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Published in Agron J 100:611-618 (2008)
DOI: 10.2134/agronj2006.0316
© 2008 American Society of Agronomy
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CROPPING SYSTEMS

Long-Term Study of Properties of a Xerofluvent of the Guadalquivir River Valley under Organic Fertilization

Sebastiana Melero Sáncheza,*, Engracia Madejónb, Juan Francisco Herenciaa and Juan Carlos Ruiz-Porrasa

a Inst. de Investigación y Formación Agraria y Pesquera "Las Torres-Tomejil" Seville (IFAPA), Carretera Sevilla-Cazalla de la Sierra Km 12.2. 41200 Alcalá del Río (Seville) Spain
b Inst. de Recursos Naturales y Agrobiologia de Sevilla (IRNAS-CSIC), Avenida Reina Mercedes 10, P.O. Box 1052, 4180 Sevilla, Spain

* Corresponding author (sebastiana.melero.ext{at}juntadeandalucia.es).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The long-term effect of inorganic and organic fertilization in a vegetable crop rotation on soil chemical and biochemical properties was investigated in a trial in southern Spain. Two crops were grown in succession, potato (Solanum tuberosum L.) and a mixed-cropped strawberry–onion (Fragaria vesca L.–Allium cepa L.). Total organic carbon (TOC), Kjeldahl-N, bicarbonate-extractable P (Olsen-P), ammonium acetate extractable-potassium (AAE-K), microbial biomass carbon (Cmic), and enzyme activity (dehydrogenase, β-glucosidase, protease, and alkaline phosphatase) were determined in soils in the seventh and eighth year of management comparison. The metabolic quotient (qCO2) and the Cmic to TOC ratio were also calculated. Organically fertilized soils showed significant increases in TOC and Kjeldahl-N, available-P, AAE-K, microbial biomass C, and enzymatic activities compared with those found under inorganically fertilized soils. The qCO2 values were greater in inorganic than in organic fertilized plots indicating a lower microbial community respired at a greater rate in inorganic fertilized soils. The Cmic to TOC ratio in organic plots was lower than in inorganic plots indicating that microorganism in inorganically fertilized soils could have a better efficiency in the conversion of C sources to microbial biomass. Long-term organic fertilization positively affected soil organic matter content, thus improving soil chemical and biological fertility under arid environmental conditions in southwest Spain. However precautions must be taken as excessive accumulation of some nutrients, particularly N and P, can arise from the long-term use of compost.

Abbreviations: AAE-K, ammonium acetate extractable-potassium • Cmic, microbial biomass carbon • EC, electrical conductivity • qCO2, metabolic quotient • SOM, soil organic matter • TOC, total organic carbon


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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.

Received for publication November 10, 2006.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
DEGRADATION OF AGROECOSYSTEMS is related to important changes in soil quality [loss of structure, increase of soil erosion, loss of essential nutrients, and soil organic matter (SOM) decrease]. Conventional farming practices in Europe lead to organic matter decrease and consequently degradation of cultivated soils (Nachtergaele et al., 2002). Specially, in the Mediterranean areas the climatic conditions produce higher organic matter oxidation. For that, Food and Agriculture Organization of the United Nations (FAO) recommends organic matter addition to soils to increase their agronomic quality. The close relation between soil organic matter content and its fertility is widely reported and universally accepted (Smith et al., 1993). Organic amendments have numerous positive effects on soil physical, chemical, and biological properties (Webber, 1978; Reganol et al., 1993; Smith et al., 1993). For that soil organic management could be managed as agronomic practices to restore and improve soil fertility in arid environmental area such as southwest Spain.

Long-term studies of physical, chemical, and biological properties have been proposed and used to determine the impact of different soil management systems (Doran and Parkin, 1996).

The understanding of microbial processes is important for the management of farming systems, particularly those that imply organic inputs of nutrients (Smith and Paul, 1990). The microbial biomass is a small and labile fraction (1–3%) of organic matter and plays an important role as nutrients reservoir for plants (Jenkinson and Ladd, 1981). Changes in the microbial biomass-C can provide an early indication of trends in the soil organic C (Bergstrom et al., 1998).

Microorganisms decompose organic substrates to obtain energy producing CO2. The CO2 production has been used to determine biological activity in soils in relation to changes in soil chemical properties and agricultural practices (Nannipieri et al., 1990).

Measurement of the soil enzymatic activities can be used as early biological indicators of soil management changes. Enzymatic activities are very sensitive to changes, which occur in the soil, and could provide rapid and accurate information on changes in soil quality (Melero et al., 2007).

However, single measurements of biological and biochemical properties have serious limitations as soil quality indicators (Gil-Sotres et al., 2005). Therefore, it would be appropriate to study the use of simple indexes (i.e., combination of two or more measured parameters into a single criterion). Among others, the most widely used simple indexes are the qCO2, which is the rate of carbon dioxide per unit of biomass and time, and the relation between Cmic/TOC (Gil-Sotres et al., 2005).

The aim of the present work was to evaluate the long-term effect of the incorporation of organic fertilizers on the chemical, biological, and biochemical properties of the soil to compare the effect of different management systems on soil quality.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Location and Management Systems
The field study was performed on a silty loam soil classified as a Xerofluvent (Soil Survey Staff, 2003. The study site (l37°8'33'' N, 5°16'4'' W) was located in the Guadalquivir River Valley (southwest Spain), at the CIFA "Las Torres-Tomejil" farm in Alcalá del Río (Seville). Soil texture and chemical characteristics are presented in Table 1 . Climatic characteristics of farm are: average annual rainfall of 650 mm, 18°C of temperature and 4 mm of average daily evapotranspiration.


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  		Table 1. Some characteristics at the beginning of the experiment (1995) at 0- to 15-cm soil depth.{dagger}

 
A completely randomized experimental design (four replicates per treatment) was conducted in eight subplots of 6 by 12.5 m. Two treatments were tested: (i) inorganic fertilization; (ii) organic fertilization. A crop succession was conducted along the experiments following Regulation (EEC) No. 2092/91. Organic and inorganic systems were managed since 1995 and crop backgrounds (from 1995–2001 yr) are in Table 2 . This study includes the results of 2 yr (from 2002–2003) over two cultivation cycles. The following crops were grown: potato (var. Spunta) (March 2002–May 2002) and strawberry–onion (mixed cropped) (var. Camarosa–var. Babosa, respectively) (November 2002–June 2003).


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  		Table 2. Fertilization from the beginning of study from 1995 until 2001.

 
Some of the most relevant characteristics of the added composts are shown in Table 3 . In the organic treatment, potato was fertilized with 30 t ha–1 of vegetal (pruning waste and crop residues) compost and strawberry–onion with 30 t ha–1 of animal (manure) compost, both applied by superficial tillage each cycle at the beginning of each cycle. Composts were chosen because they constitute low-cost representative materials for organic fertilization. We use either animal or vegetal compost depending on their availability as both are permitted in organic farming following Regulation (EEC) No. 2092/91. The inorganic treatment was fertilized with chemical fertilizers that are shown in Table 4 . Table 5 shows the doses of nutrients (N, P, and K) applied per hectare in the inorganic and organic plots. Weed control in both treatments was performed by mechanical tillage. The soil was moldboard plowed to a depth of 20 to 25 cm after each crop harvest in both treatments.


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  		Table 3. Chemical characteristics of applied compost in each crop cycle.{dagger}

 

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  		Table 4. Applied doses by inorganic fertilization in each crop cycle.

 

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  		Table 5. Applied nutrient doses in inorganic and organic fertilization in each crop cycle.

 
The first crop was irrigated three times by surface irrigation applying a total amount of 210 mm and the second crop was irrigated once a week by drip irrigation, applying a total amount of 600 mm of water. Crops also received a total rainfall of 161 and 596 mm respectively each year.

Sampling and Soil Analysis
Soil was sampled three times to a depth of 15 cm in each crop. In the potato crop, soil was collected before sowing (first sampling) (21 Feb. 2002), 2 mo after sowing (second sampling) (27 Apr. 2002) and after harvest (third sampling) (2 June 2002). In the strawberry–onion crop, soil was collected 1 mo after sowing (first sampling) (3 Dec. 2002), in the flowering period (second sampling) (8 Apr. 2003) and after harvest (third sampling) (26 June 2003).

At each sampling, three soil cores (10 cm of diameter) were randomly taken from each plot to make a composite sample. Soil samples were sieved through 2-mm sieve and divided into two subsamples. One was immediately stored at 4°C in plastic bags loosely tied to ensure sufficient aeration and to prevent moisture loss until assaying of microbiological and enzymatic activities. The other was air-dried for chemical analysis. Microbiological analysis was performed during 2 wk after each sampling.

Soil pH values were measured in a 1/2.5 soil/water extract after shaking for 1 h (Hesse, 1971.) using a pH meter (CRISON micro pH 2002). Soil EC values were determined in a 1/2.5 soil/water extract after shaking for 1 h (Hesse, 1971) using a conductivity meter (CRISON micro pH 2002). Total organic C was analyzed by dichromate oxidation and titration with ferrous ammonium sulfate (Walkley and Black, 1934). N-Kjeldahl was determined after digestion with sulfuric acid in a Digestion System Kjeldatherm (Gerhardt and after was determined by auto-analyzer (BRAN+LUEBBE, method G-188–97, BRAN+LUEBBE, Norderstedt, Germany). Available-P was measured as described by Olsen et al. (1954). Extraction using NaHCO3 and the extract was determined colorimetrically by Murphy and Riley (1962) method. Extracted K in the ammonium acetate was measured by flame photometry (Varian Spectra A-220FS).

Microbiological Analysis
Soil respiration was determined according to Anderson (1982). This field method is based on the determination of CO2 evolved from undisturbed soils. The NaOH solution is placed in an open glass jar above the soil surface and the area is covered with a metal cylinder at the upper end. After 24 h of incubation the NaOH (1 M) solution is removed and the trapped CO2 titrated with HCl (1 M).

Microbial biomass C was determined by a fumigation-extraction method (Brookes et al., 1985; Vance et al., 1987). Samples of soils were fumigated with ethanol-free CHCl3. Control samples nonfumigated were also established. After removal of the CHCl3, fumigated and nonfumigated soil samples were extracted with 0.5 M K2SO4 and organic C quantified by oxidation with 66.7 mM K2Cr2O7 and subsequent back-titration of the unreduced dichromate. Microbial biomass C content was estimated as follows: microbial Bc = 0.38:Ec, where Ec is the difference between the organic C extracted from the fumigated and nonfumigated treatments (Vance et al., 1987).

Dehydrogenase was determined according to Thalmann (1968), 5 g of soil sample were incubated at 25°C for 24 h with 5 mL of 2,3,5 triphenyl-tetrazolium chloride (TTC) as a substrate. Triphenyl formazan (TPF) produced in the reduction of TTC was extracted with acetone and measured in a spectrophotometer at 546 nm.

Protease activity was measured after incubation (50°C for 2 h) of soil (1 g) with casein (2%) and pH 8. 1. Amino acids released during the incubation period are extracted, and the remaining substrate is precipitated after the addition of trichloroacetic acid. Aromatic amino acids react with Folin–Ciocalteu's phenol reagent in an alkaline solution to form a blue complex which is measured in a spectrophotometer at 700 nm (Ladd and Butler, 1972).

Beta-glucosidase activity was measured as indicated by Eivazi and Tabatabai (1988). One gram of soil sample was incubated (at 37°C for 1 h) with p-nitrophenyl-β-D-glucopyranoside 25 mM and buffer pH6. The p-nitrophenol produced was extracted and determined at 400 nm.

Alkaline phosphatase was determined according to Tabatabai and Bremner (1969). One gram of soil sample was incubated (at 37°C for 1 h) with p-nitrophenyl phosphate disodium 15 mM and buffer pH 11. The p-nitrophenol produced was extracted and determined at 400 nm.

Results were based on oven-dry weight of soil.

Statistical Analysis
Statistical analyses were performed using the program SPSS 11.0 for Windows and results were expressed as mean values. Significant differences between management systems were declared by the students-t test at P < 0.05.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil pH and Electrical Conductivity
No significant differences in pH values were found between inorganic and organic treatments (Table 6 ).


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  		Table 6. Mean values of soil chemical properties during period of experimentation.

 
In general, electrical conductivity (EC) values were mostly not different between inorganic and organic fertilization, except in the first and second sampling in strawberry–onion crop when organic plots showed a great increase in EC values.

Total Organic Carbon and Soil Nutrient Content
In both potato and mixed crops, the highest TOC values were found in organically fertilized soils (Table 6) and significant differences between inorganic and organic treatments were observed. The TOC values increased 1.3-fold in the inorganic treatment and threefold in the organic treatment at the end of the experiment compared to the initial content in soil (Table 1).

Organically fertilized soils also had a higher soil Kjeldahl-N and Olsen-P content than inorganic fertilized soils in all sampling periods (Table 6). Kjeldahl-N and Olsen-P values increased in soil organically fertilized 2 and 2.4-fold, respectively at the end of the experiment compared to the initial content in soil (Table 1).

Ammonium acetate extractable-K means values in organic fertilized soils were higher than inorganically fertilized soils (Table 6). However, statistical differences between inorganic and organically fertilized soils were only observed in the third sampling of potatoes crop cycle and in the first sampling of the mixed crop cycle.

Metabolic Quotient, Microbial Biomass Carbon, and Microbial Biomass Carbon to Total Organic Carbon Ratio
The qCO2 showed seasonal variability of sampling along the study. In general qCO2 ratio did not show statistically significant differences between inorganic and organic treatments, except in the second and third sampling of mixed crop cycle (Fig. 1a ).


Figure 1
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  		Fig. 1. (a) Metabolic quotient (qCO2), (b) soil microbial biomass C (Cmic), and (c) Cmic to total organic carbon (TOC) ratio during the experimental period. Significant difference between treatments are indicated with asterisks (P < 0.05). Thin bars represent the standard error of the mean.

 
Microbial biomass C was significantly higher in organic than in inorganic fertilization (Fig. 1b).

In general the Cmic to TOC ratio in organic plots was lower than inorganic plots (Fig. 1c), except in the second and third sampling of potato and mixed crop cycle, respectively.

Soil Enzyme Activities and Ratios of Enzyme Activities to Microbial Biomass Carbon
In general, from the beginning of this study enzymatic activities (dehydrogenase, protease, β-glucosidase and alkaline phosphatase) showed clearly statistical difference between inorganically and organically fertilized soils. Enzymatic activities were highest in organically fertilized soils (Fig. 2a, 2b, 2c, 2d ). All enzymatic activities showed a decrease in the strawberry–onion crop in organic plots.


Figure 2
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  		Fig. 2. Soil enzyme activities during the experimental period: (a) dehydrogenase, (b) protease, (c) β-glucosidase, and (d) phosphatase alkaline. Significant difference between treatments are indicated with asterisks (P < 0.05). Thin bars represent the standard error of the mean.

 
In general, ratios of enzyme activities to microbial biomass were mostly not found to be statistically different between inorganic and organic treatments (Fig. 3a, 3b, 3c, 3d ).


Figure 3
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  		Fig. 3. Ratios of enzyme activities [(a) dehydrogenase, (b) protease, (c) β-glucosidase, and (d) phosphatase alkaline] to soil microbial biomass C (Cmic) during the experimental period. Significant difference between treatments are indicated with asterisks (P < 0.05). Thin bars represent the standard error of the mean.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Neither inorganic nor organic fertilized soils had effect in pH values due to the high carbonate content of these soils.

Increase of soil salinity in organic plots in the strawberry–onion crop can be due to the addition of nutrients and salts through the animal compost. Xiying and Chang (2003) observed an increase of EC at increasing doses of manure. It is known that application of compost can cause significant increases of EC (Madejón et al., 2001; Xiying and Chang, 2003), so care has to be taken when composts are applied to soils with restricted drainage or on unirrigated lands. However neither inorganic nor organic fertilization appeared to cause soil salinization at the end of study.

Organic management (Regulation [EEC] No 2092/91) characterized by the incorporation of organic matter through compost (animal, vegetal), crop rotation, weed control by mechanical tillage maintain soil organic matter at higher levels than inorganic fertilization (Bulluck et al., 2002; Edmeades, 2003; Melero et al., 2006). Long-term studies report in organic fertilized soils had higher organic matter content than inorganic fertilized soils (Edmeades, 2003; Melero et al., 2007). This fact is particularly important to improve fertility in Mediterranean soils, the region of our study, where the levels of organic matter in agricultural soils are characterized by low organic matter levels (Costa et al., 1991).

The increase in TOC in inorganic treatment may be explained to an indirect effect of inorganic fertilization. This occurs primarily because fertilizer N inputs increase crop production and thereby increase the amount of crop residue, including roots, returned to the soil. Indeed, Omay et al. (1997) reported that the higher input and different variety of crop residues returned to the soil under crop succession contributed to the increase in soil organic C and N content.

The application of compost may account for the highest levels of the Kjeldahl-N and available-P (Olsen-P) in the organically fertilized soils (Clark et al., 1998; Marschner et al., 2003; Melero et al., 2006). Furthermore, the addition of organic matter to calcareous soils can increase available P and decrease P-insolubilization (Braschi et al., 2003).

Nevertheless, at the end of the study Olsen-P values were normal in the inorganic treatment and very high in organic treatment (López Ritas and López Melida, 1990). Excessive accumulation of some nutrients, particularly N and P, can arise from the long-term use of compost. So take into account the increase of N and P through continued addition of compost. That accumulation in soils with low retention/or in situations where organic nutrients are leach could contribute to runoff or leaching losses of N and P. Several authors have found in organically fertilized soils a higher K availability than inorganic fertilized soils (Clark et al., 1998; Bulluck et al., 2002; Edmeades, 2003). Xiying and Chang (2003) observed increase in K content in soil manure managed due to its high content in K and low mobility of K in soil. Long-term studies in organic fertilized soils (Edmeades, 2003, Xiying and Chang, 2003) reported that the use of organic residues to fertilizers can result in soils excessively enriched with some nutrients, particulary P, K, and Ca in the topsoils, as a consequence of the beneficial effect of increased soil organic matter in organic fertilized soils.

In general, the qCO2 values were higher under inorganically fertilized soils than organically fertilized soils. These results indicated that in inorganic plots, a lower microbial community respired at a greater rate. The low qCO2 values in organic fertilized soils can be related to protector effect of organic matter on microbial biomass (Pascual et al., 1997). According to Fliebbach and Mader (2000), organic management system benefits soil microbial biomass because microorganisms are using the available C more efficiently as indicated by a lower qCO2 and suggests better conditions within the soil organic matter which may contribute to nutrient mineralization and temporary storage of potentially leachable elements.

The highest qCO2 values in organic plots, in the second sampling of mixed crop cycle, can be related to high EC values in this period. The qCO2 can be used as an indicator of environmental stress since it is calculated from parameter which are very sensitive to environmental changes (Pascual et al., 1997). In situation stress microorganisms population put up defense mechanisms by increasing their respiration per unit of biomass in situation stress (Pascual et al., 1997). Several authors (Rietz and Haynes, 2003; Yuan et al., 2007) found that qCO2 was positively correlated with EC. They observed positive relationship between qCO2 and EC reflected the environmental stress as a result of salinity conditions on the soil microbial community.

Microbial biomass C was higher in organic than in inorganic plots due to greater supply of available C, which induced a better stimulation and microbial growing (Schjonning et al., 2002; Dinesh et al., 2004). In the strawberry–onion crop a progressive decrease in Cmic values was observed in organic plots which can be related with the increase of soil salinity. Several authors (Rietz and Haynes, 2003; Yuan et al., 2007) report a negative exponential relationship between EC and microbial biomass. This negative relationship demonstrates the highly detrimental effect that small increases in soil salinity have on the microbial community. In general Cmic/TOC values were slightly higher in inorganically fertilized soils, despite having lower inputs of organic C, microorganisms in inorganic plots could have a better efficiency in the conversion of C sources to microbial biomass.

Our long-term results showed a greater enzymatic activity under soil organic fertilization than soil inorganic fertilization. The addition of organic residues to soil activates microbial growth and consequently production of enzymes (Marschner et al., 2003; Dinesh et al., 2004), although organic residues also increase enzymatic contents directly. This great enzymatic activity is favored by increase in soil organic matter so organic matter plays a relevant role in the protection of extracellular enzymes in soil humic–mineral complex (Tabatabai, 1994).

The decrease of enzymatic activities in organic plots in the strawberry–onions crop could be related to salinity. Several authors (Gianfreda and Bollag, 1996; Rietz and Haynes, 2003) have found a significant negative exponential relationship between EC and enzymatic activities. This decreased enzyme activity is partially because the smaller and less active, microbial biomass is releasing less enzymes. In addition, high salt concentrations tend to reduce the solubility and denature enzyme proteins through disruption of the tertiary protein structure which is essential for enzymatic activity (Rietz and Haynes, 2003).

In general, ratios of enzymatic activities by microbial biomass values were not different between inorganic and organic treatments, indicating that inorganic and organic fertilized soils have same activity per unit of microbial biomass. These results can be related to a lower rate of enzyme production by this larger microbial biomass content in organic fertilized soils.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
These results showed that long-term organic fertilization ameliorate soil chemical and biological fertility. Organic residues added to the soil promote microbial biomass and enzyme activities improving biochemical properties, aspect of great importance in the turnover of organic matter and availability of nutrients in soil. All of the above comments play an important role in improving and restoring of arid land of southwestern Spain.

However, continuous additions of compost to the soil can increase soil nutrients content, particularly N and P that should be taken into account to avoid runoff and leaching of P and N. Therefore, these results could indicated that after continuous application of compost to soil for more than 5 yr, one or more residual years (no compost addition) would be adequate to profit the pool of the nutrient and organic matter accumulated in soil. Furthermore, incorporation compost, especially animal, to soil must be made with great caution to prevent increase in soil salts content, which can have negative effect on yields.


   ACKNOWLEDGMENTS
 
We thank the European Commission and the Spanish Science and Technology Ministry and Andalusia Government for financial support to the project (FEDER AGL00-0493-C02-02)(P.I.A 13.01.1), which allowed this work to be carried out.

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.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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