Tracing sources and turnover of soil organic matter in a long-term irrigated dry forest using a novel hydrogen isotope approach

The contribution from foliage, roots, and fungi to soil organic matter (SOM) is still largely unknown. Since stable hydrogen isotope ratios (δ²H_n) can differ among sources, we explored δ²H_n to elucidate sources contribution to SOM. We also assessed whether addition of ²H-depleted water allows tracing new organic H incorporation into SOM.

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

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 POM, but to 5-25% of mineral associated organic matter (MOM).

The soil water ²H-depletion with irrigation was paralleled by 11‰ lower δ²H_n in coarse POM of irrigated vs dry plots, 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 organic H turnover of about 30 years.

Our study showed that δ²H_n allowed a clear differentiation among SOM sources. Long-term ²H-labelling by irrigation indicated higher organic H turnover in coarse vs fine fractions, offering a novel tool to identify SOM cycling.