Does X-Ray Computed Tomography Affect Soil Respiration and Microbial Activity?.

Application of X-ray computed micro-tomography (CT) to soil science research has a great potential in improving our understanding of the influence of soil physical micro-environments on a wide range of physical, chemical and biological soil processes. However, in order to use the CT tools in analysis of soil biological processes, it is imperative to understand possible artifacts that treatment with X-ray CT might have on soil microorganisms. The objective of this study was to investigate the effects of CT scanning on soil respiration and microbial community composition in soil aggregates of different sizes. Soil respiration was measured in aggregates of the following sizes: 1-2, 0.5-1, 0.105-0.5, 0.053-0.105 and <0.053 mm. Two replications of aggregate samples treated with X-ray CT and four replications without X-ray CT treatment were subjected to 120 days incubations. Our results suggest that CT scanning can have an effect on microbial activity leading to soil respiration; however, the magnitude of its influence depends on soil pore structure. Specifically, we observed that as a result of scanning, the rate of soil respiration was significantly reduced in 0.053-0.105 and <0.053 mm aggregate samples in the first of week of the incubation period; however, no differences between soil respiration in scanned and un-scanned aggregates of the studied sizes were observed in the rest of the incubation period. Microbial community composition was measured in the 4-6 mm aggregates. The results indicated that while the overall microbial communities of the scanned and unscanned aggregates as assessed by principal component analysis and Bray-Curtis dissimilarity measures were similar to each other, some of the microbial groups differed. Specifically, scanned aggregates tended to have lower percentages of Fimicutes, Planctomycetes, and Verrucomicrobia phyla than unscanned aggregates. We recommend that care must be exercised when deriving conclusions regarding short term biological processes or individual microbial groups based on samples subjected to CT scanning.