Mechanistic understanding of the effect of soil carbonates and organic amendments on soil structure and biological activity.

Calcareous soils are common in arid and semi-arid regions and account for around half of the earth surface. In addition to the more widely studied soil organic carbon (SOC) pools, these soils also hold a large stock of soil inorganic carbon (SIC) surpassing SOC stocks. Nonetheless, despite their relevance, the effect of the interplay between SOC and SIC in calcareous soils is still poorly understood.

Soil carbonates can dissolve and re-precipitate in soil pores in short periods of time, dynamically changing the soil pore space, causing direct and indirect impacts on the SOC cycle that increase organic matter turnover rates (Fernández-Ugalde et al., 2011) than in soils of similar characteristics (climate, clay content, etc) without carbonates. This can be partially caused by the fact that carbonates dynamics (dissolution-precipitation) contributes somehow to a slower mineralization of organic matter. Several hypotheses exist to explain this positive effect for the stabilisation of SOC. One is the abundance of Ca that would favour mineral-mineral and organo-mineral interactions and associations. Another is that carbonates can protect SOC from further degradation by cementation. This can be related to carbonate crystals interfering with SOC mineralization by microorganisms.

We use a combination of 3D X-ray Computed Tomography and new mechanistic modelling to determine the relationship between the presence of carbonates in soil (and their dynamics) on the SOC mineralization rates (modelled). Preliminary results will be presented in this contribution.

Soil samples subject to different treatments were obtained from two soil sites: Arazuri (Navarra, Spain) and Rodezno (Rioja, Spain). The Arazuri soil supports a long-term experiment assessing the effect of the continuous application of sewage sludge on agricultural soil quality and productivity. Two contrasting fertilisation treatments corresponding to a baseline (mineral fertilization) and a high organic fertilisation treatment (80 t ha^-1 of sewage sludge) were selected. Rodezno samples were obtained in an agricultural field subject to identical historical agricultural management for decades but naturally presenting two types of soils, differing in their carbonate content in their upper horizon (none and 20% equivalent calcium carbonate) due to their position on the landscape. Air-dried soil aggregates were scanned using a Nikon XT H 225ST X-ray CT system at two voxel resolutions 5 µm (2-5 mm aggregate size) and 25 µm (> 5 mm aggregate size). In parallel, a spatially-explicit mechanistic model of the SOC dynamics (Portell et al. 2018) considering explicitly the role of soil bacteria was expanded to take into account the modifications of the soil architecture due to the presence of soil carbonates as observed in the scanned samples.

Image-analysis of the X-ray CT data allowed to quantify the effect of calcium and organic fertilisation in the pore space distribution and connectivity. In addition, the combination of imaging data and the mechanistic model allowed to estimate mineralisation rates and link them to the calcium carbonate content and fertilisation treatment. Overall, our research provides a deeper understanding of the soil carbon organic and inorganic cycles.

References: Fernández-Ugalde et al. (2011). Geoderma,164: 203-214; Portell et al. (2018) Front. Microbiol. 9:1583.