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Pore‐scale view of microbial turnover: Combining 14C imaging, μCT and zymography after adding soluble carbon to soil pores of specific sizes

The location of microorganisms and substrates within soil pore networks plays a crucial role in organic carbon (C) processing, and its microbial utilization and turnover, and has direct consequences for C and nutrient cycling. An optimal approach to quantify responses to new C inputs from microorganisms residing in specific pores is the addition of new C to pores of target sizes in undisturbed soil cores. We used the matric potential approach to add 14C‐labelled glucose to small (< 40 μm, root free) or large (60–180 μm, potentially inhabited by roots) pores of undisturbed soil cores. Localization of glucose‐derived C via 14C imaging was related to pore size distributions and connectivity, characterized via X‐ray computed microtomography (μCT), and to β‐glucosidase activity, characterized via zymography. After 2‐week incubations, 1.3 times more glucose was mineralized (14CO2) when it was added to the large pores; however, more 14C remained in microbial biomass when glucose was added to the small pores. Consequently, although utilizing the same amounts of easily available C, the microorganisms localized in the large pores had faster turnover compared to microorganisms in small pores. Stronger associations between β‐glucosidase activity and glucose‐derived C were observed when glucose was added to the large pores. We conclude that (a) the matric potential approach allows placing, albeit not exactly, of soluble substrates into pores of target diameter range, and (b) microorganisms localized in large pores respond to new C inputs with faster turnover, greater growth and more intensive enzyme production compared to those inhabiting the small pores.