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Published in Agron J 99:1597-1606 (2007)
DOI: 10.2134/agronj2007.0019
© 2007 American Society of Agronomy
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Waste Management

Application of Different Organic Wastes on Soil Properties and Wheat Yield

M. Tejadaa,* and J. L. Gonzalez{dagger}

a Dep. de Cristalografía, Mineralogía y Química Agrícola, E.U.I.T.A. Univ. de Sevilla, Crta de Utrera km. 1, 41013 Seville, Spain
b Dep. de Química Agrícola y Edafología, Univ. de Córdoba, Campus de Rabanales, Edificio C-3, Crta N-IV-a, km. 396, 14014 Córdoba, Spain

* Corresponding author (mtmoral{at}us.es)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Fresh and composted organic wastes [beet vinasse (BV), sewage sludge (SS), and a cotton gin crushed compost (CCGC)], were applied for 4 yr to a Typic Xerofluvent in dryland conditions near Seville, Spain. Organic wastes were applied at rates of 5, 7.5, and 10 Mg organic matter ha–1, respectively. The effect on the soil's physical properties, soil microbial biomass, and six soil enzymatic activities (dehydrogenase, urease, protease, ß-glucosidase, arylsulfatase, and alkaline phosphatase activities) and the yield parameters of wheat (Triticum aestivum cv. Cajeme) were ascertained. The application of CCGC compost improved the soil's physical (structural stability, bulk density), chemical (exchangeable sodium percentage, ESP), and biological properties (microbial biomass, soil respiration, and enzymatic activities) and the wheat yield parameters; however, the application of SS adversely affected the soil biological properties and reduced the wheat yield, probably because high of amounts of heavy metals. The application of fresh BV also adversely affected the soil's physical, chemical, and biological properties and the wheat yield, probably because high amounts of Na and fulvic acids were introduced into the soil by the vinasse, which destabilized its structure. Wheat yield decreased 22.5% in BV with respect to CCGC-amended soil, 13.6% in SS with respect to CCGC-amended soil, and 7.9% in BV with respect to SS-amended soil. These results suggest that the chemical composition of the three organic wastes notably influenced the soil properties and therefore the wheat yield parameters. Of the three organic wastes studied, alone the application of CCGC originated a positive effect in the soil and in the wheat yield parameters, while the application of BV and SS originated a negative effect in the soil properties and therefore in the wheat yield parameters.

Abbreviations: BV, fresh beet vinasse • CCGC, crushed cotton gin compost • ESP, exchangeable sodium percentage • SS, sewage sludge

Application of Different Organic Wastes on Soil Properties and Wheat Yield

M. Tejadaa,* and J. L. Gonzalez{dagger}

a Dep. de Cristalografía, Mineralogía y Química Agrícola, E.U.I.T.A. Univ. de Sevilla, Crta de Utrera km. 1, 41013 Seville, Spain
b Dep. de Química Agrícola y Edafología, Univ. de Córdoba, Campus de Rabanales, Edificio C-3, Crta N-IV-a, km. 396, 14014 Córdoba, Spain

* Corresponding author (mtmoral{at}us.es)

Received for publication January 13, 2007.
Fresh and composted organic wastes [beet vinasse (BV), sewage sludge (SS), and a cotton gin crushed compost (CCGC)], were applied for 4 yr to a Typic Xerofluvent in dryland conditions near Seville, Spain. Organic wastes were applied at rates of 5, 7.5, and 10 Mg organic matter ha–1, respectively. The effect on the soil's physical properties, soil microbial biomass, and six soil enzymatic activities (dehydrogenase, urease, protease, ß-glucosidase, arylsulfatase, and alkaline phosphatase activities) and the yield parameters of wheat (Triticum aestivum cv. Cajeme) were ascertained. The application of CCGC compost improved the soil's physical (structural stability, bulk density), chemical (exchangeable sodium percentage, ESP), and biological properties (microbial biomass, soil respiration, and enzymatic activities) and the wheat yield parameters; however, the application of SS adversely affected the soil biological properties and reduced the wheat yield, probably because high of amounts of heavy metals. The application of fresh BV also adversely affected the soil's physical, chemical, and biological properties and the wheat yield, probably because high amounts of Na and fulvic acids were introduced into the soil by the vinasse, which destabilized its structure. Wheat yield decreased 22.5% in BV with respect to CCGC-amended soil, 13.6% in SS with respect to CCGC-amended soil, and 7.9% in BV with respect to SS-amended soil. These results suggest that the chemical composition of the three organic wastes notably influenced the soil properties and therefore the wheat yield parameters. Of the three organic wastes studied, alone the application of CCGC originated a positive effect in the soil and in the wheat yield parameters, while the application of BV and SS originated a negative effect in the soil properties and therefore in the wheat yield parameters.

Abbreviations: BV, fresh beet vinasse • CCGC, crushed cotton gin compost • ESP, exchangeable sodium percentage • SS, sewage sludge


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
TWO IMPORTANT CHARACTERISTICS of the soils of the Mediterranean region is they are subjected to erosion and desertification processes and that they have a low organic matter content. Intensive cultivation, continuous plowing, and forest fires, combined with years of unsuitable agricultural practices, have had an important effect on humification processes and on properties associated with degradation. This has lead to a dramatic decrease in the quantity of organic remains contributed to the soil, while the humus is undergoing a process of accelerated mineralization as a result of tilling. The result is a progressive decrease of the organic matter content and the negative consequences entailed (Ortiz, 1990; Tejada et al., 2001).

For this reason, in the last decade the application of organic wastes with a high organic matter content, such as animal manure (Haynes and Naidu, 1998), SS (Fließbach et al., 1994; Albiach et al., 2001), city refuse (Giusquiani et al., 1995; Eriksen et al., 1999), compost (Sikora and Enkiri, 1999; Tejada and Gonzalez, 2003a), crop residues (De Neve and Hofman, 2000; Trinsoutrot et al., 2000), and byproducts (Madejon et al., 2001; Tejada and Gonzalez, 2003b, 2004) to soil is a current environmental and agricultural practice for maintaining soil organic matter, reclaiming degraded soils, and supplying plant nutrients.

However, the influence of organic matter on soil physical properties depends on amount, type, and size of added organic materials (Nelson and Oades, 1998; Barzegar et al., 2002). The effect of each organic material on soil physical properties depends on its dominant component.

Since many enzymes respond immediately to changes in soil fertility status, they can be used as potential indicators of soil quality for sustainable management (Garcia et al., 2000). Enzymes may react to changes in soil management more quickly than other variables and therefore may be useful as early indicators of biological changes (Bandick and Dick, 1999; Masciandaro et al., 2004). In fact, they may also indicate the soil's potential to sustain microbiological activity (Paul and Clarck, 1989).

Oxidoreductases and hydrolases enzymes act on the basic processes of organic matter decomposition. In this respect, dehydrogenase activity is an oxidoreductase enzyme which has been used as a measurement of overall microbial activity (Garcia et al., 1997; Pascual et al., 1998; Masciandaro et al., 2004), since it is an intracellular enzyme related to the oxidative phosphorylation process (Trevors, 1986). Other hydrolytic enzymes involved in the cycling of principal nutrients (such as ß-glucosidase, urease, phosphatase, and arylsulfatase linked to C, N, P, and S) are sensitive indicators of management-induced changes in soil properties due to their strong relationship with soil organic matter content and quality (Pascual et al., 1998; Masciandaro and Ceccanti, 1999; Masciandaro et al., 2004). These parameters are the most sensitive to the changes which occur in a soil, provide rapid and accurate information on changes in soil quality, and will help determine the best ways of maintaining sustainable productivity.

The objectives of this study were to evaluate the effects of using three organic wastes (BV, CCGC, and SS) as soil amendment at different rates on soil physical and biological properties, soil microbial activity, and the yield of wheat grown in a semiarid Mediterranean agro-ecosystem.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil and Organic Wastes
The study was conducted from October 1998 to October 2002 near Seville, Spain (Guadalquivir Valley, Andalusia), on a Typic Xerofluvent with a 2% slope. The general properties of this soil (0–25 cm) are shown in Table 1 . The organic wastes applied were BV, a CCGC, and a SS. The general properties of the organic wastes used are shown in Table 2 . Organic matter content was determined by dry combustion method (MAPA, 1986). To determine amounts of humic and fulvic acid-C, BV was extracted with 0.1 M sodium pyrophosphate and 0.1 sodium hydroxide. The supernatant was acidified to pH 2 with HCl and allowed to stand for 24 h at room temperature. To separate humic acids from fulvic acids, the solution was centrifuged and the precipitate containing humic acids was dissolved with sodium hydroxide (Yeomans and Bremner, 1988). The carbon content of humic acid and fulvic acids was determined by the method of Sims and Haby (1971). For BV, inorganic soluble P (PO4H2– principally) was determined by the Willians and Stewart method, as described by Guitian and Carballas (1976). For CCGC and SS, inorganic soluble P was determined by the Willians and Stewart method, as described by Guitian and Carballas (1976) after nitric and perchloric acid digestion. For BV, K and Na were determined by an atomic emission spectrometer, and Ca, Mg, Fe, Cu, Mn, Zn, Cd, Ni, Cr, and Hg were determined by atomic absorption spectrometer. For CCGC and SS, Ca, Mg, Fe, Cu, Mn, Zn, Cd, Pb, Ni, Cr, and Hg were determined by atomic absorption spectrometer after nitric and perchloric acid digestion. Potassium and Na were determined by atomic emission spectrometer after nitric and perchloric acid digestion.


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  		Table 1. Main soil characteristics at the beginning of the experiment (data are the means of six samples). Standard error in parentheses.{dagger}

 

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  		Table 2. Characteristics of organic wastes used (data are the means of six samples, standard error in parentheses).{dagger}

 
Experimental Layout and Treatments
The experimental layout was in a randomized complete block design with 10 treatments and three replicates per treatment. The plot size was 9 by 5 m. The treatments were the following: (1) nonfertilized control plot; (2, 3, and 4) fertilized with 14.04 Mg ha–1 of CCGC (5 Mg fresh organic matter ha–1), 21.06 Mg ha–1 of CCGC (7.5 Mg fresh organic matter ha–1), or 28.08 Mg ha–1 of CCGC (10 Mg fresh organic matter ha–1); (5, 6, and 7) fertilized with 12.56 Mg ha–1 of BV (5 Mg fresh organic matter ha–1, 1424 L ha–1), 18.84 Mg ha–1 of BV (7.5 Mg fresh organic matter ha–1, 6141 L ha–1), and 25.12 Mg ha–1 of BV (10 Mg fresh organic matter ha–1, 8188 L ha–1); (8, 9, and 10) fertilized with 12.79 Mg ha–1 of SS (5 Mg fresh organic matter ha–1), 19.19 Mg ha–1 of SS (7.5 Mg fresh organic matter ha–1), and 25.59 Mg ha–1 of SS (10 Mg fresh organic matter ha–1).

The organic wastes were applied to the soil surface on 13 Oct. 1998, 12 Oct. 1999, 14 Oct. 2000, and 11 Oct. 2001, and incorporated to a 25-cm depth by chisel plowing and disking the day after application. The quality of the byproducts was the same for the three experimental seasons. In this respect, the byproducts were kept refrigerated at 0°C after its application in the first experimental season, so that there were no problems of mineralization of the organic compounds from these byproducts. Also, the plots received 150 kg N ha–1 (as NH4NO3) surface broadcast on 10 Oct. 1998, 9 Oct. 1999, 10 Oct. 2000, and 9 Oct. 2001.

The experiment was conducted under dryland conditions. The test crop, wheat (Triticum aestivum cv. Cajeme) was seeded at a rate of 150 kg ha–1, which is the common practice in the area. The sowing date was 5 Nov. 1998, 8 Nov. 1999, 7 Nov. 2000 and 9 Nov. 2001. The herbicides 2,4-D (2,4-dichlorophenoxyacetic acid) (27.5% w/v) and MCPA [(4-chloro-2-methylphenoxy) acetic acid] (27.5% w/v) were applied at a rate of 1 L ha–1 to prevent suboptimal plant growth conditions due to weed diseases.

Soil Sampling and Analytical Determinations
Soil samples (0–25 cm) were collected from each plot over a period of 4 yr (1999–2002) with a gauge auger (30-mm diam.) (11 Oct. 1999, 13 Oct. 2000, 10 Oct. 2001, and 15 Oct. 2002). Three subsamples were collected from each plot. After drying, the soil samples were ground to pass a 2-mm sieve and stored in sealed polyethylene bags at 4°C until chemical analysis.

Soil pH was determined in distilled water with a glass electrode (soil:H2O ratio 1:1). Soil electrical conductivity was determined in distilled water with a glass electrode (soil:H2O ratio 1:5). Soil texture was determined by the Robinson's pipette method (Soil Survey of England and Wales, 1982). Soil organic carbon was determined by oxidizing organic matter in soil samples with K2Cr2O7 in sulfuric acid (96%) for 30 min, and measuring the concentration of Cr3+ formed (Sims and Haby, 1971). Soil total N was determined by the Kjeldahl method (MAPA, 1986). Soil structural stability was determined by the Hénin and Monnier (1956) method. The aggregate size fraction < 2 mm was used. The proportions (%, w/w) of stable Ag, Aga and Agb aggregates (corresponding to untreated, alcohol-treated, and benzene-treated aggregates, respectively) were calculated, and the instability index, Is, was obtained using the equation

Formula 1[1]
where (% < 20 µm)max indicates the largest proportion of suspended particles < 20 µm determined for the three samples treatments, and %CS is the largest proportion of coarse sand (the 0.2–2 mm fraction) forming part of the stable aggregates.

Soil bulk density was determined using the core method. Metal cores of 6.1-cm length and 7.6-cm diam. were used to collect soil-core samples. The soil was weighed and dried at 105°C for 48 h before determining bulk density as the ratio between soil dry weight and the ring volume, according the official methods of the Spanish Ministry of Agriculture (MAPA, 1986).

The ESP was determined using the formula

Formula 2[2]
where Nax is the exchangeable sodium (cmol kg–1) and CEC is the cation exchange capacity of the soil (cmol kg–1). Exchangeable sodium (Nax) was determined with 1 M ammonium acetate at pH 7 (Richards, 1954), and the cation exchange capacity was determined with 1 M ammonium chloride solution in ethanol/water (60:40 v/v) at pH 8.2 (Tucker, 1954). Extracted Na was determined by flame photometry.

Soil microbial biomass was determined using the CHCl3 fumigation-extraction method (Vance et al., 1987). Samples of moist soil (10 g) were used, and K2SO4–extractable C was determined using dichromate digestion. Microbial biomass-C was calculated (Vance et al., 1987) using the equation

Formula 3[3]
where EC = (organic-C in K2SO4 from fumigated soil) – (organic-C in K2SO4 from unfumigated soil).

The levels of six enzymatic activities in the soil were measured. Dehydrogenase activity was determined by the method of Garcia et al. (1993). In this procedure, 0.1 g of soil was exposed to 0.2 mL of 4% INT (2-P-iodo-3-nitrophenyl 5-phenyl tetrazolium chloride) in distilled water for 20 h at 22°C in darkness. The INTF (iodonitrotetrazaolium formazan, 2-P-iodo-3-nitrophenyl) formed was extracted with 10 mL of mixture 1:1.5 ethylene chloride/acetone by shaking vigorously for 2 min. The INTF was measured spectrophotometrically at 490 nm. Controls were prepared without susbstrate.

Urease activity was determined by the buffered method of Kandeler and Gerber (1988). In this procedure, 0.5 mL of a solution of urea (0.48%) and 4 mL of borate buffer (pH 10) were added to 1 g of soil in hermetically sealed flasks, and then incubated for 2 h at 37°C. The ammonium content of the centrifuged extracts was determined by a modified indophenol-blue reaction. Controls were prepared without substrate to determine the ammonium produced in the absence of added urea.

Protease activity in the form of N-{alpha}-benzoyl-L-argininamide (BBA) protease was measured by a modification of the method proposed by Nannipieri et al. (1980). Phosphate buffer (2 mL, pH 7) and 0.5 mL of substrate (0.03 M N-{alpha}-benzoyl-L-argininamide) were added to 0.5 g of soil. Again, controls were prepared without substrate.

Alkaline phosphatase activity was measured by the method of Tabatabai and Bremner (1969) except that incubation was at 30°C in maleate buffer (2 mL, pH 6.5) for 90 min and 0.5 mL of substrate (0.115 P-nitrophenyl phosphate) were added to 0.5 g to soil. Controls were prepared without substrate.

ß-glucosidase activity was determined using 2 mL of 0.1 M maleate buffer (pH 6.5) and 0.5 mL of 50 mM P-nitrophenyl-ß-D-glucopyranoside (PNG) at 0.5 g of soil. The rest of method was the same as for alkaline phosphatase activity (Masciandaro et al., 1994).

Arylsulfatase activity was determined by the method of Tabatabai and Bremner (1970). Four milliliters of acetate buffer (pH 5.8) and 1 mL of P-nitrophenylsulphate were added to 1 g of soil and then incubated for 1 h at 37°C. One milliliter of 0.5 M CaCl2 and 10 mL of 0.5 M NaOH were then added. P-nitrophenyl was determined in a spectrophotometer at 410 nm. Again, controls were prepared without substrate.

In the laboratory, and in the samples at the end of the experiment, soil respiration for all treatments was measured by incubation for 0, 3, 7, 15, 30, 45, 60, 90, and 120 d. Total CO2–C collected in the NaOH flasks was determined by the addition of an excess of 1.5 M BaCl2 followed by titration with standardized HCl using a phenolphthalein indicator (Zibilske, 1994).

Statistical Analysis
Analysis of variance was performed using the Statgraphics v. 5.0 software package (Statistical Graphics Corporation, 1991, p. 105). The ANOVA was based on the LSD criterion (least significant differences between means using Student's t), considering a significance level of P < 0.05 throughout the study. For the ANOVA analysis, the triplicate data were used for each fertilizer treatment and every experimental season, although the values in the tables are the average of the triplicate. To compare the effects of each byproduct applied to the soil, three correlation matrices (for each organic waste) between all the parameters measured at the end of each experimental season were calculated, and the significance of the correlation coefficients is shown by using *, **, and *** to indicate the 0.05, 0.01, and 0.001 probability levels, respectively.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Structural Stability, Bulk Density, and ESP in Soils
Table 3 shows the structural stability and bulk density evolution in soils amended with the three byproducts. Structural stability increased in soils amended with CCGC and SS, but decreased in soils amended with BV. At the end of the experimental period and at the highest rates, structural stability increased 27.2 and 11% in CCGC- and SS-amended soils with respect to the control soil, and decreased 27.9% in BV-amended soils. Statistical analysis showed differences between the treatments at the end of experimental period. Soil bulk density decreased in CCGC and SS-amended soils, and by the end of experimental period and for the high rate, this property decreased by 19.9 and 17.8%, respectively, compared with the control soil. Soil bulk density increased in BV-amended soil. At a higher rate and at the end of the experimental period, soil bulk density increased 19.2% compared with the control soil. Again, the statistical analysis showed differences between the treatments at the end of experimental period.


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  		Table 3. Instability index (log 10 Is) and bulk density in soils amended with organic wastes. Standard error in parentheses.{dagger}

 
Because the same amount of organic matter was added to the soils, the variability in the results obtained in the soil structural stability and soil bulk density must have been due to the different chemical nature of the organic wastes added to the soil (Table 2).

The ESP increased more in BV-amended soils than in the CCGC and SS-amended soils during the experimental period (Table 4 ). For CCGC- and SS-amended soils, ESP did not reach the critical sodicity value of around 15 established by Richards (1954). However, for BV-amended soils at the end of the experimental period ESP did reach critical sodicity values (ESP = 15.4). The control soil showed the lowest ESP value of all treatments assayed.


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  		Table 4. Exchangeable sodium percentage (ESP) in soils amended with organic wastes. Standard error in parentheses.{dagger}

 
Soil Microbial Properties
Table 5 shows the cumulative CO2–C values determined in the soil during the experimental period, and Table 6 shows the soil microbial biomass. Initially, the results seem contradictory since the data for the soil microbial biomass and soil respiration rates are very different for the same dose of organic matter applied to the soil. At the end of the experimental period and at the highest rates, soil microbial biomass increased 75.5 and 46.5% in CCGC- and SS-amended soils compared with the control soil, whereas it decreased 65% in BV-amended soils with respect to the control soil. Soil respiration increased 63.6 and 12% in CCGC- and SS-amended soils compared to the control soil, whereas it decreased 53% in BV-amended soils compared with the control soil.


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  		Table 5. Cumulative CO2–C (mg kg soil–1) during incubation in soils amended with organic wastes. Standard error in parentheses.{dagger}

 

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  		Table 6. Microbial biomass (µg C g–1 dry soil) in soils amended with organic wastes. Standard error in parentheses.{dagger}

 
With respect to the soil enzymatic activities analyzed (Tables 7 Go– 9) , the highest values were observed at the end of experimental period for CCGC-amended soils, followed by SS- and BV-amended soils, respectively. Also, soil enzymatic activities decreased from the third experimental season for SS-amended soils and decreased from the second experimental season for BV-amended soils. For CCGC-amended soil, soil enzymatic activities increased during the experimental period.


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  		Table 7. Dehydrogenase and urease activities in soils amended with organic wastes. Standard error in parentheses.{dagger}

 

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  		Table 8. N-{alpha}-benzoyl-L-argininamide (BBA) protease and ß-glucosidase activities in soils amended with organic wastes. Standard error in parentheses.{dagger}

 

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  		Table 9. Arylsulfatase and alkaline phosphatase activities in soils amended with organic wastes. Standard error in parentheses.{dagger}

 
Crop Yield Parameters
Tables 10 and 11 show crop yield parameters for the different treatments during the experimental period. As can be seen, the yield characteristics and yield parameters increased with increasing amounts of CCGC applied to soil. These values are similar to values reported by Gonzalez et al. (1992) for the same wheat variety fertilized with pig slurry compost in the Guadalquivir Valley (similar pedoclimatic conditions). Also, wheat yield parameters of the fourth experimental season were better than those of the third, second, and first experimental seasons respectively, due to the residual effect of the organic matter from each organic waste application. At the end of the experimental period, the number of grains spike–1 decreased 23% in BV with respect to CCGC-amended soils, 10% in SS with respect to CCGC-amended soils, and 11.6% in SS with respect to BV-amended soils. At the end of the experimental period, the application of BV led to a significant decrease in the number spikes m–2 (4.7% compared with CCGC, and 2.3% compared with SS). In addition, wheat yield decreased 22.5% in BV with respect to CCGC-amended soil, 13.6% in SS with respect to CCGC-amended soil, and 7.9% in BV with respect to SS-amended soil.


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  		Table 10. Number of wheat grains per spike and number of spikes per square meter in soils amended with organic wastes. Standard error in parentheses.{dagger}

 

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  		Table 11. Wheat yield in soils amended with organic wastes. Standard error in parentheses.{dagger}

 
Correlation Matrices
To compare the effects of each organic waste applied to the soil, three correlation matrices between all the parameters measured were calculated to obtain correlations between these parameters.

Table 12 shows positive correlations between instability index (log 10 Is) with soil bulk density (P < 0.001), and negative correlations with the soil enzymatic activities (P < 0.001) and wheat yield (P < 0.01) for CCGC-amended soils. Positive correlations between soil enzymatic activities, microbial biomass, and crop yield were observed (P < 0.001).


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  		Table 12. Correlation matrix for all parameters determined in crushed-cotton-gin-compost-amended soil (n = 36).{dagger}

 
Positive correlations between instability index (log 10 Is) with soil bulk density and ESP (P < 0.001) were observed in Table 13 for BV-amended soils. A negative correlation between ESP and yield crop P < 0.05) as observed. Also, positive correlations between soil enzymatic activities, microbial biomass, and crop yield were observed (P < 0.001).


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  		Table 13. Correlation matrix for all parameters determined in fresh-beet-vinasse-amended soil (n = 36).{dagger}

 
For the SS-amended soils, positive correlations between instability index (log 10 Is) with soil bulk density (P < 0.001) were observed (Table 14 ). No correlations between ESP and crop yield were observed. Positive correlations between soil enzymatic activities, microbial biomass, and crop yield were also observed (P < 0.001).


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  		Table 14. Correlation matrix for all parameters determined in sewage-sludge-amended soil (n = 36).{dagger}

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Physical and Chemical Properties
Our results obtained for CCGC and SS-amended soils (in which the organic matter increased the soil structural stability, especially at high organic waste doses and at the end of the experimental period) agree with those of Chenu et al. (2000), Puget et al. (2000) and Tejada and Gonzalez (2003b, 2004), who found that a good soil structure depended on the content and nature of the organic matter added. Organic matter promotes flocculation of clay minerals, which is an essential condition for the aggregation of soil particles. In addition, bulk density is decreased as a result of dilution of the denser soil mineral fraction, and soil aeration increases because of the increase in soil porosity and increased structural stability. This increase was especially evident for high rates of CCGC and SS at the end of the experimental period and was in line with the results reported by Kay and VandenBygaart (2002) and Tejada and Gonzalez (2003b, 2004).

However, the data on soil structural stability and soil aeration indicate better physical properties in CCGC-amended soils than for SS-amended soils. This may be due to the different humic and fulvic acid concentration in the organic wastes. In this respect, it is well known that the less-oxidized, higher-molecular-weight humic matter is more important in the process of aggregate stabilization than more-oxidized humic substances of lower molecular weight. Chaney and Swift (1984) and Piccolo and Mbagwu (1990) suggested that the aggregate stability is significantly correlated with humic but not fulvic acid concentration because the humic acids are directly involved in the clay-organic complex formation, whereas the fulvic acids are not. Sewage sludge is a byproduct with the lowest humic acid-C and highest fulvic acid-C concentrations. Fulvic acids are macromolecules with a lower polymerization index than humic acids. Hence, the applied organic matter may not have flocculated with clay minerals (an essential condition for the aggregation of soil particles), which resulted in less stable aggregates formed (Piccolo and Mbagwu, 1990; Porta et al., 1994).

The adverse effect which BV had on the soil structure may be due to its composition, since BV contains a high concentration of Na+ and fulvic acids. This interpretation would agree with Haynes and Naidu (1998) and Grahan et al. (2002), who also found that adding large quantities of organic manures to soils destroyed the soil structure presumably because of the high amounts of Na+ transported into the soil. We assume that with the high doses of BV, not only the monovalent cations but also the fulvic acids were responsible for the degradation of the soil structure. Furthermore, ESP increased during the experimental period at the highest BV doses. According to Mamedov et al. (2002), this increase points to dispersibility and aggregate disintegration in these soils.

Microbial Activity in Amendment Soil
The supply of readily metabolizable C in the organic byproduct is likely to have been the most influential factor contributing to the biomass-C increases. In this respect, and according to De Neve and Hofman (2000) and Tejada and Gonzalez (2003a,b, 2004), soil microbial biomass responds rapidly, in terms of activity, to additions of readily available C. The positive effect on microbial biomass observed in the soils amended with compost is due to a direct (microbial growth in these composts, Pascual et al., 1998) and indirect effect (improvement of plant growth).

Our results showed that an increase in soil microbial biomass lowered the soil instability index (log 10Is). Several studies have indicated that soil microbial processes are directly and indirectly influenced by soil structure. The presence of small pores reduces accessibility of organic materials to decomposers, causing the physical protection of C and a reduction in N mineralization (Van Veen and Kuikman, 1990). The spatial distribution of microbes and soil mesofauna has been shown to be partially associated with the size distribution of aggregates (Jastrow and Miller, 1991).

Soil enzymes are biological catalysts of specific reactions and these reactions, in turn, depend on a variety of factors (Burns, 1978), such as the presence or absence of inhibitors, type of amendment, and crop type. Soil enzymes are good markers of soil fertility since they are involved in the cycling of the most important nutrients. The incorporation of organic amendments influences soil enzymatic activities because the added material may contain intra- and extracellular enzymes and may also stimulate microbial activity in the soil (Goyal et al., 1993; Pascual et al., 1998). The development of microbial populations, which is favored by the root exudates of plants, may also be responsible for dehydrogenase activity stimulation. The greater dehydrogenase activity noted at the high dosage suggests that the added compost did not include compounds which were toxic for this activity (Pascual et al., 1998).

The stimulation of urease and protease activity BAA related to the N cycle suggests that the treatment used (CCGC) does not include compounds toxic for this activity, or that microbial growth and/or the addition of microbial cells or enzymes with the amendment counteract any inhibitory effect due to toxic compounds. The demand for N by both plants and soil microorganisms was probably responsible for the increase of this enzyme activity. Garcia et al. (1994) studied the influence of some toxic compounds contained in organic amendments such as municipal solid wastes on soil microbial activity in semiarid zones; they found that the positive effect of the organic matter on biological soil quality counteracted the negative effect produced by these toxic compounds. Organic amendment by the compost studied had a positive effect on the activity of these enzymes, particularly when the amendment was at the high dose, probably due to the higher microbial biomass produced in response.

Also, soil arylsulfatase activity and soil phosphatase activity was higher in the CCGC-amended soils than in SS-amended soils. The demand of P by plants and soil microorganisms can be responsible for the stimulation of the synthesis of this enzyme (Garcia et al., 1994). In addition, the processes related to degradation of organic matter may be followed through hydrolases such as phosphatase. According to Rao and Tarafdar (1992), increases in phosphatase activity (as we have detected in the treated soils) indicate changes in the quantity and quality of soil phosphoryl substrates. The supply of readily metabolisable C in the organic byproduct is likely to have been the most influential factor contributing to the soil arylsulfatase activity and soil phosphatase activity increases.

For BV- and SS-amended soils, the effect on soil parameters is very different from compost-amended soils, with fresh BV and SS organic matter addition decreasing the soil microbial biomass, soil respiration, and soil enzymatic activities. Previous research indicates the application of organic wastes decreases soil microbial biomass. Brendecke et al. (1993), Fließbach et al. (1994), and Filip and Bielek (2002) reported a decrease of soil microbial biomass after a 10-yr application of 5 and 15 Mg ha–1 yr–1 of fresh organic matter (SS). These authors indicated that the presence of high amounts of heavy metals (e.g., Cd, Cr, Hg, Pb) in this byproduct may counterbalance the positive effects of organic matter in soil microbial biomass. Possibly the important heavy metal concentration in the SS is the cause by which the data of microbial soil biomass, soil respiration, and soil enzymatic activities obtained for SS-amended soils are inferior to the ones obtained for CCGC-amended soils. The BV analyses indicated very low concentrations of Cd, Cr, Hg, and Pb; therefore, the observed inhibition cannot be due to the heavy metal content. Perhaps this inhibition could be caused by labile organic toxic compounds and/or an increase of the electrical conductivity in soil with the BV addition (Garcia and Hernandez, 1996).

Our data indicated that increasing the dose of BV to the soil decreases the soil structural stability, soil microbial biomass, and soil respiration, whereas soil bulk density increased. According to Tate (2002), oxygen concentration in soil can affect the metabolic status of the enzyme-producing cells. Disruption of soil aggregates negatively alters the oxygen diffusion rate from the atmosphere above the soil into the soil matrix as well as the rate of its consumption.

To understand why the enzyme activities are inhibited by BV addition is not easy. For example, phosphatase inhibition could be caused either by an excess of inorganic P (Nannipieri et al., 1990), or by heavy metals incorporated into the soil with the organic waste. But, in our case, the BV does not have a high quantity of either P or heavy metals. We think this inhibition is probably due to the adverse effect on structural soil (probably by an excess of labile organic matter), and the increase of salinity.

Our results indicate that the application of BV to the soil exerts a greater inhibiting effect than that exerted by SS. Poor soil structure and therefore decreased soil aeration negatively affects the soil microbial biomass more than high heavy metal concentrations.

Wheat Yield Crop
Since soil enzymatic activities are responsible for important cycles such as C, N, P, and S, wheat yield parameters increased significantly when a higher dose of CCGC was applied to the soil.

Possibly, the high levels of heavy metals in SS exert an inhibiting effect in soil biological properties. Many authors have reported the negative effects of heavy metal contamination on the microbiological characteristics of soils; for example, Hattori (2000) found that CO2 production was significantly depressed in soils contaminated with heavy metals. Also, Kizilkaya et al. (2004) found that microbial biomass carbon and some enzyme activities decrease in heavy metal contaminated soils. Since the decrease in microbial biomass may reduce microbial functionality of soil and therefore decrease the N, P, and S available levels to plants, wheat yield parameters decreased significantly when a higher rate of SS was applied to the soil. These results are in agreement with Guiller et al. (1998), who found a decrease in subsequent activity of soil microbial communities and therefore a decrease in plant quality and yield.

However, the degradation progressive of soil structure after the application of BV caused a greater inhibiting effect of soil biological properties than that exerted by SS. For this reason, the SS wheat yield parameters were lower than that obtained in the BV-amended soils.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The influence of organic matter on soil properties and crop yield depends on their chemical composition. The application of CCGC to the soil at rates studied under dryland conditions produced an improvement in the soil physical and biological properties as well as in the production and quality of wheat. However, the application of SS at rates studied caused a decrease in soil biological properties and wheat yield parameters. The higher amounts of heavy metals may be responsible for this result. Also, the application of fresh BV to the soil caused a decrease in soil physical and biological properties and wheat yield parameters, in spite of having a high organic matter content. Perhaps the increase in Na+ and labile organic matter (possible inhibitor compounds) contributed to these observations. These results suggest that soil microbial biomass is affected negatively by the soil physical properties (structure and aeration) more than by exogenous amounts of heavy metals that can be applied to the soil.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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