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 layers (69 ± 15%) and coarse particulate organic matter (POM) at 0-5 cm (65 ± 12%). In contrast, roots dominated fine POM at 2-5 cm (58 ± 12%). Fungal mycelia contributed only to 1-5% of coarse and fine POM, but to 5-25% of mineral associated organic matter (MOM) at 0-5 cm.
The soil water ²H-depletion with irrigation (-76‰ vs -68‰ at 0-10 cm) was paralleled by lower δ²H_n in coarse POM of irrigated vs dry plots (-122‰ vs -111‰ at 0-5 cm), indicating organic H turnover of less than 17 years. In contrast, δ²H_n in fine POM and MOM decreased only by 3‰ with irrigation, implying mean residence times of approximately 30 years.
While δ¹³C and δ¹⁵N differed by a maximum of 1‰ between plant sources, δ²H_n were 30-39‰ higher in roots than foliar litter, thus improving SOM sources differentiation. Long-term ²H-labelling by irrigation indicated higher organic H turnover in coarse vs fine soil fractions, offering a novel tool to identify SOM cycling and microbial H incorporation into SOM pools.