Effects of Repeated Application of Municipal Sewage Sludge on Soil ...

Effects of Repeated Application of Municipal Sewage Sludge on Soil Fertility, Cotton Yield, and Nitrate Leaching Vasilios Samaras,* Christos D. Tsadilas, and Stamatis Stamatiadis

The effects of sludge application on soil properties and cotton (Gossypium hirsutum L.) response were investigated in an Inseptisol in central Greece. Digested sludge was incorporated in the 0 to 15 cm soil depth at rates of 10, 30, and 50 Mg ha–1, and repeated for four consecutive years. Sludge treatments were compared to an inorganic fertilizer application and an untreated control in a completely randomized design with four replications. Sludge application increased soil organic matter, associated nutrients and improved physical properties. However, soil electrical conductivity increased with increasing sludge application to levels that may affect growth of salt-sensitive crops and warns against long-term application that may impair essential soil functions. The multifold increase of Olsen P and nitrate N beyond crop needs is a reason of concern for surface runoff and nitrate leaching below the root zone at the higher sludge application rates. Sludge application of 10 Mg ha−1 was sufficient to improve soil chemical properties with less risk of water contamination. Cotton responded to sludge application by increased nutrient uptake and yield, which indicated that sludge could replace inorganic fertilizer needs even at the lower application rate. However, fluctuations of nutrient uptake and yield between growing seasons were of greater magnitude than those caused by sludge application. Multiple regression analysis revealed that P uptake was the major limiting factor for determining cotton yield.

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mprovement of soil physical and chemical properties is among the benefits of sewage sludge application to agricultural land. As a soil conditioner, sludge reduces bulk density and increases porosity, improves structural stability, and enriches soil with organic carbon (Pagliai et al., 1981; Metzger and Yaron, 1987; Tester, 1990; Sort and Alcaniz, 1999; Marinari et al., 2000). These changes result generally in increased water retention capacity in coarse-textured soils and, in the long-term, in enhanced water transmission properties and resistance to soil erosion (Khalee et al., 1981; Metzger and Yaron, 1987). In association with its high organic matter content, sludge also contains appreciable amounts of N and P with significant fertilizer replacement value, although its K content is low for meeting crop requirements (Smith, 1996; Warman and Termeer, 2005). The pools of soluble nutrients in sludge are initially small, and plant uptake must await mineralization of organic constituents (Petersen et al., 2003). The extent and temporal dynamics of nutrient release and uptake are less variable than those of livestock manures, but will still depend on sludge characteristics, the method and timing of application, V. Samaras and C.D. Tsadilas, National Agricultural Research Foundation, Institute of Soil Classification and Mapping, 1 Theophrastos St., 41335 Larissa, Greece; S. Stamatiadis, Soil Ecology and Biotechnology Lab., Gaia Environmental Research and Education Center, Goulandris Natural History Museum, 13 Levidou St., 145 62 Kifissia, Greece. Received 17 May 2007. *Corresponding author ([email protected], [email protected]). Published in Agron. J. 100:477–483 (2008). doi:10.2134/agronj2007.0162 Copyright © 2008 by the American Society of Agronomy, 677 South Segoe Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

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soil type and properties, and environmental conditions (Smith, 1996; Smith et al., 1998). Crop response to soil amendment with sewage sludge results in yields often equal to or higher than those resulting from recommended fertilizer applications (Epstein, 2003) unless sludge has a high C to N ratio, excess metals, high soluble salts or is applied at extremely high rates (Warman and Termeer, 2005). These beneficial effects make sludge application an attractive option for eroded soils of dry Mediterranean climates that often have low organic matter content. Relatively large quantities of sludge, of the order of 30 Mg ha–1, are generally required to raise soil N content significantly and have a measurable effect on soil physical properties (Hall and Coker, 1983; Metzger and Yaron, 1987). Comparable quantities of sludge cakes are commonly applied to degraded soils of olive orchards in Spain (Gasco and Lobo, 2006) or by certain recycling operations in England (Smith et al., 1998). These rates, however, exceed crop N requirements and may cause undesirable changes in soil chemical properties leading to environmental contamination. Such effects include ammonia volatilization and denitrification, excessive soil acidification from nitrification of ammonia, accumulation of nitrates in sludge-treated profi les, and increased nitrate leaching from susceptible loamy soils (Powlesland and Frost, 1990; Smith and Doran, 1996). Even when sludge is applied at rates consistent with the N requirement of the crop, P is supplied in excess because the P requirement of crops is only about 10 to 25% that of N while sludges generally contain half as much P as they do N (Smith, 1996). Accumulation of P in sludge-amended soil can result in benefits for P nutrition of future crops but also potentially impact water bodies through surface runoff and leaching (Sui et al., 1999; Maguire et al., 2000; Penn and Sims, 2002). In this study, the effects of repeated sludge application were investigated at rates that are reportedly required for a measurable improvement of soil fertility in a fine loamy soil of the 2008

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Biosolids

ABSTRACT

Table 1. Annual variation of the chemical composition of sewage sludge of the city of Volos. Chemical property pH† EC,†§ mS cm –1 OM, % CaCO3, % Total N, % Total P, % K, % Fe, % Cl, % Na, % Zn, mg kg –1 Pb, mg kg –1 Cu, mg kg –1 Mn, mg kg –1 Ni, mg kg –1 Cd, mg kg –1

1996 7.00‡ 7.27 44 9.2 2.86 2.76 0.12 5.73 0.89 0.64 1805 376 255 238 52 8

1997 6.99 5.41 44 7.3 2.94 2.32 0.16 4.14 1.19 0.46 1756 456 283 231 68 6

1998 7.09 5.90 45 6.1 3.06 2.07 0.11 2.60 1.00 0.59 1620 208 213 139 55 2

1999 7.01 6.03 37 4.8 4.14 2.90 0.17 4.47 0.71 0.52 1667 223 270 182 55 4

Mean 7.02 6.15 43 6.8 3.25 2.51 0.14 4.23 0.95 0.55 1712 315 255 197 57 5

† 1:5 sludge-to-water mixture. ‡ Values represent the mean of at least 5 samples within each year. § Electrical conductivity.

semiarid Mediterranean region. The cultivated crop, cotton, requires high levels of N with measured uptakes as much as 230 kg N ha–1 under irrigation (Constable and Rochester, 1988). Higher rates of sludge application than those applied in this study produced cotton yields that were comparable to those obtained from fertilizer addition in the semiarid southern Arizona (Watson et al., 1985). The specific objectives were the evaluation of the effects of digested sludge application on (i) improvement of soil physical and chemical properties, (ii) its potential to replace inorganic fertilizer for cultivation of cotton, and (iii) environmental contamination in terms of nutrient loss potential. MATERIALS AND METHODS Experimental Design and Sampling The experimental field was located 1 km west of the village of Rizomilos and 20 km NW of the city of Volos, central Greece (39°26´04.04˝ N, 22°42´49.64˝ E). The soil is classified as a Typic Xerochrept that is a clay loam with a high cation exchange capacity (33 cmol 100 kg–1). Part of the field (30 by 37 m) was divided into 20 plots (5 by 5 m) with a 2-m spacing between them. Each plot was randomly assigned to receive one of five treatments as follows: three rates of sewage sludge (10, 30, and 50 Mg dry solids ha–1 yr–1), one rate of inorganic fertilizer (160 kg NH4 –N ha–1 yr–1 and 80 kg P2O5 ha–1 yr–1) and an unfertilized control with no organic or inorganic amendments. The complete randomized block design had four replicates (plots) for each treatment. The digested sludge was derived from the municipal treatment plant of the city of Volos after treatment of sewage in aerobic tanks and dewatering. Phosphorus was chemically precipitated in the tanks by addition of FeClSO4. Sludge was distributed manually and incorporated to approximately 15 cm by rotovation and fertilizer was sprinkled uniformly on the soil surface 2 wk before sowing (beginning of April) for four consecutive years from 1996 to 1999. Planting of cotton took place in the middle of April with 22 kg seed ha–1. Seed variety was not the same each year, but all 478

seed varieties were selected for early maturity characteristics and high yield potential. The domestic variety Korina 01256 (Cotton and Industrial Plant Institute, National Agricultural Research Foundation) was planted in 1996, DP 50 (Delta & Pine Land Co.) in 1997 and 1998, and ST 453 (Stoneville) in 1999. The following pesticides and herbicides were applied to the crop in all plots: phorate to the seed (10 kg ha–1), prometryne (10 kg ha–1) on the soil surface after sowing, endosulfan (3 kg ha–1) at first bloom on 20 July and two to three sprays of pyrethrine in combination with acaricides thereafter. Groundwater was supplied to the plants by drip irrigation 11 times during the growing season up to 25 August with an average of 370 m3 ha–1 time–1. Leaf and soil samples were taken from the middle rows of each plot. Leaf samples were taken at full bloom (end of July) by removing well-developed leaves from the middle of the canopy (Sabbe and Zelinski, 1990). Composite soil samples were taken at the same time from 0- to 25- and 25- to 50-cm depth for the determination of chemical and physical properties (pH, EC, extractable P and K, organic matter, and total N). Selected soil physical properties were only measured the third year of the experiment. Additional composite soil samples were taken at the beginning (early June) and end (mid-September) of the growing season in the last 3 yr to assess the vertical distribution of nitrate N at four soil depths (0–25, 25–50, 50–75, 75–100 cm). Cotton yield (lint + seed dry weight) was measured by harvesting each 5 by 5 m plot at the end of each growing season. Soil samples sealed in plastic bags, and leaf samples in paper bags were transported to the laboratory in a portable cooler. Soil samples were mixed and passed through a 2-mm sieve. Soil analysis included pH (1:5 soil to water ratio), electrical conductivity (1:5 soil to water ratio), nitrate content in soil extracts of 2 M KCl using a colorimeter (Keeney and Nelson, 1982). Carbonate content was determined using the Bernard method (Nelson, 1982), organic matter by the Walkley–Black method of wet oxidation (Nelson and Sommers, 1996) and total N by the Kjeldahl wet oxidation method (Bremner and Mulvaney, 1982). Soil K was extracted with 1 nM ammonium acetate at pH 7 (Knudsen et al., 1984), while P was extracted from soil with 0.5 M sodium bicarbonate at pH 8.5 as proposed for calcareous soils (Olsen and Sommers, 1984). Bulk density, gravimetric water content and infi ltration were measured in the third year (1998) of the experiment using a portable soil quality kit (Liebig et al., 1996). Leaf samples were ground to a fine powder and heated at 500°C for 5 h. Ash was digested with 1 M HCl. Metal concentrations were determined using an atomic absorption spectrophotometer and K and P concentrations with a flame photometer (Benton Jones et al., 1991). Leaf N content was determined by the Kjeldahl wet oxidation method (Bremner and Mulvaney, 1982). The soil methods were used for sludge analysis in the case of pH, EC, carbonates, organic matter and total N content. Total P, K, and Na were determined by heating sludge in a furnace at 500°C for 4 h, dissolution of ash with aqua regia and measurement with a spectrophotometer (Benton Jones et al., 1991). The chemical composition of sludge remained more or less constant throughout the 4 yr of the experiment (Table 1). Based on an average N content of 3.25%, annual N inputs were approximately 325, 975, and 1625 kg ha–1 for each sludge treatment, Agronomy Journal

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respectively. Assuming 40% N mineralization during the first year of application (Watson et al., 1985), the lower application rate provided approximately 130 kg N ha–1 which is less than the reported maximum N uptake by cotton under irrigation (230 kg N ha–1, Constable and Rochester, 1988).

Table 2. Analysis of variance of soil properties for the sludge experiment conducted in Greece from 1996 to 1999. F values of type 3 tests Fixed effect Treatment Year Treatment × year Depth Treatment × depth Year × depth Treatment × year × depth

Electrical Extractable Exchangeable Organic pH conductivity P K matter Total N 20.3*** 26.8*** 38.2*** 0.5 14.2*** 26.3*** 8.0** 6.4* 30.4*** 56.1*** 90.1*** 20.2*** 2.5* 1.2 5.2*** 1.9 2.8* 1.8 27.0*** 2.8 45.5*** 289.1*** 206.8*** – 1.9 2.0 6.7* 0.2 9.4** – 1.2 4.5* 8.1* 5.2* 13.3*** – 1.8 2.3* 2.4* 1.5 1.4 –

* Significant at the 0.05 probability level. Statistical Analysis Data analysis included analysis ** Significant at the 0.001 probability level. *** Significant at the 0.0001 probability level. of variance (general linear models), correlation analysis, and multiple regression. The linear mixed model for Table 3. Effect of sludge and fertilizer application on selected soil properties 4 mo after application (July). Means are averaged across years. soil variables was a special case of split plot Treatment in time that used restricted maximum likeSludge, Mg ha –1 yr –1 lihood (REML) for the estimation of fi xed Soil depth Soil property Fertilizer Control 10 30 50 and random effects. Treatment was the 0–25 cm pH‡ 7.88 a† 7.89 a 7.83 a 7.75 b 7.65 c whole-plot fi xed effect while year and soil 0.20 c 0.18 c 0.21 c 0.25 b 0.30 a EC, mS cm –1‡§ depth were treated as split-plot fi xed effects Extractable P, mg kg –1 7.9 c 7.0 c 13.2 c 29.1 b 44.2 a relative to treatment. The random effects Exchangeable K, mg kg –1 372 378 379 388 384 of the model were plot, year, and soil depth Organic matter, % 2.48 c 2.37 c 2.52 c 2.89 b 3.11 a that were all nested within treatment. The Total N, % 0.13 cd 0.13 d 0.14 c 0.15 b 0.18 a LSD test was used to detect differences 25–50 cm pH 7.92 ab 7.98 a 7.86 bc 7.81 c 7.78 c between means of the fi xed effects at P < EC, mS cm –1 0.19 bc 0.17 c 0.21 ab 0.24 a 0.26 a 0.05 and to compute standard errors using Extractable P, mg kg –1 5.9 c 4.3 c 9.0 bc 16.3 b 27.7 a their root mean square errors. The same Exchangeable K, mg kg –1 278 290 270 283 290 model was used for the analysis of plant Organic matter, % 2.12 2.05 2.05 2.12 2.17 Total N, % – – – – – variables, but with the absence of the depth † Within rows, means followed by different letters are significantly different according to LSD (P < 0.05). effect. In stepwise multiple regression, the ‡ 1:5 soil to water mixture. independent variables of the model were § Electrical conductivity. selected by allowing at least 10 degrees of freedom for the estimation of the error term. The regression model was checked by appropriate diag(Table 3). The reduction of soil pH with increasing sludge nostic procedures (collinearity and influence diagnostics). Data application may be partly attributed to the lower pH of sludge, but also to microbial nitrification of ammonium contained analysis was conducted using Statistical Analysis System softin sludge (Patriquin et al., 1993; Smith and Doran, 1996). ware, version 6 (SAS Institute, 1990). Reduction of soil pH was in the order of 0.4 units in the high RESULTS AND DISCUSSION rate of sludge application in comparison to the control over the Soil Properties course of the 4 yr of the experiment. This pH reduction toward Soil properties were strongly affected by treatment and soil more neutral values appears to improve soil fertility. However, depth (Table 2). The unamended soil (control) was slightly long-term sludge application may lead to excessive soil acidification and reduction of crop yields in other soils of low buffering alkaline (Table 3, 0–25 cm) with 378 mg kg–1 of exchangeable K that is sufficient for crop growth (Allan and Killorn, 1996). capacity (Smith and Doran, 1996). Soil EC increased with Extractable P was relatively low (7 mg kg–1) in that a P fertilincreasing rate of sludge application and reached 0.30 mS izer application would probably cause a yield response (Olsen cm–1 (1:5 soil-to-water mixture) in the highest sludge applicaand Sommers, 1984; Allan and Killorn, 1996). The application tion rate (Table 3). This value equates to 1.5 mS cm–1 for a 1:1 of ammonium fertilizer did not result in any changes of soil soil-to-water mixture. The rise of EC was caused by the high properties in comparison to the control despite the fact that NaCl content of sludge (Table 1). Possible sources of NaCl in nitrification effects, such as acidification and a rise in EC, are sewage are the fish preservation industry in the area, the use of known to occur after inorganic fertilization of agricultural water softeners and the intrusion of sea water from leakage of soils (Patriquin et al., 1993; Smith and Doran, 1996). pipes. Cotton is a salt-tolerant crop with soil threshold values Sludge application decreased soil pH and increased all other that exceed 4 mS cm–1 (Smith and Doran, 1996). However, measured chemical properties except soil K (Table 3). The vala fine textured soil with an EC value of 1.5 mS cm–1 (1:1) is ues of soil EC, organic matter, extractable P and K were higher considered slightly saline for salt sensitive species such as bean in the surface 0 to 25 cm than in the 25 to 50 cm, but the rate (Phaseolus vulgaris L.), most legumes, maize (Zea mays L.), of change due to sludge application was similar in both depths pepper (Capsicum annuum L.), and tomato (Lycopersicon escu-

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lentum Mill.). Furthermore, such EC values may have adverse effects on microbial mineralization processes and increase ammonia volatilization (Smith and Doran, 1996). On the contrary, the increase of organic matter, total N, and extractable P content indicated the beneficial effects of sludge application on soil fertility. A significant increase in both organic matter and total N content appeared in the treatments of 30 and 50 Mg ha–1. These results agree with suggested rates of 20 to 30 Mg ha–1 to have a measurable effect on soil organic matter and physical properties (Hall and Coker, 1983; Metzger and Yaron, 1987; Smith, 1996). However, surface Olsen P increased to 60 mg kg–1 in the 50 Mg ha–1 treatment in the last year of the experiment (Fig. 1). This value is four times above those considered high for application of P fertilizers in Minnesota soils (12–16 mg kg–1; Allan and Killorn, 1996). Such high P values after repeated sludge application are posing risks for dissolved phosphates in runoff. A similar increase in extractable P occurred in the 25- to 50-cm depth (Table 3, Fig. 1) and indicated significant leaching despite the general immobility of P due to fi xation and precipitation processes in soil (Smith 1996; Petersen et al., 2003). An increase of total P was also found at the 5- to 25-cm depth after 6 yr of continuous biosolids application to poplars (Sui et al., 1999). The low content of K in sludge did not result in any differences between treatments. Significant treatment effects were also detected in soil physical properties as measured in the third year of the experiment (Table 4). Soil bulk density decreased and field water holding capacity increased with increasing rate of sludge application. Even though the water infi ltration rate doubled in the 50 Mg

ha–1 sludge application in comparison to the control, it was still very low and made this soil susceptible to surface erosion. Even the 10 Mg ha–1 treatment brought about significant improvement in soil bulk density and water holding capacity (Table 4). These results confirm findings of previous studies about improvement of soil physical properties on long-term sludge application (Smith, 1996). Several authors (Reganold and Palmer, 1995; Ellert and Bettany, 1995; Doran and Parkin, 1996) caution against gravimetric comparisons between soil management systems of differing bulk densities because their biological or ecological relevance may be misrepresented. In our study, the reduction of bulk density due to sludge application reduced the magnitude of differences between treatments, but the reduction was not great enough to alter the statistically significant differences between means (Table 4). Soil Nitrates Additional sampling at the beginning and end of the growing season provided evidence of nitrate leaching during the last 3 yr of the experiment. The nitrate N content of the control soil early in the growing season (Table 5, 0–25 cm) appeared Table 4. Physical soil properties and conversion of gravimetric to volumetric units for chemical soil properties measured at the end of July 1998 (0- to 25-cm depth). Sludge application rate, Mg ha –1 yr –1 Soil property Physical property Bulk density, g cm –3 Infiltration, cm h –1 Water holding capacity, g g –1, % Gravimetric units Total N, % Extractrable P, mg kg –1 Extractrable K, mg kg –1 Volumetric units‡ Total N, kg ha –1 Extractrable P, kg ha –1 Extractrable K, kg ha –1

0

10

30

50

1.41 a† 1.95 b 27.5 c

1.32 b 1.95 b 29.5 b

1.30 b 3.60 a 30.0 b

1.27 b 4.05 a 33.8 a

0.14 c 8c 391

0.16 bc 26 b 421

0.18 b 56 a 419

0.20 a 63 a 431

3922 c 22 c 1099

4196 bc 68 b 1105

4649 ab 145 a 1082

5046 a 157 a 1078

† Within rows, means followed by different letters are significantly different according to LSD (P < 0.05). ‡ kg ha –1 = (mg kg –1) × (BD) × (soil depth, cm) × (0.0795).

Table 5. Nitrate-N levels (mg kg –1) at the beginning and the end of the growing season. Treatment† Sludge, Mg ha –1 Month Early June

September

Depth cm 0–25 25–50 50–75 75–100

Fertilizer

Control

10

30

50

67 c‡ 46 b 24 b 22 bc

20 d 15 c 10 c 13 c

56 c 41 b 23 bc 16 c

88 b 48 b 34 a 30 b

113 a 68 a 39 a 47 a

0–25 25–50 50–75 75–100

40 c 23 b 15 17

8d 13§ 4 4

57 bc 19§ 13 13

73 ab 43 ab 16 23

87 a 57 a 25 25

† Values represent means over the last three growing seasons.

Fig. 1. Mean annual concentration of extractable P (±SE) as affected by treatment in two soil depths.

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‡ Within rows, means followed by different letters are significantly different according to LSD (P < 0.05). Means with no letters are not significantly different. § Statistical comparisons were not possible due to missing data.

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Table 6. Analysis of variance of leaf nutrients and yield for the sludge experiment conducted in Greece from 1996 to 1999. F values of type 3 tests Fixed effect Treatment Year Treatment × year

Leaf N 13.6*** 113.1*** 4.2**

Leaf P % 9.4** 195.5*** 3.3*

Leaf K 3.4* 96.7*** 1.0

Yield Mg ha –1 5.9* 63.9*** 0.8

* Significant at the 0.05 probability level. ** Significant at the 0.001 probability level. *** Significant at the 0.0001 probability level.

Table 7. Effect of growing seasons on leaf nutrients and yield. Growing season Plant property Leaf N, % Leaf P, % Leaf K, % Yield, Mg ha –1

1996 3.83 a† 0.21 a 1.22 c 3.93 a

1997 2.85 c 0.14 d 1.64 b 2.99 c

1998 3.38 b 0.15 c 2.13 a 2.92 c

1999 3.37 b 0.19 b 1.64 b 3.68 b

† Within rows, means followed by different letters are significantly different according to LSD (P < 0.05).

at the end of the growing season (Fig. 2), thus posing a threat for surface runoff and water contamination given the low infi ltration rates of this soil (Table 4). Ojeda et al. (2006) detected relatively high concentrations of mineral N in the runoff 5 mo after sludge application in a loamy soil under similar climatic conditions characterized by irregular and intense rainfall events. Nitrate-N content declined rapidly with depth but its concentration in the high sludge applications (30 and 50 Mg ha–1) was higher than that of the other treatments even at depths considered to be below the root zone (75–100 cm, Table 5). The elevated nitrate levels in this depth provide further evidence of leaching and reason for concern of groundwater contamination. Residual nitrates at the end of the growing season were generally lower than those in June (Table 5), but their concentrations remained excessive and similar to those in June in the last year of the experiment (Fig. 2). The low sludge application rate (10 Mg ha–1) had, on average, nitrate concentrations similar to those of the inorganic fertilizer at all depths and similar to those of the control at depths below 50 cm (Table 5). However, nitrates accumulated to excessive levels in the surface soil the final year of the experiment. Therefore, the 10 Mg ha–1 application appears to be acceptable from an environmental point of view provided that repeated annual applications are avoided.

Fig. 2. Vertical distribution of soil nitrates within and after the growing season. Annual means ( ± SE) are averaged across treatments.

adequate for growing crops such as corn without the need of N amendments (Bundy and Meisinger, 1994). Sludge or fertilizer application increased soil nitrate N to excessive levels in the surface soil. Nitrate levels increased with increasing rate of sludge application up to six times the level of the control in the 50 Mg ha−1 sludge application (50 Mg ha–1, Table 5). An expected pattern of increased nitrate N with year of sludge application was disrupted in 1998. Compared to June of 1997 and 1999, nitrate levels in 1998 were reduced in the surface soil (0–25-cm depth) and increased in greater depths (Fig. 2). This was indicative of increased nitrate leaching and was explained by unusually high precipitation in May of that year (119 mm) as recorded by a meteorological station 45 km away. Nitrate concentration reached a maximum in the last year of the experiment (0–50 cm, Fig. 2) being three to four times higher in the 50 Mg ha–1 treatment than sufficiency levels for corn during early growing season (20–25 ppm, Bundy and Meisinger, 1994). These high concentrations in the top soil did not decline Agronomy Journal

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Leaf Nutrients and Yield Compared to most soil properties, nutrient uptake and yield were influenced to a greater extent by growing season than by sludge application (Table 6). Leaf N and P content followed strong annual patterns (Table 7). Increased leaf P content occurred in the 1996 and 1999 growing seasons in all treatments and was closely followed by yield. Differences between cotton varieties were not expected to be the cause of these large annual variations because all varieties were selected for their high yield potential and early maturity characteristics. Similar annual differences in yield were also obtained with a single cotton cultivar in sludge-amended plots in southern Arizona (Watson et al., 1985). It is possible that the increased uptake of P was caused by increased soil P availability as a result of higher rainfall in March and April of these years. Air tempera2008

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Table 8. The effect of treatment on leaf nutrients and yield over four growing seasons. Treatment Sludge, Mg ha –1 yr –1 Plant property Fertilizer Control 10 Leaf N, % 3.27 c† 3.15 d 3.36 bc Leaf P, % 0.16 b 0.17 b 0.17 b Leaf K, % 1.65 ab 1.53 b 1.68 a Yield, Mg ha –1 3.36 a 3.09 b 3.37 a

30 3.46 ab 0.18 a 1.68 a 3.48 a

50 3.54 a 0.19 a 1.75 a 3.60 a

† Within rows, means followed by different letters are significantly different according to LSD (P < 0.05).

ture could not be implicated in annual differences of nutrient uptake since summer monthly temperatures showed a remarkable similarity between years (data not shown). Nitrogen uptake did not exactly follow the annual pattern of P and yield in that a great reduction of leaf N content occurred only in 1997 in the sludge-treated plots (Table 7). Despite the strong effect of growing seasons, cotton yield increased with sludge and fertilizer application, but not by more than 1.2 times that of the untreated control (Table 8). Differences in yield between sludge and inorganic fertilizer treatments were not evident (Table 8) indicating that sludge application, even at the lower rate of 10 Mg ha–1, can successfully replace inorganic fertilizer needs from the first year of application. In a previous 3-yr field experiment with cotton, Watson et al. (1985) used higher rates of repeated sludge application (20, 40, and 60 Mg ha–1) on a clay loam in southern Arizona. Otherwise, their results were similar to ours in terms of (i) the proportional yield increase in sludge-amended plots relative to the untreated control and (ii) the nonsignificant difference in yield between sludge treatments and the fertilized control. Leaf nutrient content increased with increasing rate of sludge application and even above the level of the inorganic fertilizer in the case of N (Table 8). Leaf K differences between treatments were relatively small reflecting the lack of differences in soil K content (Table 3). In any case, leaf K concentration was within the sufficiency range between peak bloom and first open boll (Reuter et al., 1997; Mengel, 2007). Multiple regression analysis indicated that P uptake was the major limiting factor for determining cotton yield when soil properties and leaf nutrients were used as independent variables of the model. Leaf P concentration accounted for 64% of yield variation in the control and sludge treatments and resulted in a positive linear relationship (Fig. 3). The increased P uptake was partly explained by the high concentration of available soil P in the sludge-amended plots (Table 3). Although relatively weak, the correlation between leaf P and Olsen P in the root zone (25–50-cm depth) was significant (r = 0.50, n = 60). Positive correlations between NaHCO3–extractable P and uptake of P by cotton and other crops have been obtained in the past (Olsen and Sommers, 1984). However, even at the excessive levels of extractable soil P in the higher rate of 50 Mg ha–1, leaf P content remained below the sufficiency limit of 0.3% at flowering as reported by Reuter et al. (1997) and Sanchez (2007) for cotton. Soil nitrate levels ranged from adequate to excessive during the growing season (Fig. 2). Consequently, leaf N concentrations were generally above the critical deficiency level for this growth stage (leaf N >3%, Reuter et al., 1997) and fell below that only during the second growing season (Table 7). 482

Fig. 3. Relationship between leaf nutrients and yield as affected by sludge application.

Therefore, N uptake was not a major factor in determining yield response as evidenced by their weak correlation (Fig. 3). CONCLUSIONS Repeated sludge application over four growing seasons improved soil fertility by means of increased soil organic matter, associated nutrient content (except K) and improvement of soil physical properties. However, soil EC increased with increasing sludge application to levels that may affect growth of salt-sensitive crops and warns against long-term application that may impair essential soil functions. In addition, the multifold increase of available soil P and nitrate N beyond crop needs is a reason of concern for surface runoff and leaching below the depth of the root zone. These processes are likely to result in surface- and ground-water contamination during and after the growing season due to high sludge application rates. The lower rate of sludge application (10 Mg ha–1) was sufficient to improve soil chemical properties with less risk of phosphate runoff and nitrate leaching below the depth of the root zone. This rate of application appears to be an acceptable practice from an environmental point of view provided that it is not repeated annually. Cotton responded to sludge application by increased nutrient uptake (N and P) and yield and indicated that Agronomy Journal

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sludge can replace inorganic fertilizer needs even at the lower application rate. However, fluctuations of nutrient uptake and yield between growing seasons were of greater magnitude than those caused by sludge application. Multiple regression analysis of all data revealed that P uptake was the major limiting factor for determining cotton yield. ACKNOWLEDGMENTS This project was funded by the Municipal Wastewater Treatment of Magnesia. Special thanks are extended to Prof. Kent Eskridge (University of Nebraska) for assistance in the statistical analysis of the data. REFERENCES Allan, D.L., and R. Killorn. 1996. Assessing soil nitrogen, phosphorus and potassium for crop nutrition and environmental risk. p. 187–201. In J.W. Doran and A.J. Jones (ed.) Methods for assessing soil quality. SSSA Spec. Publ. 49. SSSA, Madison, WI. Benton Jones, J., Jr., B. Wolf, and H. Mills. 1991. Plant analysis handbook: A practical sampling, preparation, analysis, and interpretation guide. Micro-Macro Publ., Athens, GA. Bremner, J.M., and C.S. Mulvaney. 1982. Nitrogen-total. p. 595–624. In A.L. Page et al. (ed.) Methods of soil analysis. Part 2. Chemical and microbiological properties, 2nd ed. ASA and SSSA, Madison, WI. Bundy, L.G., and J.J. Meisinger. 1994. Nitrogen availability indices. p. 951–984. In R.W. Weaver et al. (ed.) Methods of soil analysis. Part 2. SSSA Book Ser. 5. SSSA, Madison, WI. Constable, G.A., and I.J. Rochester. 1988. Nitrogen application to cotton on clay soil: Timing and soil testing. Agron. J. 80:498–502. Doran, J.W., and T.B. Parkin. 1996. Quantitative indicators of soil quality: A minimum data set. p. 25–37. In J.W. Doran and A.J. Jones (ed.) Methods for assessing soil quality. SSSA Spec. Publ. 49. SSSA, Madison, WI. Ellert, B.H., and J.R. Bettany. 1995. Calculation of organic matter and nutrients stored in soils under contrasting management regimes. Can. J. Soil Sci. 75:529–538. Epstein, E. 2003. Land application of sewage sludge and biosolids. Lewis Publ., New York. Hall, J.E., and E.G. Coker. 1983. Some effects of sewage sludge on soil physical conditions and plant growth. p. 43–61. In G. Catroux et al. (ed.) The influence of sewage sludge application on physical and biological properties of soil. D. Reidel, Dordrecht, the Netherlands. Gasco, G., and M.C. Lobo. 2006. Composition of a Spanish sewage sludge and effects on treated soil and olive trees. Waste Manage. 27(11):1494–1500. Keeney, D.R., and D.W. Nelson. 1982. Nitrogen-inorganic forms. p. 643–698. In A.L. Page et al. (ed.) Methods of soil analysis. Part 2. Chemical and microbiological properties, 2nd ed. ASA and SSSA, Madison, WI. Khalee, R., K.R. Reddy, and M.R. Overcash. 1981. Changes in soil physical properties due to organic waste applications: A review. J. Environ. Qual. 10:133–141. Knudsen, D., G.A. Peterson, and P.F. Pratt. 1984. Lithium, sodium and potassium. p. 225–246. In A.L. Page et al. (ed.) Methods of soil analysis. Part 2. Chemical and microbiological properties, 2nd ed. ASA and SSSA, Madison, WI. Liebig, M.A., J.W. Doran, and J.C. Gardner. 1996. Evaluation of a field test kit for measuring selected soil quality indicators. Agron. J. 88:683–686. Maguire, R.O., J.T. Sims, and F.J. Coale. 2000. Phosphorus solubility in biosolids-amended farm soils in the Mid-Atlantic region of the USA. J. Environ. Qual. 29:1225–1233. Marinari, S., G. Masciandaro, B. Ceccanti, and S. Grego. 2000. Influence of organic and mineral fertilisers on soil biological and physical properties. Bioresour. Technol. 72:9–17. Mengel, K. 2007. Potassium. p. 91–120. In A.V. Barker and D.J. Pilbeam (ed.) Handbook of plant nutrition. Taylor & Francis, London.

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Metzger, L., and B. Yaron. 1987. Influence of sludge organic matter on soil physical properties. p. 141–163. In B.A Stewart (ed.) Advances in soil science, Vol. 7. Springer, New York. Nelson, D.W., and L.E. Sommers. 1996. Total carbon, organic carbon, and organic matter. In D.L. Sparks (ed.) Methods of soil analysis. Part 3, Chemical methods. SSSA, Madison, WI. Nelson, R.E. 1982. Carbonate and gypsum. p. 181–197. In A.L. Page et al. (ed.) Methods of soil analysis. Part 2. Chemical and microbiological properties, 2nd ed. ASA and SSSA, Madison, WI. Ojeda, G., D. Tarrason, O. Ortiz, and J.M. Alcaniz. 2006. Nitrogen losses in runoff waters from a loamy soil treated with sewage sludge. Agric. Ecosyst. Environ. 117:49–56. Olsen, S.R., and L.E. Sommers. 1984. Phosphorus. p. 403–430. In A.L. Page et al. (ed.) Methods of soil analysis. Part 2. Chemical and microbiological properties, 2nd ed. ASA and SSSA, Madison, WI. Patriquin, D.G., H. Blaikie, M.J. Patriquin, and C. Yang. 1993. On-farm measurement of pH, electrical conductivity and nitrate in soil extracts for monitoring coupling and decoupling of nutrient cycles. Biol. Agric. Hortic. 9:231–272. Penn, J.C., and J.T. Sims. 2002. Phosphorus forms in biosolids-amended soils and losses in runoff : Effects of wastewater treatment process. J. Environ. Qual. 31:1349–1361. Pagliai, M., G. Guidi, M. La Marca, M. Gichetti, and G. Lucamante. 1981. Effect of sewage sludges and composts on soil porosity and aggregation. J. Environ. Qual. 10:556–561. Petersen, S.O., J. Petersen, and G.H. Rubaek. 2003. Dynamics and plant uptake of nitrogen and phosphorus in soil amended with sewage sludge. Appl. Soil Ecol. 24:187–195. Powlesland, C., and R. Frost. 1990. A methodology for undertaking BPEO studies of sewage sludge treatment and disposal. WRc Rep. 2305-M/1, WRc Medmenahm, Marlow. Reganold, J.P., and A.S. Palmer. 1995. Significance of gravimetric versus volumetric measurements of soil quality under biodynamic, conventional, and continuous grass management. J. Soil Water Conserv. 50:289–305. Reuter, D.J., D.G. Edwards, and N.S. Wilheman. 1997. Temperate and tropical crops. p. 125–133. In D.J. Reuter and J.B. Robinson (ed.) Plant analysis: An interpretation manual. 2nd ed. CSIRO Publ., Coolingwood, Australia. Sabbe, W.E., and L.J. Zelinski. 1990. Plant analysis as an aid in fertilizing cotton. p. 469–493. In R.L. Westerman (ed.) Soil testing and plant analysis. SSSA Book Ser. 3. SSSA, Madison, WI. Sanchez, C.A. 2007. Phosphorus. p. 51–90. In A.V. Barker and D.J. Pilbeam (ed.) Handbook of plant nutrition. Taylor & Francis, London. SAS Institute. 1990. SAS/STAT user’s guide, version 6, 4th ed. SAS Inst., Cary, NC. Smith, J., and J.W. Doran. 1996. Measurement and use of pH and electrical conductivity for soil quality analysis. p. 169–185. In J.W. Doran and A.J. Jones (ed.) Methods for assessing soil quality. SSSA Spec. Publ. 49. SSSA, Madison, WI. Smith, S.R. 1996. Agricultural recycling of sewage sludge and the environment. p. 155–206. In Nutrients. CAB Int., Wallingford, UK. Smith, S.R., V. Woods, and T.D. Evans. 1998. Nitrate dynamics in biosolids-treated soils. I. Influence of biosolids type and soil type. Bioresour. Technol. 66:139–149. Sort, X., and J.M. Alcaniz. 1999. Modification of soil porosity after application of sewage sludge. Soil Tillage Res. 49:337–345. Sui, Y., M.L. Thompson, and C.W. Mise. 1999. Redistribution of biosolids-derived total phosphorus applied to a Mollisol. J. Environ. Qual. 28:1068–1074. Tester, C.F. 1990. Organic amendment effects on physical and chemical properties of a sandy soil. Soil Sci. Soc. Am. Proc. 54:827–831. Warman, P.R., and W.C. Termeer. 2005. Evaluation of sewage sludge, septic waste and sludge compost applications to corn and forage: Yields and N, P and K content of crops and soils. Bioresour. Technol. 96:955–961. Watson, J.E., I.L. Pepper, M. Unger, and W.H. Fuller. 1985. Yields and leaf elemental composition of cotton grown on sludge-amended soil. J. Environ. Qual. 14:174–177.

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