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The contribution from foliage, roots, and fungi to soil organic matter (SOM) represents a key unknown in SOM dynamics. Since stable hydrogen isotope ratios (δ²H_n) can greatly differ among these sources, we explored δ²H_n to elucidate sources contribution to SOM. Moreover, we assessed whether addition of ²H-depleted water – providing ²H-signal that gets incorporated into organic matter – allows tracing of new organic H and thus assessing SOM turnover.

In a 17-year-long irrigation experiment in a dry pine forest, we measured δ²H_n and stable carbon and nitrogen isotope ratios (δ¹³C, δ¹⁵N) in SOM sources, bulk SOM and particle-size fractions. Relative source contribution to SOM was estimated by Bayesian mixing models including all three isotopes.

The δ²H_n increased from foliar litter (−153‰) to fine roots (−118‰) and fungal mycelia (−81‰). Mixing models indicated that foliar litter dominated organic …