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TECHNICAL REPORTS
Bioremediation and Biodegradation

Irrigation of Broccoli and Canola with Boron- and Selenium-Laden Effluent

USDA-ARS Water Management Research Lab., 9611 S. Riverbend Ave., Parlier, CA 93648

* Corresponding author (gbanuelos{at}fresno.ars.usda.gov)

Received for publication December 27, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Selenium (Se), boron (B), and salinity contamination of agricultural drainage water is potentially hazardous for water reuse strategies in central California. To demonstrate the feasibility of using plants to extract Se from drainage water, Se accumulation was determined in canola (Brassica napus L.) and broccoli (Brassica oleracea L.) irrigated with drainage effluent in the San Joaquin Valley, California. In the 2-yr field study, both crops were irrigated with a typical drainage water containing Se (150 µg L^-1), B (5 mg L^-1), and a sulfate dominated salinity (EC of 7 dS m^-1). Total dry matter yields were at least 11 Mg ha^-1 for both canola and broccoli, and plant tissue Se concentrations did not exceed 7 mg kg^-1 DM for either crop. Based on the amount of soluble Se applied to crops with drainage water and the estimated amount of soluble Se remaining in soil to a depth of 90 cm at harvest, both canola and broccoli accumulated at least 40% of the estimated soluble Se lost from the soil for both years. Applied Se not accounted for in plant tissue or as soluble Se in the soil was presumably lost by biological volatilization. This study suggests that irrigating two high value crops such as canola and broccoli with Se-laden effluent helps manage Se-laden effluent requiring treatment, and also produces economically viable Se-enriched crops. Future research should focus on managing residual salt and B in the soil for sustaining long time water reuse strategies.

Abbreviations: EC, electrical conductivity • DM, dry matter • CIMIS, California Irrigation Management Information System • NIST, National Institute of Standards and Technology • SRM, standard reference materials

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
THERE IS AN increasing trend in using poor quality waters for irrigated agriculture. Water-use strategies are under consideration in central California and many regions of the western USA due to growing municipal and environmental demands for good quality water, as well as the need for wastewater disposal. Sufficient evidence exists supporting the successful use of saline water originating from drainage or from shallow ground waters (Grattan and Rhoades, 1990; Rhoades et al., 1992; Oster, 1994; Shalhevet, 1994; Shennan et al., 1995; Rhoades, 1999). Because subsurface drainage water can be defined as infiltrated water that has percolated through the root zone, the resulting salinity and sodicity levels in drainage water generally determine the feasibility of its reuse. A successful adoption of reuse will require integrated management practices related to irrigation, crop, and soil to control both the impact of salinity on planted fields and the movement of inorganic constituents (Shannon et al., 1997, 1998, 1999).

Soils on the western side of central California are derived from Cretaceous shale rocks that contain high levels of Se and other salts (McNeal and Balisteri, 1989). Trace elements such as Se, arsenic (As), and molybdenum (Mo) are of particular concern because they were reported to cause toxicity in many biological ecosystems, including waterfowl ponds at Kesterson Reservoir, California. Selenium in discharged drainage water that is collected and channeled into evaporation ponds may build up to a high-toxic concentration and pose a threat to wildlife (Ong et al., 1997). Subsequently, soluble Se released from these irrigated agricultural soils into drainage waters sites has been strictly monitored for growers in western central California (San Joaquin Valley Drainage Program, 1990). Reuse of Se-laden drainage water is one management option for reducing the volume of drainage water (San Joaquin Valley Drainage Program, 1990; Cervinka et al., 1999). With a reuse strategy, Se deposited onto soils after irrigation with Se-laden drainage water must be managed to minimize its detrimental effects on the biological environment where it is discharged. Earlier research showed that certain exotic plants, e.g., Indian mustard [Brassica juncea (L.) Czern. & Coss.], extract and accumulate Se added to soils as soluble sodium selenate (Bañuelos and Meek, 1990). Identifying nonexotic plants that accumulate Se under high saline and B conditions and incorporating them as part of the crop rotation with typically grown cotton (Gossypium hirsutum L.) or alfalfa (Medicago sativa L.) may hinder the buildup of Se in soils irrigated with Se-laden drainage waters.

Successful phytoextraction of Se by plants is dependent on the crop and its acceptance and widespread use by growers, who generally prefer economic viability crops (Parker and Page, 1994). In this regard, two members of the mustard family have been considered—canola (Bañuelos et al., 1997) and broccoli (Bañuelos and Meek, 1989)—as recipients of Se-laden drainage water. Canola is typically grown in Canada and in Europe as an oil-producing crop, whereas broccoli is grown in both central and northern coastal California. There is a paucity of information on growing these two crops for in situ reduction of and accumulation of Se under a reuse drainage water strategy in field conditions.

Our objective was to evaluate biomass productions and Se accumulation in canola and broccoli irrigated with saline water containing Se and B under typical field conditions in western central California.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
A 2-yr field study was conducted in 1998 and 1999 with Brassica napus var. Hyola (canola) and Brassica oleracea var. Marathon (broccoli) on two different 10-ha field sites at Red Rock Ranch (courtesy of Mr. John Diener, Five Points, CA). Both Brassica crops were selected because Bañuelos et al. (1996) have reported on the ability of this family to accumulate Se under a chloride-dominated salinity and moderately high B level. The soil in the experimental area was classified as an Oxalis silty clay loam (fine montmorillonitic, thermic Pachic Haploxeral with a well-developed salinity profile). The site was drained by subsurface 15-cm diameter plastic drains, which were installed at depths ranging from 2.5 to 3 m with lateral spacings of 200 to 300 m.

For both years, canola was directly seeded on 7.5 ha in June to a plant density of 100 000 ha^-1 on 200 m long, 1 m wide beds, and broccoli was planted as 4- to 6-wk old transplants on 2.5 ha in August to a plant density of 112 000 ha^-1. Each bed contained two planted rows spaced 0.3 m apart. Two water sources were available for irrigation—canal water from the California aqueduct and drainage water produced on Red Rock Ranch. The canal water had concentrations of Se < 0.01 mg L^-1, B < 1 mg L^-1, and a salinity (electrical conductivity) [EC] < 1 dS m ^-1 (Table 1). The drainage water was pumped from a drainage sump, and had a range of Se from 0.100 to 0.175 mg L^-1, B from 4 to 7 mg L^-1, and a sodium sulfate–dominated salinity (EC) of 5 to 8 dS m^-1 (Table 1). For irrigation with drainage water, the water was routed through a central distribution manifold. If the EC of the drainage water was greater than 6 dS m^-1, then the canal water valve was turned on to release water for limited blending. Once the EC of drainage water was about 6 dS m^-1 (determined by sampling and measurement of EC), the blended water was released for furrow irrigation on the desired field sites. Pratt and Suarez (1990) list the recommended maximum concentration of 15 trace elements in irrigation waters that provide for the long-term protection of plants and animals (FAO, 1990).

View this table: [in this window] [in a new window]   Table 1. Quality of canal and drainage water used for irrigating canola and broccoli during the 1998 and 1999 growing seasons.

Collected soil samples, free from plant residues, were thoroughly mixed and sieved with a 2-mm screen. Water-soluble Se and B, and EC were determined in a soil water extract of 1:1 (Bañuelos and Meek, 1990). The different plant organs were washed with deionized water, dried at 50°C for 7 d, and weighed and ground in a stainless steel Wiley mill equipped with a 0.83-mm screen. Plant tissues were acid digested with HNO₃–H₂O₂–HCl as described by Bañuelos and Akohoue (1994). Selenium and B in soil and plant samples were analyzed by an atomic absorption spectrophotometer (Thermo Jarrell Ash, Smith Hieftje 1000, Franklin, MA) with an automatic vapor accessory (AVA 880) and inductively coupled plasma spectrometer (Perkin Elmer Plasma 2000 Emission Spectrometer, Norwalk, CT), respectively. The National Institute of Standards and Technology (NIST) coal fly ash [Standard reference materials (SRM) 1633; Se content of 10.3 ± 0.6 mg kg^-1, with a recovery of 93%] and NIST wheat flour (SRM 1567, Se content of 1.1 ± 0.2 mg kg^-1, with a recovery of 94%) were used as external quality control standards.

A general mass balance for soluble Se applied to the soil with drainage water was created based on the following variables combined from both years to obtain a general indication of the fate of Se:

1. Estimate Se applied (Volume of drainage water applied x Average Se concentration)
   
2. Subtract Se accumulated by plants (Dry matter yield x Se concentration)
   
3. Subtract Se residing in soil to a depth of 90 cm [Differences in soluble Se concentration between postsprinkler and postharvest x Calculated soil mass (a bulk density of 1.3 g cm^-3 was determined for each 30-cm increment of soil)]
   
4. Results in estimated losses of soluble Se applied to crops with drainage water
   

Any losses of soluble Se via surface water runoff, percolation beyond 90 cm, biological volatilization, or even reduction of soluble Se to elemental Se, were not measured and thus not considered in the general mass balance. These four potential variables, in addition to knowledge of the spatial variability of soil Se under field conditions, are acknowledged as essential for creating a more accurate mass-balance expression. The Statistical Analysis System (SAS) Version 6.03 was used for the data analyses (SAS Inst., 1988), and Duncan's multiple range test was applied to treatment means at the P < 0.05 probability level (Gomez and Gomez, 1984).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Yields of Canola and Broccoli
Stand establishment for both crops was excellent; this was likely due to the canal water applied with the sprinkler at time of planting and transplanting for both canola and broccoli. Most of the plants did not exhibit typical symptoms of leaf burn/necrosis from excessive salt or B buildup in the soil. Stunted plants and necrotic leaves were, however, observed for both canola and broccoli in isolated areas near the ends of furrows used for irrigation, where there was an obvious soil surface accumulation of salts. Overall, total fresh and dry biomass yields were slightly greater with canola (Table 3). Because Se is observed to accumulate in canola leaves (Bañuelos et al., 1997), canola was grown for its vegetative biomass and not for seed. Canola, which is normally grown for seed and its subsequent oil had vegetative yields that were comparable to vegetative yields reported by Bañuelos et al. (2002) with good quality water. Compared with typical yields from broccoli grown in central California and irrigated with canal water, fresh weight floret yields were on the average 30 to 40% lower after irrigation with drainage water. On the average for both crops (except broccoli in 1999), 62% of total water applied was drainage water. Broccoli received only 50% of its total water applied from drainage water in 1999 (Table 2), and hence floret yields were also significantly higher than in 1998 (Table 3).

View this table: [in this window] [in a new window]   Table 3. Fresh and dry weights and tissue Se concentrations for canola and broccoli irrigated with drainage water during the 1998 and 1999 growing seasons.

View this table: [in this window] [in a new window]   Table 4. Changes in water soluble Se and B, salinity (EC), and pH in soils from canola and broccoli during the 1998 and 1999 growing seasons.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

In western soils of central California, a water reuse and Se management strategy with canola and broccoli is dependent on the ability of the crops to accumulate Se under increasing sulfate–salinity conditions and to tolerate increasing soil B concentrations in the soil (Rhoades, 1984). As salt and B concentrations increase above a threshold level, both growth rate and ultimate size of plants will progressively decrease (Maas and Grattan, 1999). Compared with canola, broccoli is not considered as tolerant to high salt and B levels in the soil (see Table 3.1 in Maas and Grattan, 1999). Canal water applied to canola and broccoli helped maintain the salinity and B levels to nontoxic levels within the soil profile for each growing season. Floret yields were likely higher in 1999, because 12% more canal water was applied to broccoli than in 1998. Using our present data and other salt tolerance data in conjunction with equations developed by Maas and Grattan (1999), crop yield responses for canola and broccoli can be estimated under increasing salinity and B levels in the soil. This information would be economically useful for growers who need to determine threshold yields of canola and broccoli grown under saline conditions. If yields are economically unacceptable, then leaching with canal water and rotating to more salt-tolerant plant species such as sugarbeet (Beta vulgaris L.) or barley (Hordeum vulgare L.) may be eventually be needed on sites previously irrigated with drainage water. The potential phytotoxicity of B, its relative immobility in the root zone, and boron's interaction with salinity are major limitations to water reuse programs where irrigation water contains high B at concentrations 5 mg L^-1 or greater. Careful salt management, especially of excessive B, will be essential for maintaining long-term drainage water reuse strategies on the same sites (Grattan and Rhoades, 1990; San Joaquin Valley Drainage Program, 1990; Shennan et al., 1995; Shannon et al., 1998; Mitchell et al., 2000).

A high sulfate concentration in the drainage water likely inhibited plant uptake of the analogous form of soluble selenate (Mikkelsen et al., 1989; Bell et al., 1992). Despite the relatively low plant concentration of Se (not greater than 7 mg kg^-1 DM in either crop), the general mass balance indicates that both crops extracted and accumulated up to 18% of soluble Se applied with the drainage water. Soluble soil Se concentrations increased at all depths with continued irrigation with drainage water for both crops. For this reason, it is important to consider using alternative growing sites for long-term use with Se-laden drainage water on previously irrigated sites. The planting of rainfed cover crops may be useful for extracting residual Se without applying additional Se, B, and other soluble salts to the soil. Moreover, high salinity levels in the soil will eventually restrict use of high value crops and limit the economical reuse of drainage waters.

Plant accumulation of applied Se was likely not the only means for removing Se from soils irrigated with drainage water. Losses of applied soluble Se were also noted for Se not recovered in canola and broccoli. The processes of plant and/or microbial volatilization of Se may have contributed significantly to the unaccounted losses of Se estimated in Table 5 (Terry et al., 2000). In this regard, Frankenberger and Karlson (1994) have reported volatilization of Se due to microbial activities as high as 800 µg Se m^-2 h^-1 in soil amended with citrus peel, N, and Zn, whereas Terry et al. (1992) reported that canola and broccoli can volatilize up to 2.4 mg Se kg^-1 DM d^-1 under ideal conditions. Although it is unlikely that canola and broccoli volatilized Se at such reported rates under the tested field conditions, both plants and microbial activities likely contribute to the volatilization of Se to some degree (field measurements of Se volatilization are presently being investigated by Bañuelos and Terry et al., unpublished data, 2002). The role of biological volatilization takes on more importance for removing applied Se with sulfate-rich irrigation waters, because of the competitive effect sulfate exerts on Se uptake by some plants (Zayed and Terry, 1994; Lin et al., 2000)

Phytoextraction of Se by canola and broccoli not only removes Se that has accumulated onto soils after irrigation with Se laden drainage water, but harvesting the Se-enriched crops produces products of potential economical importance for the grower. Selenium is an essential trace element for normal nutrition and health of animals, and Se deficiencies are generally a far greater problem than Se toxicities in animals in the USA (Mayland, 1994). In this regard, Bañuelos and Mayland (2000) improved the Se status of animals by carefully mixing canola used in the phytoextraction of Se with other animal feedstuffs, and feeding it to lambs (Ovis aries) and cattle (Bos taurus). Canola has long been used as a forage crop as grazing or silage (Bell, 1995); some forage canola cultivars may contain up to 22% of digestible protein, which is comparable to alfalfa. Using Se-rich vegetative canola as a blend in animal feed not only supplements the animal's diet with Se, but also provides growers with a potential disposal option for plants irrigated with Se-laden effluent. Generally, cattle and sheep may consume seleniferous plant tissues up to 5 mg kg^-1 DM without suffering from Se toxicity (Mayland et al., 1989).

The Se concentrations measured in the broccoli florets (<5 mg kg^-1 DM) were well below potentially toxic concentrations for human consumption. Because Se is also an essential trace nutrient for humans (FDA recommends approximately 200 µg Se on a daily basis), and it is known as an antioxidant and has reported anticancer activity (Clark et al., 1996), growing Se-enriched broccoli with Se-laden effluent may make this high value crop a potential source of supplemental Se for humans. Extensive research on metabolism, health, and benefits of supplemental Se supplied as Se-enriched broccoli is presently being conducted by J.W. Finley at the Grand Forks Human Nutrition Research Center in Grand Forks, ND (work in progress, 2001).

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Many growers in the San Joaquin Valley of California are reusing drainage water, at least temporarily, to reduce drainage volume and to meet recently imposed discharge restrictions related to loads of Se leaving the farms. The attenuation of Se accumulation in the soil will be a critical management component for the sustainable reuse of drainage water. The development of irrigation–drainage water reuse projects should include within the design criteria a component that considers planting moderately salt-tolerant and B-tolerant crops, e.g., canola and broccoli, to help remove Se deposited by drainage water reuse. Irrigation of canola and broccoli crops with Se-laden drainage effluent reduces amount of drainage water to dispose and simultaneously produces viable Se-enriched agronomic products. Planting broccoli would likely be a preferred choice by growers in western central California for disposal of Se-laden effluent because broccoli is more known to be a high value crop. Broccoli's survival rate and yields would, however, decrease with repeated plantings and irrigation with drainage water at the same site because of increased soil salinity and B levels. Thus, intermittent use of canal water and rotating field sites are necessary approaches to minimize yield decreases. Reclamation strategies will be necessary, however, for lowering B levels in the soil with long-term use of saline effluent.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 

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