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Soil & Water Management & Conservation

Effects of Cover Crops on Soil Aggregate Stability, Total Organic Carbon, and Polysaccharides

^a College of Resources and Environmental Science, Shanxi Agricultural Univ., Taigu, Shanxi, P.R. China, 030801
^b Eastern Cereal and Oilseed Research Centre, Agric. and Agri-Food Canada, 960 Carling Ave., K.W. Neatby Building, Ottawa, ON, Canada, K1A 0C6
^c Dep. of Soil Science, 2357 Main Mall, Univ. of British Columbia, Vancouver, BC, Canada, V6T 1Z4

^* Corresponding author (liua{at}agr.gc.ca)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Structural degradation of silt clay loam soils in Delta, British Columbia, has resulted from intensive cultivation of vegetable crops. A field experiment and a laboratory incubation study were conducted to assess the ability of nonleguminous winter cover crops, spring barley (Hordeum vulgare L.), fall rye (Secale cereale L.), and annual ryegrass (Lolium multiflorum Lam.), to affect soil organic C, total and dilute acid extractable polysaccharides, and aggregate stability, expressed as mean weight diameter (MWD). The field experiment included four treatments: three cover crops (spring barley, fall rye, and annual ryegrass) and control (bare soil) arranged in a randomized complete block design. Annual ryegrass and fall rye increased MWD, and all of the cover crops increased soil dilute acid extractable polysaccharides. In the incubation experiment, starch (2.68 g C kg^–1 soil) or chopped shoots and coarse roots of fall rye (single- [4.14 g C kg^–1 soil] and double-dose [8.28 g C kg^–1 soil]) and annual ryegrass (4.62 g C kg^–1 soil) were added to a soil from the cover-crop site and incubated for 2, 4, and 8 wk. Cover crop and starch amendments increased soil organic C, dilute acid–extractable polysaccharides, and soil MWD. After 2-wk incubation, the starch amendment had the greatest MWD in all the treatments, increasing by 25, 44, and 45%, compared with the annual ryegrass, double-dose fall rye, and fall rye amendments, respectively (P < 0.05). After 8-wk incubation, however, the MWD in the starch amendment containers decreased by 18% compared with that in the double-dose fall rye amendment treatments (P < 0.05). All the cover crop amendments increased MWD and percentages of water stable 2- to 6-mm aggregates at all incubation periods (P < 0.05). Soil aggregate stability highly correlated with dilute acid-extractable polysaccharides in the field and in incubation experiments. This study suggests that the dilute acid-extractable polysaccharide fraction represents active binding agents under short-term cover crops. It has been shown that soil aggregate stability can be increased under 8-mo nonleguminous cover crops in the intensively cultivated soils.

Abbreviations: MWD, mean weight diameter • WSA, water-stable aggregates

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
IN DELTA, BRITISH COLUMBIA, Canada, there is a warm rainy winter with a temperate climate. Many soils are subjected to frequent, intensive tillage and cultivation, especially the soils where vegetables are grown. To increase land use benefit, vegetable farmers increase multiple crop index; to reduce pests, diseases, and weeds and to make soils soft, fine, and flat, they usually increase tillage (five or six times of tillage with different equipment, such as rototiller, plow, or disk) after each harvest. After the long-term intensive tillage and cultivation, the soils exhibit compaction, low infiltrability, ponding, and a decrease in the stability of soil aggregates (Hermawan and Bomke, 1996, 1997).

Soil aggregation can be increased by various types of cover crops or relay crops (Calkins and Swanson, 1998; Kabir and Koide, 2000; Sainju et al., 2003). Two years after an arable soil was converted to pasture, soil aggregate stability significantly increased though the amounts of organic C, and total acid-hydrolyzable carbohydrate did not (Haynes and Swift, 1990). Roberson et al. (1991) showed that cover crops could rapidly increase the stability of soil macroaggregates. Similarly, Hermawan and Bomke (1997) found greater structural stability, as indicated by MWD, following winter cover crops on lowland soils on the Fraser River delta of British Colombia.

It is generally known that the formation of stable aggregates involves organic binding agents. Tisdall and Oades (1982) suggested that plant roots and fungal hyphae bind microaggregates (<0.25 mm in diameter) into macroaggregates (>0.25 in diameter), and they considered three groups of organic binding agents: transient, temporary, and persistent. Water-extractable carbohydrates have been identified as a fraction that responds quickly to increases in C input by cover crops and can be an important factor in aggregate stabilization (Metzger et al., 1987; Kinsbursky et al., 1989; Haynes and Swift, 1990; Roberson et al., 1995; Sainju et al., 2003). More research is needed to study the changes in organic binding agents and aggregate stability under different cover crops in the short-term, such as within a growing season.

Soil aggregate stability is positively correlated with soil organic matter content or total soil organic C (Tisdall and Oades, 1980); however, there is evidence that water-stable aggregates are more strongly correlated with water-extractable and/or acid-extractable carbon fractions than with total organic matter or total carbohydrate C (Haynes et al., 1991; Degens, 1997). It is likely that C fractions extracted by water or acid can better represent soil organic binding components, which are directly involved in stability of soil aggregates (Degens, 1997). Roberson et al. (1995) reported that soil aggregate stability was significantly correlated with soil heavy fraction carbohydrate content after two winter cover crops.

Weaker correlations between water- or concentrated acid-extractable C fractions and aggregation stability have been reported (Degens et al., 1994). Thus more research is needed on soil active C fractions, which may be strongly correlated with soil aggregate stability. Dilute acid-extractable polysaccharides represent the labile polysaccharides, the easily hydrolyzable carbohydrates, originating from soil micro-organisms (Aoyama et al., 1999), cover crop exudates, and large parts of crop residues, which may have contributed to aggregation. Our hypothesis is that dilute acid-extractable polysaccharides derived from winter cover crops are soil labile polysaccharides, which are good soil binding agents, and can form water stable aggregates. The dilute acid-extractable polysaccharides, including polymers such as starch, hemicellulose, and pectins most active in soil aggregation other than cellulose, were determined by hydrolysis with a phenol-sulfuric acid reagent.

The objectives of this project were (i) to determine the effects of winter cover crops during their late growing period on soil organic binding agents (total soil organic C, total and dilute acid-extractable polysaccharides) and soil aggregate stability and (ii) to assess the effects of incorporating cover crop top and root material on soil organic binding agents (total soil organic C, total and dilute acid-extractable polysaccharides) and subsequent generation of aggregate stability.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Cover-Crop Trials
Cover-crop trials were conducted on a farm near Delta Municipality about 30 km south of Vancouver, BC, Canada. The location is in a temperate climate region with warm, rainy winters and relatively cool, dry summers. Annual average precipitation is 1160 mm, 80% of which occurs during the cover-crop growth period. The soil is a Westham silt clay loam and is classified as a Rego Humic Gleysol (Luttmerding, 1981). The soil had a pH of 5.4, and total organic C, total N, and Mehlich-III extractable P (Mehlich, 1984) in the 0- to 20-cm soil were 17.8, 1.64, and 0.073 g kg^–1, respectively. The soil texture and organic C contents in different depths are summarized in Table 1. Potatoes were the preceding crop. The fields were plowed on 15 July 1994.

Incubation Trial
To evaluate the role of cover crops on soil aggregation in low organic soil with relatively low MWD, a bulk soil sample was collected from the subsurface layer (40- to 60-cm depth) at the control plots (no cover crops) of the field treatment site. The subsoil had a pH of 5.5, and total organic C, total N, and Mehlich-III extractable P (Mehlich, 1984) were 12.9, 1.25, and 0.057 g kg^–1, respectively. Other soil properties are described in Table 1. At field moisture content, clods were gently broken by hand and passed through a 6-mm sieve. The incubation study included five treatments: control (no cover-crop amendment), annual ryegrass (4.62 g C kg^–1 soil), fall rye (4.14 g C kg^–1 soil), double-dose fall rye (8.28 g C kg^–1 soil), and starch equal to 80% of the average standing biomass C of the three field cover crop treatments (2.68 g C kg^–1 soil). Fresh green shoot and coarse root materials of cover crops, which were grown in the field cover crop site, were chopped to 2-mm length or less. The cover-crop addition rates were selected to simulate mixing the standing biomass of shoots and coarse roots found in the field plots to a depth of 10 cm in the soil. Each amendment was mixed with 1000 g of soil in a covered 1-L plastic container with 10.5 cm of upper diameter and 17 cm of height. The soil moisture content was gravimetrically adjusted to field capacity (250 g kg^–1) with water. The incubations were conducted in the laboratory under aerobic conditions at room temperature (19–22°C) for 2, 4, and 8 wk. The container lids were opened for aeration three times a day for a total of 3 h. Containers were weighed three times weekly to determine water loss, and an appropriate amount of water was added to adjust soil moisture content to field capacity. The five amendment treatments and three incubation periods formed a factorial design with three replicates for each treatment (5 x 3 x 3 = 45 pots). At the end of each incubation period, the soil from 0 to15 cm was sampled.

Wet-Sieving Methods
A modification of the wet-sieving method of Yoder (1936) was used. Moist soil samples (containing about 20% moisture) were gently sieved with a 6-mm sieve, and clods were gently crushed by hand. The soil samples that passed through a 6-mm sieve were sieved with 2-mm sieves again for the 2- to 6-mm aggregate stability analysis. The 2- to 6-mm aggregates were collected and air-dried. Before wet-sieving, 20 g of the 2- to 6-mm aggregates were put on the uppermost of a set of three sieves having 2.0-, 1.0-, and 0.25-mm aperture mesh, respectively, and wetted to saturation in a vaporizer. The vaporized samples with the set of sieves were installed on the wet-sieving apparatus. The water level in the tub was adjusted so that the aggregates on the 2.0-mm sieve were just submerged at the highest point of the oscillation. The oscillation rate was 31 cycles per minute, the amplitude of the sieving action was 3.5 cm, and the period of sieving was 15 min. The aggregates remaining on each sieve were collected onto a preweighed coffee filter, oven dried at 105°C for 24 h, and weighed to calculate moisture content. To remove sand in the aggregates, the aggregates were suspended in 50 mL of 0.5% sodium hexametaphosphate in a 250-mL Erlenmeyer flask and shaken for 45 min to disperse sand particles >0.25 mm, and the suspension was poured through a sieve with the same mesh size as the one from which the aggregates were collected. The sand remaining on each sieve was oven dried at 105°C for 24 h and weighed. The proportion of water-stable aggregates (WSA) in each size fraction (WSA_i) was calculated from Eq. [1]:

[1]

where i is the ith size fraction, Aggregate is the oven-dry mass of water-stable aggregates collected on each sieve, Sand is the oven-dry mass of sand collected on each sieve, Soil is the oven-dry mass of total 2- to 6-mm aggregates sieved, and Moisture is the gravimetric moisture content.

The MWD of aggregates was calculated from Eq. [2]:

[2]

where i is the ith size fraction, and X is the mean diameter of each size fraction, based on the mean intersieve size.

Chemical Analysis
Total and dilute acid extractable polysaccharides, soil organic C, and total N analyses were performed on bulk soil samples from the field and incubation experiments and on the 2.0- to 6.0-, 0.25- to 2-, and <–0.25-mm WSA size fractions of the field experiment.

The total and dilute acid-extractable polysaccharides were analyzed with the technique modified from Whistler and Wolfrom (1962) by Lowe (1994). For analysis of the total polysaccharides, a 0.5-g subsample of a soil fraction was put in an Erlenmeyer flask, and 4.0 mL of 12 M H₂SO₄ was added to the flask. The flask was covered with a large glass sphere and left to stand for 2 h. The H₂SO₄ in the flask was diluted to 0.5 M by adding 92 mL distilled water, and the flask was autoclaved for 1 h (103 kPa, 121°C). After cooling, the polysaccharides in the flask were filtered with 9 cm Whatman #2 filter paper, and distilled water was added to the filtrate to a volume of 100 mL for analysis. One milliliter of the filtrate was pipetted into a cuvette, and 1 mL phenol solution and 5 mL of concentrated H₂SO₄ were added. After standing for 10 min, the cuvette was placed in a water bath at 25 to 30°C for 25 min. Absorbance at 490 nm of the aliquot was read on a spectrophotometer. A standard curve was prepared by dilution of stock glucose solution to determine polysaccharide concentration in the sample. The procedures of dilute acid-extractable polysaccharide analysis were the same as those of total polysaccharides except that 100 mL 0.5 M H₂SO₄ (not 12 M H₂SO₄) was added directly into the flask with the soil sample.

The total N was determined by a semi-micro-Kjeldahl apparatus (AACC, 1983). The soil organic C was analyzed with a LECO induction furnace and carbon analyzer (Furnace Model 521 and Analyzer Model 572-200; LECO Equipment Corporation, St. Joseph, MI). The lack of carbonates in the soil permitted equating total soil C with soil organic C.

Statistical Analyses
Data were analyzed by ANOVA using the SAS statistical package (SAS Institute, 1990). Statistical significance was determined at P 0.05. Means of main effects were compared using the least significant difference test after a significant ANOVA test. Linear correlations between MWD and each organic component, including soil organic C, total polysaccharides, dilute acid-extractable polysaccharides, and total N, were calculated.

	RESULTS

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Cover Crop Trial
In the fall of 1994, the fall rye had significantly smaller shoot biomass than the spring barley and annual ryegrass treatments, which did not differ from each other (Table 2). In the April of 1995, shoot biomass was greater in the annual ryegrass treatment than in the fall rye and spring barley (Table 2). Soils under the annual ryegrass and fall rye had greater MWD than the control, whereas the MWD for the spring barley treatment was similar to the control (Table 3). Similarly, the percentage of water-stable 2- to 6-mm aggregates in the annual ryegrass and fall rye treatments were greater than in the control, whereas aggregate stability in the spring barley plots did not differ from the control (Table 3). The annual ryegrass treatment had significantly greater total organic C content than the control, but fall rye and barley did not (Table 3). Total organic C contents of the treatments followed a trend similar to MWD; however, the correlation between total organic C content and MWD was not significant (Table 4).

There were no statistically significant differences in the contents of total organic C, total N, total polysaccharides, and dilute acid extractable polysaccharides between 2- and 6-mm aggregates and 0.25- and 2-mm aggregates, but the contents of these organic components in macroaggregates (>0.25 mm) in the cover crop treatments were significantly greater than those in microaggregates (<0.25 mm) (Fig. 1) . There were no significant differences in these organic components between treatments in any of the three aggregate fractions (data not shown). The total organic C, total polysaccharide, dilute acid-extractable polysaccharide, and total N contents for the aggregate fractions followed the order: (2–6 mm) = (0.25–2 mm) > (<0.25 mm) (Fig. 1).

View larger version (20K): [in this window] [in a new window]   Fig. 1. Means of total organic C content, total and dilute acid-extractable polysaccharides, and total N content in different water-stable aggregate-size classes isolated after cover crop growing period in the field study. Vertical bars represent standard error of the means. The means with same letters above the error bars in the same organic components are not significantly different at p < 0.05. TOC, total organic C; TPS, total polysaccharides; DAEPS, dilute acid-extractable polysaccharides; TN, total N.

View larger version (35K): [in this window] [in a new window]   Fig. 2. Influence of amendments on soil mean weight diameter after different laboratory incubation times. Vertical bars represent standard error of the means. The means with same letters above the error bars are on the same sampling date not significantly different at p < 0.05.

View larger version (29K): [in this window] [in a new window]   Fig. 3. Influence of amendments on proportion of 2- to 6-mm water stable aggregates at different laboratory incubation times. Vertical bars represent standard error of the means. The means with same letters above the error bars on the same sampling date are not significantly different at p < 0.05.

The contents of total N for all of the green cover crop amendments were significantly greater than for the starch amendment and control (Table 5). Mean weight diameter was weakly correlated with total N contents (Table 4).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The dilute acid-extractable polysaccharide fraction measured in this study seems to represent an active binding agent. Although total polysaccharides were considered to be important for soil aggregate stability, the correlation of soil aggregate stability with total polysaccharide content was sometimes not as strong as with organic C content (Haynes et al., 1991; Haynes and Francis, 1993; Degens, 1997). Baldock et al. (1987) concluded that if carbohydrate materials are involved in changes in aggregate stability that occur over relatively short time periods, then a specific pool of carbohydrate must be involved. The total polysaccharide content does not differentiate between active and inactive carbohydrate binding agents. On the other hand, hot water-extractable polysaccharides are considered to be soil labile polysaccharides; however, they represent only a few percent of total polysaccharides, which may be only a part of labile polysaccharides, and they were not significantly correlated with soil aggregate stability in the short-term grass pasture (Haynes, 1999). In this study, dilute acid-extractable polysaccharides account for about 75 to 95% of total polysaccharides, depending on the cover crop species. This fraction represents the labile polysaccharides, the easily hydrolyzable carbohydrates, originating from soil microorganisms (Aoyama et al., 1999), cover-crop exudates, and large parts of crop residues, which may have contributed to aggregation. Increased dilute acid-extractable polysaccharides and soil aggregate stability with cover crops also suggest that the polysaccharides exuded by cover crops may be good soil-binding agents.

This study showed that the two winter cover crops, annual ryegrass and fall rye, increased soil aggregate stability. Spring barley was winter-killed, whereas fall rye and annual ryegrass remained vegetative over the winter months and continued growth in early spring. The latter cover crops develop densely ramified root systems, and their root systems remain active (Gaborcik et al., 2000; Isse et al., 1999). Large quantities of organic materials are supplied to soils from the roots of cover crops during their growing periods (Goodfriend et al., 2000). The major source of the organic matter is root senescence and exudation (Shamoot et al., 1968; Goodfriend et al., 2000; Lu et al., 2002). The exudates and other organic constituents result in the production of large amounts of active polysaccharide binding agents in the surface soil (Haynes et al., 1991; Degens, 1997). Greater organic C supplied by the cover crop roots in the soil could encourage greater microbial activity and biomass, which would produce extracellular mucilaginous polysaccharide materials that also have the capacity to stabilize soil aggregates (Lynch and Bragg, 1985; Roberson et al., 1995). The results in this experiment are consistent with those of Dapaah and Vyn (1998) and Kabir and Koide (2000). Dapaah and Vyn (1998) found that soil aggregate stability was greater following cover crops than where no cover crops were used. After 54 d of maize growth in the field, both cover crops, winter wheat and dandelion, increased water stable aggregates (Kabir and Koide, 2000).

Annual ryegrass had greater shoot biomass than fall rye and has been shown to have notably greater root mass density and root length densities than other crops (e.g., prairie grass, phacelia, pea, and maize) in the 0- to 10- and 10- to 20-cm soil layers (Haynes and Francis, 1993). Thus, annual ryegrass would have produced more metabolic organic products, including transient binding agents, by its root system than fall rye (Shamoot et al., 1968). Biologic activity of soil micro-organisms was also likely more vigorous under annual ryegrass than under fall rye because there was more substrate (Tisdall, 1994; Guggenberger et al., 1999). Soil micro-organisms under annual ryegrass could, therefore, synthesize more microbial extracellular polysaccharides and increase soil aggregate stability and 2- to 6-mm WSAs. Also, the larger root system under annual ryegrass likely increased soil aggregate stability by directly entangling soil particles to form aggregates (Tisdall and Oades, 1979).

That cover crops increased soil polysaccharide content but did not increase N suggests that cover crops input less N than C through their root systems during their growing periods. The result was not surprising because those cover crops could not fix atmospheric N, and the amount of N released to the soil in crop root exudates and by root senescence was less than that taken up from the soil.

The effectiveness of added organic materials in increasing the stability of soil aggregates is determined by their amount, composition, and decomposability (Albiach et al., 2000, 2001). Although green cover crops are rapidly decomposable materials, they are not as easily decomposable as starch. Thus, after 2 wk of incubation, the aggregate MWD of the soil to which starch, a plant polysaccharide, had been added was the greatest of all the treatments and was significantly greater than the cover crop amendments (Fig. 2). The results agree with other studies; for example, Guggenberger et al. (1999) found that at 15 d of incubation, the proportion of macroaggregates by soil mass reached a maximum when starch was added in a cultivated grassland soil.

After 8 wk of incubation, the aggregate MWD and the dilute acid-extractable polysaccharide content of starch-amended soils were reduced due to starch decomposition, but the green cover crop amendments may continue to release polysaccharides and other organic binding agents into soils. This might lead to greater MWD under double-dose fall rye than in starch amendment. The double-dose fall rye amendment reached the greatest MWD and proportion of 2- to 6-mm WSAs of all the treatments because of the larger quantities of organic materials, including dilute acid-extractable polysaccharides, supplied to the soil by this cover-crop amendment.

Comparison and interpretation of the results of the cover-crop field trial and the incubation experiment should be tempered by the facts that the field measurements were made in the 0- to 20-cm depth of the plow layer and the incubation trial was conducted with subsoil from the 40- to 60-cm layer. The MWD for the subsoil was lesser, and a greater response to soil amendments would be expected. Although all of the cover-crop amendments in the incubation study increased MWD, the spring barley treatment did not increase MWD in the field soil before spring cultivation. The significant correlation between MWD and total organic C content in the incubation study differed from the lack of correlation in the field study. However, the correlations between MWD and dilute acid-extractable polysaccharide contents were consistent between the field and incubation experiments. This can be explained by differences in organic C sources between the incubation and field experiments. Total soil organic C may be divided into two parts: (i) old soil organic matter, which is mainly humic substances and is relatively stable, and (ii) new input organic C, which comes from cover crops (shoots, roots, root senescence, and exudates) and is easily decomposed and is thus a major microbial C source also resulting in increase of production of microbially derived binding agents (Baldock, 2001; Six et al., 1999). In the incubation experiment, all of the parts of cover crops, including tops and coarse roots, which contained a variety of carbon materials (including a large proportion of acid dilute-extractable polysaccharides), were added to the soil. Also, the soil in the incubation experiment was sampled from sublayers, which contained much less humic matter than the top layers in the field experiment. Therefore, a larger proportion of total organic C in the incubation study was in dilute acid-extractable polysaccharides than in the field where cover crops were grown. If dilute acid-extractable polysaccharide content was a better index for soil aggregate stability, MWD would be correlated with dilute acid-extractable polysaccharide contents (high correlation level) and total organic C (low correlation level) because of the large proportion of dilute acid-extractable polysaccharides in the total organic C. In the field experiment, however, input of organic C material came only from root exudates and root dieback materials during the cover crops' growing period. This resulted in lower quantities of input of organic C and in much lower percentages of labile polysaccharide content to total soil organic C in the field studies than after the incorporation of cover-crop aboveground biomass into the soil in the incubation study. Therefore, in the field, MWD was correlated with dilute acid-extractable polysaccharides and not with total organic C. This implies that soil dilute acid-extractable polysaccharide content was a better indicator of soil aggregate stability than total soil organic C in the agricultural soil with intensive cultivation.

This study also showed the concentrations of total organic C and dilute acid-extractable polysaccharides in macroaggregates were greater than those in microaggregates. This further confirms the results of Elliott (1986) and Puget et al. (1999). After analyzing C, N, and P in different aggregates, Elliott (1986) found that macroaggregates had considerably greater organic C, N, and P than microaggregates. Total carbohydrate-C also increased with aggregate size (Puget et al., 1999). Plant polysaccharides, extracellular polysaccharides, and fungal hyphae produced and/or induced by cover crops may have led to holding macroaggregates together (Degens, 1997; Oades and Waters, 1991; Tisdall and Oades, 1982). These organic binding agents plus cover-crop debris may lead to the increase of the organic components in macroaggregates observed in this experiment.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
In the 8-mo cover-crop field and 8-wk incubation experiments, soil aggregate stability was better correlated with soil dilute acid-extractable polysaccharides than with soil total polysaccharides. During the winter cover-crop growing period, fall rye and annual ryegrass increased dilute acid-extractable polysaccharide content, which contributed to the increased stability of 2- to 6-mm aggregates and increased aggregate MWD. The incubation experiment showed that incorporation of green cover crops into soil increased soil MWD, total organic C, total N, and dilute acid-extractable polysaccharides. This experiment suggests that 1-yr winter cover cropping can increase soil aggregate stability through the input of dilute acid-extractable polysaccharides in the soils with intensive cultivation. Also, annual ryegrass was a good winter cover crop for soil aggregation.

Received for publication January 25, 2005.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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