Abstract
Aggregates play a key role in protecting soil organic carbon (SOC) from microbial decomposition. The objectives of this study were to investigate the influence of pore geometry on the organic carbon decomposition rate and bacterial diversity in both macro- (250–2000 μm) and micro-aggregates (53–250 μm) using field samples. Four sites of contrasting land use on Alfisols (i.e. native pasture, crop/pasture rotation, woodland) were investigated. 3D Pore geometry of the micro-aggregates and macro-aggregates were examined by X-ray computed tomography (μCT). The occluded particulate organic carbon (oPOC) of aggregates was measured by size and density fractionation methods. Micro-aggregates had 54% less μCT observed porosity but 64% more oPOC compared with macro-aggregates. In addition, the pore connectivity in micro-aggregates was lower than macro-aggregates. Despite both lower μCT observed porosity and pore connectivity in micro-aggregates, the organic carbon decomposition rate constant (Ksoc) was similar in both aggregate size ranges. Structural equation modelling showed a strong positive relationship of the concentration of oPOC with bacterial diversity in aggregates. We use these findings to propose a conceptual model that illustrates the dynamic links between substrate, bacterial diversity, and pore geometry that suggests a structural explanation for differences in bacterial diversity across aggregate sizes.
Highlights
Aggregates play a key role in protecting soil organic carbon (SOC) from microbial decomposition
There appears to be a feedback loop between organic carbon concentration and pore geometry of soil[26]. Both organic matter concentration and pore geometry regulates the carbon decomposition rate and microbial diversity in soil, but it is still unclear whether the influence of pore geometry on carbon decomposition and microbial diversity is stronger compared with carbon concentration in soil
Since we used a small dataset in the structural equation modelling (SEM) to evaluate the relationship of Ksoc with pore geometry, we took the non-significant negative association between Ksoc and pore connectivity to imply a potential indication that pore connectivity played a regulatory role in organic carbon decomposition in soil aggregates
Summary
Aggregates play a key role in protecting soil organic carbon (SOC) from microbial decomposition. The objectives of this study were to investigate the influence of pore geometry on the organic carbon decomposition rate and bacterial diversity in both macro- (250–2000 μm) and micro-aggregates (53–250 μm) using field samples. At spatial scales relevant to biological processes (i.e. carbon decomposition) and soil management (i.e. tillage, land use change), pore geometry controls oxygen diffusion rate, water flow and nutrient supply for microbial communities and vascular plants[2]. The objective of the present study was to investigate the influence of pore geometry on organic carbon decomposition rate and bacterial diversity in soil aggregates. We sought to analyze functional relationships among pore geometry, organic carbon decomposition and bacterial diversity in soil aggregates using structural equation modelling (SEM)
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