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

Abstract

The contribution from foliage, roots, and fungi to soil organic matter (SOM) represents a key unknown in SOM dynamics. Since stable hydrogen isotope ratios (δ2Hn) can greatly differ among these sources, we explored δ2Hn 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 δ2Hn 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 δ2Hn 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 δ2Hn 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, δ2Hn 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, δ2Hn 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.