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DIVISION S-6—NOTES

Soil aggregate stability and organic matter in clay and fine silt fractions in urban refuse-amended semiarid soils

Dep. of Soil and Water Conservation and Organic Waste Management, Centro de Edafología y Biología Aplicada del Segura (CSIC), P.O. Box 4195, Campus de Espinardo 30100-Murcia, Spain

* Corresponding author (fcb{at}cebas.csic.es)

	ABSTRACT

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Organic refuse application to soil may improve soil structure and thus soil quality and productivity. Two semiarid, structureless soils with <10 g kg^-1 organic C (OC), differing in clay mineralogy, were amended annually at rates of 30 g kg^-1 fresh or composted refuse to a total of 60 g kg^-1 in a greenhouse experiment lasting 2 yr. Fresh refuse additions significantly increased the soil aggregate stability in water by 17%. Composted refuse application only increased aggregate stability in the beidellitic clay-dominated soil (13%). In both soils, the greatest increase in OC and humin following refuse additions occurred in the fine silt fraction, which contained less decomposed organic matter than the clay fraction. In the beidellitic clay soil, this increase in OC was larger than in the illitic clay soil. The aggregate stability of the beidellitic clay soil was significantly correlated with OC and humin of fine silt fraction.

Abbreviations: OC, organic carbon • Soil A, Typic Haplargid • Soil B, Xeric Torriorthent

	INTRODUCTION

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WATER EROSION, LAND USE CHANGES, AND HIGH DECOMPOSITION RATES in semiarid regions lead to a reduction of organic matter and microbiological activity, resulting in deterioration of soil structure (García et al., 1997). The regeneration of soil structure is a prerequisite for improving the quality and productiveness of a soil and to control soil degradation and desertification.

Soil aggregate stability depends on the physical–chemical interactions of organic compounds with clay and fine silt particles. The quantity and quality of organic and mineral compounds are a key factor in aggregate stabilization and formation. Thus, the addition of organic matter improves soil structure, although there is not agreement on which components of the organic matter are most effective. Roldán et al. (1996) showed that composted organic refuse was less effective in aggregate stabilization than uncomposted refuse. On the other hand, Kemper et al. (1987) indicated that clay content was correlated with aggregate stability in arid and semiarid soils. The size and stability of a soil aggregate is related to its clay mineral composition and content. Soils dominated by clay minerals with a high specific surface area have a high capacity for adsorbing humic substances and hence for stabilizing aggregates (Parfitt et al., 1997). Elustondo et al. (1990) found a close relationship between the proportion of stable macroaggregates and soil clay content, as well as between aggregation and organic matter associated with the clay fraction.

The effect of land use on the composition and distribution of organic matter in size fractions is well known (Puget et al., 1999). Generally, cultivation leads to a substantial loss of organic matter associated with size fractions >50 µm, resulting in a relative shift of soil organic matter from coarser to finer fractions. However, the contribution of organic matter associated with clay and silt fractions to the improvement of aggregate stability in soils treated with organic materials is scarce (Angers et al., 1997).

Assuming that (i) organic amendments of a soil increase the OC content associated with the clay and fine silt fractions and improve soil structure, and (ii) the effectiveness of this improvement depends on the surface area of the soil minerals and on the composition of the organic matter found in the soil, this study deals with the use of urban refuse at two different stages of composting for the regeneration of two degraded soils with different mineralogical composition and calcium carbonate contents in a semiarid area. The specific objectives were: (i) to determine OC and humin contents associated with the clay and fine silt fractions following the addition of urban refuse at two stages of composting, and (ii) to relate OC associated with the clay and fine silt fractions to aggregate stability.

	Materials and Methods

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Two agricultural soils from southeastern Spain (Murcia) were evaluated in this experiment. The semiarid environment of Murcia had an average precipitation of 300 mm and an average temperature of 19.2°C. Both structureless soils were characterized by low organic matter content (<10 g kg^-1). The soils mainly differed in their clay mineralogy and calcium carbonate content. Illite was the dominant clay of the Typic Haplargid (Soil A), while beidellite dominated the clay fraction of the Xeric Torriorthent (Soil B). Soil A contained 60 g CaCO₃ kg^-1 and Soil B contained 600 g CaCO₃ kg^-1.

The solid urban refuse (fresh and composted urban waste) used in this experiment came from the municipal waste treatment plant in Murcia. The fresh urban refuse had undergone 10 to 15 d of natural maturation and contained 196 g C kg^-1, 11 g N kg^-1, and 44 g C as carbohydrate kg^-1. The composted urban refuse was mechanically produced by mixing the waste heap daily under aerobic conditions by fast fermentation (60 d). Composted urban refuse applied contained 109 g C kg^-1, 9 g N kg^-1, and 14 g C as carbohydrate kg^-1.

Subsamples (25 kg) from the A horizon (0–20 cm) of both soils were placed in 33 L (35-cm diameter, 34-cm height) containers. Fresh or composted urban refuse was incorporated annually into the soil for 2 yr at a rate of 30 g kg^-1 each, for a total of 60 g kg^-1. Urban refuse incorporations were made in April 1994 and in April 1995, and the experiment was carried out in a greenhouse. An untreated soil was used as the control. The containers had drainage holes and were irrigated at a rate of 2 to 3 L per container each week using drip irrigation. During the 2 yr of the greenhouse experiment, annual rotations of tomato (Lycopersicon esculentum Mill.) and oat (Avena sativa L.) crops, which are typical for semiarid areas, were grown consecutively in the soils. Untreated and treated soils received N-P-K (17-6-17) fertilizer during tomato crops. Mineral fertilizer was not applied to any of the containers during Avena crops. The soils were sampled before the second application of urban refuse in April 1995 and at the end of the experiment in April 1996. At each sampling, the contents of each container were mixed and a homogeneous 300-g sample from each container was taken. The soil was air-dried, one subsample was sieved at 2 mm for particle-size fractionation, and another at 0.2 to 4 mm for aggregate stability measurements.

Aggregate stability in water was determined by the method of Lax et al. (1994). An aliquot of 4 g from the sieved soil between 0.2 to 4 mm was placed on a small 250 µm sieve, wetted using an atomizer, and after 15 min, subjected to an artificial rainfall of 150 mL with an energy of 270 J m^-2. The soil remaining on the sieve was put in a previously tared capsule (T), dried at 105°C, and weighed (weight of residue = P1). Then, the former residue was soaked in distilled water and, after 2 h, passed through the same 250 µm sieve with the assistance of a small stick to break the remaining aggregates. The residue remaining on the sieve, which was made up of plant debris and sand particles, was dried at 105°C and weighed (weight of 2nd residue = P2). The percentage of stable aggregates (P1 - P2) with regard to the total aggregates was calculated by

Particle-size fractions were isolated according to Tippkötter (1994). Briefly, 60-g soil samples were dispersed ultrasonically in 300 mL of water (400 W for 30 min) using a Branson-450 probe-type sonifier. The sand fraction was separated by wet sieving. Clay- and fine silt-size fractions were obtained by gravity sedimentation.

The size-fraction samples were extracted with 0.1 M sodium pyrophosphate at pH 9.8, and insoluble residue from this extraction is referred to as humin. Organic C, humin, and Total N were measured by pretreatment with (1:1) HCl to remove carbonates, followed by combustion at 1020°C in a Carlo Erba NA 1500 analyzer (Navarro et al., 1991). The OC content of the <2 µm and 2 to 20 µm size fractions were corrected to account for carbonate removed during sodium pyrophosphate extraction. The carbohydrate content of urban refuse was determined by the anthrone colorimetric method of Brink et al. (1960) after hydrolysis with 0.5 M H₂SO₄. Mineralogical characterization of the clay and fine silt fractions was performed by X-ray diffractometry using a Philips PW1710. Detailed information on the dominant clay minerals of the different size fractions from both soils is given by Caravaca et al. (1999).

The experiment design was a randomized complete block with four replicates. Experimental factors were urban refuse and soil type, fraction size, and their interactions. Where significant treatment effects were observed (P < 0.05), Tukey's analyses were performed to permit separation of means. Statistical procedures were carried out with the software packages Statgraphics for Windows 7.0 (Software Technology Support Center, 1993).

	Results and Discussion

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The fine silt fraction contained more OC and humin per kg soil than the clay fraction in both the treated and untreated soils (Tables 1, 2). Leinweber and Reuter (1990) also found that the >2-µm fractions were enriched with OC following the incorporation of fresh and composted manure. Values of OC and humin within the clay and fine silt fractions from Soil B were always greater than those from Soil A at both sampling dates. The organo-mineral fractions of a specific particle size exhibit different properties with respect to their OC and humin contents, depending on the soil clay mineralogy. In Soil A, dominated by illitic clay having a low specific surface area, one application of urban refuse raised the OC content of the fine silt fraction. However, the clay fraction of Soil A required two applications of compost to increase OC (Table 1). In contrast, in Soil B (dominated by swelling clay with a high specific surface area) the increases observed in the OC contents were quite marked in the fine silt fraction and less pronounced, though also significant, in the clay fraction. However, the soil OC remained associated with the clay and fine silt fractions when both types of refuse were added repeatedly to both soils for 2 yr. This agrees with the results obtained by Christensen (1986) in a soil treated with wheat straw. In both soils, the C/N ratios of the soil fractions decreased markedly with particle size, suggesting the presence of some less-decomposed organic matter in the fine silt fraction after both additions of urban refuse (Table 1). This trend had also been reported by Angers et al. (1997), who followed the incorporation of C and N derived from crop residues in several size fractions.

The clay and fine silt fractions contribute substantially to the soil's capacity to protect the organic matter incorporated in it, and play a significant role in the stabilization of soil aggregates. However, it would be interesting to conduct a field experiment to corroborate these results in a natural environment.

	ACKNOWLEDGMENTS

 
F. Caravaca acknowledges a grant from Caja de Ahorros del Mediterráneo (CAM).

Received for publication November 29, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and Methods Results and Discussion REFERENCES

 

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