Carbon mineralisation and pore size classes in undisturbed soil cores

Abstract

Soil pore network effects on organic matter turnover have, until now, been studied indirectly because of lack of data on the 3D structure of the pore network. Application of X-ray computed tomography (X-ray CT) to quantify the distribution of pore neck size and related pore sizes from undisturbed soil cores, with simultaneous assessment of carbon (C) mineralisation, could establish a relationship between soil organic matter (SOM) decomposition and soil pore volumes. Eighteen miniature soil cores (diameter 1.2 cm, height 1.2 cm) covering a range of bulk densities were incubated at 20°C for 35 days. Respiration was modelled with a parallel first- and zero-order kinetic model. The cores were scanned at 9.44 µm resolution using an X-ray CT scanner developed in-house. Correlation analysis between the slow pool C mineralisation rate, ks, and pore volume per pore neck class yielded significant (P < 0.05) positive correlations: r = 0.572, 0.598, and 0.516 for the 150–250, 250–350, and >350 µm pore neck classes, respectively. Because larger pores are most probably mainly air-filled, a positive relation with ks was ascribed to enhanced aeration of smaller pores surrounding large pores. The weak and insignificant relationship between the smallest pore neck class (<9.44 µm) and ks could be explained by obstructed microbial activity and mobility or diffusion of exo-enzymes and hydrolysis products as a result of limited oxygen availability. This study supports the hypothesis that the impact of soil structure on microbial processes occurs primarily via its determination of soil water distribution, which is possibly the main driver for the location of C mineralisation in the soil matrix.