A stable soil aggregation is an important factor for a number of ecosystem processes related to soil structure, organic matter turnover, soil chemical and biological fertility. Soil aggregates have been used as basic units of soil habitat to understand the dynamics of soil organic matter, nutrient cycling and biological functions. Since the proposal of aggregate hierarchy model by Tisdall and Oades (1982) considerable advancements have been made in our understanding of soil organic matter-aggregation relationships.

Roots and decomposing crop residues form centres for the formation of macroaggregates in soils. The process of aggregation is dynamic, in particular for macroaggregates, with short-term changes to their structure and chemical constituents. Microbiota communities (microflora and protozoa) associated with macroaggregates are distinct and known to be influenced by the source of labile organic matter. Any changes to the quality of this labile organic matter source have the potential to influence the composition and activity of associated microbial communities. In this study, we measured the effect of labile organic matter sources, i.e. roots and residues, on the composition and activity of microbial communities associated with rhizosphere macroaggregates.

To determine the short-term dynamics of microbiota communities as influenced by macroaggregate environment, surface soil samples (0-10 cm) were collected from Pasture-Pasture-Wheat rotation plots in the Waite long-term trial located in Adelaide, South Australia. Macroaggregates from the rhizosphere zone were separated from field moist soils using a dry sieving technique. Two types of macroaggretages were hand picked i.e. 1-2 mm diameter aggregates directly associated with roots, and 0.5 to 1.0 mm diameter aggregates with no visible root/crop residue fraction. Moisture levels were equilibrated to -10kPa and the aggregates were incubated at 25oC. The composition of bacteria, fungi and protozoa were measured in fresh aggregates and after two weeks of incubation using 16S / 18S rDNA–DGGE, microscopy techniques, and C substrate utilization profiles. Aggregates were also analysed for microbial biomass C and microbial activity levels.

Prior to incubation, the two types of rhizosphere macroaggregates differed in their bacterial community level physiological profiles (CLPP; C substrate utilization patterns). Similarly, the diversity of bacteria, fungi and protozoa were strongly influenced by the proximity of aggregate to the root, with bacteria being less diverse and fungi more diverse in root aggregates. Dissolved organic C (DOC) levels were similar in both types of aggregates but microbial biomass C was higher in aggregates associated with roots.

Incubation of aggregates for a two week period caused significant reduction in the level of microbial biomass and DOC in both types of aggregates, although the magnitude of reduction was greater in the root-associated aggregates. Incubation also caused significant changes in the diversity of bacteria and fungi, in particular with diversity increasing in the root-associated aggregates. Changes in the CLPP of bacterial communities due to incubation were evident in both types of macroaggregates, with incubated aggregates utilizing less substrate overall.

Our investigation of the interaction between the aggregate environment and carbon availability in relation to associated microbial activity showed that accessibility to C source is the key factor regulating the level of microbial activity in macro-aggregates of increasing size.

Tisdall, J.M., Oades, J.M., (1982) Organic matter and water-stable aggregates in soils. J. Soil Sci. 62, 141–163.