Contributions of biotic and abiotic processes to the presence of an aggregate hierarchy in three different mineral soils in an experimental incubation study

Abstract

This study aims to investigate the presence of aggregate hierarchy during the decomposition of rice straw in a subtropical soil (Ultisol) with large quantities of Fe and Al oxides and in two temperate soils with a large amount of montmorillonite (Vertisol) or with less organic matter but a large content of CaCO3 (Inceptisol) and to explore the relationship between relative contributions of biotic and abiotic processes in soil aggregation and the presence of aggregate hierarchy. A 120-day incubation experiment with or without 13C-labeled straw was conducted in the above-mentioned three soils previously sieved at < 0.25 mm (i.e., without macroaggregates). During incubation, the aggregate size distribution, aggregate associated organic C (OC), straw-derived 13C, soil respiration, soil microbial biomass C and N, and phospholipid fatty acids were measured several times. At the end of each experiment, both macro- and microaggregates were present. Higher OC and 13C concentrations in macroaggregates than in the fractions < 0.25 mm were observed within the first 15 days in all three soils. However, after 60 days, only the Vertisol could still show a slightly higher OC concentration in the macroaggregates. Same as the temperate Vertisol and Inceptisol, more macroaggregates could form by organic binding agents than by inorganic binding agents in the Ultisol at the early incubation stage. While similar to the subtropical Ultisol, macroaggregates could also mainly form by inorganic binding agents than by organic binging agents in the Vertisol and Inceptisol at the later stage. Despite the microbial community structure being different from each other, the macroaggregation was significantly related to the microbial biomass and activity in all three soils at the early incubation stage. A high degree of aggregate hierarchy can present in all three soils at the early decomposition stage of rice straw, while at the later stage, no aggregate hierarchy or only a small degree of aggregate hierarchy can present in the Ultisol and Inceptisol or in the Vertisol. Consequently, instead of soil type, the expression of aggregate hierarchy is essentially determined by the relative contributions of biotic and abiotic processes or of organic and inorganic binding agents in soil aggregation.