Soil carbon sequestration arises from the interplay of carbon input and stabilization, which vary in space and time. Assessing the resulting microscale carbon distribution in an intact pore space, however, has so far eluded methodological accessibility. Here, we explore the role of soil moisture regimes in shaping microscale carbon gradients by a novel mapping protocol for particulate organic matter and carbon in the soil matrix based on a combination of Osmium staining, X-ray computed tomography, and machine learning. With three different soil types we show that the moisture regime governs C losses from particulate organic matter and the microscale carbon redistribution and stabilization patterns in the soil matrix. Carbon depletion around pores (aperture > 10 µm) occurs in a much larger soil volume (19–74%) than carbon enrichment around particulate organic matter (1%). Thus, interacting microscale processes shaped by the moisture regime are a decisive factor for overall soil carbon persistence.

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Water-extractable organic carbon (WEOC) is considered as the most important carbon (C) source for denitrifying organisms, but the contribution of individual organic matter (OM) fractions (i.e., particulate (POM) and mineral-associated (MOM)) to its release and, thus, to denitrification remains unresolved. Here we tested short-time effects of POM and MOM on potential denitrification and estimated the contribution of POM- and MOM-derived WEOC to denitrification and CO2 production of three agricultural topsoils. Suspensions of bulk soils with and without addition of soil-derived POM or MOM were incubated for 24 h under anoxic conditions. Acetylene inhibition was used to determine the potential denitrification and respective product ratio at constant nitrate supply. Normalized to added OC, effects of POM on CO2 production, total denitrification, and its product ratios were much stronger than those of MOM. While the addition of OM generally increased the (N2O + N2)-N/CO2-C ratio, the N2O/(N2O + N2) ratio changed differently depending on the soil. Gas emissions and the respective shares of initial WEOC were then used to estimate the contribution of POM and MOM-derived WEOC to total CO2, N2O, and N2O + N2 production. Water-extractable OC derived from POM accounted for 53–85% of total denitrification and WEOC released from MOM accounted for 15–47%. Total gas emissions from bulk soils were partly over- or underestimated, mainly due to nonproportional responses of denitrification to the addition of individual OM fractions. Our findings show that MOM plays a role in providing organic substrates during denitrification but is generally less dominant than POM. We conclude that the denitrification potential of soils is not predictable based on the C distribution over POM and MOM alone. Instead, the source strength of POM and MOM for WEOC plus the WEOC’s quality turned out as the most decisive determinants of potential denitrification.

<p>Denitrification usually takes place under anoxic conditions and over short periods of time and depends on readily available nitrate and carbon sources. Variations in CO<sub>2</sub> and N<sub>2</sub>O emissions from soils amended with plant residues have mainly been explained by differences in their decomposability. Another factor rarely considered so far is water-extractable organic matter (WEOM) released into soil during residue decomposition. Here, we examined the potential effect of plant residues on denitrification with special emphasis on WEOM. A range of fresh and leached plant residues was characterized by elemental analyses, <sup>13</sup>C-NMR spectroscopy, and extraction with ultrapure water. The obtained solutions were analyzed for the concentration of organic carbon (OC), organic nitrogen (ON), and by UV-VIS spectroscopy. To test the potential denitrification induced by plant residues or three different OM solutions, these carbon sources were added to soil suspensions and incubated for 24 hours at 20 °C in the dark under anoxic conditions; KNO<sub>3</sub> was added to ensure unlimited nitrate supply. Evolving N<sub>2</sub>O and CO<sub>2</sub> were analyzed by gas chromatography and acetylene inhibition was used to determine denitrification and its product ratio. The production of all gases as well as the molar N<sub>2</sub>O+N<sub>2</sub>-N/CO<sub>2</sub>-C ratio was directly related to the water-extractable OC (WEOC) content of the plant residues and the WEOC increased with carboxylic/carbonyl C and decreasing OC/ON ratios of the plant residues. Incubation of OM solutions revealed that the molar N<sub>2</sub>O+N<sub>2</sub>-N/CO<sub>2</sub>-C ratio and share of N<sub>2</sub>O are influenced by the WEOM’s chemical composition. In conclusion, the effect of plant residues on potential denitrification is governed by their composition and the related production of WEOM.</p>