Abstract
While microbial-mineral-organic matter interactions are key features controlling the fates of low molecular-weight compounds in soils, direct investigations of how they control their fine-scale spatial distribution are scant. Here, we addressed how microbial transformations affect the retention of 13C/15N-labeled glycine in a forest topsoil 8 h after application. We assessed the contribution of soil microorganisms to glycine-derived 13C and 15N retention using γ-irradiated and non-irradiated soils. We tracked down the glycine-derived 13C and 15N at the surface of particles randomly isolated from soil density fractions using nano-scale secondary ions mass spectrometry (NanoSIMS) imaging. Eight hours after addition, 7 % of the glycine-derived 13C and 15N initially applied was recovered among soil density fractions, mainly via the activity of soil microorganisms (>85 % of total retention). Glycine-derived 13C and 15N distribution among density fractions was correlated with that of soil organic matter (SOM) determined by NanoSIMS (R ≥ 0.85), suggesting that the spatial patterns of the mineral-attached SOM controls the spatial distribution of the glycine-derived 13C and 15N. NanoSIMS images showed largely decoupled glycine-derived 13C and 15N spots preferentially attached to aggregated particles. We speculate that the glycine-derived 13C was principally found within or in the vicinity of microbial cells, whereas the glycine-derived 15N was mostly found as NH4 + and/or exoenzymes spread across soil surfaces. The C:N ratios determined by NanoSIMS suggest that local chemical properties of mineral-attached SOM drive glycine-derived 15N attachment, with the preferential attachment to mineral-attached SOM rich in N (C:N ratios mostly 2.65 g cm−3).
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