Decomposition of bacterial polymers in soil and their influence on soil structure

Summary

The adherence of soil particles into stable aggregates increases with the addition of monosaccharides or polysaccharde polymers to soil, either as plant residues, microbial metabolites, or as simple carbohydrates. Microbial polysaccharides are one of the most effective organic agents that promote soil aggregate stability, but the effectiveness of these polymers in stabilizing soil particles varies dramatically between microbial strains, the amount present and the prevailing environmental conditions. We conducted glasshouse and laboratory studies to determine the effectiveness of selected microbial polymers in stabilizing soil aggregates. The addition and thorough mixing of 1.0 mg microbial polymer C g⁻¹ soil of seven bacteria strains (Arthrobacter viscocus, Azotobacter indicus, Bacillus subtilus, Chromobacterium violaceum, Pseudomonas aeruginosa, Pseudomonas strain I, and Pseudomonas strain II), three deuteromycete strains (Cryptococcus laurentii, Hansenula holstii, and Mucor rouxii), and two reference compounds (hydroxyethyl guar and glucose) to an Arlington coarse-loamy soil resulted in stimulated soil respiration, increased aggregate stability, and decreased soil bulk density and modulus of rupture when incubated from 1 to 12 weeks. The monosaccharides present in the added polymers were rapidly decomposed and the sacchride content of the polymer-treated soil returned to the level of the soil control (with no polymer addition) after 2 weeks of incubation, while the maximum increase in soil aggregate stability was noted during the 3rd and 4th weeks of incubation. Statistical analyses showed that the glucose content of the polymers added was significantly correlated with soil aggregation [weeks 1 (r=0.78^***) and 2 (r=0.61^*)], but the extractable soil saccharides were not significantly correlated with increased aggregate stability or decreased soil bulk density during this study. When microbial extracellular polymers were added to soil only a transient increase in soil stability was measured upon decomposition of the added saccharides. This finding suggests that the stabilization of soil aggregates is a result of other microbial processes or metabolites rather than the direct binding effects of the added polysaccharides.