring to 17% in fall). However, N uptake by ryegrass (Lolium perenne L.) in Year 2 showed that some nitrogen came from the previous year's urine application. The largest leaching losses of urine N resulted from the late application date. These losses mainly occurred during the first winter despite the small amount of water drainage. Soil type largely determined 15N losses. The granitic Brunisol was the most freely draining and had the greatest leaching (up to 35% recovery of urinary N). In contrast, leaching in the silty loam Neoluvisol remained under 4% of 15N applied. The Calcosol appeared to be susceptible to all kinds of N losses with intermediate unaccounted-for N pool and leaching fractions and lesser utilization of urinary N by grass. Immobilization in soil organic matter, roots and litter, and stubble pools were not markedly influenced by the date of application or soil type. They amounted to 25 to 33, 2, and 2% of N applied as urine, respectively. In these climatic conditions with moderate drainage, leaching of water poor in quality for nitrate only occurred for late-season grazing or on the granitic Brunisol, which was very vulnerable to leaching.

Abbreviations: DM, dry matter • SOM, soil organic matter


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE RETURN OF nitrogen as excreta is an important part of the N cycle in grazed pastures. Depending on animal type, production levels, and the N concentration in the herbage, between 80 and 95% of N ingested is excreted and a major proportion is found in urine (Lantinga et al., 1987). Cattle urine spots cover about 0.40 m2 (Lançon, 1978a,b; Williams and Haynes, 1994) and the quantity of N returned within the area of a urine patch varies from 20 to 97 g m-2 (Ball et al., 1979; Ledgard et al., 1982). The area affected by urine has been reported to be more than twice as large as the area actually wetted, because of diffusion in the soil and lateral spread of roots (Doak, 1952). Nitrogen returned as urine to the sward ranges from 10 to 40 g m-2 (Delaby et al., 1997). Once in the soil, urea N is rapidly converted to mineral forms and becomes available for plant uptake and undergoes transformations and processes such as leaching, volatilization, and denitrification (Hack-ten Broeke et al., 1996; Whitehead and Bristow, 1990; De Klein and van Logtestijn, 1994). Previous studies have stressed the positive effects of excretal returns on herbage yields (Norman and Green, 1958) while more recent reports have demonstrated that there is a large potential for losses (Ball and Ryden, 1984). Different soil types have been shown to have a major influence on the fate of urinary N in intensive systems (Clough et al., 1998; Stout et al., 1998). In the present study, the behavior of urinary N was examined in a lysimeter experiment with moderate N fertilization, on three different types of soils and applied in three contrasting seasons. The objectives were to study the recovery of 15N from urine by plants, determine the effects of soil type and season of application on the fate of this source of nitrogen (with special regards to N leaching), and establish its mass balance in the soil–plant–water system. The experiment was conducted over 24 mo as it is essential to know the short- (one year) and longer-term turnover in the soil of N derived from urine to maximize crop utilization and minimize losses. Differences due to the soil type characteristics were also examined.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experiment was conducted at Le Robillard, France (48°59' N, 0°0' W), where annual climatic averages (1971–2000) are 642 mm rainfall, which is evenly distributed over the year, and 10.5°C mean annual air temperature. Daily weather data were recorded using an automatic meteorological station, located 200 m from the experimental area. In 1993, a set of nine lysimeters was filled with 0.9 m of soil at the top and 0.1 m of quartz gravel at the base. The cross-sectoral area of each lysimeter area was 2 m2. Three types of soil were used in this experiment with three lysimeters per soil type. Typical properties for each soil are presented in Table 1 . One was a Calcosol developed from the underlying calcareous rock (Soil C), another was a Brunisol developed on granitic arena (Soil B), and the last one was a Neoluvisol laying on schists alterite (Soil L), according to Association Française pour l'Etude des Sols (1995). Volumetric soil water contents were measured in situ during the experiment using time domain reflectometry (Trime FM; IMKO GmbH, Ettlingen, Germany). The water-holding capacity was calculated as the difference between the soil water content at field capacity and the minimum value of soil water content observed during the experiment. The lysimeters, and the soil surrounding them, were sown with ryegrass (cv. Magella; 25 kg ha-1) in the spring of 1994. The investigation was performed from 1996 to 1998. Until 1996, the whole area was mowed five times at a height of 4 cm and top dressed with 15 g m-2 N fertilizer every year. Over a 10-d period, about 200 L of urine were collected (and immediately frozen) from a cow kept in a metabolism crate and fed exclusively on silage corn (Zea mays L.). Initial chemical characteristics of the urine are shown in Table 2 . Before use, the urine was mixed and enriched with a solution of 15N-labeled urea to obtain the following characteristics: total N = 7 mg L-1, urea N = 4.61 mg L-1, and 15N excess = 2.4725%.


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  		Table 1. Main physical and chemical properties of the soils used in lysimeters.

 

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  		Table 2. Chemical characteristics of the urine collected.

 
Aboveground biomass was assessed with five cuts per year made on 4 May 1996, 5 June 1996, 15 July 1996, 28 Aug. 1996, 23 Oct. 1996, 15 Apr. 1997, 13 May 1997, 18 June 1997, 22 Sept. 1997, 15 Oct. 1997, and 30 Mar. 1998. The 15N-labeled urine was applied to three lysimeters, each containing a different soil in 1996 after herbage cutting on 4 May 1996 (spring application). Three more lysimeters were treated on 15 July 1996 (summer application), and the final three lysimeters were treated on 23 Oct. 1996 (fall application). Each lysimeter therefore received urine on only one date, which led to nine treatments (three soils x three dates). A watering can was used to spray 3 L of urine (52.5 g N m-2) over a 0.4-m2 square area in the center of the lysimeters. As the treated area represented 20% of the total area of the lysimeter, 10.5 g N m-2 was applied over the whole lysimeter. On each occasion, each lysimeter that did not receive urine, received instead 3 L of water to maintain comparable water balance. On all lysimeters, grass was fertilized with 15 g N m-2 yr-1 applied in five dressings of ammonium nitrate. Potassium and phosphorus fertilizers were applied on 13 Mar. 1996 and again on 13 Mar. 1997 at 15 g K m-2 and 4 g P m-2 to ensure good fertility conditions for growth. Herbage was harvested separately from the central 0.4-m2 area (treated with urine = Zone A), an intermediate 0.63-m2 zone (a square area 20 cm beyond the treated area = Zone B), and the outer 0.97-m2 area (Zone C). The sward was cut to a height of 4 cm and samples were dried at 60°C, ground, and analyzed for total N and 15N, after dry matter determination. Drainage water was collected and the volume measured, and then the solution was analyzed weekly for N and 15N. On the final occasion, in spring 1998, different components of the soil–plant–water system were sampled, including 4 cm of aboveground plant material, drainage water, and soil cores. The latter were collected from the lysimeters using an auger 48 mm in diameter x 40 cm in length. Four replicate soil cores were extracted, from each of the three different zones (A, B, and C) of each lysimeter. At the top of the cores, stubble was cut with scissors from 4 cm height down to the ground (stubble), then litter was hand-separated and collected. Roots were separated from the soil both manually and mechanically by dispersion in water. Roots and litter were then mixed to form a similar pool (roots + litter). All vegetative samples were washed, dried, weighed for determination of dry matter, and ground before analysis. The remaining soil samples (free of vegetative material) were kept. Inorganic N was extracted by shaking a 150-g fresh soil subsample with 350 mL of 0.5 M KCl for 30 min. Ammonium and nitrate were measured in the soil extracts using autoanalyzer colorimetric techniques (Bran+Luebbe GmbH, Norderstedt, Germany). Soil remaining after extraction of the inorganic N was dried, ground, and analyzed for total organic N and the 15N content of the soil organic matter (SOM) pool. All N and 15N analysis of the experiment was performed with a Carlo Erba (Milan, Italy) NA100 linked to an isotope ratio mass spectrometer (Europa Scientific, Crewe, UK).

When collected by zone, calculations of DM, N, and 15N uptake of any compartment on the lysimeter were weighted by the zone area contribution to the whole lysimeter area. At the end of the experiment, the fate of urinary 15N was assessed in the harvested herbage, soil organic matter, leachate, stubble, and roots + litter pools. Urinary N balance was obtained by summing these pools and generally resulted in a shortfall. The imbalance was assumed to correspond to measurements and analytical errors as well as gaseous losses that were not measured directly.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Climate
Over the two experimental years, mean air temperature varied between 10°C in 1996 (Year 1) and 11°C in 1997 (Year 2). To prevent the swards from drying out during the very dry summer in Year 1 (May–August 1996), to ensure that normal drainage resulted from typical rainfall events during the first winter (January–April 1997), and to maximize soil leaching during the last winter (October–December 1997), we applied 90 mm of irrigation water during each of these three periods. This resulted in an annual precipitation (752 and 803 mm for Years 1 and 2, respectively) being greater than the long-term mean of 642 mm.

Dry Matter Yields and Total Nitrogen Uptake
In Year 1, the fall treatment lysimeters, which did not receive any urine until the end of the season, were used as controls for dry matter production (approximately 5 Mg ha-1 for Soils C and B and 6 Mg ha-1 for Soil L (Table 3) . Compared with these same controls, spring application of urine increased DM yields in Year 1 by +17, +21, and +33% in Soils C, L, and B, respectively. This influence was mainly observed during the two cuts that followed urine deposition, and effects were no longer statistically significant (P > 0.05) in Year 2. For the summer application treatment, the increase in DM production was smaller than for spring application: approximately 10% in Soils B and C and approximately 15% in Soil L. This was probably due to the small amount of DM accumulation in the last months of the year associated with the large soil water deficits. In Year 2, DM accumulation on Soils L and B was much less affected by the application of urine. In Year 2, fall treatments did not affect DM yields in Soils C and B, however in Soil L it increased by 1 Mg ha-1 (Table 3). Over the whole experimental period, the date of urine application had no significant influence on DM accumulation. However, in Year 1, DM harvested from the fall application treatment was less than that resulting from the two other treatment dates (P < 0.009), whereas on average it was greater in Year 2 (P < 0.05). The DM production on Soil L was larger than on the other two soils (P < 0.002) in both years.


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  		Table 3. Dry matter production, total N uptake, and urinary N uptake in harvested herbage for the whole area of the lysimeters in Years 1 and 2.

 
In Year 1, N uptake from Soils B and C was similar on the fall treatments (Table 3). For Soil L there was a slightly greater uptake associated with a larger DM production (Table 4) . Irrespective of soil type, total N uptake from lysimeters without urine application was less than the N fertilization applied.


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  		Table 4. Average total N uptake in harvested herbage according to the three types of soil and the three dates of urine application in Years 1 and 2.

 
For urine treatments of spring and summer application, total N uptake into the herbage was greater than for fall application, and values were similar for Soils B and C. Uptake by grass on Soil L was greater than that from the other soils after spring and summer applications. However, because of differences in DM production, the concentration of N in plants grown in Soil L was smaller than that in plants from Soils B and C (Table 3). This underlines the fact that the poorer production on Soils B and C was due mainly to water deficit or to other growth-limiting factors rather than to limited N availability.

Nitrogen uptake in Year 2 varied according to application date (Table 4). The same pattern was observed in Soils B and C, where later applications of urine (summer and fall application treatments) led to an increase in the N harvested (15% more than for the spring application treatment). For Soil L, there was a greatly enhanced N uptake (+28%) following the fall application but this did not occur on the other treatments. Nitrogen content herbage dry matter was mainly related to soil type. The greatest N content in the biomass was found in Soil B (2.35% compared with 2.07 and 2.18% for Soils C and L, respectively).

On the treated area (Zone A), the sward received a total of 67.5 g N m-2 (as fertilizer and urine), which exceeded the ability of the sward to take up N (40 g m-2 in our growing conditions). However, in Year 1, the total utilization of the N applied to the lysimeter was good (77% in spring, 63% in summer, and 47% in fall).

Urinary Nitrogen Uptake
In Year 1, urinary N uptake by grass was similar for each of the three soils when the treatment was applied in spring (Table 3). Urinary N applied in summer was better utilized by grass grown in Soil L. In Year 2, only a small amount of the urinary N was harvested except for the summer application treatment in Soil B and the fall application treatment in all soils (Table 3). After two years, cumulative uptake of urinary N for the summer application was approximately 40% of that applied irrespective of the soil; in spring it reached 52, 57, and 60% in Soils C, B, and L, respectively. For the fall application treatment, uptake only occurred in Year 2, and was very poor in Soil C (16% of urinary N applied), whereas it reached 35 and 41% in Soils B and L, respectively. In both years the date of application was the factor that most influenced urinary N uptake (Table 5) .


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  		Table 5. Average urinary N uptake in harvested herbage according to the three dates of urine application in Years 1 and 2.

 
Over the experimental period, urinary N harvested in Zone C remained less than 0.5 g m-2 (Fig. 1) for all treatments. In Zone B, urinary N uptake of spring application was approximately 5 g m-2 (Fig. 1) and that of summer application of approximately 3 g m-2 (data not shown), irrespective of the soil type. Urine from the fall application treatment showed large uptake differences in Zone B (0.3, 2, and 3 g m-2 in Soils C, B, and L, respectively). These observations indicated that the urinary N did not diffuse much beyond 20 cm, irrespective of the soil type or the season of urine application for our observed climatic conditions.



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  		Fig. 1. Cumulative urinary N uptake (g m-2) in harvested grass for (a) spring, Soil C; (b) spring, Soil B; (c) spring, Soil L; (d) fall, Soil C; (e) fall, Soil B; and (f) fall, Soil L. Symbols: •, Zone A; {blacksquare}, Zone B; {blacktriangleup}, Zone C.

 
In Zone A, most of the uptake by grass occurred during the first three cuts after spring treatment and occurred at the end of Year 1 and beginning of Year 2 after summer treatment (data not shown), and mostly during the first three cuts of Year 2 for fall treatment (Fig. 1). In Year 2, there was little urinary N recovered in the biomass for spring and summer (approximately 4 and 10%, respectively) and only a moderate amount (approximately 30%) from the fall application of urine. Urinary N uptake for spring and summer applications in the marginal zones (B and C) was very similar in every soil. The date of urine application markedly influenced the recovery of labeled urinary N in harvested plants. In Zone A, recovery followed expected trends for application date with greater values in spring (32–40%) than in summer (14–26%) in Year 1 (Fig. 1). Differences in urinary N uptake between soil types were observed in both Zones A and B for the latest application date.

Nitrogen Leaching Losses and Drainage Water Nitrate Concentration
During the first winter, the amount of drainage water was small (approximately 100 mm for Soils B and C), lasted a short period, and was late in the season (13 Jan. 1997–24 Mar. 1997), that is, there was a long period between fall mineralization and drainage occurrence. The amount of drainage water was approximately 40 mm less in Soil L than in the two other soils (Table 6) . A difference of approximately 30 mm was also observed in Year 2 for Soil L. However, in Soils B and C, drainage began earlier (16 Nov. 1997–31 Mar. 1998), lasted longer, and was much greater than the previous year with about 250 mm drainage water. In the first drainage period, after spring treatments, total N and urinary N leached and nitrate concentration in drainage water remained small (Table 7) irrespective of the soil type. This was also the case for Soil L for summer treatment. Under freely draining soils with poor water-holding capacity (i.e., Soils B and C), the later the treatment date, the greater the N amount lost by leaching (Table 8) . The Brunisol showed the greatest loss of both total and urinary N (Table 8). Comparing the amounts of 15N-labeled leached with the total N in the drainage water, it was clear that throughout the first drainage period, 15N originating from urine was an important constituent of the N leached. The relative contribution of 15N and 14N in the leachate was highly dependent on application date and on the extent of native soil organic N mineralization during the drainage period. The greatest contribution of labeled urinary N occurred in Soil B with 34, 68, and 77% for spring, summer, and fall, respectively. Soil C had intermediate values of urinary N leaching (4, 57, and 68%) while Soil L had the smallest proportions (6, 4, and 43%).


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  		Table 6. Average water drainage amounts of the three soils for Year 1 and 2 drainage seasons.

 

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  		Table 7. Total and urinary N leached and nitrate N concentration in water in the whole area of the lysimeters in Years 1 and 2.

 

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  		Table 8. Average total and urinary N leached and nitrate N concentration in water in the whole area of the lysimeters in Years 1 and 2, according to type of soil and date of urine application.

 
Despite the small amounts of N lost compared with the quantity applied, high nitrate concentrations in the drainage water were observed, with the mean annual nitrate content exceeding the 11.3 NO-3–N mg L-1 maximum acceptable concentration for drinking water (Table 7). Typically, the nitrate content of drainage water followed a pattern of an initial increase followed by a decrease after a peak concentration had been reached. In Year 1 (Fig. 2) , nitrate concentrations significantly increased in treatments with late urine application, that is, summer and fall in Soils B and C and fall only for Soil L. However, the very small drainage volume did not permit the completion of the pattern, and drainage stopped when concentrations were at their maximum. In Year 1, peak concentrations reached 40 to 50 mg N L-1 for summer and fall treatments whereas it remained very small (less than 10 mg N L-1) for the spring application.



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  		Fig. 2. The NO-3–N concentration of drainage water (mg L-1) during Year 1 and 2 winter periods for (a) Soil C, (b) Soil B, and (c) Soil L. Symbols: •, fall, total N; {blacksquare}, summer, total N; {blacktriangleup}, spring, total N; {circ}, fall, urinary N; {square}, summer, urinary N; {triangleup}, spring, urinary N.

 
During the second winter, the nitrate concentration in the drained water remained at a low level in Soil C irrespective of the urine application date (Fig. 2), with very small losses of total and urinary N (Table 7). Late season application of urine in fall had a long-term effect on the total amount of N leached (Table 8) in Soil B but the direct contribution of urine was very small. Nevertheless, mean annual nitrate concentration exceeded the 11.3 mg N L-1 drinking water standard in Soils B and L. In Soil L, the long-term influence of urine application was present for fall only (Table 8), as we observed a greater amount of N leached than for spring and summer treatments. This N is mainly constituted of soil N, with the direct contribution of urinary N to N leached being only 6%.

When averaging the flow-weighted mean NO-3–N contents of the three application dates, to provide an estimate for a pasture grazed throughout the year, Soil B gave the greatest concentration in each year (18.1 and 10.7 mg N L-1 in Years 1 and 2, respectively), and Soil C had 12.7 mg N L-1 in Year 1 and 4.1 in Year 2. Both soils showed a similar attenuation of water quality with date of urine application. Soil L had mean NO-3–N concentrations of about 5.2 mg N L-1 for both years.

Fate of Urinary Nitrogen
There was no significant difference of application date or soil type on root DM, total N, and urinary N with mean values of 2.4 Mg DM ha-1, 2.7 g total N m-2, and 0.15 g urinary N m-2. The same was observed for the stubble except for Soil B, where a greater DM production of stubble was associated with greater total and urinary N. Mean values were 1.6 Mg, 2.6 g, and 0.1 g for DM, total N, and urine N m-2, respectively. When cumulated, roots + litter and stubble compartments presented similar proportions of urinary N: 2% in spring and 3% in summer and fall treatments. Table 9 presents the mean urinary N recovery values for the other measured pools and the 15N balance shortfall at the end of the experimental period (i.e., 17, 20, and 22 mo after urine application on lysimeters) according to type of soil and date of application. On the whole, the largest proportions of 15N were observed in harvested herbage, soil organic matter, and the balance shortfall.


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  		Table 9. Average percentages of 15N-labeled urine recovery in several soil–plant–water system compartments, two years after deposition for the three different types of soil and the three dates of urine application.

 
Urinary N recovery in harvested herbage varied according to soil type (with a smaller recovery for Soil C than for other soils). The season of urine application significantly influenced urinary N recovery in herbage: spring and summer applications had recoveries enhanced by 75 and 32%, respectively, compared with fall application. Urinary N recovered in leachate varied according to soil type and date of application of urine, with the value for Soil L being significantly smaller than that for other soils and the largest proportion being associated with the fall treatment.

In our experiment, the recovery of urinary N in the soil organic matter did not vary largely with the type of soil or the season of urine application. The largest observed differences remained lower than 6%. Moreover, the very small standard error of means indicated that the processes leading to accumulation of urinary 15N in SOM might have been very stable during the experimental period or else were mainly climate dependent. Soil L had a significantly smaller proportion of 15N in SOM than did the other soils (Table 9). Immobilization of N from an early urine application (spring) averaged 25% for SOM urinary N content, which was significantly less than that for later application dates.

At the end of the experiment, all treatments showed an imbalance of unaccounted N, which must have been due either to gaseous losses, or to storage pools situated below the 40-cm depth in the soil or else to measurements and analytical errors. In contrast with SOM, the proportion of 15N unaccounted for presented a wide range of values (5–23%). Moreover, this pool exhibited a high standard error of means. This indicated that, besides the fact that our experimental design had no replication, a combination of several factors and their interaction might have influenced the balance shortfall. The unaccounted-for proportion of urinary N largely depended on the type of soil, with Soils L and C showing higher values than Soil B, but was not significantly related to date of application.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
We performed a lysimeter study because it is considered to be more reliable than drained plots or ceramic cups for leaching assessment (Addiscott, 1990). However, to reproduce urine deposition over a large paddock (and thus create a nonpoint source of N), we established a urine patch that was an average in terms of volume and N content, covering 20% of the lysimeter area and the average for the area covered by urine (12–31%; Cuttle et al., 2001; Hack-ten Broeke et al., 1996) in swards grazed by cattle. In most of the experiments reported in the literature on this topic, treatments are repeated over several years, leading to cumulated yearly and long-term effects of the treatment. In our experiment, we intended to study the direct and long-term effects of a single grazing sequence. Significantly, our climatic conditions induced very small annual amounts of drainage (150 mm in our situation whereas values of 500 mm have often been reported in other experiments), which is typical of what happens over large geographic areas.

Nitrogen Vulnerability to Leaching in Relation to Nitrogen Uptake by Herbage
Uptake by herbage of the N sources applied to grassland largely helped to reduce the amount of mineral N remaining vulnerable to leaching at the end of the growing season. The influence of a urine patch on plants has often been described, both within and beyond the treated area. Typical effects include enhanced biomass production and increased N and K uptake, but Deenen and Middelkoop (1992) noted that no influence was observed beyond 15 cm from the urine patch (on a sandy soil). Similarly, our results indicated a very low influence of urinary N beyond 20 cm in all treatments. Despite the small diffusion of urinary N, the apparent recovery of urinary nitrogen (ANR) from a whole lysimeter were much greater in our study for spring and summer treatments than those reported by others (e.g., 20%, Ledgard et al., 1982; 16%, Deenen and Middelkoop, 1992).

Our results indicated a marked influence of urine application date on the recovery of labeled urinary N. Though with greater recovery levels, Cuttle and Bourne (1993) and Stout et al. (1998) reported a similar influence of the time of application. Urinary N recovery in the biomass in Year 2 was greater for fall treatments than for spring and summer applications (approximately 4 and 10%, respectively). As a result, there was overall a high level of urinary N uptake, and there was little N susceptible to leaching at the end of the growing season in either year, with the exception of the fall application in Year 1.

An influence of soil type on N uptake and recovery was observed irrespective of the urine treatment date. Soil L had the greatest N uptake, N utilization, and urinary N recovery. Previous reports in the literature are conflicting. Sorensen and Jensen (1996) indicated greater recovery from Soil L than from Soil B, whereas Stout et al. (1998) observed the opposite. Year 1 in our experiment was characterized by a very small annual rainfall, little drainage in winter, and a cold and dry growing season. In these conditions, the greater water-holding capacity in Soil L probably resulted in greater grass growth and consequently less N leaching in winter than for the other soils.

Nitrogen Leaching after Urine Application
Each winter, the nitrate content of drainage water followed an often-described pattern (Steele et al., 1984; Heng et al., 1991) with an initial increase followed by a decrease after a peak concentration was reached. The observed maximum values were greater than results reported previously where larger N fertilization rates were applied (Scholefield et al., 1993) or where sheep (Ovis aries) were grazing (Heng et al; 1991), or else where the herbage was white clover (Trifolium repens L.) and ryegrass (Steele et al., 1984). The NO-3–N content of the drainage water from the fall application treatment exceeded the maximum acceptable concentration for drinking water even in Year 2. Our experimental design allowed us to show how the effect of urine deposition on water quality was buffered over several years. Based on comparable levels of N fertilization, the amounts of N leaching losses reported in the literature for grazed situations remained moderate, that is, between 1 to 5.8 g N m-2 yr-1 (Scholefield et al., 1988; Watson et al., 1992; Decau and Le Corre, 1994). In Year 1, nitrate losses by leaching in Soil L were small with an extremely small contribution of labeled urinary N irrespective of the treatment date. A combination of factors may explain the low level of losses observed from Soil L. Although drainage water quantity does not necessarily influence the mean annual NO-3–N concentration, it influenced the overall leaching loss (Jarvis, 1992). Every year, the drainage from Soil L was less than from the other soils due to greater water use associated with a larger dry matter production, and a greater water-holding capacity. Within soil treatments, there was a seasonal effect as N leaching losses were greatest for fall, intermediate for summer, and smallest for spring-applied urine. As also found by Stout et al. (1998), little or no plant growth occurred following the fall urine application treatment and most urinary N would therefore be vulnerable to leaching during the winter interval. Cuttle and Bourne (1993) and Sherwood (1986) estimated that when urine N was deposited before September, only 3% of the N applied was lost by leaching whereas after September it amounted to between 30 and 66%. When calculating the amount of N susceptible to leaching in each lysimeter less the N removed by uptake into the herbage, we observed that direct urine contribution to leaching amounted to 1 to 2% in spring, 8 to 15% in summer, and 15 to 29% in fall for Soils B and G, respectively. Two reasons might contribute to the small percentage remaining for N leaching compared with other reports. On the one hand, even with fall-applied urine, a 60-d delay occurred between application and beginning of drainage, during which time N applied might have undergone other transformations or loss pathways. On the other hand, the small drainage volume did not leach all the inorganic N remaining in the soil after the growing season. Our results and observations of other authors (e.g., Whitehead and Bristow, 1990; Cuttle and Bourne, 1993; Clough et al., 1998) confirmed that leaching in the second winter drainage period did not contain much N originating from the urine applied in the previous year. The increase of N leaching with date of urine application did not parallel the simple calculation of N applied minus N removed in herbage and was always higher. This suggested that a second factor related to increasing N leaching is a function of the time delay between application and drainage. The longer the delay, the smaller the increase in N leaching attributable to urine application. We assumed that other N transformation processes had a better opportunity to occur with a longer N applied residence time.

Nitrogen Leaching in Urinary Nitrogen Budget at the End of the Experiment
The recovery of labeled N in the stubble showed no significant trend for any soil type or application date, consistent with results reported by Cuttle and Bourne (1993) for a one-year study. On average, 1 to 2% of the quantity of N applied was recovered in stubble. These values were small compared with the 8 to 9% reported on a yearly basis by Kimura and Kurashima (1991). Even if larger values had been reached at the end of the first year of our experiment, this remaining N would have been recycled through the plant during the second growing season, which may explain why we observed no difference between treatment and the small recovery of urinary N in this compartment at the end of Year 2. The root compartment followed exactly the same pattern and the same recovery levels. This agrees with the results of Clough et al. (1998), who indicated that soil type did not influence the recovery of urinary N in roots one year after application. No urinary N was observed in the inorganic soil nitrogen pool. This was already established by Clough et al. (1998) and Cuttle and Bourne (1993) for the end of the first year, except for late season applications when they found 66% of urine N in the inorganic N pool. However, after two winter drainage periods and a second growing season we considered it to be normal. Clough at al. (1998) noted that labeled N immobilized in the soil organic matter was independent of soil type. However, in our experiment, Soil L had a significantly smaller proportion of 15N in SOM than did the other soils. Data consistent with our observations can be found in the literature with recoveries in soil organic matter plus root pool ranging between 20 and 34% (Kimura and Kurashima, 1991; Fraser et al., 1994).

The N that was unaccounted for varied markedly according to soil type. Values were very small for Soil B (5%), but reached 22% for the other soils. This pool necessarily covers all the errors associated with measurements and analysis. However, given the level of N that was unaccounted for in all treatments, the pool must also have represented a large part of the gaseous losses. Loss due to volatilization of ammonia usually occurs within hours of urine application and usually continues for the next few days. Typically, 12 to 25% of urinary N may be lost by volatilization from grassland in the first 14 d (Whitehead and Raistrick, 1993). Denitrification usually occurs if soil is depleted in oxygen (i.e., in waterlogged soil). Poorly drained soils were more susceptible to denitrification. A wide range of values have been reported for the loss by denitrification: from 1 to 5% (Monaghan and Barraclough, 1993) up to 16% (De Klein and van Logtestijn, 1994) of urinary N applied on grassland. Occurring over winter, this process of NO-3–N transformation is in competition with leaching. Waterlogging (assessed by time domain reflectometry measurements) occurred in Soil L at the beginning of winter of Year 1 and the end of spring of Year 2. Thus, denitrification might have contributed to the unaccounted pool of N in this soil. In the C Soil, which was freely draining, denitrification should be smaller than in Soil L but its high CaCO3 content (11% in the 0- to 40-cm layer) suggests that this soil is more vulnerable to volatilization. This gaseous loss might have largely contributed to the shortfall of the N budget in this soil. Soil B had the smallest pool of N that was not accounted for and consequently the lowest gaseous losses.

Based on our observations in the two years after urine application, the urinary N recovered in leachate remained less than 16% except for the fall application treatment on Soil B (30%). Under the conditions of the experiment, leaching was only the fourth most important fate of urinary N (after herbage, other losses, and immobilization in SOM).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Under our experimental conditions (maritime oceanic climate with moderate drainage), we studied the influence of the season of deposition and of the type of soil on the fate of urinary N applied to grassland, giving particular attention to leaching. Some soil–plant pools (soil organic matter, roots plus litter, and stubble) did not vary much over the long term within the treatments. However, the earlier in the season the urine was applied, the better it was utilized by the sward and the smaller were all pathways of loss, regardless of the soil type. Soil type influenced the fate of urinary N, mainly for summer and autumn applications. Based on the amount of N observed or the proportion of the applied N, N leaching under grazed grassland in these conditions did not seem to be harmful for water quality. We assumed that small drainage amounts resulted in part of the small inorganic N amounts remaining in the soil at the end of the season and after drainage, and that it was readily taken up by the grass sward in the second year. However, several results also suggested that this general trend did not always hold true. The very poor dilution of leached excess N generated by N deposition in summer and fall led to water with very high NO-3–N concentration reaching the environment and significantly influenced the long-term water NO-3–N concentration. Moreover, in the Brunisol, where other loss pathways were minimized, leaching was the fate of a large proportion of urinary N.


   ACKNOWLEDGMENTS
 
We are grateful to the Conseil Régional de Basse Normandie for funding the building of the lysimeters. We are also in debt to D. Perrin for his skilled and permanent assistance with field work. L. Delaby, J.L. Peyraud, and R. Vérité from SRVL INRA Rennes deserve special mention for urine collection and valuable advice and comments on the experimental design.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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