Localization of mineralizable organic carbon using X-Ray CT
Loading...
Date
2010
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Ghent University Vrije Universiteit Brussel
Abstract
The sensitivity of organic C (OC) to decomposition depends not only upon its chemical composition but also on its location within the soil matrix. However, the precise location of OC mineralization is not clearly known. This Master thesis research's objective was to investigate the role of soil pore structure on the organic matter (OM) decomposability in a model sandy loam soil. To achieve this, the influence of different artificial operations
(compaction, artificial change in texture, acidification, and OM application) on soil pore size distribution, decomposer group community (assessed by the fungal: bacteria ratio) and C mineralization was studied. Two major experiments were included. In the first one, manipulated soil samples were incubated in larger tubes for five weeks. A second incubation experiment was set up to follow C mineralization of added C-sources (grinded sawdust or grass particles) in artificially manipulated soil with differing pore size distribution and decomposer community. Soil samples were incubated in small tubes (1cm diameter* 1cm height) for five weeks and the evolving CO2 gas was analyzed by a Gas chromatography method. Samples were then scanned after incubation by X-ray computed tomography (CT) and following image processing, the total pore volume and % volume of different pore size classes were calculated. Correlation analysis was used to investigate the relation between the % pore volume of five pore classes (10-200, 210-400, 410-600, 610-800 and >800 pm equivalent sphere diameter) and the cumulative C mineralization after 5 weeks. Artificial manipulation of the sandy loam soil, through compaction at BD 1.6 g cm -3 and artificial change in texture from coarse sand: fine sand: silt and clay (CS: FS: S&C) ratio of 10:40:50 to 15:50:35 and 20:60:20 was found to reduce the total soil porosity. Soil compaction at BD 1.6 g cm'3 reduced the proportion of macropores (pores with pore neck diameter 30-300 pm) while artificial change in soil texture found to affect mainly the distribution of micropores (pores with pore neck diameter 0.2-15pm) and to a lesser extent the macropores. Soil amendment with grass material found to increases the total soil porosity. Artificial change in soil texture, soil acidification to pH 4.3 and additions of sawdust material
compared to grass changed the microbial community towards more fungi oriented. This suggests these artificial soil operations to be usable for manipulation of soil pore structure and the microbial community. In the second experiment, very pronounced interaction effects on C mineralization between substrate type and the artificial changes in soil pore structure were found. First an interaction effect between soil compaction at BD 1.6 g cm'3 compared to BD 1.3 g cm'3 and substrate type was present as the reduction on C mineralization was more pronounced in grass amended soil than in sawdust amended soil. Similarly, a very pronounced interaction effect between artificial change in texture and substrate type was also noted as addition of sawdust strongly reduced the net substrate derived C mineralization, while grass addition did not. C mineralization from native SOM positively correlated to % pore volume of the 200-600 pm, class implying the dominance of mineralization in intermediate sized pores. On the other hand, a positive correlation between C mineralization and % volume of 610-800 pm pores with both grass and sawdust addition demonstrates the importance of larger pores for substrate decomposition. An inhibiting effect of soil compaction on C mineralization was observed from pF and X-ray CT data and this was likely related to the effect of macropore reduction and probably aeration. The negative influence on C mineralization of artificial change in soil texture and the associated reduction in the microporosity only with addition of sawdust but not with grass seems to be related to observed differences in microbial community involved in decomposition of both substrates. Interestingly but complex interactions between soil pore structure and substrate type were demonstrated in this thesis. Pronounced negative or positive correlations between individual pore size class volumes and C mineralization could be established and this indicates their role in the OM decomposition process.
Description
Dissertation
Keywords
C mineralization, soil pore structure, organic carbon, sandy loam soil