Abstract
We investigated the effects of long-term cattle slurry application on soil aggregation and the distribution of microbial populations by characterizing soil structural properties, extraction patterns of chloroform-labile carbon over increasing periods of fumigation and microbial direct counts using the wash-sonic method in soil samples from an upland field with three levels of slurry application, 60, 150 and 300 Mg ha−1. The soil samples could be categorized based on the extraction patterns of chloroform-labile carbon. Soil from the 60 Mg ha−1 plot released carbon at a constant rate under chloroform fumigation for 0.5 and 24 h. The chloroform-labile carbon in the 300 Mg ha−1 plot significantly increased over the same period, while that in the 150 Mg ha−1 plot increased for 1 h only. The bacterial count and the overall length of fungal hyphae were similar in the three plots. Indirect assessments of the relative abundance of bacterial and fungal populations using the wash-sonic method suggested that water-stable soil aggregates in the 300 Mg ha−1 plot were stabilized by hyphae to a higher degree than aggregates from the 60 and the 150 Mg ha−1 plots. The bulk densities significantly decreased and the maximum water-holding capacity significantly increased with the slurry application level. These observations confirm that slurry application enhances water-stable soil aggregation. A combined approach using the partial-sterilization and wash-sonic methods revealed that the relative dominance of fungal populations in the water-stable soil aggregates resulted in a higher amount of labile carbon in the time-dependent extraction by chloroform in the 300 Mg ha−1 plot compared with the 60 and 150 Mg ha−1 plots. The wash-sonic method has the potential to characterize microbial distribution between water-dispersed soil components and water-stable soil aggregates.
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