INTRODUCTION
The environmental problems emphasize the urgent need to evaluate the
available natural resources and their requirement by a rapidly growing
human population (Hardin, 1993). All the basic natural resources including
the land, water, energy and biota have been inherently limited for human
use (Lubchenco, 1998). As the human population continues to grow, the
water is being divided amongst the increasing number of people.
The rapid population growth in many municipalities in the arid eastern
Iran continues to place increasing demands on limited fresh water supplies.
Many cities and districts are struggling to balance water use among municipal,
industrial, agricultural and recreational users. Treated or recycled wastewater
appears to be the only water resource that is increasing as other sources
are dwindling. Use of Recycled Waste Water (RWW) for irrigating landscapes
is often viewed as one of the approaches to maximize the existing water
resources. Often, recycled wastewater contains different levels of dissolved
solids, nutrients (N and P) and other elements. Nitrogen, P and K are
three important elements in maintaining a healthy turf stand with N producing
the greatest growth response.
Research done in the central Iran has indicated that dense, well-managed
turf grass areas are among the best bio-filtration systems available for
removal of excess nutrients and further reclamation of recycled wastewater.
Pasture was the earliest and currently the leading urban landscape users
of RWW in southeastern Iran. Recently, this reuse practice has been extended
to include some of the large area, open spaces and greenbelts. There is
limited information available in the arid eastern Iran concerning effects
of irrigation with RWW on soil chemical characteristics. Most research
addressing this issue has been conducted in the center Iran where the
soil type and climate conditions are quite different from Sistan region.
Research is needed to examine the impact of different term RWW irrigation
on soil chemical properties in hot regions. In this study, we examine
the soil chemical properties of 5 part of Sistan that have been irrigated
with RWW for 3 to 14 year, in comparison with 5 pasture with similar age
ranges, soil texture, landscape management regimes and plant species,
but use surface water for irrigation and assess changes in soil chemical
properties after 3 to 5 year of RWW irrigation on two selected part of
pasture.
MATERIALS AND METHODS
Study sites: This research study was conducted from 21st October,
2003 to 03 December, 2007 (but some data available from 1990) in Sistan,
Iran. Ten sites were selected from Sistan region pasture, for the study.
Sistan has an arid climate. The average annual precipitation is 6 and
7 cm in East and West of the Sistan, respectively. The main soil series
and surface texture for the 10 sites were obtained with the assistance
of the Natural Resources Center (NRC) of the Sistan (Table
1). All of the pasture was in a hyperthermic soil temperature regime.
Among these pasture, five had been irrigated with RWW for 3, 5, 8, 11
and 14 year, respectively, in 2005 (Table 1). On all
recycled wastewater irrigation sites, RWW from wastewater treatment plants
are stored in irrigation ponds and used exclusively as the irrigation
source. Turf grass grown on pasture were perennial ryegrass (Lolium
perenne L.), Kentucky blue-grass (Poa pratensis L.), Cyperus
sp., Cynodon dactylon pasture received approximately 55 cm of RWW.
| Table 1: |
Age, years of recycled wastewater irrigation (RWI) and
surface texture soil of pasture selected for the study to evaluate
Recycled Waste Water (RWW) irrigation on soil properties and landscape
plant health |
 |
| Table 2: |
Average water quality values of ditch water and Recycled
Waste Water (RWW) from advanced wastewater treatment plants in Sistan |
 |
Five parts of pasture with similar ranges in age, soil texture, landscape
management regimes and plant species, but irrigated with surface water
were selected as controls (Table 1). Most of the surface
water comes from Helmand River good quality (Table 2).
The Helmand River rises in the mountainous region of north Afghanistan
and flows in a westerly direction towards Iran-Afghanistan border to meet
the Hamoun lakes. Pasture received approximately 70 cm of irrigation water
annually. The average water quality values of water and recycled wastewater
used in the 10 selected parts of pasture are presented in Table
2.
Sampling from study area: A total of 120 soil samples (60 samples
were from pasture with RWW irrigation and 60 were from pasture with surface
water irrigation) were collected to a depth of 15 cm from these pasture
points in 2004-2005 to test soil chemical properties. This sampling depth
is very common in pasture where turf is mowed to less than 3.5 cm height
and majority of turfgrass roots are concentrated in the surface 10 cm.
Soil samples were tested by Zabol University Laboratory. Parameters of
each soil sample tested included pH; extractable salt content (Ca, Mg,
K, Na, Fe, Mn, Cu, Zn, P and B); base saturation percent of Ca, Mg, K
and Na; Soil Organic Matter (SOM) content and Cation Exchange Capacity
(CEC). Zabol University Laboratory soil-testing lab provided information
on analytical methods. Soil pH was analyzed using a saturated paste extract.
Sieved soil samples were extracted using the Mehlich III extractant (0.015
M NH4F+0.20 M CH3COOH+0.25 M NH4NO3+0.013
M HNO3+0.0005 M EDTA chelating agent) to determine Ca, Mg,
K, Na, Fe, Mn, Cu, Zn, B and P by inductively coupled plasma-emission
spectrophotometry instrumentation. Mehlich III extracted Ca, Mg, K and
Na plus soil buffer pH data are used to calculate CEC. Base saturation
percent of Ca, Mg, K and Na was calculated by dividing the extracted Ca,
Mg, K and Na by the calculated CEC, respectively. Base saturation percent
of Na is considered the Exchangeable Sodium Percentage (ESP). Soil organic
matter was determined by reaction with Cr2O72–
and sulfuric acid. The remaining unreacted Cr2O72–
is titrated with FeSO4 using ortho-phenanthroline as an indicator
and oxidizable organic matter was calculated by the difference in Cr2O72–
before and after reaction (Nelson and Sommers, 1982). In 2006, three additional
soil samples from each site were collected to measure soil EC and SAR
of saturation paste in the Soil, Plant and Water Analytical Lab at Zabol
University. Electrical conductivity of soil saturation paste extract was
determined with a conductivity meter. Cation (Ca, Mg and Na) concentrations
of saturation paste extracts were analyzed by inductively coupled plasma-emission
spectrophotometry instrumentation and SAR was calculated.
Soil tests before and 3 or 5 years after recycled wastewater irrigation:
At pasture I and pasture II, soil samples were collected before (1998
and 1990, respectively) and 3 or 5 year after the commencement of recycled
wastewater for irrigation (2005 and 1995, respectively). Soil sampling
depth and soil analysis protocols were the same as previously described.
The before and after comparisons provide indications about the impacts
of RWW irrigation on soil chemical properties. However, one of the disadvantages
of the before and after comparison of soil chemical properties is the
inability to separate the effects of RWW constituents and the effects
of irrigation water itself.
Data analysis: Data were subjected to analysis of variance (SAS,
1991) to test the effect of irrigation water source on individual soil
chemical characteristics. Significant differences in soil chemical properties
before and 3 or 5 year after RWW irrigation were also determined using
an analysis of variance (p<0.05).
RESULTS AND DISCUSSION
Comparison of reuse sites surface water irrigated sites: Sites
irrigated with RWW exhibited an average soil salinity 4.7 mhos cm-1
that was 212% higher than site irrigated with surface water EC 1.7
mmhos cm-1 (Table 3).Variations in increase
in EC under RWW irrigation appeared to relate to soil texture and drainage
effectiveness. Previously, Qian et al. (2001) reported that the
salinity levels that caused 25% shoot growth reduction were 3.2 (mmhos
cm-1) for a salt-sensitive Kentucky bluegrass cultivar and
4.7 mhos cm-1 for a salt-tolerant Kentucky bluegrass cultivar.
It is apparent that the salinity build-up in sites irrigated with RWW
would result in growth reduction of salt sensitive Kentucky bluegrass
cultivars that may slow the recovery of erosion from over grazing and/or
other biotic and abiotic stresses. We have observed salinity stress for
sites with long-term RWW irrigation, especially for sites with fine soil
texture and poor drainage. Several sites on IV of the pasture, which had
been irrigated with RWW for 14 year, were replaced by more salt-tolerant
grass, such as alkaligrass Puccinellia distans, Aeluropus sp.,
Jgphacea sp., Typha sp. and Phragmites communis. Soils
from sites with RWW for irrigation exhibited 188% (279 mg kg-1)
higher concentration of extractable Na and 28% higher concentration of
extractable Ca than sites irrigated with surface water (Table
3). The high Na content reflected the greater than six fold increase
in Na via RWW. The average Na concentration of over 35 RWW samples collected
was 102 mg L-1, ranging from 32 to 180 mg L-1 (Table
2). Runoff of P was likely to be minimal from turf sites due to the
dense vegetation cover that could effectively prevent P runoff. Soil pH
was higher (approximately 0.5 units) in RWW-irrigated sites than in the
control sites. Increases in soil pH under land application of wastewater
have been previously reported by Schipper et al. (1996), Pepper
and Mancino (1992) and Qian and Mecham (2005). In New Zealand, Schipper
et al. (1996) found an increase in soil pH by 0.8 units after
applying tertiary-treated domestic wastewater to a forest site for 3 year
at 4.9 cm week-1. In addition, in Colorado, Qian and Mecham
(2005) found an increase in soil pH by 0.3 units after applying tertiary-treated
domestic wastewater to a Golf Course site for 5 year in via RWW. The researcher
suggested that the rise in soil pH was likely related to a high rate of
denitrification that produced hydroxyl ions. Pepper and Mancino (1992)
found that effluent water irrigation increased soil pH by 0.1 to 0.2 units
when compared with potable water irrigation. The soil pH increase in present
study likely resulted from the 0.5 unit higher pH and higher bicarbonate
concentration in RWW than surface water. The average bicarbonate concentration
in the RWW was 118 mg L-1. The small magnitude of increase
in soil pH in this study suggests the effectiveness of management in controlling
soil pH.
| Table 3: |
Mean soil chemical properties pasture courses with recycled
wastewater irrigation vs. surface water irrigation |
 |
*Significantly different from surface water-irrigated
sites at p ≤ 0.005.
**Significantly different from surface water-irrigated sites at p
≤ 0.05.
***Significantly different from surface water-irrigated sites at p
< 0.001 |
Soil B content was about 34% higher in the RWW-irrigated sites than
in surface water-irrigated sites. Although the average B concentration
in the RWW was only 0.16 mg L-1, lower than the permissible
limits for the allowable concentration of B in irrigation water presented
by Van der Leeden et al. (1990), we consistently observed an increase
in B content in the soil. Likely the accumulation of B was associated
with the borate adsorption by soil. With increasing soil pH, B adsorption
by soil would increase, reaching the maximum B adsorption by soil at a
pH of 9 (Ayers and Westcot, 1985).
Despite the fact that Mg content was two fold higher in RWW than surface
water (Table 2), soil Mg content was 18% lower in RWW-irrigated
sites than the control sites (Table 3). The cation exchange
site occupied by Mg was reduced, reflecting the replacement of this element
with Na.
The ESP and SAR for RWW irrigated sites was 224 and 638% higher than
the surface water-irrigated soil, respectively (Table 3).
Soil ESP and SAR would have continued to increase without the regular
amendment of Ca products. In soil collected from the rough at pasture
II that was not amended with Ca products, the ESP rose to as high as 17.0.
Although the ESP and SAR values on pool are not high enough to be classified
as a sodic soil, Halliwell et al. (2001) stated that the dispersion
and deflocculation effects of sodicity might be evident in soils that
are well below reported threshold values. Long-term uses of RWW with marginal
high SAR may result in reductions of soil infiltration and
permeability in clayey soils and for sites with high traffic and compaction
pressure. Further research is needed to monitor soil hydraulic properties
for sites irrigated with RWW. Our results indicated predominant differences
in soil SAR; EC; ESP; extractable soil Na, Ca, P, B and Mg concentration;
and soil pH between RWW-irrigated and surface water-irrigated sites (p<0.05).
Differences in CEC, SOM and K content between the two types of irrigation
sites were not significant.
Soil analysis at two parts of pasture before and 3 to 5 years after
recycled wastewater irrigation: At pasture I, we observed an increase
of Na by 372 mg kg-1 (135%) after 3 year of irrigation with
RWW (Table 4). This is higher than the Na increase observed
in a sandy loam soil at pasture II. Pepper and Mancino (1992) reported
an increase of water-extractable Na content by 155 mg kg-1 after
3.2 year of using RWW in a bermudagrass [Cynodon dactylon (L.)
Pers.] fairway in Arizona. The increase in soil Zn (by 82%), B (by 55%)
and P (by 88%) content after 3 year of RWW irrigation were also significant
(Table 4). The increased Na, Zn, B and P in the soil
solution reflected the characteristics of RWW. We did not observe significant
differences in soil pH or K content. The addition of an S burner in the
irrigation system appeared to effectively prevent the increase of soil
pH by RWW for irrigation.
At pasture II, soil Na, Mg, Mn, P, B and K content were increased by
192, 137, 13, 43, 0.3 and 262 mg kg-1, respectively, after
5 year irrigation with RWW (Table 5). These corresponded
to 85, 42, 20, 116, 13 and 80% increases. We also observed an increased
SOM after 5 year And of RWW irrigation, which likely resulted from input
of organic C from turfgrass roots.
| Table 4: |
Mean soil chemical properties from pasture No. 1 before
and 3 year after recycled wastewater irrigation |
 |
*Significantly different from surface water-irrigated
sites at p ≤ 0.005.
**Significantly different from surface water-irrigated sites at p
≤ 0.05.
***Significantly different from surface water-irrigated sites at p<0.001 |
| Table 5: |
Mean soil chemical properties from pasture No. II before
and 5 year after recycled wastewater irrigation |
 |
*Significantly different from surface water-irrigated
sites at p ≤ 0.005.
**Significantly different from surface water-irrigated sites at p
≤ 0.05.
***Significantly different from surface water-irrigated sites at p<0.001 |
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