How does pore structure affect microbial respiration on the aggregate-scale?

Small-scale drivers of soil organic matter (SOM) mineralization can have important implications for larger scale soil carbon dynamics. SOM affects soil aggregation and structure formation. In turn, the soil pore network can impact SOM mineralization as it governs soil aeration and determines the physical accessibility of SOM to microbes. However, it remains elusive if structural or chemical soil properties determine the SOM turnover in soil aggregates. The aim of this study was to assess how SOM mineralization is affected by soil structure and chemical properties on the aggregate scale. For this purpose, we combined microbial respiration essays with 3-D X-ray micro-computed tomography (CT) scans of single aggregates. We expected that microbial respiration is driven by the co-location of SOM and soil pores and their connectivity but further influenced by the quality of organic matter and its stabilizing complexation with soil minerals. For this purpose, we measured the microbial respiration in soil aggregates (4-6 mm) from soils of two long-term field trials from central and southern Sweden with differing soil textures (sandy loam and clay loam) and organic matter sources and qualities (bare fallow, mineral fertilization, straw addition). Basal respiration rates were measured for moist single aggregates using MicroResp^TM (µg CO₂-C g^-1 SOC h^-1). We characterised the internal pore networks of aggregates using X-ray micro-CT with a voxel edge length of 5 µm, to assess the influence of pore size distribution, pore volume, pore surface area and connectivity on microbial respiration rates. In addition, we mapped particulate organic matter to compare its spatial distribution in relation to the pore network as an indicator of physical accessibility to microbes. For estimating the chemical accessibility of SOM we determined its composition using solid state ¹³C nuclear magnetic resonance spectroscopy and its potential chemical complexation with amorphous aluminium hydr(oxides). The highest respiration rates were observed for aggregates from straw-amended soils, even after normalization for carbon content. Our results will contribute to a better understanding of the small-scale mechanisms of SOM turnover that affect larger-scale organic carbon stocks in soils.

Acknowledgements: The study was funded by FORMAS  grant no. 2022-00225.