Persistence of dissolved organic matter explained by molecular changes during its passage through soil

Abstract

Dissolved organic matter affects fundamental biogeochemical processes in the soil such as nutrient cycling and organic matter storage. The current paradigm is that processing of dissolved organic matter converges to recalcitrant molecules (those that resist degradation) of low molecular mass and high molecular diversity through biotic and abiotic processes. Here we demonstrate that the molecular composition and properties of dissolved organic matter continuously change during soil passage and propose that this reflects a continual shifting of its sources. Using ultrahigh-resolution mass spectrometry and nuclear magnetic resonance spectroscopy, we studied the molecular changes of dissolved organic matter from the soil surface to 60 cm depth in 20 temperate grassland communities in soil type Eutric Fluvisol. Applying a semi-quantitative approach, we observed that plant-derived molecules were first broken down into molecules containing a large proportion of low-molecular-mass compounds. These low-molecular-mass compounds became less abundant during soil passage, whereas larger molecules, depleted in plant-related ligno-cellulosic structures, became more abundant. These findings indicate that the small plant-derived molecules were preferentially consumed by microorganisms and transformed into larger microbial-derived molecules. This suggests that dissolved organic matter is not intrinsically recalcitrant but instead persists in soil as a result of simultaneous consumption, transformation and formation.

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Fig. 1: Molecular changes in soil DOM based on FT-ICR mass spectra.
Fig. 2: Variation partitioning for potential drivers of DOM transformation.
Fig. 3: Shift of molecular DOM masses during soil passage.
Fig. 4: Proposed mechanisms for spatial and temporal evolution of DOM molecular structures during soil passage.

Data availability

The compiled dataset used in our analyses is available at https://doi.org/10.17617/3.28 and root standing biomass at https://doi.org/10.1594/PANGAEA.880324. The raw data are available from the corresponding author (M.L.) on request.

Code availability

The codes used for this study are available on request.

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Acknowledgements

We thank U. Gerighausen for sampling and K. Klapproth for technical support with FT-ICR-MS measurements. This work was supported by the Zwillenberg-Tietz Stiftung and the Deutsche Forschungsgemeinschaft as part of the Critical Zone Observatory ‘AquaDiva’ (CRC 1076) and the Jena Experiment (FOR 1451, GL 262/14 and GL 262/19). The International Max Planck Research School for Global Biogeochemical Cycles (IMPRS-gBGC) provided the funding for the PhD scholarships of P.G.M.-V. and C.S.

Author information

V.-N.R. and G.G. conceived and designed the study. M.L., V.-N.R. and G.G. wrote the main text. V.-R.N. and T.D. measured and processed MS data, N.H. obtained the NMR data. M.L. and V.-R.N. analysed the data. V.-N.R. and S.B. performed the supplementary decomposition experiment and C.S. measured, processed and analysed the data from the supplementary sites. L.M., N.J.O. and A.W. provided root standing biomass data, P.G.M.-V. provided data on microbial biomass, and R.I.G. and T.G. provided data on microbial diversity. All authors reviewed and edited the manuscript.

Correspondence to Markus Lange.

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