Microbial contributions to the aggregation of a cultivated grassland soil amended with starch

Abstract

The activity of microorganisms, especially fungi, is thought to play a key role in the formation and stabilization of macroaggregates in many soils. Our aim was to follow the aggregate formation and stabilization of organic matter in a controlled laboratory experiment, where microaggregates (53–250 μm) obtained by slaking of a cultivated Duroc loam (fine–silty, mixed, mesic Pachic Haplustoll) were incubated with 13C-labelled granular starch equivalent to 8 mg starch-C g −1 soil. Over 71 d we measured the size distribution of water-stable aggregates, followed the dynamics of native and labelled C within the aggregate-size classes, and estimated the development of the microbial biomass by chloroform fumigation–extraction and by direct counting. After an initial flush of starch decomposition, at d 4 macroaggregates (250–8000 μm) represented 56% of the soil mass and contained 6.3 mg starch-derived C g −1 soil compared with 1.6 mg starch-derived C g −1 soil in the 53–250-μm microaggregates. The concentration of starch-derived C isolated in microbial biomass at d 4 was 3270 μg g −1 soil in the macroaggregates and 630 μg g −1 soil in the microaggregates and declined thereafter in both size classes. At d 4, fungal biomass predominated the microbial community in the macroaggregates, in all other samples bacterial biomass predominated. The mass of soil found in macroaggregates did not follow the decline in fungal and bacterial biomass with incubation-time but remained constant. Except at the last sampling date, we observed no formation of macroaggregates in the control samples (incubated without starch amendment), and the undecomposed starch did not glue microaggregates into macroaggregates either. We suggest that microhabitats enriched in substrate acted as `hot spots' for fungal growth towards and on these sources. In accordance with previous studies, the resulting filamentous entanglement of primary particles and microaggregates formed macroaggregates. However, the stabilization of the macroaggregates throughout the experiment may be attributed to other factors that persist after cell death of the microorganisms. In situ mineralization rate constants of labelled C were much lower for the microaggregates, compared with those of the macroaggregates, indicating a better stabilization of starch-derived C within the microaggregates, and thus supporting the macroaggregate–microaggregate conceptual model.