Growth of Atriplex canescens, A. lentiformis, A. nummularia and Prosopis velutina irrigated with saline cooling tower water

ABSTRACT

As part of a revegetation effort, four-wing saltbush, Atriplex canescens, quailbush, Atriplex lentiformis, old man saltbush, Atriplex nummularia and velvet mesquite, Prosopis velutina, were irrigated with saline water generated by the cooling system of an electrical generating station in Tempe, Arizona. A. nummularia was grown in lysimeter basins while the other plants were planted directly in the soil of experimental revegetation plots. A. nummularia grew from 5 to 160.0 cm over 12 months. A. canescens grew from 47.5 to 174.5 cm over 16 months. A. lentiformis grew from 38.5 to 134.0 cm over 20 months. P. velutina grew from 126 to 221cm over 20 months.

Soil moisture levels were determined at several depths in the lysimeter basins, in two experimental plots and two control plots. The northeast experimental plot had higher soil moisture levels than other plots. The northwest control plot had lowest soil moisture levels. Southeast experimental and southwest control plots were intermediate. Northeast experimental and southwest control plots had a distinct high moisture layer that was variable between 2.8 m and 3.5m below ground surface (10 feet ).

Soil salinity in the lysimeter study increased from an electrical conductivity of 0.23 to 2.35 to 8.93 dS m-1, depending on where in the profile the soil was sampled. Soil salinity in the field study increased from 0.23 to 0.42 to 1.13 dS m-1. The study determined saline waste water from the cooling system of the generating station could be used to grow several species for revegetation.

INTRODUCTION

Several species of Atriplex, saltbushes, are used for revegetation projects in arid regions (Jusaitis and Pillman 1997; Pillman and Jusaitis 1997; Squires and Ayoub 1994; El-Din 1993; Lailhacar et al. 1989). They are well adapted to thriving in desert regions and have the added benefit of being salt-tolerant (halophytic). In western Texas, halophytes have been used for revegetation over buried drilling fluids, which have an EC of 155-185 dS/m (McFarland et al. 1992a,b).

Prosopis velutina, velvet mesquite, another plant used in revegetation projects, is not considered a halophyte, but will grow when irrigated with low salinity water (refs.) Blowdown from generating stations may be an important resource which could provide water to revegetation projects. Engel et al. (1985a,b) examined the effect of saline cooling tower water on crops and soils. Water with an EC of 2.2 dS/m was used to irrigate alfalfa (Medicago sativa) and corn (Zea mays) over three years. The crops were not adversely affected by the salinity. Salts accumulated in the soil over the growing season to 3.0 - 4.0 dS/m but precipitation between growing seasons returned soil salinity in the root zone (to 75 cm) to initial conditions each year.

Materials and Methods

Lysimeters - Atriplex nummularia were purchased from a commercial nursery and planted, October 1993, into 8 gravity drain lysimeters (1.25 m diameter, 1.36 m deep) at Arizona Public Services, Ocotillo Electric Generating Station in Tempe, Az. Lysimeters were constructed of cylindrical, cardboard forms with a 0.05 cm PVC liner. After excavation, the soil was placed inside the lysimeter, each of which was fitted with a slotted drainpipe in the bottom that collected leachate. Four lysimeters were flood irrigated with ei blowdown water from the station cooling towers while the other four were flood irrigated with storm drainage water collected in a pond. Water drained to one of two central sumps and was collected into buckets. The plants were allowed to grow for two years in the lysimeters before harvesting. A second crop was planted in December of 1995 and harvested in September 1996. The mean salinity of the pond water was 1149 mg l-1 (SE=76, N=37 blowdown water over the three years was 4100 mg l-1 (SE=155, N=45). Lysimeter basins were flood irrigated weekly from March through September, and every 2 or 3 weeks during the rest of the year. Field capacity of the lysimeter soil was determined at the start of the experiment by measuring volumetric soil moisture at 30 cm 24 h after irrigation. Values ranged from 25 to 27% among the 8 lysimeters. Before each irrigation, the neutron probe was used to measure volumetric soil moisture content at 4 depths (30,60,90 and 120 cm). If the basin was found to be less than 75% of field capacity, irrigation water would be added. The irrigation volume needed was determined by calculating the amount needed to return the basin to field capacity. Volumes required ranged from 40 to 250 l per basin.

Field plots - Atriplex canescens var. occidentalis were transplanted to an adjacent site in April 1996. These plants were placed directly into the ground. Some plants were purchased from a commercial nursery, others were taken from stocks maintained at the Environmental Research Lab of the University of Arizona. Both stocks were from the same population originally collected from Marana, Arizona. A. lentiformis and P. velutina were purchased from the same nursery and also planted in April 1996. Water from the station cooling system was used to irrigate the experimental plots from April 1996 to October 1997. Mean salinity of the blowdown water over the 18 months was 1307 mg l-1 (SE=928 , N=111). Salinity of irrigation water and leachate was determined in the field with an electrical conductivity meter calibrated with a 5 mg l-1 solution of NaCl. Field plots were irrigated on a weekly basis. Volume was ad libitum, based on the volume of water available in the storage pool. Volumes per irrigation event ranged from 533-18,655 l.

Soil samples were collected from each lysimeter at the start of the experiment and each October thereafter. Samples were collected from the field plots before planting (March 1996) and at the end of the trial (October 1997). Soil salinity was determined on a 1:1 soil:water (w/w) extract. Soil moisture was determined using a neutron hydroprobe. The hydroprobe was calibrated using gravimetric moisture determinations of soil samples taken adjacent to probe ports.

Plant heights of A. nummularia in the lysimeter basins were determined on a monthly basis. Plants in the field plots were sampled every three months.

Results

In the lysimeter basins, A. nummularia increased average plant height each month through the first year. During the second year, plant heights decreased over the winter but increased again from March to the next October when the plants were harvested. During the third year, plants increased in height from December through October (Figure 1).

In the field plots, A. canescens, A. lentiformis and P. velutina displayed similar patterns. Rapid growth would occur from March till October. Through the winter months, average plant height would regress slightly, followed by rapid growth in the spring and summer months (Figure 2).

In the lysimeter basins, soil moisture levels were continually brought back to field capacity. In the field plots, the soil rarely, if ever, reached field capacity. Irrigation rarely amounted to more than a centimeter of water on the surface. Soil moistures reached xxxxxxxx

Soil moisture levels did follow a pattern which indicates the presence of a clay layer which contributes to a high soil level at a depth of xx meters.

Table 1. A. nummularia

Table 2. Mesquite

DISCUSSION

A. canescens var. occidentalis from Marana was compared to fifteen other accessions in a common garden trial at an electrical generating station near Farmington, New Mexico.

At the New Mexico location this variety grew from 25.4 cm to 62.8 cm over 16 months. At the Ocotillo station var. occidentalis grew from 47.5cm to 174.5 cm over 16 months. Experimental plots in New Mexico were more heavily irrigated , 1.3 m3 water m-2, compared to 0.7 m3 water m-2, at the Ocotillo site.

At the New Mexico site TDS average was 7200 mg/L. At Ocotillo it was 1307 mg/L TDS.

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