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

The activity of microorganisms, especially fungi, is thought to play a key role in the formation and stabilization of macroaggregates in many soils. O ur aim was to follow the aggregate formation and stabilization of organic m atter in a controlled laboratory experiment, where microaggregates (53-250 mu m) obtained by slaking of a cultivated Duroc loam (finesilty, mixed, mes ic Pachic Haplustoll) were incubated with C-13-labelled granular starch equ ivalent to 8 mg starch-C g(-1) soil. Over 71 d we measured the size distrib ution of water-stable aggregates. followed the dynamics of native and label led C within the aggregate-size classes, and estimated the development of t he microbial biomass by chloroform fumigation-extraction and by direct coun ting. After an initial flush of starch decomposition, at d 4 macroaggregate s (250-8000 mu m) represented 56% of the soil mass and contained 6.3 mg sta rch-derived C g(-1) soil compared with 1.6 mg starch-derived C g(-1) soil i n the 53-250-mu m microaggregates. The concentration of starch-derived C is olated in microbial biomass at d 4 was 3270 mu g g(-1) soil in the macroagg regates and 630 mu g g(-1) soil in the microaggregates and declined thereaf ter in both size classes. At d 4, fungal biomass predominated the microbial community in the macroaggregates, in all other samples bacterial biomass p redominated. The mass of soil found in macroaggregates did not follow the d ecline in fungal and bacterial biomass with incubation-time but remained co nstant. Except at the last sampling date, we observed no formation of macro aggregates 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 s pots' for fungal growth towards and on these sources. In accordance with pr evious 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 fa ctors that persist after cell death of the microorganisms. In situ minerali zation rate constants of labelled C were much lower for the microaggregates , compared with those of the macroaggregates, indicating a better stabiliza tion of starch-derived C within the microaggregates, and thus supporting th e macroaggregate-microaggregate conceptual model. (C) 1999 Published by Els evier Science Ltd. All rights reserved.