Tracing sources and turnover of soil organic matter in a long-term irrigated dry forest - a non-exchangeable hydrogen isotope approach
Authors: Guidi Claudia , Lehmann Marco , Meusburger Katrin , Saurer Matthias , Vitali Valentina , Peter Martina , Brunner Ivano , Hagedorn Frank 
Published: February 2023

Soil organic matter (SOM) originates from various sources such as foliar litter, roots and microbial (e.g. fungal) components. The relative sources contribution represents one of the key unknowns in SOM dynamics. Our study aimed to explore whether stable isotope ratios of non-exchangeable hydrogen (Hn) bound to organic matter can be used to differentiate SOM sources, since natural 2Hn abundance can strongly differ between root and foliar tissues. We also investigated if long-term irrigation with 2H-depleted water in a pine forest can be used to track Hn incorporation into organic matter inputs and eventually in the soil pools.In a 17-year-long irrigation experiment in a dry pine forest, we assessed variations in natural abundance of 2Hn, 13C, and 15N in SOM sources (foliar litter, fine roots, fungal mycelia), decomposing litter, soil (organic layers and uppermost 5 cm-mineral soil) and particle-size fractions. We then applied a Bayesian mixing model (including &#948;2Hn,&#948;13C, and &#948;15N) to estimate the relative sources contribution to SOM.Natural 2Hn abundance was significantly higher in roots vs. foliar litter (up to +39&#8240;), and in fungal mycelia vs. roots (up to +41&#8240;). Results from Bayesian mixing model suggest that foliar litter contributed to approximately 68 &#177; 10% of SOM in organic layers and in coarse particulate organic matter (POM). Foliar litter and roots contributed similarly to upper 2 cm of mineral soil (46 &#177; 11%), while 2-5 cm of mineral soil were largely derived from roots (61 &#177; 13%). Fungal mycelia contributed to 18 &#177; 8% of mineral-associated organic matter (MOM), while only to 1-2% of coarse and fine POM. Bayesian mixing models provided only a general indication of the sources contribution to SOM, also considering that isotopic signatures shifted during decomposition. Measurements of isotope signatures in microbial necromass might allow a more accurate assessment of the different SOM sources contribution.The &#948;2Hn depletion of soil water under irrigation was paralleled by a comparable decrease in &#948;2Hn of roots (~12&#8240;). In comparison, the natural 2Hn abundance in fresh needles and foliar litter decreased less strongly (~ 7&#8240; and 4&#8240;, respectively), likely due to photosynthetic adjustments that may have counterbalanced the irrigation water 2H-depletion. Similar to soil water 2H-depletion, &#948;2Hn values in coarse POM were 11&#8240; lower in irrigated vs. dry plots, suggesting that nearly all organic Hn turned over or exchanged with soil water in less than two decades. In contrast, &#948;2Hn values in fine POM and MOM decreased only by 3&#8240; under irrigation, which indicate that these fractions comprise slower cycling Hn pools.Our study showed that the natural 2Hn abundance represents a promising tool to differentiate among SOM sources. While 13C and 15N did not clearly separate between roots and foliar litter, Hn isotopic signatures allowed a good discrimination between SOM sources. In addition, long-term irrigation can provide a potential in situ 2H-labelling of SOM, which may help to examine organic Hn turnover rates across SOM pools.