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Published online 1 May 2008
Published in J Environ Qual 37:1288-1298 (2008)
DOI: 10.2134/jeq2007.0414
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Leachability and Phytoavailability of Nitrogen, Phosphorus, and Potassium from Different Bio-composts under Chloride- and Sulfate-Dominated Irrigation Water

Zahoor Ahmada,*, Sadahiro Yamamotob and Toshimasa Honnab

a Dep. of Bio-environment Science, The United Graduate School of Agricultural Sciences, Tottori Univ., Tottori. Japan
b Lab. of Soil Science, Faculty of Agriculture, Tottori Univ., Tottori, Japan

* Corresponding author (zahoor112{at}hotmail.com).

Received for publication August 6, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Concerns over increased phosphorus (P) application with nitrogen (N)-based compost application have shifted the trend to P-based composed application, but focusing on one or two nutritional elements does not serve the goals of sustainable agriculture. The need to understand the nutrient release and uptake from different composts has been further aggravated by the use of saline irrigation water in the recent scenario of fresh water shortage. Therefore, we evaluated the leachability and phytoavailability of P, N, and K from a sandy loam soil amended with animal, poultry, and sludge composts when applied on a total P–equivalent basis (200 kg ha–1) under Cl– (NaCl)- and SO42– (Na2SO4)-dominated irrigation water. Our results showed that the concentration of dissolved reactive P (DRP) was higher in leachates under SO42– than Cl– treatments. Compost amendments differed for DRP leaching in the following pattern: sludge > animal > poultry > control. Maize (Zea mays L.) growth and P uptake were severely suppressed under Cl– irrigation compared with SO42– and non-saline treatments. All composts were applied on a total P–equivalent basis, but maximum plant (shoot + root) P uptake was observed under sludge compost amendment (73.4 mg DW–1), followed by poultry (39.3 mg DW–1), animal (15.0 mg DW–1), and control (1.2 mg DW–1) treatment. Results of this study reveal that irrigation water dominated by SO42– has greater ability to replace/leach P, other anions (NO3–), and cations (K+). Variability in P release from different bio-composts applied on a total P–equivalent basis suggested that P availability is highly dependent on compost source.

Abbreviations: DRP, dissolved reactive phosphorus • DW, dry weight • EC, electrical conductivity


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
NITROGEN (N), phosphorus (P), and potassium (K) are not only important in agriculture, but these nutrients also play a vital role in aquaculture from an environmental point of view. Continuous application of different bio-wastes in agricultural fields has increased the concentration of N and P in many freshwater bodies, creating severe environmental concerns. Nitrogen accounts for about 80% of the total mineral nutrients absorbed by plants (Marschner, 1995). To meet crop N demand, soil N concentration is maintained at levels greater than those required (Wilkinson et al., 1999). Simultaneously, N is one of the most mobile plant nutrients in soil, and it has been reported that diffuse N pollution from agricultural land is the major source of N load to surface waters and ground water in many regions (Kronvang et al., 1996; Stålnacke, 1996). Because of the mobility of N, the availability from organic sources must be known for efficient management of N inputs. It is difficult to predict the pattern and amount of available N from organic sources during the growing season because these are influenced by biological decomposition, chemical composition, and climate. In some cases, the abundance of N from amendments has resulted in the implementation of P-based application rates. However, little attention has been paid to the availability of N when compost is applied based on P.

Phosphorus desorption is of great interest from the standpoint of plant nutrition and water quality. Because P is considered to be relatively immobile in the soil system (Johnson et al., 1997), less attention has been paid to P subsurface movement/leaching. The movement of P through surface runoff can be controlled via implementation of conservation and nutrient management practices. However, the control of P contamination through subsurface flow is effective only if P concentrations in soil are reduced to levels that are not considered to be a risk for local water bodies. Several studies have shown that organic materials and their decomposition can reduce P fixation in soils (Iyamuremye et al., 1996; Kwabiah et al., 2003). From an environmental standpoint, reduced P fixation increases the P concentration in soil solutions, which increases the chance of P mobility to the surface or subsurface. Therefore, the application of organic and artificial fertilizer should use a budget approach to prevent environmental pollution but also increase the crop production.

The agricultural sector is estimated to be responsible for two thirds of global water withdrawals, accounting for 90% of total water consumption (Shiklomanov, 2007). In the scenario of fresh water shortage, irrigation with saline water is one of the options to meet crop water demands. There are many different situations where saline water is being used for irrigation. One situation is where high-quality water is available during the early growing season but is either too costly or too limited in supply to meet the entire season's requirements. This situation is common in many parts of India and Pakistan. Rhoades (1988) has also demonstrated a strategy that uses saline (electrical conductivity of water [ECw] {approx} 4.0 dS m–1) and low-saline (ECw {approx} 1.25 dS m–1) water in rotation and found no reduction in crop yields. Application of organic fertilizers to soil is a common practice, especially in saline environments, due to the ameliorative effects of organic matter on soil conditions and plant growth. Many studies have focused on the effect of saline irrigation on plant growth, the response of plants to different organic materials, and nutrient leaching under fresh water irrigation, but our knowledge about nutrient leaching and phytoavailability from different compost amendments under SO42–– or Cl––dominated irrigation water is scarce. As some studies have shown, increased inputs of surface water high in chloride or sulfate ion concentrations have increased the availability of nutrients, which is a major cause of increases in "internal eutrophication" (Beltman et al., 2000; Beltman et al., 2005). Therefore, further studies are needed to address the phenomenon of increased P desorption/release from soil particles when they are exposed to water dominated by SO42– or Cl–. A large number of salinity studies on agronomic crops used NaCl as the sole salinizing agent. Likewise, the majority of salinity studies have used Cl– as the sole salinizing anion, yet most soil solutions contain a substantial amount of SO42– and HCO3– (Grattan and Grieve, 1992). The current study was designed with the following objectives: (i) to evaluate the phytoavailability of P, N, and K from different bio-composts applied on a total P–equivalent basis, (ii) to evaluate NPK leachability under saline water irrigation (iii), and to evaluate the comparative effect of Cl– and SO42– saline water on phytoavailability of NPK from different bio-composts. We hypothesized that NPK release and uptake from different bio-composts are influenced by water dominated by SO42– or Cl–.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Soil and Compost Analysis
Masa soil was used as a test soil in this study. According to the United Soil Classification System of Japan (2002), the soil is Terrestrial Regosol. The physicochemical properties of the soil were determined by using standard laboratory procedures (Table 1 ). The soil was air dried and passed through a 4-mm sieve. Soil texture was determined by the pipette method (Gee and Bauder, 1986). Soil electrical conductivity (EC) and pH were measured in soil–water (1:5; w/v) suspensions. Total C and total N were determined by the dry combustion method using a SumiGraph NCH-21 analyzer (Model MT 700; Sumika Chemical Analysis Services, Tokyo, Japan). Soil samples were digested in a nitric–perchloric acid (5:1) mixture to measure the total elemental concentration of P (IBSRAM, 1994). Phosphorus contents in the acid-digest were measured colorimetrically by using the sulfo-molybdo-phosphate blue color method (Murphy and Riley, 1962) on a spectrophotometer (Model U 2001; Hitachi Corp., Tokyo, Japan) at 710 nm.


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  		Table 1. Selected characteristics of the soil and bio-composts used for the study.

 
Three bio-composts were used in this study: animal (cow) compost, composted poultry droppings, and sludge compost. Composted poultry droppings and animal compost were collected from the feedlot yard of Tottori University, and a commercial composted sludge was used. All composts were air dried, crushed, and passed through a 0.5-mm sieve. Total C, total N, EC, and pH of the compost samples were measured by the same procedures used for soil sample analysis (Table 1). To measure the total elemental concentration of P, Ca, K, Fe, and Al in the composts, samples were digested in a nitric-perchloric acid mixture (5:1). Phosphorus concentration was measured colorimetrically, and total elemental concentrations of cations (K, Ca, Al, and Fe) were recorded on a Polarized Zeeman Atomic Absorption spectrophotometer (Model Z-2300; Hitachi).

Salinity and Compost Treatments
Three and half kilograms of each soil sample were weighed into Wagner pots (height 30 cm, diameter 18 cm). Before putting in the soil, pots were bedded with an equal amount of gravel to avoid blockage of the drainage hole. Animal compost (66.6 ton ha–1), composted poultry droppings (33.3 ton ha–1), and composted (sewage) sludge (24.1 ton ha–1) were applied on a total P–equivalent basis at the rate of 200 kg ha–1 each along with unamended soil (control). Each compost sample was mixed well with the soil, and the weight of each pot was recorded after putting in the amended soil.

Artificial saline water for irrigation was prepared by dissolving two salts (NaCl [Cl–] and Na2SO4 [SO42–]) each at the rate of 60 mmolc L–1 along with the non-saline control (deionized water). Electrical conductivity of the irrigation was recorded before application (NaCl = 6.51 ± 0.1 dS m–1; Na2SO4 = 5.75 ± 0.1 dS m–1). Irrigation was performed by the weighing method. Two pots from each treatment were weighed on a daily basis to determine the water loss. The required amount of irrigation water was calculated from the difference between pot capacity plus leaching requirement (25% of applied water) and the actual weight of each individual treatment. After each irrigation event, the leachate was collected for the entire 24-h period and immediately transferred to the laboratory for chemical analysis. The amount of water leached out from each treatment was recorded. A total of nine leaching events were observed during the growth span of maize crop. Leachates were collected after 8, 12, 18, 22, 26, 29, 32, 38, and 47 d after sowing, which are referred to as leaching event 1, 2, 3, 4, 5, 6, 7, 8, and 9, respectively. To measure dissolved reactive P (DRP), leachate was passed through a cellulose acetate membrane filter (0.2 µm). Thereafter, the concentration of DRP in the filtrate was measured colorimetrically. The concentration of NH4–N in leachate was measured by using the indophenol blue color method, and NO3–N was measured using the salicylic acid–sulfuric acid method (IBSRAM, 1994). Concentrations were recorded on a spectrophotometer. Potassium concentration in leachate was recorded on an Atomic Absorption spectrophotometer.

Pots were arranged in randomized complete design on greenhouse benches under natural light and temperature conditions. Pots were reshuffled after every 48 h to avoid micro-climatic effects. Four composts and three saline irrigation treatments in triplicate gave a total of 36 treatments. Eight maize (Zea mays L., variety: Yellow dento) seeds were sown (May 2005) in each pot and thinned to four plants after germination. Pots were irrigated with deionized water (without causing the leaching of water) during the first week of sowing to encourage seed germination. Maize plants were harvested at two growth stages: The first harvest (two plants) was taken after the 3 wk of saline irrigation, and the second harvest (two plants) was taken after 6 wk of saline irrigation. Roots were collected after the second harvest. After the harvest, plant tissues were rinsed with distilled water and separated into shoot (stem + leaves) and roots. Plant samples were oven dried at 65°C for 48 h, and dry matter yield was recorded. Thereafter, plant samples were crushed into powder and digested in a nitric–perchloric acid (5:1) mixture for total P and K analysis (IBSRAM, 1994). Nitrogen contents were recorded on an NCHS analyzer (Elementar Vario EL-III; Elementar Analysensysteme, Hanau, Germany). Nutrient uptake was calculated by multiplying the concentration by the total root or shoot dry weight of each pot (mg DW–1). Apparent recovery percentage of nutrients was calculated by using following formula:

Formula

Statistical Analysis
The data collected during the study were statistically analyzed using StatView software (SAS, 1999). A probability level of <0.05 was considered significant, and means were separated by Fisher's LSD test.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
EC and pH of the Leachate
A significant variation in EC and pH was observed in the water leached out from different bio-composts under saline irrigation (Table 2 ). Electric conductivity (dS m–1) of leachate increased steadily under all compost treatments except in the animal compost amendment, in which the increase was abrupt at first and then became steady (Fig. 1 ). Irrespective of the compost amendments, the EC of leachates increased with both saline irrigations as compared with the non-saline treatment, and the increase in EC was higher in NaCl- than in Na2SO4–irrigated pots. The pH of leachates varied greatly under all compost amendments. The pH of leachates collected from the animal compost amendment increased with saline irrigations, whereas pH decreased in sludge amended pots with non-saline irrigation. However, the pH of water collected under the poultry compost treatment remained around neutral (pH 7). Similar to compost types, pH values decreased under saline irrigation. However, the decrease in pH was greater under both saline irrigations as compared with the non-saline treatment (Fig. 1).


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  		Table 2. Summary of two-way ANOVA for nutrient uptake.

 

Figure 1
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  		Fig. 1. Effect of compost types and saline irrigation on electrical conductivity and pH of leachate. Leaching events 1, 2, 3, 4, 5, 6, 7, 8, 9 refer to 8, 12, 18, 22, 26, 29, 32, 38, and 47 d after sowing, respectively (LSD at P = 0.05; n = 3).

 
Lower EC values in leachate from early irrigation events might result from the retention of dissolved salts in irrigation water, which are subsequently transported by later irrigation events. Noble et al. (1987) has also reported that the EC of soil increased with an increase in salinity level of irrigation water over time. Increase in EC under animal compost amendment might be due to the presence of salts in the compost (Alva et al., 1999) and the mineralization of different ions after compost application. An abrupt increase in EC under animal compost treatment (Fig. 1) suggests that soils amended with animal composts can easily enhance the soil salinity when exposed to impaired irrigation water. Electric conductivity of leachate decreased from sludge compost–amended pots. Paramasivam et al. (2006) has also reported that the EC of leachate collected from different sludge-amended soils decreased or first increased and then decreased to a steady level.

The pH of leachate increased under the animal compost treatment, decreased in non-saline and sludge compost treatments, and remained around neutral in poultry compost–amended treatments. An increase in soil pH with animal manure application has been reported in some studies. Bickelhaupt (1989) found that the application of composted lime-treated horse manure to a slightly acidic soil (pH 5.7) increased the soil pH to 6.7 and 7.3, and the effect was undiminished even after 12 yr of manure application. Similarly, chicken manure has been reported to raise soil pH (Hue, 1992). Higher pH under animal compost treatment might be partially due to buffering from carbonates because much of the CaCO3 is added to cattle diets. The compounds like organic acids with carboxyl and phenolic hydroxyl groups also have an important role in buffering soil acidity and increasing the pH of acid soils amended with manure (Whalen et al., 2000). Sui et al. (1999) observed a decrease in pH from the soil amended with two increasing rates of biosolids, and the decrease in pH was greater with an increase in the bio-solid application rate. Bajwa et al. (1986) has reported that the use of saline water increased the pH, electric conductivity of the soil saturated extract, and sodium absorption ratio of the saturation extract in the entire soil profile as compared with non-saline treatment.

Dry Matter Yield
The effect of saline irrigation and compost type on maize dry matter yield is shown in Fig. 2 . Saline irrigation reduced plant growth under all compost amendments. Among salt types, Cl––dominated irrigation water severely reduced plant growth and development compared with SO42– irrigation. A similar pattern of plant growth was observed in all plant tissues (shoot and roots) in both harvests. However, compared with SO42–, reduction in shoot growth under Cl– irrigation was higher in the second harvest (20.1%) than in the first harvest (12.7%). Plant growth varied greatly under all compost amendments (Fig. 2). Pots amended with sludge compost resulted in the highest shoot and root biomass.


Figure 2
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  		Fig. 2. Effect of compost types and saline irrigation on maize dry matter yield. Small bars refer to SE. DW, total shoot or root dry weight per pot.

 
Plant growth was suppressed by saline irrigation. The adverse effect of salinity on plant growth and development has been reported by many researchers (Mer et al., 2000; Ramoliya and Pandey, 2003). The mechanism of this effect involves reduced water absorption, reduced metabolic activities as a result of salt toxicity, and nutrient imbalances (Yeo, 1983). At the same time, increased Na concentrations in rooting media can impair plant growth and nutrient uptake. This damage has been attributed mainly to excessive accumulation of Na+ and Cl– in plant tissues, causing nutritional imbalances (Zekri and Parsons, 1992). Similarly, uptake of major nutrients, sch as N (Aslam et al., 1984), P, and K, decreased under Na salts (Adams, 1991; Sonneveld and De Kreij, 1999), which affects plant growth. The adverse effect of SO42– salt on the plants was less apparent than Cl– because sulfur is considered an essential plant nutrient. Our previous study (Zahoor et al., 2007) also showed that wheat (Triticum aestivum L.) growth was more affected by Cl–– than SO42––saturated soil. Similarly, Manchanda et al. (1982) found that plants treated with high levels of SO42– yielded 3 to 6 times more dry matter of wheat and barley than with Cl– treatment.

Plant growth under different bio-compost amendments also differed greatly. Differences in plant growth under different bio-composts can be attributed to the different nature and nutritional status of composts. Nutrient availability from a particular organic source is dependent on many factors, such as type or source of compost, method of composting, N and P mineralization rate, plant type, and clay content of the soil media. Salinity complicates the influence of these factors. Plant nutrition and salinity has been reported as a complex phenomenon, and the form in which fertilizer is applied to salt-stressed plants may influence this salinity–nutrient relationship (Lewis et al., 1989; Martinez and Cerda, 1989), which ultimately affects plant growth.

Nutrient Uptake by Maize
Plant samples were analyzed for NPK content at the final harvest. All the parameters were significantly affected by compost type and saline irrigation (Table 2). A diverse behavior of N uptake by plant tissues was observed under saline irrigation and compost amendments (Fig. 3 ). Irrespective of compost amendments, N uptake by shoot increased more under both saline irrigations than with non-saline treatments, whereas N uptake by roots decreased (Fig. 3). Among salt types, mean N uptake by shoot was higher in Cl– (4.2%) than in SO42– saline irrigation, whereas N uptake increased in roots under SO42–– (22.6%) compared with Cl––dominated saline irrigation. Nitrogen recovery was relatively higher under both saline irrigations as compared with non-saline treatment. Nitrogen recovery and uptake by plant tissues varied significantly (p < 0.05) under different compost amendments (Table 2). Irrespective of saline irrigation, maximum N uptake by shoot and roots was recorded under sludge compost–amended pots, followed by poultry, animal, and control treatment. Similarly, N recovery was higher under sludge-amended pots.


Figure 3
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  		Fig. 3. Effect of saline irrigation and compost amendments on nutrient uptake and recovery. Small bars refer to SE.

 
Salt types significantly affected the shoot P uptake and recovery by maize tissues under all compost amendments (Fig. 3). Irrespective of compost type, P uptake by shoots and roots was higher under SO42– saline irrigation as compared with Cl– and non-saline treatments. Similarly, P recovery also increased with SO42– saline irrigation under all compost amendments. Phosphorus was applied on a total P–equivalent basis, but P uptake varied significantly (p < 0.05) under different compost amendments (Table 2). Higher P uptake by shoot was observed under sludge compost–amended pots, whereas root P uptake was higher under poultry compost–amended treatments (Fig. 3). Mean P recovery for compost types varied in the following order: sludge (20.6%) > poultry (10.9%) > animal compost (4.0%).

Potassium recovery and uptake by plant tissues was significantly affected by both salt types and compost amendments (Table 2). Potassium uptake by shoots and roots decreased with both saline irrigations as compared with non-saline treatment under all compost amendments (Fig. 3). Shoot and root K uptake decreased under Cl– than SO42– treatment, but the difference was not significant statistically. Mean K recovery was higher under non-saline treatment (43.4%), followed by SO42– (40.3%) and Cl– (30.9%) treatments. Among the compost amendments, K uptake by shoots and roots was high under poultry and sludge compost–amended pots as compared with animal compost and unamended treatments. Potassium uptake by shoot increased in sludge-amended pots with non-saline and SO42– saline irrigation, whereas root K uptake was higher in poultry compost with non-saline irrigation (Fig. 3). Analogously, mean K recovery from different compost amendments varied in the following order: sludge (75.6%) > poultry (28.0%) > animal (10%).

The influence of salinity on nutrient accumulation in plants is variable and depends on many plant and experimental conditions (Irshad et al., 2002). Nitrogen uptake by shoot was relatively higher under Cl– than with the SO42– treatment. Pessarakli and Tucker (1985) also reported significantly higher N concentration in cotton under NaCl stress.

Contrary to N uptake by stem, mean P uptake under SO42– salinity increased by 77.4% in shoot and 62.7% in roots as compared with Cl– treatment. Current results showed that plants vary not only in the rate by which they absorb an available nutrient but also in the manner in which they spatially distribute elements within the plant tissues. Higher P uptake and recovery under SO42––saturated pots compared with Cl– has also been reported by Zahoor et al. (2007). Manchanda et al. (1982) reported that P availability and its absorption were more adversely affected by excess Cl– than SO42– salt. Ravikovitch and Yoles (1971) observed that increasing NaCl salinity suppressed the P content and yield of clover, which markedly improved with an increase in P contents of soil. Some researchers have also reported an antagonism between Cl– and P absorption in tomato plants (Cerda and Bingham, 1978).

Potassium uptake by shoot and roots decreased more under saline irrigation than non-saline treatment. Previous studies have also shown that K uptake decreased with an increase in salt concentration in rood media (Khan et al., 1995; Irshad et al., 2002). The potassium content of chickpea (Manchanda et al., 1991) and pea (Mor and Manchanda, 1992) decreased significantly regardless of salinity treatments; shoots grown under the SO42– system contained more K than Cl–. Similarly, Meiri et al. (1971) observed that K concentrations in bean leaf sap increased with increasing salinity and that the increase was more marked in SO42– than Cl– salinity.

Nutrient uptake and recovery greatly differed according to the compost type. Among the compost types, overall NPK uptake and plant growth were higher under sludge compost, followed by poultry and animal compost. Variability in nutrients from different organic materials has been reported by many researchers, and many factors have been assumed to be responsible for these differences. Eneji et al. (2001) reported that not only manure type but also soil type can affect the efficiency of manure. In addition to the soil type, the methods used to produce biosolids and the C/N ratios of biosolids affect the mineralization of N (Barbarika et al., 1985; Douglas and Magdoff, 1991) and P (McCoy et al., 1986). However, relatively low crop growth and nutrient uptake by animal compost might be due to the abrupt increase in soil pH due to salt accumulation during the early growth stages of plants. Plant response to salt types differed under different compost types; in particular, the response of non-saline treatment was inconsistent under different compost treatments. In fact, plant nutrients and salinity is a very complex phenomenon, and the form in which fertilizer is applied to salt-stressed plants may influence this salinity–nutrient relationship (Lewis et al., 1989; Martinez and Cerda, 1989).

Effect of Saline Irrigation on Nutrient Leaching
The concentration of nitrate nitrogen (NO3–N) and ammonia nitrogen (NH4–N) in leachate varied greatly with each saline irrigation event (Table 3 ). The concentration of NO3–N and NH4–N in leachate decreased steadily, but after the fifth leaching event, only trace amounts of NH4–N were detected in leachates (Fig. 4 ). It was observed that NO3–N concentration in leachates was higher in SO42––dominated irrigation water as compared with Cl– treatment (Fig. 4a), whereas NH4–N leaching increased more under Cl– than under SO42– saline irrigation (Fig. 4b). Irrespective of saline irrigation, NO3–N and NH4–N concentrations in leachate were higher from animal and sludge compost amended pots, respectively.


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  		Table 3. Summary of two-way ANOVA for electrical conductivity, pH, and nutrient concentration in leachate.

 

Figure 4
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  		Fig. 4. Effect of Cl–– and SO42––dominated saline irrigation water on NO3–N (a) and NH4–N (b) leaching from different compost amendments. Leaching events 1, 2, 3, 4, 5, 6, 7, 8, and 9 refer to 8, 12, 18, 22, 26, 29, 32, 38, and 47 d after sowing, respectively (LSD at P = 0.05; n = 3).

 
The concentration of DRP was relatively steady during the early three leachings and then started to decrease under both saline conditions (Fig. 5 ). The concentration of DRP increased more under SO42–– than Cl––dominated saline irrigation. Salt types differed for DRP release in the following order: non-saline {approx} SO42– > Cl–. Non-saline and SO42– saline irrigation substantially increased the DRP concentration in leachate from animal and sludge-amended pots (Fig. 5).


Figure 5
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  		Fig. 5. Effect of Cl–– and SO42– –dominated saline irrigation water on dissolved reactive phosphorus concentration in leachate from different compost amendments. Leaching events 1, 2, 3, 4, 5, 6, 7, 8, and 9 refer to 8, 12, 18, 22, 26, 29, 32, 38, and 47 d after sowing, respectively (LSD at P = 0.05; n = 3).

 
The concentration of K in leachate was increased under both saline irrigations as compared with non-saline treatment (Fig. 6 ). Potassium release was enhanced by SO42––dominated irrigation water, but no consistent pattern was observed for K release under Cl– and SO42– saline irrigations. Potassium leaching was substantially high from animal compost–amended pots under all saline irrigations (Fig. 6).


Figure 6
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  		Fig. 6. Effect of Cl–– and SO42––dominated saline irrigation water on potassium leaching from different compost amendments. Leaching events 1, 2, 3, 4, 5, 6, 7, 8, and 9 refer to 8, 12, 18, 22, 26, 29, 32, 38, and 47 d after sowing, respectively (LSD at P = 0.05; n = 3).

 
Results of the current study reveal that SO42––dominated irrigation water enhanced NO3–N, P, and K leaching as compared with Cl– treatment. Some studies showed that N mineralization increases under SO42– and Cl– salinity (Koerselman et al., 1994). Higher NO3–N concentrations in leachate under SO42––dominated irrigation water might be due to the high desorption capacity of sulfate ions. Sulfate, being a divalent, has more ionic strength to replace HPO42– or PO43– than Cl– (monovalent ion) (Zahoor et al., 2008a), and the same assumption can be taken for NO3–N (monovalent ion) release. Leaching of cations like K, Ca, and Mg was enhanced by SO42––dominated irrigation water (the data of Ca and Mg are not shown here) (Zahoor et al., 2008b).

Among the salt types, DRP leaching was enhanced by SO42––dominated water more than by Cl– treatment. The increase in DRP release by SO42– might be due to the reduced sulfur (S2–) formed after the input of SO42––rich water and to the precipitation of FeS, which releases P adsorbed to iron into the water column (Smolders and Roelofs, 1995). Similarly, Zahoor et al. (2008a) observed a significantly higher amount of P release from soil presaturated with SO42– as compared with Cl– salts. Most of the researchers suggested that the mechanisms of SO42– and PO4 adsorption are similar and that both ions compete for the same sorption sites (Couto et al., 1979; Pasricha and Fox, 1993). Although SO42– does not compete strongly with PO4 for adsorption sites, there is likely some competition for sorption between these anions that may cause comparatively more P release by SO42– than Cl– salt (Zahoor et al., 2008a).

Nutrient Leaching from Bio-composts
Nutrient leaching from different compost amendments greatly varied with each irrigation event (Table 3). Maximum NO3–N concentration in leachates was recorded from animal compost–amended pots, especially in the first few leaching events, followed by sludge compost, poultry compost, and control (Fig. 4a). Nitrate concentration from animal compost was relatively higher under SO42– and non-saline treatments. The concentrations of NH4–N leached from sludge compost was higher (about ninefold) during first five leaching events as compared with the other compost amendments (Fig. 4b).

Despite the fact that all organic amendments were applied on a total P–equivalent basis, the amount of DRP release varied greatly from different bio-composts (Table 3). The concentration of DRP in leachates was substantially higher from sludge compost and animal compost than from poultry compost or unamended soil (Fig. 5). The concentrations of DRP leached from sludge and animal compost amended pots were about sixfold and threefold higher than poultry compost, respectively.

The leaching of K was also strongly affected by amendments. Animal compost released a substantial amount of K in leachate (Fig. 6). Potassium concentration in animal compost increased up to the third leachate (248 mg L–1) and thereafter decreased steadily. The concentration of K from poultry and sludge compost amended treatments was relatively steady during the all leaching events.

Nitrate was the dominant form of N leached out from the composts, especially under the animal and sludge compost-amended pots. Carefoot and Whalen (2003) also reported higher NO3–N concentration through piezometers when soil was amended with an organic or inorganic form of N fertilizer. In the corn production system, mean NO3–N concentrations in drainage discharge may range from 4 to 43 mg NO3–N L–1 (Jaynes et al., 2001), which is consistent with the NO3–N concentrations we found in leachate under different bio-amendments. Contrary to NO3–N, NH4+ can be sorbed to the cation exchange capacity, incorporated (fixed) into clay and other complexes within the soil, released by weathering back into the available mineral pool, or immobilized into organic form by soil microbial processes, which might have resulted in less NH4–N leaching. Nitrogen release or mineralization from organic sources depends on many factors, such as soil type, C/N ratio, and method of composting (Barbarika et al., 1985; Douglas and Magdoff, 1991), which ultimately affect its leachability. Nitrogen (especially NO3–N) release from poultry compost treatments was lower and consistent as compared with other compost amendments. This might be due to the fact that the composting process immobilizes N in the litter and produces humus, which can be used as a source of organic material and which slows the release of nutrients (Paul and Clark, 1996) and reduces the adverse environmental effects of N leaching from the soil (Chang and Janzen, 1996).

Higher DRP release was observed from sludge and animal compost than poultry compost and control treatments. Griffin et al. (2003) reported that after 84 d of incubation at constant soil water status, water-extractable P from sandy loam soil was lower from poultry manure than from dairy and beef manure. Similarly, Dou et al. (2000) observed that 53 and 64% of P in poultry and dairy manure, respectively, was soluble by repeated water extraction. Generally, the quantity of P transported via subsurface pathways has been reported to remain high if the site is fertilized with organic residues because organic P is sorbed less strongly than inorganic P and thus may be leached (Zheng et al., 2001). High variation in DRP leaching from different bio-composts might be due to the difference in the nature of each bio-compost. It has also been reported that P release depends more on the type of P source than the amount of P applied (Zahoor et al., 2008a). Similarly, the literature has defined a large range of the relative effectiveness of biosolids P as compared with chemical fertilizers, which may range from 10 to 100%, as determined in different green house studies (De Haan, 1980; Coker and Carlton-Smith, 1986). A further complication in predicting P release from biosolids is the P retention characteristics of the soil being amended. A given rate of soluble fertilizer P results in different P phytoavailabilities and leaching tendencies in different soils, and reactions of biosolids-borne P should be similarly complex.

Potassium release was higher under animal and poultry compost. Potassium is believed to be lost from soils by two processes: (i) removal of harvested plant material and (ii) leaching to below the root zone. The amount of K leaching is determined by several factors, including drainage (Shepherd and Bennett, 1998) and fertilization (Wulff et al., 1998). Soil water K concentrations vary significantly among different soil types (Ulén, 1999). Simmelsgaard (1996) observed K concentrations from different soil types as 0.5 to 1.7 mg K L–1 in clay soils, 5 to 7 mg K L–1 in loamy sand, and 10 to 15 mg K L–1 in coarse sandy soil. However, the values of K leaching in the current study are about 10 times higher than the K values from coarse sandy soil. Higher K values in leachate in the current study are due to the higher native K status of the composts in addition to the coarse textured soil.

Indicators of P Variability from Different Bio-composts
Although P in organic wastes is present in organic and inorganic forms that display a complex continuum of solubility, several researchers have suggested that the water-extractable inorganic P provides a simple indicator of the potential release of dissolved P into agricultural runoff (Dou et al., 2000; Sharpley and Moyer, 2000). However, if we also use water-soluble P as an indicator for P leachability of bio-composts, then the assumption is true for sludge compost (747 mg kg–1). However, water-soluble P was higher from poultry compost (434 mg kg–1) than animal compost (268 mg kg–1), which is in contrast to P leached from these bio-composts (Fig. 5). Relatively low P release from poultry compost might be due to the high Ca content of poultry compost compared with other composts. It has been reported that high Ca contents of some manures may be responsible for modifying the P sorption characteristics of manured soils (Robinson and Sharpley, 1996). In addition to Ca, Al and Fe affect P availability because soil P forms precipitates with these elements (Barber 1995). De Haan (1980) reported that among 15 biosolids, the material with higher total Al + Fe concentrations had less P availability. Pastene (1981) recommended the molar ratio of (Al + Fe) to P as an indicator of the P-supplying power of biosolids. He suggested that ratio values of <1 were characteristic of biosolids that are capable of supplying large quantities of soluble P, whereas ratio values of >1 indicated poor sources of P supply. In the current study, it was also observed that total P to total Al + Fe ratio was >1 in poultry compost (12.0) and was well below this in animal and sludge compost (0.4).


   Conclusions
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 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
The relative effectiveness factor tacitly acknowledges that all bio-composts have different nutrient leachability and phytoavailability. Results from the current study revealed that nutrient availability from different bio-composts is complex and that nutrient release or uptake depends on many factors under impaired irrigation water quality. Aluminum, Fe, and Ca content plays an important role on P fate in soil and is mainly responsible for P variability in different bio-composts. Therefore, it is necessary that when composts are applied on a P-equivalent basis, in addition to N and K, concentrations of Al, Fe, and Ca should be taken into account for assessing optimum nutrient availability from composts to mitigate nutrient leaching. Further studies are needed to focus on P release in drainage water induced by Cl–– or SO42––dominated irrigation water.

Plant growth was severely affected by saline irrigation, but the effect of Cl––dominated irrigation water was more severe on plant growth compared with SO42–. However, it was observed that SO42––dominated irrigation water leached more anions (NO3– and PO43–) and cation (K+) than Cl–. Plant growth under saline and normal (non-saline) conditions was much better under sludge-amended pots. From an agronomic point of view, sludge compost can best meet the maize nutrient requirements under saline and non-saline conditions. However, from an environmental point of view, application of sludge compost under impaired water quality would not be a pragmatic approach due to the high release of N and P. If poor irrigation water is the only available choice, then composted poultry droppings would be relatively safer to use. In areas where agricultural lands are the ultimate place for animal and sludge manure disposal, a full insight over compost characteristics and quality of irrigation water is needed. Proper management practices should be adopted in areas receiving high dosages of bio-amendments and irrigated with poor-quality water. One possible management practice could be the split application of P sources to reduce higher initial P losses from the soil. The combined application of organic and inorganic N and P sources could also help to reduce the problem. The current study has important implications from an environmental perspective because it reveals that irrigating water dominated with SO42– ions can pose a threat to the environment under particular circumstances. Analogously, recommendation of P-based compost application, without a full insight over compost nutritional status, will aggravate the problem.


   ACKNOWLEDGMENTS
 
The authors are grateful to Dr. M. Irshad and Mr. Faridullah for their technical support during the study. Thanks are also due to the Ministry of Education, Science, Sports and Culture, Japan, for financial support. The authors also acknowledge the cooperation of Japanese Society for the Promotion of Science under Global Centers of Excellence Program (GCOE).


   NOTES
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