Winter 1994 (v6n1)


Cover crop management of polysaccharide-mediated aggregation in an orchard soil.

Emily B. Roberson, Shlomo Sarig and Mary K. Firestone

Soil sc. Soc. Amer. J . 55: 734- 73s . 1991

Cover cropping may affect soil structure by supporting the growth of soil microbes. Some of these microorganisms produce polysaccharides that in turn promote the formation of water-stable aggregates, particles of soil resistant to erosion by water. This phenomenon leads to improved soil structure and increased rates of water infiltration. In the present study, various understory-management regimes were applied to experimental plots in a flood-irrigated prune orchard in Gridley, Butte County, California, in order to determine various effects on the loam soil. Understory management regimes were as follows: 1) permanent cover with perennial ryegrass, Lolium perenne; 2) annual cover occasionally mowed, using the fall-sown cereals barley, Hordeum vulgare, or wheat, Triticum aestivum; 3) fall-sown barley or wheat killed in May using glyphosate (this substantially decreased the biomass provided by the cereals); and 4) clean-cultivation using occasional disking to break surface crusts and reduce weeds.

Soils were sampled after the treatments had been applied for two growing seasons. Soil samples were taken in the crusted layer (0-2 cm deep), and in a deeper stratum (2-10 cm deep). The following response variables were assessed in the laboratory:

  1. saturated hydraulic conductivity (a measurement of water permeability);
  2. mean weight diameter of soil particles (the weighted average of soil mass occurring in four size classes);
  3. aggregate slaking resistance of particles ranging in diameter from 0.3-2.0 mm (this property was assessed using two methods, both of which gauge the proportion of soil remaining in a 0.3-mm sieve after water is poured over the soil);
  4. organic carbon (all forms, including carbohydrates);
  5. microbial biomass carbon;
  6. heavy-fraction carbohydrates (which are believed to be to a large extent of microbial origin and to include a high proportion of the polysaccharides associated with aggregates);
  7. light-fraction carbohydrates (removed by flotation from the heavy-fraction carbohydrate, and including most of the undecomposed plant residue).

Data were assessed using appropriate analysis of variance (ANOVA) models followed by mean separation by single degree of freedom linear contrasts. Results are summarized in table 1. Response variables are listed in column headings; treatments followed by the same letter are deemed not significantly different for the relevant response variables. Results indicated that permanent cover and mowed barley led to especially high values for saturated hydraulic conductivity, aggregate slaking resistance, and heavy-fraction carbohydrate. Correlation analysis showed that both microbial biomass carbon and heavy-fraction carbohydrates were highly correlated with aggregate slaking resistance and saturated hydraulic conductivity. Multiple regression showed that heavy-fraction carbohydrates were a better predictor of aggregate slaking resistance and saturated hydraulic conductivity than were light-fraction carbohydrates, organic carbon, or microbial biomass carbon.

The higher amounts of carbon available to microbes in the permanent cover and mowed barley treatments appear to translate into improved soil structure. This effect was related to short-term increases in the heavy-fraction carbohydrates, but was not reflected in statistically higher concentrations of total organic carbon or microbial biomass carbon.

The loam soil in this study had low organic matter, little structure, and was very prone to crusting. Thus, it was more susceptible than some other soils to rapid improvement through additions of organic matter Also, in other soil types, clay, humified organic matter, or iron oxides may be more important in the aggregation process.

For more information write to: M. Firestone, Department of Soil Science, 108 Hilgard Hall, University of California, Berkeley, CA 94720.


Table 1. Effect of orchard floor management practices on soil response variables as indicated by single degree of freedom linear contrasts. Treatment effects followed by the same letter are not significantly different for the relevant response variable (a = 0.05).

Treatment		 Saturated Hydraulic Conductivity Mean Weight Diameter Aggregate Slaking Resistance Organic Carbon Organic Microbial Biomass Heavy- Fraction Carbohydrates Light- Fraction Carbohydrates
Permanent Covera a a a a a a
Mowed Barleya a a a a a a
Clean Cultivated a ba ba a b a
Herbicided Barley ba a ba a ba

(DEC.51 7)

Contributed by Robert Bugg
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