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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Organic Carbon Leaching from Effluent Irrigated Lysimeters as Affected by Residence Time

^a Institute of Soil, Water and Environmental Sciences, the Volcani Center, ARO, P.O. Box 6, Bet Dagan 50250, Israel
^b Unit de Science du Sol, INRA, Route de Saint Cyr, 78026 Versailles, France
^c Institute of Field and Garden Crops, the Volcani Center, ARO, P.O. Box 6, Bet Dagan 50250, Israel

* Corresponding author (finep{at}volcani.agri.gov.il)

	ABSTRACT

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

 
Under irrigation with secondary effluent, it is often assumed that reduced leaching would delay the transport of effluent organic matter (OM) from within the root zone for long enough for biodegradation to practically eliminate it. This hypothesis was tested in a lysimeter study which compared tap water with low-quality secondary effluent (total organic C [TOC] = 190 mg C L^-1) for Eucalyptus irrigation. Lysimeters (200-L drums) were packed with dune sand or one of two clayey soils (A horizons of a Calcic Haploxeroll and a Typic Palexeralf) and were either not planted or planted. Leaching treatments were leaching fraction 1 (LF 1) (not planted) and LF 0.2, intermittent leaching and prolonged deficit irrigation. These provided residence times of the water in the soil that ranged from 0.8 to over 200 d. While root organic C (OC) made a negligible net contribution to leachate OC, the effluent-derived OC was rather recalcitrant as was indicated by: (i) the concentrations of TOC in the leachate increased with decreasing LF to over 250 mg C L^-1; (ii) the recovery of the applied TOC in the leachate was significant (10–20%) even at RTs over 20 d (LF 0.06 in the sand-packed lysimeters). Most all the dissolved and colloidal organic C (DOC) was smaller than 30 kDa. We expect these concentrations of low-molecular-weight mobile OC to govern many aspects of the biological and chemical behavior in solutions of effluent-irrigated soils, especially under reduced-leaching regimes.

Abbreviations: COD, chemical O₂ demand • DOC, dissolved and colloidal organic C (0.45 µm) • DOM, dissolved and colloidal organic matter • EC, electrical conductivity • EC₂₅, electrical conductivity at 25°C • ET, evapotranspiration • LF, leaching fraction • OC, organic C • OPE, oxidation ponds effluent • OM, organic matter • PET, potential evapotranspiration • RT, residence time • SAR, Na adsorption ratio • TOC, total organic C • TOM, total organic matter

	INTRODUCTION

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

 
SEWAGE EFFLUENT is becoming the main, if not the only, source of water available for irrigation in semiarid regions of the world. However, effluents contain organic and inorganic constituents, the concentrations of which depend on the quality and type of sewage and the extent of treatment (Feigin et al., 1991; Soffer et al., 1997). The rates of uptake and degradation of these constituents are often slower than those of their addition in the irrigation water. Thus, effluent-irrigated fields become a source of organic and inorganic contaminants in soils and aquifers (Amiel et al., 1990).

Irrigation with effluents, especially at lower degree of treatment, adds OC to the soil, part of which is DOC (Feigin et al., 1991). Effluent DOC can influence soil properties and behavior (e.g., soil redox potential, Patrick and Jugsujinda, 1992; Eshel and Banin, 2002), and can interact with microcontaminants to enhance their transport in the soil, the unsaturated zone, and the groundwater (e.g., Mingelgrin and Biggar, 1986; Gooddy et al., 1995; Han and Thompson, 1999). Siebe and Fischer (1996) found that irrigation with low-grade sewage effluent in the central Mexico valley greatly increased the contents of TOC and DOC in the upper soil layers and enhanced metal solubility and transport down the soil profile. The rate of migration of heavy metals was enhanced by the reductive dissolution of Mn oxides in the soil. Lensing et al. (1995) showed that DOC induced Cd mobility in aquifer material derived mainly from DOC-Cd complexes. Dunnivant et al. (1992) measured a rapid breakthrough of Cd from soil columns treated with a solution with 60 mg L^-1 of dissolved and colloidal organic matter (DOC). Lamy et al. (1993) found migration of Cd to a depth of 80 cm in the field (loam; pH 6.4–6.9) following stepwise application of 1000 m³ ha^-1 of liquid sludge (containing 1.1% solids); the mobility was correlated with high concentrations of DOM in the soil solution. Vulkan et al. (2000) showed that DOM from unaerobically digested, waste-activated sludge increased the solubilities of Cu and Zn in sand columns, and that almost all the soluble Cu was associated with recalcitrant low-molecular weight (<1 kDa) DOM. In a lysimeter study, similar to the one in the present study, Fine et al. (1999) found a straight linear correlation between the concentrations of Zn and TOC in the leachate from sand-packed lysimeters that were amended with biosolids compost and irrigated with secondary effluent.

Graber et al. (1995) examined the distribution of atrazine (C₈H₁₄ClN₅) in the 4-m profile of a vertisol, following 4 yr of irrigation with low-grade secondary effluent. They showed that effluent irrigation substantially enhanced the transport of this organic micropollutant, which was applied to the field at agronomic rates, compared with the transport under tapwater irrigation. Although Graber et al. (1999) and Seol and Lee (2000) considered the possible effect of effluent DOM on the sorption of triazines to soil to be small, they suggested that slight but persistent suppression of sorption, together with alteration of other properties of the soil solution by effluent water components, may be important in pesticide mobilization in soils. Bouwer (1987) thought that the biological breakdown of toxic organics might be more complete under restricted soil leaching, because of the extension of the RT within the actively biodegrading zone. Fine et al. (1999) and Graber et al. (1999) suggested that reduced soil leaching under effluent irrigation might enhance transport of inorganic and organic micropollutants, because plant water uptake can concentrate effluent-derived DOM in the soil solution. We used a lysimeter experimental design which utilized effluent and freshwater irrigation of Eucalyptus to address the question of how restricted leaching regimes affect the fate of effluent OM in soils.

	MATERIALS AND METHODS

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

 
Container Construction
Data were collected in a lysimeter setup comprising forty-eight 200-L drums in six treatments. The containers were positioned on metal frames (3.0 by 0.6 by 0.6 m) in groups of three. Each container was lined with 0.1-mm-thick polyethylene. A 0.1-m layer of limestone pebbles at the bottom of the container, on top of the liner, was overlain by a nylon mesh (1-mm openings) and covered with a 0.7-m sand or soil column. The surface area of the soil in the container was 0.25 m². A drainage device (5 cm long, 20-mm i.d.) was fitted to the bottom of the container and the liner.

Treatments
The main variables were: (i) type of irrigation water–secondary effluent vs. tap water (Table 1); (ii) type of soil (Table 2); (iii) presence or absence of a Eucalyptus camaldulensis tree, and (iv) LF. Planted treatments were in six replicates and not-planted treatments were in two replicates. The treatments (and amounts of irrigation and tree yields) are summarized in Table 3. The data reported here are from the second year of the experiment, between April and November 1996 (TOC data were not collected in the first year of the experiment).

Irrigation and Leaching
Low-quality secondary effluent, with TOC concentration of 192 ± 11 mg L^-1 and a DOC/TOC ratio of 0.20 ± 0.03, was pumped directly from a facultative oxidation pond at the Dan Region sewage treatment plant, Israel (Table 1). Fertigation was by irrigation with tap water that was supplemented with N, P, K, Fe, Mn, Cu, Zn, and Mo at concentrations similar to their average concentrations in the effluent in the preceding year. The microelements were added as chelates with EDTA (the overall concentration of EDTA was 4.3 mg L^-1).

Irrigation water was applied, three times daily, to the soil surface in each container by two 8-L regulated drippers. The water was evenly spread via eight semi-regulated distribution rod-drippers. All irrigation processes and the admixing of tap water with fertilizer solution were computer controlled and monitored. The volumes of irrigation and drainage water were monitored for each lysimeter separately on a daily to weekly basis. The amounts of water that were applied to the lysimeters (m³ lysimeter^-1) are also shown as water heads (m ha^-1). The conversion was according to the soil surface area (Table 3).

Sampling of the two types of water for chemical analysis was done once in 1 to 3 wk. These samples were used to calculate the recoveries of the constituents of interest. Drainage was sampled by connecting a 1-L bottle to the lysimeter outlet. Entrapped air was released via a 5-mm i.d. tube that opened under water. The atmosphere was confined to minimize changes in the chemical composition of the sample. Drainage accumulation took 6 to 12 h.

Irrigation water was applied with or without leaching. Without a tree in the lysimeter, leaching was usually 90 to 100% of the amount of irrigation water (LF 1). Planted lysimeters were tested under three leaching treatments: (i) constant leaching of 20% of the amount applied (LF 0.2); (ii) intermittent leaching, where individual water heads were adjusted to compensate for the actual potential evapotranspiration (PET), trying to avoid leaching; and (iii) deficit irrigation, where irrigation was with a fixed amount of water that was below the PET.

Water heads were adjusted for each treatment (often even daily) to produce the desired LF (e.g., Fig. 1) . If needed, finer tuning of the amount of irrigation water applied to individual lysimeters was done by changing to drippers of lower capacities. This caused some differentiation between individual lysimeters within each treatment and among the treatments.

View larger version (25K): [in this window] [in a new window]   Fig. 1. Average water consumption of Eucalyptus camaldulensis trees grown in the sand-packed lysimeters under leaching fraction 0.2 (LF 0.2) and intermittent-leaching regimes with the secondary effluent.

Deficit Irrigation Experiment
In January 1998, 30 Eucalyptus trees with trunk diameter of 15 to 20 cm at base, were cut out of their drums. Only a 30-cm long trunk piece and the very top part of the main root system were left. These stumps were replanted in new sand-packed drums. The trees were fertigated until mid June with 10 L tree^-1 d^-1. By mid June, almost all the trees had grown enough to stop all drainage. Fertigation of six of these trees was continued until the end of December, while 12 others were switched to secondary effluent (Table 3). In mid September, six of the effluent-irrigated trees were switched back to fertigation. In October, the water supply was reduced to 5 L tree^-1 d^-1. Irrigation continued until December (with very little interference from rain—that winter was extremely dry). In January 1999, the soil solution was displaced by flushing with 50 L of tap water (about two volumes of soil water).

Residence Time Calculation
The RT of water within the soil under deficit irrigation treatment was 6 to 7 mo. The RTs for the treatments with leaching were calculated in a similar way to the calculation of average LF. Under the various leaching regimes, individual RTs were estimated as RT = V/(V_L/t), in which V equals the volumetric moisture content of the lysimeter at soil field capacity, V_L equals the cumulative leachate volume per container; and t equals time (d). The average (±SE) RTs for the leaching treatments, on a whole-season basis, are presented in Fig. 2 as functions of soil type and leaching treatment. By manipulation of the LF, we were able to maintain residence times of the irrigation water within the 70-cm deep soil layer, that ranged from <1 d to more than 40 d.

View larger version (29K): [in this window] [in a new window]   Fig. 2. Average (and standard error) yearly residence times of soil solution within the lysimeters as affected by soil type, irrigation regime, and presence of a Eucalyptus tree.

View larger version (25K): [in this window] [in a new window]   Fig. 3. Average (and standard error) residence times (RTs) of the soil solution within the lysimeters under the leaching fraction 1 (LF 1) regime (no tree in the lysimeter) as related to the soil type and the time of the year.

Organic C in leachate and in effluent water was analyzed with a Formacs, combustion TOC analyzer (Skalar, De Breda, The Netherlands). The sample was acidified to pH 3.5 to facilitate carbonate removal prior to the determination of OC. Disolve and colloidal OC was determined on samples that passed a 0.45-µm pore-size polypropylene syringe-mounted filter. The filter was prechecked to confirm that it did not emit OC. When not available, chemical O demand (COD) analyses were performed by high-temperature potassium dichromate oxidation, with prepacked reagent vials from HAACH, and colorimetric measurement on a HAACH DR/2000 spectrophotometer (HAACH Co., Loveland, CO). For the effluents and leachate studied, COD was related to TOC by the near-theoretical conversion factor TOC COD/2.65.

	RESULTS AND DISCUSSION

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

 
Eucalyptus Yield and Water Consumption
Eucalyptus plantlings were planted one per lysimeter, in October 1994, and the irrigation treatments were started immediately. The trees were cut at 40 cm above ground in December 1995 and the irrigation was stopped until regrowth started in April 1996, when we also started to collect and analyze leachate samples. The canopies were harvested again in December 1996. The canopy yields and the amounts of irrigation water used are shown in Table 3. In the present paper, we report only the data on leaching of TOC from the lysimeters; the data on tree growth and water uptake are presented only to demonstrate the performance of the lysimeter setup. The canopy yields were averaged for combinations of type of soil and type of irrigation water. In the case of the effluent-irrigated sand-packed lysimeters, the yields were averaged for the 12 trees in the LF 0.2 and intermittent leaching regimes. The average fresh weight of the canopies ranged from 11 to 15.9 kg tree^-1. This rather sizeable regrowth for the 1-yr-old plantlings resulted from the combination of favorable climatic conditions, spacious containers and application of ample water and nutrient elements.

Figure 1 shows an example of the seasonal pattern of average water uptake by the trees in the secondary-effluent-irrigated sand-packed lysimeters. Data from two treatments are shown: trees that were irrigated at LF 0.2 for the whole period and trees that were irrigated at LF 0.2 until August 1st and then shifted to intermittent leaching. In midsummer, the average water consumption of the 2-yr-old trees was more than 40 L d^-1 per tree. Under intermittent leaching, the rate of water uptake soon decreased, most probably as a result of the gradual salinization of the soil solution (below).

Concentration of Total Organic Carbon in Leachate from Lysimeters
The average (±SE) concentrations of TOC in the leachate from the lysimeters are presented in Fig. 4 for the three soils, two types of irrigation water, and two or three leaching regimes. The average concentration of TOC in the leachate from the effluent-irrigated not-planted sand-packed lysimeters was 41 ± 10 mg L^-1 (Fig. 4A). This was 20% of the original concentration in the irrigation water. The TOC in the leachate from the planted sand-packed lysimeters was 4 to 6 times more concentrated: 159 ± 21 mg L^-1 and 250 ± 31 mg L^-1 under the LF 0.2 and intermittent-leaching regimes, respectively. The increase could be because of a concentrating effect driven by the uptake of water by the trees, exudation of OC-containing compounds by tree roots, or both.

View larger version (41K): [in this window] [in a new window]   Fig. 4. Average (and standard error) concentrations of total organic C (TOC) in leachate from planted and not-planted lysimeters as affected by the type of irrigation water (A, effluent; B, fertigation), type of soil, and leaching regime.

Nonetheless, we conclude that the net contribution of OC from the roots of the Eucalyptus trees to the leachate was negligible. The increase in the concentration of TOC in the leachate from the sand-packed effluent-irrigated lysimeters was caused by the accumulation and concentration of partially decomposed effluent OC. However, this does not imply that the amounts of root exudates and sloughed-off parts were insignificant. The trees were quite large (Table 3); their root systems (not shown) were extensive and they spread through the entire soil volume. Thus, the rather small net contribution of the tree roots to the TOC of the leachate suggests that the rate of OC turnover in the soil was high; at least as high as the rate of discharge of OC from the roots to the soil solution.

The concentrations of the TOC in the leachates from the fertigated and effluent-irrigated soils were usually greater than that in their sand counterparts (Fig. 4). This was probably because of the OC contribution from the organic matter of the soils (Table 2). As mentioned above, the two soils were excavated from the top layer of virgin soils. The salinization of the soil solution at LF 0.2 could perhaps also enhance the dissolution of soil organic matter (SOM) (Reemtsma et al., 1999). Thus, it is perhaps surprising that the differences between the OC concentrations in the leachates from the soils and from the sand were not larger than those actually found. One plausible explanation is that although the LFs were the same, the actual RTs were considerably longer in the soils than in the sand (Fig. 2). Thus, it might be that larger contributions of OC from the OM pool of the soils were counteracted, at least in part, by the longer residence in the biodegrading soil environment.

The individual observations of leachate TOC concentrations from all the lysimeters (total of 145 observations) are shown in Fig. 5 with respect to the EC_leach/EC_irr ratio (EC_leach; electrical conductivity of the leachate; EC_irr; EC of the irrigation water), which is the solution concentration factor relative to the irrigation water. Also depicted in Fig. 5 is the power regression line that was calculated for the leachate data from the sand-packed effluent-irrigated lysimeters (58 observations):

[1]

View larger version (26K): [in this window] [in a new window]   Fig. 5. Concentration of total organic C (TOC) in the leachates from the lysimeters of all treatments with respect to the electrical conductivity ratio (ECleachate/ECirrigation ratio) of the leachate samples. The power regression fit was calculated only for the TOC in leachates from the sand-packed effluent irrigated lysimeters.

Recovery of Total Organic Carbon in the Leachate
The recovery rates of applied TOC in the leachate from all the effluent-irrigated lysimeters are shown in Fig. 6 with respect to the RT of the water in the soil profile. The recoveries were calculated on the basis of the measured TOC concentrations and volumes of leachate and irrigation water. Evidently, the recovery of OC in the leachate decreased with increasing RT of the irrigation water. However, following 20 d of residence, which was the average RT in the intermittent leaching regime in the sand-packed lysimeters (Fig. 2), the leachate still contained some 20% of the applied OC. Following 60 d of residence, the OC discharge in almost all of the leachate was equal to 5% of the amount applied in the effluent water.

View larger version (21K): [in this window] [in a new window]   Fig. 6. Recovery of effluent total organic C (TOC) in the leachates from the three soils under all effluent-irrigation regimes. The recoveries are presented with respect to the estimated residence time of the leachate water in the soil column. The power regression line was calculated for the data from the sand-packed lysimeters using [Eq. 2].

[2]

The same power function was fitted to the entire data set from all three soils (165 observations), and yielded similar coefficient of determination but a steeper decay rate and a larger ‘intercept’:

[3]

The larger ‘intercept’ of the latter function reflects the OC discharged from the soil itself. The r² values are not too bad considering that the RT and the recovery parameters were both calculated values, and that the RT was a parameter of the irrigation water rather than of the TOC itself. We conclude: (i) that 55% of the effluent TOC was intercepted within the 70-cm sand column even at high leaching rates (residence times shorter than 1 d); and (ii) that recalcitrant TOC, almost all of effluent origin, would eventually leach below the root zone even following extended residence times.

Relationships between the Dissolved Organic Carbon/Total Organic Carbon Ratio and the Residence Time
The DOC/TOC ratio was examined in leachate samples from all leaching regimes in the sand-packed lysimeters (Fig. 7) . Evidently, the average size of the leachate OC particles decreased with increasing RT. The average (±SE) DOC/TOC ratio gradually increased from 0.20 ± 0.01 in the effluent water to 0.30 ± 0.05, 0.47 ± 0.05, 0.75 ± 0.04, and 0.93 ± 0.01, respectively, in the leachates obtained under the LF 1, LF 0.2, intermittent leaching, and deficit irrigation regimes. In addition, the DOC/TOC ratio in the leachate from LF 1 and LF 0.2 was inversely related to the TOC concentration in the leachate sample (respective r² values of the power regression lines are 0.60 and 0.68, p < 0.001). Hence, a more thorough removal of the added TOC also resulted in a greater reduction in the effective size of the remaining OC particles.

View larger version (25K): [in this window] [in a new window]   Fig. 7. Dissolved and colloidal organic C/total organic C (DOC/TOC) ratio in the oxidation pond effluent and in the leachates from the sand-packed lysimeters as affected by the leaching treatments.

It should be mentioned that the increased salinization of the soil solution with reduced leaching could also change the configuration of the DOM in the soil solution, by reduction of the effective size distribution. Ghosh and Schnitzer (1980) showed that samples of humic and fulvic substances extracted from soils were flexible linear colloids at low ionic strength (and also low sample concentration and neutral or higher pH). The extended configuration was because of charge separation and intramolecular repulsion. As the ionic strength increased (0.05 M NaCl), these macromolecules spontaneously assumed coiled configurations similar to those of rigid uncharged spherocolloids. Similarly, Tsutsuki and Kuwatsuka (1984) demonstrated a reduction in the effective mean size distribution of humic materials that were extracted from a soil. The effective size decreased from 210 and 310 kDa to 2.3 and 4.5 kDa, respectively, following an increase of the ionic strength from 0.03 to 0.5 M NaCl. To avoid this effect, we diluted the leachates with deionized water before DOC determination. In addition, the TOC concentration of the leachates was always well below the value of 1700 to 2500 mg C L^-1 that Ghosh and Schnitzer (1980) found to be the critical concentration for spontaneous coiling.

Recovery and Size Distribution of Total Organic Carbon under Deficit Irrigation
Deficit irrigation was applied to test the hypothesis that the OC from the effluent water will eventually be biodegraded within the soil profile if the soil microflora are allowed enough time to degrade it. Three treatments were tested (Table 4): continuous irrigation with either (i) secondary effluent or (ii) tap water, and (iii) switching from effluent to tap water at mid season. The total amounts of irrigation water and TOC (and nutrients) that were applied were approximately one-third of the amount that had been applied under LF 0.2 two years previously. Consequently, tree growth was considerably less (not measured). Nonetheless, deficit irrigation is a realistic scenario for future wastewater disposal in industrialized, short-cycled, Eucalyptus plantations in semi-arid regions (e.g., Myers and Lehane, 1999).

The DOC/TOC ratio in the displaced soil solutions ranged from 0.84 to 0.97 (Table 4). We studied the size distribution of the DOC by ultrafiltration. The TOC was sequentially separated into the following size fractions: OC > 0.45 µm; 0.45 µm > DOC > 300 kDa; 300 kDa > DOC > 30 kDa; 30 kDa > DOC > 1 kDa; and DOC < 1 kDa. Figure 8 shows striking similarity between the size distribution of the DOC in the effluent water itself and those in the leachate from the effluent irrigated sand treatments. This was true also for the case in which effluent irrigation was replaced by fertigation 3 mo before sampling. Almost all the DOC (>90%) was smaller than 30 kDa and 40% was smaller than 1 kDa. The fertigated soil had a somewhat different size distribution but the difference was not statistically significant. The similarity of size distributions of the DOCs does not necessarily mean that their chemical nature was also similar; this is the subject of ongoing studies. Of the low molecular organic acids (citric and smaller), only citric acid was present in the effluent water at low concentrations (<1 mg L^-1) but it was not found in the leachate. Adipic, succinic, malonic, fumaric, and oxalic acids were not found in the secondary effluent (B. Artzi, unpublished data, 1996).

View larger version (38K): [in this window] [in a new window]   Fig. 8. Size fractions of dissolved and colloidal organic C (DOC) as percentages (average and SE) of total DOC in oxidation pond effluent (OPE) and in soil solutions displaced from sand-packed lysimeters. The lysimeters were irrigated for 6 to 7 mo with either (i) OPE or (ii) tap water or (iii) OPE (for 3 mo) and than with tap water. Irrigation was under the deficit-irrigation regime. The size fractions were separated sequentially with membranes having the depicted nominal cutoffs (in µm or kDa).

	SUMMARY AND CONCLUSIONS

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

 
A strict control of water application under effluent irrigation is important in attempting to reduce overall loads of contaminants in the plant-soil-aquifer system. We tested the hypothesis that reduced soil leaching and extended residence time of the wastewater components in the soil profile would enhance the interception of organic matter loads. The treatments provided a wide range of LFs and RTs of the water and solutes within the 70-cm soil profile. The range of RT with respect to the irrigation water was between 0.8 and >200 d.

The recovery of effluent OC in the leachate decreased exponentially with increasing RT. However, it was still substantial (20% of the applied TOC) even at RTs exceeding 20 d. Inasmuch as the percentage of the applied OC recovered in the leachate eventually decreased below 5% at 60 or more days of residence, the salinity of the soil solution (under the conditions of our experiment) became too high for most crops, even in the more clayey soils. Furthermore, the concentration of the residual OC in the soil solution increased as the LF diminished. In the sand-packed effluent-irrigated lysimeters, the TOC concentrations in the leachate were 40, 150, and 250 mg C L^-1 at LF 1, 0.2, and 0.06 (0.85, 5.4, and 21.5 d of residence), respectively. This occurred in an effectively biodegrading soil environment that mineralized, or otherwise intercepted, almost all root exudates. Root-derived DOM was a significant component of soil DOC only under the deficit-irrigation regime. We suggest that the rate of accumulation of the recalcitrant components of the effluent DOC was controlled by the weighted effects of the rate of application of effluent OC, the degree of soil leaching and the rate of biodegradation.

Thus, rather recalcitrant components of the effluent water DOC seemed gradually to concentrate in the soil solution, following the reduction of soil leaching. The net effect was a gradual increase in the DOC/TOC ratio in the leachate, from 0.20 in the typical effluent, to 0.30, 0.47, 0.75, and 0.93, respectively, in leachates obtained under the LF 1, LF 0.2, intermittent-leaching, and deficit-irrigation regimes. The DOC that accumulated in the soil solution and that in the effluent had very similar size distributions. The DOC was probably rather hydrophilic, as would be expected of DOC that comprised, almost entirely, aquatic humics and fulvics (Chiou et al., 1986). The role of the effluent and soil solution DOC that is smaller than 1 kDa, in all soil processes that involve DOM interactions, should not be overlooked (Seol and Lee, 2000). This DOC formed a substantial proportion (40%) of the DOC of both the effluent and the irrigated soil solution.

We suggest that the concentration of low-molecular-weight mobile DOC in the soil solution, combined with high salinity, sodicity, and bicarbonate concentration (Fine, unpublished data, 1997), might govern many aspects of the biological and chemical behavior in the soil profile under effluent irrigation (e.g., leaching of heavy metals; Fine et al., 1999), especially under reduced-leaching regimes. Hence, the characteristics of the DOM that forms in the soil profile are very relevant in studies that attempt to predict the behavior of xenobiotic micropollutants or even that of indigenous soil constituents.

Finally, the combination of the gradual increase of OC concentration in the leachate, the gradual decrease of OC recovery and the reduction of the DOC/TOC ratio with the lengthening of the RT suggests that bypass flow or sidewall flow (Corwin, 2000) was minimal under the conditions of the experiment.

	ACKNOWLEDGMENTS

 
The authors thank the Chief Scientists of the Ministry of Agriculture and the Ministry of the Environment, Israel, the Texas Israel Exchange Program (TIE), and the Jewish National Fund, for supporting the project, and to ‘Mekorot - Israel Water Company’ and the Dan Region Cities Sewage Association, for making available the site and water. Special thanks to Vasiliy Solopanov, Shoshi Suriano, Rivka Rosenberg, and Sara Davidov for their invaluable technical help.

Received for publication June 29, 2001.

	REFERENCES

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

 

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