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Journal of Environmental Quality 32:1204-1211 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Atmospheric Pollutants and Trace Gases

Gaseous Nitrogen Emissions and Mineral Nitrogen Transformations as Affected by Reclaimed Effluent Application

Y. Master*,a, R. J. Laughlinb, U. Shavita, R. J. Stevensb and A. Shaviva

a The Faculty of Agricultural Engineering, Technion-IIT, Haifa 32000, Israel
b Department of Agriculture and Rural Development, Agricultural and Environmental Science Division, Newforge Lane, Belfast BT9 5PX, Northern Ireland, UK

* Corresponding author (master{at}tx.technion.ac.il)

Received for publication October 31, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Irrigation with reclaimed effluent (RE) is essential in arid and semiarid regions. Reclaimed effluent has the potential to stimulate gaseous N losses and affect other soil N processes. No direct measurements of the N2 and N2O emissions from Mediterranean soils have been conducted so far. We used the 15N gas flux method in a field and a laboratory experiment to study the effect of RE irrigation on gaseous N losses and other N transformations in a Grumosol (Chromoxerert) soil. The fluxes of N2, N2O, and NH3 were measured from six Grumosol lysimeters following application of either fresh water or RE. The N fertilizer was applied either as 15NH4 or 15NO3. Only up to 0.3% from the applied N fertilizer was lost as N2O + NH3. Reclaimed effluent enhanced the losses of NH3, but did not affect those of N2O. Nitrification and denitrification were equally important to N2O production. Laboratory incubations were performed to both confirm the influence of the irrigation water type and to test the effect of moisture content. Significant quantities of N2 and N2O (up to 3.1% of the applied fertilizer) were emitted from saturated soils. Reclaimed effluent application did not induce higher N2O emissions, yet significantly more (approximately 33%) N2 was emitted from RE-irrigated soils. Denitrification contributed up to 75% of the N2O amounts emitted from saturated soils. Reclaimed effluent application inhibited nitrification in the Grumosol by 15 to 25% and induced NO2 accumulation in soils incubated at a field-capacity moisture content.

Abbreviations: FW, fresh water • IRMS, isotope ratio mass spectrometer • OM, organic matter • RE, reclaimed effluent


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IRRIGATION WITH TREATED WASTEWATER (commonly referred to as RE) became a necessity for maintaining and developing agriculture in arid and semiarid regions. The main advantage of RE utilization in agriculture is the redirection of the scarce fresh water (FW) to domestic and industrial uses while securing food production and maintaining the ecological and recreational values of irrigated land. The relatively high nutrient (N, P, and K) content of RE offers additional agronomic and economical advantages. The potential nitrogen supply with RE can be as much as 150 to 200 kg N ha-1 either directly as NH4 or indirectly as organic N (Ministry of Agriculture and Rural Development, 2001). There are several agronomic and environmental risks associated with RE application, such as the adverse effects on N transformations. In a lysimeter study with three local representative soils, the N recovery in RE-irrigated soils was 20 to 30% lower than in those irrigated with FW (Master, 2002). The large amounts of available organic matter (OM) and N present in both the applied RE and RE-irrigated soils are known to enhance the denitrification process and thus the losses of N2 and/or N2O (Fillery, 1983). Feigin et al. (1981) also found that the losses of tagged fertilizer N from soils irrigated with a secondary municipal effluent were significantly higher than those from FW-irrigated soils. Although they did not conduct direct measurements of the products, the enhanced losses were attributed to the stimulation of denitrification and NH3 volatilization in RE-irrigated soils. The application of high-pH substances rich in OM and NH4 (e.g., cattle slurry and liquid manure) usually results in elevated N2 and N2O emissions (Clayton et al., 1997; Stevens and Laughlin, 2001a,b). The potential for NH3 losses from RE-irrigated Mediterranean soils increases due to the high pH of both the soils and the applied RE. Little information is available regarding the N2 and N2O evolution from Mediterranean soils and the pathways of their formation as affected by RE irrigation or high loading of OM.

Koch et al. (2000) estimated that the yearly emission of the greenhouse gas N2O from agricultural soils in Israel amounts to 1650 Mg. Nitrification and denitrification are the two major processes responsible for N2O production (Hutchinson and Davidson, 1993; Stevens et al., 1997). Reclaimed effluent application may enhance N2O production by either process. Provided NO3 is present, the addition of readily available C in the RE will create conditions conductive to N2O production via denitrification. On the other hand, excessive amounts of NH4-N supplied with the RE increases the potential for nitrification and thus nitrification-derived N2O fluxes. The N2 and N2O fluxes and the processes affecting their evolution can be studied using 15N labeled fertilizers (Mosier and Klemedtsson, 1994). A better understanding of processes other than denitrification (e.g., NO2 formation, mineralization, nitrification) may be obtained by using 15N as a tracer (Barraclough, 1991; Burns et al., 1996).

This paper describes a short-term lysimeter (field) and a laboratory incubation experiment. The main aim of the study was to assess the short-term influence of RE irrigation on the losses of N2, N2O, and NH3 and its interference with other N transformations in a Grumosol (Chromoxerert) soil. Additional aims were to evaluate the main source of N2O under local environmental conditions and to assess the effect of moisture content on the emissions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Lysimeter Study
Site Description and Experimental Setup
The fluxes of N2 and N2O from six Grumosol (Table 1) lysimeters planted with corn (Zea mays L.) were measured 40 d after planting in August 2001. The lysimeters (surface area of 0.5 m2 and height of 120 cm) were located in the Acre Agricultural Experimental Farm in Western Galilee, Israel, and are described in more detail by Oved et al. (2001). Three lysimeters were irrigated with FW and three with a secondary RE from a nearby reservoir. The rates of N, P, and K application were according to plant demand and were similar for both FW and RE treatments. Gases were collected using 25-cm-long PVC cylindrical enclosures (10-cm i.d.). The enclosures were inserted 20 cm into the soil leaving 5 cm of the upper ends to protrude above the soil surface. The installation took place 2 d before the beginning of the flux measurements. The bottom ends of the enclosures (10 cm) were cut in a V-notch shape to facilitate penetration into the root-dense soil and to allow better interaction between root exudates and the added fertilizer. There were two such enclosures in each lysimeter, giving six enclosures per treatment. The temperatures prevalent during the experiment were in the range of 28 to 35 and 26 to 34°C for air and soil (up to 15 cm deep), respectively.


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  		Table 1. Physical and chemical properties of the Grumosol soil used in the experiments.

 
Treatments
Each lysimeter received two labeled treatments, either 99 atom % (15NH4)2SO4 (with no NO3 added) or 99 atom % K15NO3 (with no NH4 added). Solutions applied to FW-irrigated lysimeters were prepared from FW and those applied to RE-irrigated lysimeters were prepared from RE (BOD5 = 28 mg L-1, where BOD5 is five-day biological oxygen demand). Three-hundred milliliters of fertilizer solution containing approximately 180 mg N (either as NH4 or NO3) L-1 were applied to each enclosure, being equivalent to 67 kg N ha-1. The enrichments of the NO3 and NH4 pools after the fertilizer addition were in the range of 27 to 41 atom % to ensure the determination of the 29N2/28N2 and 30N2/28N2 ratios with adequate sensitivity using an isotope ratio mass spectrometer (IRMS) (Stevens et al., 1993). Fertilizer solutions were pipetted uniformly over the soil surface within each enclosure to ensure pool homogeneity (Stevens et al., 1997).

Dinitrogen and Nitrous Oxide Measurement
The fluxes of N2 and N2O were determined seven times at 3, 8, 11, 27.5, 34.5, 51.5, and 57.5 h after fertilizer application. At each sampling time, the lids (fitted with gas sampling ports) were fitted to the base section using a rubber band to form a gas-tight seal. The headspace volume into which the gases were emitted was approximately 400 mL. At the end of the headspace closure period (4–5 h), 12-mL headspace samples were transferred to evacuated septum-capped vials (Exetainers; Labco, High Wycombe, UK). When the lids were closed, a white polystyrene foam was placed on top of them to avoid overheating from direct sunlight.

The samples were analyzed for the isotopic composition of N2 and the concentration and isotopic composition of N2O by continuous-flow isotope-ratio mass spectrometry (Stevens et al., 1993) within 30 d. For N2, the ion currents at m/z 28, 29, and 30 enabled molecular ratios 29R (29I to 28I) and 30R (30I to 28I) to be determined. Differences between the molecular ratios in enriched and normal atmospheres were calculated as {Delta}29R and {Delta}30R. The flux of N2 was calculated using data for {Delta}29R and {Delta}30R to calculate the enrichment of the denitrifying pool (15XN) and the N2 flux according to Mulvaney and Boast (1986). The enrichment of N2O was determined using the ratios 45R(45I to 44I) and 46R(46I to 44I) measured by IRMS, and the concentration was calculated using these ratios and a reference gas of known concentration (Stevens et al., 1993). The flux was calculated from the change in N2O concentration with time, assuming linear accumulation during the closure period. The cumulative fluxes were calculated by integration, assuming linear change in flux rates between observation times. The headspace volume was corrected at each sampling time using the average air temperature during the sampling period (Mosier and Klemedtsson, 1994).

Source of Nitrous Oxide
When the NO3 pool was labeled, the flux of N2O was partitioned between nitrification and denitrification using the procedure of Arah (1997). The procedure calculates the apparent enrichment of the denitrifying pool (ap) and the fraction of the N2O derived from that pool (d) using the ratios 45R(45I to 44I) and 46R(46I to 44I). The major assumption of this procedure is the homogeneity of the NH4 and NO3 pools. The contribution of each process to the total cumulative flux was calculated using the N2O flux and the value of d at each sampling occasion.

Evaluation of Ammonia Loss
The relative potential of NH3 loss following irrigation was assessed in September 2001. The NH3 flux was determined during the 72-h period following application of approximately 75 kg N [as (NH4)2SO4] ha-1. The NH3 was sorbed into acid traps that consisted of a Petri dish (10-cm diameter) containing 5 mL of 0.5 M H2SO4. The traps were placed on a plastic tripod approximately 5 cm above the soil surface and covered with an enclosure (plastic bucket, 18-cm i.d.), that was gently pressed into the soil. Twenty-four hours later the enclosures were taken out of the soil, the acid was renewed, and the enclosures were reinserted for an additional 48 h. Blank dishes were placed near the lysimeters to account for the background NH3 concentration. The NH3 losses were evaluated on the basis of NH4 determination in the collected solutions.

Laboratory Study
Soil and Treatments
The laboratory incubations aimed at simulating the soil conditions (temperature, prevalent concentrations of mineral N species, and moisture content) during the lysimeter study. The soils used in the incubations had different management histories. The soil used for FW treatments had been irrigated with FW and the soil used for RE treatments had been irrigated with RE for the last 3 yr. The soil samples (Table 1) were air-dried to a moisture content of 10% w/w and passed through a 9.5-mm sieve. Ten milliliters of water were added to each sample 2 d before the onset of the incubation for the purpose of conditioning. The soil samples (equivalent to 100 g on an oven-dry basis) were incubated in 590-mL screw-top jars in a controlled chamber at 30°C. Full factorial experiments were performed for: (i) two types of water source (RE and FW), (ii) two moisture contents (field capacity, 40% w/w; and saturation, 75% w/w), and (iii) two types of 15N labeling (15NH4 and 15NO3).

Nitrate was added as natural abundance KNO3 or as 99 atom % K15NO3 and ammonium was added as natural abundance (NH4)2SO4 or as 99 atom % (15NH4)2SO4. The final (added + native in soil) concentrations of mineral N in each jar were approximately 50 mg NO3–N and approximately 100 mg NH4–N kg-1. The final enrichment of the labeled pool (either NO3 or NH4) inside each jar after the addition of labeled fertilizer was approximately 60 atom % excess. Reclaimed effluent used for RE treatments contained 19.2 mg NH4–N L-1 and had a BOD5 of 100 mg L-1. The amount of N applied to each jar was between 130 and 140 mg N kg-1 (depending on the amounts of N in the native soils, which differed slightly between treatments).

Dinitrogen and Nitrous Oxide Measurement and Soil Analyses
The fluxes of N2 and N2O were measured six times (3, 6, 9, 22, 31, and 48 h after fertilizer application) during the 48-h incubation period. A destructive sampling was performed at the end of each gas-sampling occasion (except at the 31-h sampling) to determine the concentrations and enrichments of the soil mineral N species. There were three replicate jars per treatment per sampling occasion, yielding a total of 120 jars for the experiment. The lids (fitted with gas sampling ports) of the jars were tightly closed 1 h before each sampling time. At other times the lids were loosely fitted on the jars to allow adequate aeration while avoiding excessive moisture escape. After the headspace closure period of 2 h, a 12-mL sample was transferred to an exetainer and analyzed by IRMS within 30 d. The N2O concentrations and enrichments were measured and calculated as in the lysimeter experiment. The N2 flux was calculated only for the 15N labeled NO3 treatment using the equations of Mulvaney and Boast (1986).

The Bunsen coefficient of N2O at 30°C and solution volume inside the jars were used to calculate the dissolved amount of gas in the soil solution (Tiedje, 1994). After the headspace samples were taken, the soil was immediately extracted with 200 mL of 2.5 M KCl. Jars containing soil KCl slurries were shaken for 1 h in an orbital shaker, filtered, and stored at 4°C. Concentrations of NH4, NO3, and NO2 in the extracts were determined using a Lachat (Milwaukee, WI) Quikchem 8000 autoanalyzer. The 15N contents of the NH4 and NO3 were determined by methods based on their conversion to N2O (Stevens and Laughlin, 1994; Laughlin et al., 1997).

The Sources of Nitrous Oxide
The flux of N2O was partitioned between nitrification and denitrification using the procedure of Stevens et al. (1997). The fraction of the N2O flux derived from the denitrification pool (d) was calculated using the measured 15N atom fractions of NH4, NO3, and N2O. The fraction of N2O flux derived from the nitrification pool (equivalent to NH4 pool) is equal to (1 - d). The above procedure assumed that mineral N pools were homogeneous in 15N.

Gross N Transformation Rates
The gross rates of mineralization and nitrification during the 48-h experiment were estimated using the sizes and dilutions of the NH4 and NO3 pools at the beginning and end of the experiment (Barraclough, 1991).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Lysimeter Study
Nitrous Oxide Losses
Nitrous oxide fluxes from the lysimeters are shown in Fig. 1 . The fraction of the flux attributed to either nitrification or denitrification, the apparent enrichment of the denitrifying pool (ap), and the N2O atom % excess at each sampling time are presented in Fig. 2 . Table 2 shows the cumulative amounts of N lost during the 57.5 and 72 h following irrigation for N2O and NH3, respectively.



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  		Fig. 1. The N2O flux from the Grumosol lysimeters irrigated with fresh water (FW) or reclaimed effluent (RE) and labeled either with 15NO3 or 15NH4. Data represent means and standard deviations (n = 3).

 


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  		Fig. 2. The fraction of the flux attributed to either nitrification or denitrification, the apparent enrichment of the denitrifying pool (ap), and the N2O atom % excess at each sampling time in lysimeters labeled with 15NO3 and irrigated with (a) fresh water (FW) and (b) reclaimed effluent (RE). Data represent means and standard deviations (n = 3).

 

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  		Table 2. Cumulative amounts of gaseous N, as N2O and NH3, lost from the Grumosol lysimeters during the 57.5 and 72 h following irrigation. Numbers in parentheses are the standard deviations (n = 3).

 
The effect of RE irrigation on N2O flux could not be clearly determined in the field experiments, partly due to the high variance between the lysimeters (Fig. 1). The cumulative N2O losses did not display consistency, either (Table 2). There were no significant differences between treatments in denitrification contribution to the N2O flux during the experiment (Fig. 2). The contribution of denitrification to the total emitted amounts of N2O was also nearly equal to that of nitrification in both treatments (54 and 50% in FW- and RE-treated soils, respectively). The soil in the lysimeters (both in FW- and RE-irrigated) had large quantities of readily available OM after three seasons of corn growth and had very high levels of NO3 (Table 1). The small amount of OM supplied with the RE did not, presumably, significantly raise the denitrification potential of the soils; therefore, no significant difference was found in denitrification contribution to the N2O flux. On the other hand, soil and climatic conditions during the field experiment were optimal for N2O production via nitrification. High soil temperatures and water-filled pore space (WFPS) of 50 to 65% were reported to be optimal for N2O production via nitrification (Linn and Doran, 1984). Soil temperature during the experiment was in the range of 26 to 34°C and WFPS was approximately 48% at the end of the experiment. It is likely that slightly higher WFPS values had prevailed during the experiment. There was evidence for the inhibition of nitrification in RE-irrigated soils. The values of ap in FW treatments declined consistently during the experiment from an average initial value of 18.3 to 9.5% (Fig. 2a), whereas in RE treatments, the initial and final ap values were nearly equal (Fig. 2b). Since this value denotes the apparent mean enrichment of the soil NO3 pool, ap dilution in the case of FW treatments indicates that nitrification took place, while in RE treatments, nitrification was less active. This observation is consistent with the findings of the laboratory studies (see later). Stevens and Laughlin (2001b) also observed that the onset of nitrification was delayed when cattle slurry was applied to a grassland soil. This was attributed to the presence of readily available volatile fatty acids in the cattle slurry, which could create a temporary O2 stress. In our case, the O2 stress could be caused by the consumption of the available OM in the RE by heterotrophs.

Dinitrogen Losses
Dinitrogen emissions were not detected in the field experiment. It is evident that denitrification did produce N2O (Fig. 2), yet it seems that the soil O2 status at the upper lysimeter layer was not low enough for its further reduction to N2 (Fillery, 1983). Elevated concentrations of both O2 and NO3 (which was abundant in the lysimeters) have been shown to inhibit the N2O reductase, thus increasing the N2O mole fraction in the total denitrification products (Firestone et al., 1979). The reduction to N2, however, might have fully proceeded in the deeper lysimeter layers. Those layers were more saturated (Master, 2002) and their potential nitrification activity was inhibited compared with the upper layers (Oved et al., 2001). These observations could indirectly indicate the lower aeration status of the deeper lysimeter layers, which would encourage the production of unlabeled N2. Significant dilution with unlabeled N2 may have rendered the labeled N2 formed inside the enclosures undetectable by IRMS.

Ammonia Losses
Ammonia losses from the lysimeters were higher than the losses of N2O (Table 2). Reclaimed effluent application nearly doubled the NH3 losses from the Grumosol, yet the amounts lost were negligible (up to 0.3% from the applied N fertilizer). Nelson (1982) states that up to 50% of added N can be volatilized as NH3 when ammoniacal fertilizers are surface-applied to alkaline soils. Ammonia losses in this study may have been underestimated due to the physical conditions imposed by the sampling devices. Covering the traps for such long periods (24–48 h) practically prevented evaporation from the upper layer inside the enclosure surface area, as compared with the soil surface outside the enclosure. This may have restricted gaseous diffusion from the soil and underestimated the losses (Mosier and Klemedtsson, 1994). This could, to some extent, apply to N2O, although in this case the soil was allowed to aerate for longer time periods.

Total Nitrogen Losses
The highest losses of total N from both FW- and RE-irrigated Grumosol lysimeters under the experimental conditions were less than 0.5% of the applied N. These losses are considered negligible in agricultural practices from an economic standpoint, and by themselves cannot explain the frequently observed poor N recovery in RE-treated soils. Ruling out the possibility of significant N2 emissions from the upper soil layers under given conditions, there is still a potential for N losses as nitric oxide (NO). Hutchinson et al. (1993) found that NO amounts lost from a sandy loam soil under several N and water treatments were an order of magnitude higher than those of N2O. Likewise, Drury et al. (1992) found NO to be the principal end product from a sandy loam when soil moisture content was similar to the moisture content prevalent in our lysimeters. The entrapment of N gases (be they N2, N2O, NH3, or NO) within the soil profile (Clough et al., 2000) and the previously discussed restriction of gaseous diffusion from the soil could also contribute to the underestimation of the N losses in our studies.

Laboratory Study
Gaseous Nitrogen Fluxes
The fluxes, concentrations, and transformation rates of the various N species are reported per gram of oven-dry soil. The fluxes of N2 and N2O as affected by various treatments for the 48-h incubation are shown in Fig. 3 . The fluxes of N2 from the 15N-labeled NO3 treatment were only detectable at saturation, starting from the third sampling time (9 h). The N2O fluxes from soils incubated under field capacity were only detectable up to 22 h. The N2O fluxes were averaged for both NH4– and NO3–labeled treatments.



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  		Fig. 3. The (a) N2 and (b) N2O fluxes from the Grumosol soil during the laboratory experiment. The N2O fluxes are shown for both field-capacity and saturation moisture contents, whereas the N2 fluxes are shown for 15N labeled NO3 saturated treatment. Data represent means and standard deviations (n = 3 and 6 for N2 and N2O, respectively).

 
The N2 flux from RE-irrigated treatments was significantly higher than from those with FW (Fig. 3a). The cumulative N2 losses in the 48-h incubation amounted to 372 and 493 g N ha-1 from FW- and RE-irrigated jars, respectively (Table 3) . The N2O fluxes from soils incubated under field capacity were about two orders of magnitude lower than those emitted from saturated soils (Fig. 3b) and the losses equaled a negligible 0.01% of the applied N fertilizer. Nitrogen lost as N2O from FW- and RE-irrigated soils at saturation was nearly equal (324 vs. 320 g N ha-1, respectively; Table 3). The pattern of the N2O emissions was also similar, except for the faster onset of N2O production in the first 9 h in RE-irrigated jars. As in the lysimeter study, no special RE effect on the N2O losses has been shown. Substances similar to RE were usually found to enhance the N2O emissions (e.g., Clayton et al., 1997). This wastewater treatment level (BOD5 = 100 mg L-1) may have been low enough not to affect the N2O emissions in the short term. Apparently the soil water content and not irrigation water type (at the tested BOD5 levels) is the major factor influencing N2O emissions from the Grumosol in the short term.


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  		Table 3. Nitrogen losses as N2 and N2O and mole fraction of N2O in a laboratory study (48 h) at saturated conditions. The values were calculated for a jar soil surface area of 50 cm2. Numbers in parentheses are the standard deviations (n = 3 and 6 for N2 and N2O, respectively).

 
Summation of the N2 and N2O losses under saturated conditions shows that significant N quantities (2.7–3.1% from the applied fertilizer) are likely to be lost when moisture content is high (Table 3). The amounts of N lost as N2O from the lysimeters (approximately 0.14%) were higher than those lost at field-capacity incubations (approximately 0.01%) and lower than those lost from saturated soils (approximately 1.2%). The moisture content in the upper lysimeter layer was closer to field capacity than to saturation during the lysimeter study. This suggests that the majority of N2O in the lysimeters was formed either in the lower saturated layers or in the upper layer where transient saturation persisted several hours following irrigation. The relatively high enrichment of N2O supports the latter possibility. There is evidence that soil structure is deteriorated as a result of prolonged RE application (Feigin et al., 1991), which in turn may enhance the potential of water-logging and hence the gaseous N losses. In our study under saturated conditions, RE lowered the mole fraction of N2O [N2O/(N2 + N2O)] from 0.47 to 0.39, but the total N flux from the soil increased from 696 to 813 g N ha-1 (Table 3). Application of substances rich in OM have usually resulted in an increase in the mole fraction of N2O (Stevens et al., 1998; Stevens and Laughlin, 2001b). Stevens et al. (1998) attributed the decrease in the N2O mole fraction to the induction of respiration by nondenitrifying microorganisms, a further increase in the degree of anaerobiosis, and eventually a reduction of more N2O to N2. The RE could have created more anaerobic conditions in the soil that already contained excessive C amounts after three seasons of corn growth.

Sources of Nitrous Oxide
The enrichments of NO3, NH4, and N2O for saturated conditions are shown in Fig. 4 . In all cases when the N2O fluxes were too low (including all field-capacity incubations), N2O enrichment data were not available. The enrichment of N2O matched more closely the enrichment of NO3 than that of NH4 in all treatments, implying that most of the gas was formed via denitrification. Denitrification was responsible for approximately 75% of the total N2O emitted during the 48-h experiment, regardless of irrigation water type. In the lysimeter study, the relative contributions of the two processes were nearly equal, as the moisture conditions were more favorable for nitrification (water-filled pore space of approximately 48%). We sought to conduct the incubations in aerobic conditions by only sealing the lids on the jars for 2-h periods. This could explain why only 75% of the formed N2O could be attributed to denitrification. The oxygen present in the jars could have diffused through the thin soil layer (approximately 3 cm) even when the water-filled pore space was 100%. We, however, assumed that the saturated soil was less aerated than that at field capacity.



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  		Fig. 4. The NH4, NO3, and N2O enrichments in soil incubated under saturated conditions with (a) fresh water (FW) labeled with 15NO3, (b) FW labeled with 15NH4, (c) reclaimed effluent (RE) labeled with 15NO3, and (d) RE labeled with 15NH4. Error bars represent the standard deviations (n = 3) or are smaller than symbols.

 
The two moisture contents examined in the incubation study could have been too "extreme" for the RE effect to be pronounced. The field-capacity moisture content induced very low emissions of N2O in both FW and RE treatments. The limited sensibility of the IRMS did not allow us to determine the enrichment of the N2O in that case; therefore, it was not possible to compare between the two treatments. At saturated conditions, on the other hand, the predominant factor affecting the emissions seemed to be moisture content rather than irrigation water type. When the Grumosol had been incubated at a slightly lower-than-saturation moisture content, RE affected both the amount and the source of N2O (Master, 2002).

Other Nitrogen Transformations
The concentrations of NH4 declined in all cases, but no significant increase in those of NO3 was observed (data not shown); therefore, the occurrence of nitrification could not be clearly inferred from this data. However, the dilution of the 15NO3 pool (Fig. 4a,c) and the gradual enrichment when 15NH4 was applied (Fig. 4b,d) clearly showed that nitrification was occurring during the 48-h incubation. The occurrence of the mineralization could be inferred from the dilution of the 15NH4 by the unlabeled 14NH4 released form the soil organic N pool (Fig. 4b,d).

Table 4 shows the amounts of nitrified and mineralized N as calculated for the 48-h incubation. The amounts nitrified in saturated soils were 30 to 40% lower than those at field capacity for both water types. The amounts of N that were nitrified in RE treatments were 15 to 25% lower than in FW at the two moisture contents. This inhibition of nitrification in RE-irrigated soils is consistent with observations made in the lysimeter study. The O2 stress created by the oxidation of organic compounds in effluents was assumed to be responsible for lowering nitrification rates (Stevens and Laughlin, 2001b; Zaman et al., 1999). The prolonged exposure of the soil to O2 consuming conditions could also weaken the nitrifying microbial population. The nitrification potential of soils having a history of RE irrigation was found to be lower than of those with a history of FW irrigation (Oved et al., 2001).


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  		Table 4. Average gross amounts of N nitrified and mineralized over 48 h.

 
The highest amounts of N (4.5 mg N kg-1) were mineralized from the saturated RE-irrigated soil (Table 4). The positive effects of moisture content and organic C content in the effluents have been previously reported (Puri and Ashman, 1998; Zaman et al., 1999). The availability of nutrients in the dairy shed effluent and its low C to N ratio were assumed to be responsible for the higher mineralization rates in those treatments (Zaman et al., 1999). A typical C to N ratio for sewage effluents is approximately 5 or lower (Feigin et al., 1991); therefore, mineralization would be preferred. Mineralization contributed only up to 3.2% of the applied N in the 48-h incubation; however, in longer-term experiments (10–14 d), a significant (up to 30%) contribution of mineralization has been observed in the Grumosol (Master, 2002). Longer-term studies are required to both confirm nitrification inhibition by RE and to assess the various factors affecting the soil mineralization potential.

Nitrite showed a very distinct increase with RE irrigation, particularly at field capacity, reaching as much as 9 mg NO2–N kg-1 (Fig. 5) . Moreover, concentrations of up to 30 mg NO2–N kg-1 were observed in RE-irrigated Grumosol soil in subsequent longer-term incubations (Master, 2002). These values are much higher than those commonly encountered in field or laboratory studies, even at much higher fertilizer rates (e.g., Burns et al., 1996). The largest NO2 peaks in our experiment were observed at field capacity, implying that the main contribution to NO2 formation was the nitrification of NH4. Master (2002) examined the 15N content of mineral N species in longer-term incubations of Grumosol soil, and showed that the majority of NO2 was formed via nitrification. Possible reasons for NO2 accumulation during nitrification include elevated NH4 concentrations and high pH (Burns et al., 1996). Such conditions (which are commonly found in the RE and in RE-irrigated soils) are expected to raise the free NH3 concentrations, which were shown to inhibit Nitrobacter during the second nitrification step, resulting in NO2 accumulation (Smith et al., 1997). The higher NH3 fluxes emitted from RE-irrigated lysimeters support this assumption. Other possible factors that could have enhanced NO2 accumulation in RE-irrigated soils include the presence of low molecular weight soil organics (Stueven et al., 1992) and changes in the population structure of ammonia-oxidizing bacteria in soils exposed to prolonged RE irrigation (Oved et al., 2001). The issue of NO2 accumulation in RE-irrigated soils requires further study.



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  		Fig. 5. The NO2 concentrations in fresh water (FW)– and reclaimed effluent (RE)–irrigated soil at field-capacity and saturation moisture contents. Data represent means and standard deviations (n = 6).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Reclaimed effluent did not affect N2O losses in both the lysimeter and the laboratory short-term studies. The measured N2O–N losses constituted only up to 0.14% from the applied fertilizer; however, they increased to approximately 1.2% when the soil was saturated. Dinitrogen was emitted from the Grumosol only at saturation. Up to 3.1% of the applied fertilizer was lost as N2 + N2O when soil was saturated. Reclaimed effluent application stimulated the emissions of both N2 and NH3. The N emissions measured in the field could not explain the frequently observed losses induced by RE application; however, an underestimation might have occurred due to the physical conditions imposed by the sampling devices. Additional reasons could be entrapment of the formed N gases within the soil profile and production of NO.

Similar quantities of N2O were formed by nitrification and denitrification at lower moisture contents; however, the contribution of the latter became dominant at higher moisture contents (up to 75% of the total emitted N2O). There was evidence of short-term inhibition of the nitrification process (by 15–25%) in RE-irrigated soils both in the lysimeter and laboratory studies. Significant NO2 levels were observed in RE-irrigated Grumosol at field capacity, implying that the source was the nitrification of NH4. Possible reasons could be high levels of free NH3 or the presence of low molecular weight compounds in RE or RE-irrigated soils. Longer-term studies are required to examine the long-term effects of RE irrigation, the issue of NO2 accumulation, and the influence of additional parameters (e.g., RE treatment level, soil irrigation history, etc.) on various N transformations.


   ACKNOWLEDGMENTS
 
This research was supported by a grant from the Technion Grand Water Research Institute and the Israeli Ministry of Agriculture. We thank Gabbi Gera from the Acre Agricultural Experimental Farm for his assistance during the field experiments. We appreciate the constructive remarks of the anonymous reviewers.


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


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