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Published online 3 May 2006
Published in Agron J 98:749-759 (2006)
DOI: 10.2134/agronj2005.0165
© 2006 American Society of Agronomy
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Production Papers

Crushed Cotton Gin Compost Effects on Soil Biological Properties, Nutrient Leaching Losses, and Maize Yield

M. Tejadaa,* and J. L. Gonzalezb

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

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

Received for publication June 1, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
There is currently interest in the use of industrial by-products to reduce the use of synthetic fertilizers; however, most organic wastes contain relatively low N levels. Our objectives were to: (i) determine the effect of incorporating crushed cotton gin compost, with and without inorganic fertilizers, on soil biological properties during three maize crops (Zea mays L., cv. Tundra); (ii) study nutrient leaching losses from soils receiving these fertilizer treatments; and (iii) to evaluate the effect of these fertilizer treatments on nutrition and yield of a maize crop. Compost was applied at 0, 20, and 40 t ha–1 rates with and without 400 kg N ha–1 (as NH4NO3), 80 kg P ha–1 [as (NH4)H2PO4], and 120 kg K ha–1 (as K2SO4) on a Typic Xerofluvent located near Sevilla (Andalusia, Spain) for 3 yr. At the end of the study, soil microbial biomass was 32% higher in the treatment including inorganic fertilizer than in the compost-only treatment. Soil biochemical properties were greater in the fertilizer treatment than the compost-only treatment (by 61, 50, 36, and 32% for dehydrogenase, N-{alpha}-benzoyl-L-argininamide protease, arylsulfatase, and phosphatase activities). Macronutrient losses were greatest in the treatment including inorganic fertilizer, where increases of 24% for inorganic N, 31% for P, and 18.5% for K over the compost treatment were noted. Lowest N/P ratios were produced by the treatment including inorganic fertilizer, which suggest a lower eutrophication risk in drainage waters from soils treated with this fertilizer. The mineral nutrition, grain protein, and maize yield indicate that the compost plus inorganic fertilizer is adequate and has a good potential for use.

Abbreviations: CC, crushed cotton gin compost • CCM, crushed cotton gin compost plus N


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE massive use of synthetic fertilizers and management practices such as stubble burning have greatly reduced the organic matter content of soils, with direct impacts on the physical, chemical, and biological properties of the soils, and the risk of degradation of these soils. The main consequence of these agronomic practices could be the mineralization and desertification of the soil (Tejada et al., 2001).

In recent years, the application of organic wastes to soil has been demonstrated as an effective environmental and agricultural practice for maintaining soil organic matter, reclaiming degraded soils, and supplying plant nutrients. Sources of organic waste include animal manure (Haynes and Naidu, 1998), sewage sludge (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 by-products with higher organic matter content (Tejada and Gonzalez, 2003b, 2004).

Compost may improve the stability of soil aggregates and, applied as a mulch, can reduce erosion risk (Pinamonti and Zorzi, 1996). Compost may increase soil porosity and water-holding capacity (McConell et al., 1993; Giusquiani et al., 1995), decrease soil acidification (Bengtson and Cornette, 1973), and release nutrients to plants (Sikora and Enkiri, 1999; Chodak et al., 2001). These, and additional effects (e.g., suppression of pathogenic microorganisms), have positive effects on plant growth and health (Pinamonti and Zorzi, 1996).

Most composts contain relatively low levels of nutrients (10–20 g N kg–1,<10 g P kg–1) compared with complete fertilizer (Sikora and Enkiri, 1999). In addition, low mineralization rates require high application rates (40–100 Mg ha–1) to satisfy the complete N or P requirement of a crop (Sikora and Enkiri, 1999).

A balance between adequate crop fertilization and the possible environmental effects caused by overfertilization must exist. Jackson and Smith (1997) and Hansen et al. (2000) suggested the use of organic fertilizers vs. inorganic fertilizers to diminish nutrient leaching losses and thus the eutrophication of running and still waters. While greatly improving the physical properties of the soil, organic matter added via organic fertilizers needs time to mineralize and supply crop nutrients. Moreover, a large quantity of product is needed to fulfill the nutritional requirements of the crops. Some researchers have suggested the addition of mineral as well as organic fertilizers to increase nutrient density of the fertilizer material and to supply the nutrients required by plants in the early stages of development (Gonzalez et al., 1992; Baron et al., 1995; Tejada and Gonzalez, 2003a, 2003b); however, greater N rates resulting from the addition of inorganic fertilizer may increase the likelihood and magnitude of N leaching and associated environmental problems. Nitrogen leaching can occur from organic as well as inorganic N sources and is thus of great interest regardless of the N source.

For this reason, our first objective was to evaluate the effect of a crushed cotton gin compost with and without inorganic fertilizers on soil biological properties. The second objective was to study nutrient leaching losses for both fertilizer treatments, and the third objective was to test the action of these fertilizer treatments on the nutrition and yield components of a maize crop.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site and Climatic Conditions
The study was conducted from March 2001 to September 2003 in Lora del Río, near Sevilla (Andalusia, Spain). The soil of the field experiment is a Typic Xerofluvent and general properties (0–25 cm) are shown in Table 1. The composting process for crushed cotton gin compost is described in detail by Tejada et al. (2001). General properties of the crushed cotton gin compost (CC) are shown in Table 2. Organic matter was determined by dry combustion (Ministerio de Agricultura, Pesca y Alimentación, 1986, p. 221–285). Humic and fulvic acids were determined by the methods of Ministerio de Agricultura, Pesca y Alimentación (1986, p. 221–285). Total N was determined by the Kjeldhal method (Ministerio de Agricultura, Pesca y Alimentación, 1986, p. 221–285). Phosphorus content was determined by the Willians and Stewart method, described by Guitian and Carballas (1976). Calcium, Mg, Fe, Cu, Mn, Zn, Cd, Pb, Ni, Cr, and Hg were determined by atomic absorption spectrometer after HNO3 and HClO4 digestion. Potassium and Na were determined by atomic emission spectrometer after HNO3 and HClO4 digestion.


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  		Table 1. Soil physicochemical and biological characteristics, mean of five samples (standard error in parentheses).

 

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  		Table 2. Characteristics of crushed cotton gin compost used in this study (standard errors in parentheses). Data are the means of six samples.

 
The total annual rainfall for the study area was 438, 301, and 306 mm and the mean air temperature was 18.7, 19.1, and 19.1°C for 2001, 2002, and 2003, respectively. These values are typical of a Mediterranean climate.

Experimental Layout and Treatments
The experimental layout was a randomized complete block with a total of 20 plots, each measuring 10 by 6 m. Five treatments were used (four replicates per treatment): (i) control soil, no compost added; (ii) CC1: 20 t ha–1 of compost; (iii) CC2: 40 t ha–1 of compost; (iv) CCM1: 20 t ha–1 of compost plus 400 kg N ha–1 (as NH4NO3), 80 kg P ha–1 (as NH4H2PO4), and 120 kg K ha–1 (as K2SO4); and (v) CCM2: 40 t ha–1 of compost plus 400 kg N ha–1 (as NH4NO3), 80 kg P ha–1 (as NH4H2PO4) and 120 kg K ha–1 (as K2SO4).

Mineral fertilizer was applied to soil on 11 Mar. 2001, 14 Mar. 2002, and 13 Mar. 2003 and incorporated to a 25-cm depth. The compost was surface broadcast on 12 Mar. 2001, 15 Mar. 2002, and 14 Mar. 2003 and incorporated to a 25-cm depth by chisel plowing and disking the day after application. The same compost was used for all experimental seasons. The compost was stored at –18°C after application in the first experimental season to avoid mineralization of the organic compounds in this by-product.

Maize (cv. Tundra) was chosen as the test crop, and seeded at a rate of 100000 plants ha–1 in 75-cm rows, which is a common practice in the area. Sowing dates were 15 Mar. 2001, 18 Mar. 2002, and 17 Mar. 2003. Crop yield and number of grains per cob were determined for samples collected from each plot on 30 Sept. 2001, 29 Sept. 2002, and 30 Sept. 2003.

In January 2001, 20 lysimeters were installed in situ without disturbing the soil profile. The lysimeters were in place throughout the study. The installation process and characteristics of the lysimeters were described in Tejada et al. (2005) Figure 1 shows the lysimeter design.


Figure 1
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  		Fig. 1. Design of the lysimeters used in this study.

 
Each lysimeter was sampled three times in relation to the maize cycle: (i) when maize was 30 cm high (17 Apr. 2001, 18 Apr. 2002, and 19 Apr. 2003); (ii) at tasseling (30 July 2001, 29 July 2002, and 29 July 2003); and (iii) at harvest (30 Sept. 2001, 29 Sept. 2002, and 28 Sept. 2003).

Soil Sampling, Determination of Drainage Water Discharge, Plant Sampling, and Analytical Determinations
Soil samples (0–25 cm) were collected from each plot (five replicates per plot) with a gauge auger (30-mm diam.) when maize was 30 cm high, at tasseling, and at harvest. After drying, soil samples were ground to pass a 2-mm sieve and stored in sealed polyethylene bags for laboratory analysis.

Soil pH was determined in distilled water with a glass electrode (1:1 soil/H2O ratio). Soil texture was determined by the Robinson's pipette method (Soil Survey of England and Wales, 1982). Soil organic C was determined by oxidizing organic matter in soil samples with K2Cr2O7 in H2SO4 (96%) for 30 min, and measuring the concentration of Cr3+ formed (Sims and Haby, 1971). Soil organic N was determined using the Kjeldahl method (Ministerio de Agricultura, Pesca y Alimentación, 1986, p. 221–285). The C/N ratio was calculated during the three maize cycles by dividing soil organic C by soil organic N.

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
where EC = (organic C in K2SO4 from fumigated soil) – (organic C in K2SO4 from unfumigated soil).

The activity levels of five soil enzymes 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% 2-p-iodo-3-nitrophenyl 5-phenyl tetrazolium chloride in distilled water for 20 h at 22°C in darkness. The iodonitrotetrazaolium formazan (INTF) formed was extracted with 10 mL of a 1:1.5 ethylene chloride/acetone mixture by shaking vigorously for 2 min. The INTF was measured spectrophotometrically at 490 nm. Controls were prepared without susbstrate.

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 BBA) were added to 0.5 g of soil. Again, controls were prepared without substrate.

Alkaline phosphatase activity was measured using 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) was added to 0.5 g of soil. Controls were prepared without substrate.

The ß-glucosidase activity was determined using 2 mL of 0.1 M maleate buffer (pH 6.5) and 0.5 mL of 50 mMp-nitrophenyl-ß-D-glucopyranoside in 0.5 g of soil. The rest of the 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 mL 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. The p-nitrophenyl content was determined in a spectrophotometer at 410 nm. Again, controls were prepared without substrate.

Drainage water was sampled throughout the study when the drain lines were flowing. The drainage water samples were frozen at –18°C until analysis and analyzed for NH4+–N by the colorimetric method of Kempers (1974); NO3–N, by the colorimetric method of Griess and Illosvay as modified by Barnes and Tolkard (1951) and Bremner (1965); inorganic N, calculated as the combined amounts of NH4+–N and NO3–N; P by the method of Williams and Stewart as described by Guitian and Carballas (1976); and K by emission spectrometry.

Plant samples were taken from each plot at three stages of the three maize growth cycles: (i) when maize was 30 cm high, (ii) at tasseling, and (iii) at harvest, by selecting a whole plant at the first date and an ear leaf at the other two (Tejada et al., 1992). The lyophilized samples were assayed for N before (Kjeldahl method) and after mineralization (Comité Interinstitutos para las Técnicas de Diagnostico Foliar, 1969), for P (by the method of Williams and Stewart as described by Guitian and Carballas, 1976), and for K (by atomic emission spectrophotometer).

Chlorophyll and total carotenoids in the lyophilized leaf samples were measured by extraction with methanol and quantified by the method of Lichtenthaler (1987). Leaf soluble carbohydrate contents were measured using the anthrone method (Yemm and Willis, 1954). Fifty-gram samples were collected from each plot. Dried leaf samples were extracted in 5 mL of 80% (v/v) ethanol (30 min, 30°C). The extract was centrifuged (10 min, 2650 g) and the pellet was extracted again with ethanol. After centrifugation, chlorophyll was removed from the combined supernatants by chloroform extraction. The samples were analyzed colorimetrically for soluble carbohydrates using the anthrone method.

Grain protein was determined as the product of grain N content, determined by the Kjeldahl method, and 6.25, a constant factor for the N in protein (Ministerio de Agricultura, Pesca y Alimentación, 1986, p. 221–285).

Statistical Analysis
All values of each experimental year were subjected to ANOVA using the Statgraphics v. 5.0 software package (Statistical Graphics Corp., 1991), with the fertilizer treatment for each experimental year as the independent variable. For the ANOVA analysis, the triplicate values of each treatment fertilizer and every year were used. The means were separated by Tukey's test, with a significance level of P < 0.05 throughout the study. Regression analysis was performed for observed inorganic N losses during the study, with treatment and years of the study as independent variables.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Properties
Data listed in Tables 3 and 4 show the changes in the soil microbial biomass and enzymatic activities in soils during the study.


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  		Table 3. Soil microbial biomass and dehydrogenase and N-{alpha}-benzoyl-L-argininamide (BBA) protease activities at harvest for the three years of the study.

 

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  		Table 4. Soil ß-glucosidase, arylsulfatase, and phosphates activities at harvest for the three years of the study.

 
Soil microbial biomass was significantly higher when compost plus the higher rate of supplemental N was applied than for compost alone. Soil microbial biomass for CCM2 was 32% greater than for CC2 and that for CCM1 increased 33% over that for CC1 at the end of the experiment. Soil microbial biomass was significantly higher for CC2 than for CC1 (20%) and higher for CCM2 than for CCM1 (19.2%) at the end of the study due to higher application rates of organic material. The highest values of soil microbial biomass were observed for the third season, followed by the second and the first seasons.

Except for soil ß-glucosidase activity, the highest soil enzymatic activity was observed for CCM2, followed by CCM1, CC2, CC1, and the control (Tables 3 and 4). Soil enzymatic activity increased in CCM2 over CC2 (by 61%, 24%, 36% and 24% for dehydrogenase, BBA-protease, arylsulfatase, and phosphatase activities, respectively) and increased in CCM1 over CC1 (by 57, 50, 30, and 32% for dehydrogenase, BBA-protease, arylsulfatase, and phosphatase activities, respectively) at the end of the study. Soil enzymatic activity at the end of the study was significantly higher in CC2 than CC1 (by 41, 41, 20, and 17% for dehydrogenase, BBA-protease, arylsulfatase and phosphatase activities, respectively) and higher in CCM2 than CCM1 (by 47, 9, 27, and 7% for dehydrogenase, BBA-protease, arylsulfatase, and phosphatase activities, respectively). This response was probably due to the increased rate of applied organic matter. Similar to microbial biomass, the highest values for soil enzymatic activity were observed for the third season, followed by the second and first seasons.

Table 5 shows the soil C/N ratio during the three seasons of the study. Optimum C/N ratio (10–12) was observed for the CCM2 treatment; however, the values obtained for CCM1 are also near optimum for organic matter mineralization. Values for CC1 and CC2 are slightly higher than for CCM2, while the control soil had the lowest C/N ratio.


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  		Table 5. Soil C/N ratio evolution during the three maize growth cycles.

 
Nutrient Leaching Losses
Figures 2 and 3 show NH4+–N, NO3––N, and inorganic N leached from the soil. The greatest amounts of inorganic N forms for each year were observed in CCM2, followed by CCM1, the control soil, CC2, and CC1 treatments. Application of mineral fertilizers with CC increased the inorganic-N losses, due mainly to the great residual effect of these treatments (Fig. 3). This increased residual effect of N in CCM1 and CCM2 treatments using greater amounts of organic matter is very important since mineralization in subsequent years will contribute more inorganic N to the soil.


Figure 2
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  		Fig. 2. Leaching losses of NH4+–N and NO3––N from soils. Error bars represent standard deviation. CC1: 20 t ha–1 of compost; CC2: 40 t ha–1 of compost; CCM1: 20 t ha–1 of compost plus 400 kg N ha–1 (as NH4NO3); CCM2: 40 t ha–1 of compost plus 400 kg N ha–1.

 

Figure 3
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  		Fig. 3. Leaching losses of total inorganic N for each year from soils (a) as total losses and regression analysis (b) for year and (c) treatment. Error bars represent the standard deviation. CC1: 20 t ha–1 of compost; CC2: 40 t ha–1 of compost; CCM1: 20 t ha–1 of compost plus 400 kg N ha–1 (as NH4NO3); CCM2: 40 t ha–1 of compost plus 400 kg N ha–1. Treatment 1: control soil; 2: CC1; 3: CC2; 4: CCM1; and 5: CCM2.

 
The highest losses were observed for 17 Apr. 2001, 18 Apr. 2002, and 19 Apr. 2003 (41% of the total losses), with NO3––N the most strongly leached fraction. In addition, the greatest losses in the different N fractions on these early dates were exhibited by CCM2 and CCM1. This is due mainly to losses of readily available N fractions coming from the mineral fertilizer and from organic matter mineralization of the organic fertilizer.

Figure 4 shows the P and K leaching losses during the study and the total leaching losses for each year. The highest losses corresponded to CCM2, followed by CCM1, the control soil, CC2, and CC1. Similar to inorganic-N forms, increased P and K losses were noted when mineral N plus CC was applied, mainly due to the greater residual effect of these treatments. During the 3 yr, there was 59% more P lost from CCM1 than CC1, and 44% more P lost from CCM2 than CC2. Potassium losses were 25% more for CCM1 than CC1 and 19% more for CCM2 than CC2 during the 3 yr.


Figure 4
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  		Fig. 4. Leaching losses of P and K from soils. Error bars represent the standard deviation. CC1: 20 t ha–1 of compost; CC2: 40 t ha–1 of compost; CCM1: 20 t ha–1 of compost plus 400 kg N ha–1 (as NH4NO3); CCM2: 40 t ha–1 of compost plus 400 kg N ha–1.

 
The N/P ratios derived from the fertilizer treatments in this study are given in Fig. 5 . Overall, the lowest ratios were produced by the CCM2 and CCM1 treatments compared with the CC2 and CC1 treatments.


Figure 5
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  		Fig. 5. The N/P ratios of leachates, as total inorganic N and its fractions, for the different treatments used in this study. Both net and cumulative values are shown. Error bars represent the standard deviation. CC1: 20 t ha–1 of compost; CC2: 40 t ha–1 of compost; CCM1: 20 t ha–1 of compost plus 400 kg N ha–1 (as NH4NO3); CCM2: 40 t ha–1 of compost plus 400 kg N ha–1.

 
Plant Analysis
Table 6 shows the dynamics of the leaf mineral contents (N, P, and K) during the maize growth cycle, expressed on a dry-matter basis. Plots amended with 40 t ha-1 of compost plus inorganic fertilizers showed the highest values in the leaves, followed by CCM1, CC2, CC1, and the control soil, with the greatest values when maize was 30 cm high. During the maize growth cycle, leaf N, P, and K levels decreased gradually. The values obtained for the different stages suggest the correct N, P, and K nutrition of the maize crop (Tejada et al., 1992).


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  		Table 6. Nitrogen, P, and K contents of maize plants during three growth cycles (on a dry-matter basis).

 
Table 7 shows leaf pigment and soluble carbohydrate content during the maize growth cycle. The statistical analysis indicated differences in leaf pigment and soluble carbohydrate content with respect to fertilizer treatment. The highest values of chlorophyll A and B, carotenoids, and soluble carbohydrate contents were obtained in the compost-fertilized plots, mainly where there was a higher supply of compost. Leaf pigments and soluble carbohydrate content increased gradually during the maize growth cycle until harvest.


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  		Table 7. Leaf pigments and soluble carbohydrate contents (SCC) of leaf samples during three maize growth cycles.

 
Table 8 shows the number of grains per cob, grain yield, and protein content for the different treatments. The highest protein content was in the CCM2 treatment, while the lowest came from the control. The values were higher than those reported by Tejada and Gonzalez (2004) for the same maize cultivar fertilized with a by-product of the two-step olive oil mill process.


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  		Table 8. Protein content and maize yield parameters for the three years of the study.

 
Mineral fertilizer additions increased the number of grains per cob. Maize yield increased significantly with the addition of fertilizer to the compost. Grain protein and yield in the third season were higher than those of the second and first seasons, probably due to the residual effect of the organic matter after application in the first and second seasons.

Treatment influenced the number of cobs per spike, especially when the highest compost rate plus N was applied to soil. Maize yield shows significant differences with fertilizer treatment. In this respect, the lowest values are for the control treatment and the highest values for CCM2, in which there was a higher supply of organic matter and mineral fertilizer.

Compared with the control, application of CC increased grain protein content by ~4%, grains cob of ~1.5%, and maize yield by ~2% across years.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Properties
Our results indicate that the supply of readily metabolizable C in the organic by-product 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), Schaffers (2000), and Tejada and Gonzalez (2003a, 2003b), soil microbial biomass responds rapidly, in terms of activity, to additions of readily available C and application of organic wastes increased soil microbial biomass. In this respect, Brendecke et al. (1993), Fliessbach et al. (1994), and Filip and Bielek (2002) reported decreased soil microbial biomass after 10 yr of application of 5 and 15 t ha–1 yr–1 of sewage sludge. These researchers indicated that the presence of high amounts of heavy metals (Cd, Cr, Hg, Pb, etc.) in this by-product may counterbalance the positive effects of organic matter. Analysis of the compost used in this study (Table 2) indicates very low concentrations of Cd, Cr, Hg, and Pb.

According to Rao and Pathak (1996), incorporation of organic amendments in soil stimulates dehydrogenase activity because the added material may contain intra- and extracellular enzymes and may also stimulate microbial activity in the soil (Pascual et al., 1998). The microbial populations favored by root exudates from plants growing in the plots may also have contributed to the stimulation of dehydrogenase activity. The high level of dehydrogenase activity in the soil treated with compost suggests the availability of a high quantity of biodegradable substrate (which is in agreement with the higher content of labile C observed in these soils) and hence an increase in their microbial activity.

The observed stimulation of urease and BAA-protease activity (related to the N cycle) is appreciable even with high doses of organic amendment, probably due to the higher microbial biomass produced in response. Some studies have indicated that high doses of some organic materials can introduce into the soil toxic compounds such as heavy metals, which could have a negative effect on enzyme activities (Garcia et al., 1994). Cotton gin compost does not have high quantities of heavy metals so that, consequently, high doses of these materials will not cause toxicity of this type.

Soil arylsulfatase and soil phosphatase activities were stimulated at high application rates of the organic amendment. The demand for P by plants and soil microorganisms under higher yielding conditions may have been responsible for the stimulation in the synthesis of this enzyme (Garcia et al., 1994). According to Rao and Tarafdar (1992), increased phosphatase activity indicates changes in the quantity and quality of soil phosphoryl substrates. The supply of readily metabolizable C in the organic by-product is likely to have been the most influential factor contributing to the increased soil arylsulfatase and soil phosphatase activities.

Our results indicate that soil microbial biomass and soil enzymatic activities are highest when the compost and inorganic fertilizers are applied jointly to the soil. Based on C/N ratios in the CCM2 and CCM1 treatments compared with the other treatments, it is probably that compost alone resulted in immobilization of N, while supplemental N resulted in net mineralization of N from the compost material.

Nutrient Leaching Losses
Nitrogen leaching was found to be highest where compost plus mineral N were supplied (CCM1 and CCM2 treatments). Not only was more N supplied directly to the system, but mineralization of N from the treatment probably also contributed to a greater N content and thus more potential for leaching.

Quantifying P leaching losses is important because this element, together with N, causes water eutrophication. In this respect, the N/P ratio is more relevant to biological production than are the individual amounts of each element (Pizzolon et al., 1999; Quiros et al., 2002). Although the N/P ratios that reportedly cause eutrophication are widely variable—ranges from 5:1 to 60:1 are biologically productive—some researchers shorten the range to 5:1 to 15:1 for NO3–N and 6.4:1 to 25:1 for NH4–N. The N/P ratios derived from our fertilizer treatments are given in Fig. 5. The lowest ratios were produced by the CCM2 and CCM1 treatments; however, the values obtained are lower than those mentioned, which suggest a lower eutrophication risk in drainage waters from soils treated with this amendment.

The highest K losses were for the CCM2 and CCM1 treatments; however, the values obtained don't indicate a possible contamination into surface waters, since they are below the risk levels mentioned (Tejada, 1996; Tejada et al., 2005).

Plant Analysis
Leaf nutrient content during the maize growth cycle increased with time for compost plus inorganic N treatments (CCM2 and CCM1) when compared with compost alone. This may be due to increased mineralization of organic matter for the CCM2 and CCM1 treatments. This mineralization led to increased soil macrocronutrient content and an increase in macronutrient uptake by plants.

Leaf pigment and soluble carbohydrate content were also higher for CCM2 and CCM1 which, similar to nutrient uptake, reflects increased mineralization of organic matter. These results are of great importance, because if leaf pigment levels can be increased with time, photosynthesis would be as well, resulting in higher production of soluble carbohydrates and thereby increased grain quality and crop yield.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Compared with compost alone, application of compost plus inorganic fertilizer resulted in increased soil biological activity, plant nutrient uptake, and crop yield as well as improved grain quality. This is due to the fact that, when compost alone was applied, immobilization of N resulted; with the addition of mineral N, net mineralization occurred. This increase in mineralization decreased the time necessary for organic matter in the compost to break down and supply nutrients to the plant in the early stages of development.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome