In situ visualization on the mobility of mineral associated organic matter in a microfluidics-based heterogeneous soil aggregate structure

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The mobility of different types of soil organic matter (SOM), including dissolved organic matter (DOM) and mineral associated organic matter (MAOM), along the soil profile in response to infiltration flow can significantly affect the carbon pools in subsoil. In addition, the soil heterogeneity on the pore-scale, comprising pore spaces within the intra-aggregate matrix and preferential pathways within inter-aggregate zones, can also affect the mobility of SOM. This is especially the case for clay-sized MAOM. However, the quantitative understanding of the clay-sized MAOM mobility in presence of pore-scale heterogeneity remains elusive. This study designed a novel microfluidic device to track the pore-scale distribution and mobility of fluorescence labeled clay-sized MAOM in heterogeneous soil pore structures. Results showed that the MAOM mobility was significantly lower in the intra-aggregate zones compared with in the inter-aggregate zones. Only 4.8 % to 47.9 % of MAOM in the intra-aggregate region was mobilized at high leaching flow rates of 81 to 163 mm/d. Moreover, colloid mobilization induced by shearing rather than solute desorption followed by solute dilution is expected to be the major factor facilitating the mobilization of clay-sized MAOM. This study demonstrated a new experimental protocol allowing the in-situ visualization and quantification of OMs mobility in presence of pore-scale heterogeneity. Moreover, our findings also highlighted the potential importance of heterogeneous mobility of clay-sized MAOMs that facilitated by the colloid transport on the SOM turnover and distribution.