From canopy to consumer: what makes and modifies terrestrial DOM in a temperate forest

Abstract

the hypermaritime climate of the region. Condensed aromatics contributed as much as 38% relative abundance of spruce and hemlock bark leachates suggesting coniferous trees are potential sources of condensed aromatics to surface waters. Soil leachates showed soil wetness dictates DOM composition and processing, with wetland soils producing more aromatic formulae and allowing the preservation of traditionally biolabile, aliphatic formulae. Biodegradation impacted soil and tree DOM differently, and though the majority of source-specific marker formulae were consumed for all sources, some marker formulae persisted. Tree DOM was highly biolabile (> 50%) and showed compositional convergence where processing homogenized DOM from different tree sources. In contrast, wetland and upland soil leachate DOM composition diverged and processing diversified DOM from different soil sources during bioincubations. Increasing precipitation intensity predicted Abstract To investigate how source and processing control the composition of "terrestrial" dissolved organic matter (DOM), we combine soil and tree leachates, tree DOM, laboratory bioincubations, and ultrahigh resolution Fourier-transform ion cyclotron resonance mass spectrometry in three common landscape types (upland forest, forested wetland, and poor fen) of Southeast Alaska's temperate rainforest. Tree (Tsuga heterophylla and Picea sitchensis) needles and bark and soil layers from each site were leached, and tree stemflow and throughfall collected to examine DOM sources. Dissolved organic carbon concentrations were as high as 167 mg CL −1 for tree DOM, suggesting tree DOM fluxes may be substantial given Responsible Editor: Penny Johnes.

Trees in the Stream: Determining Patterns of Terrestrial Dissolved Organic Matter Contributions to the Northeast Pacific Coastal Temperate Rainforest

“…Instead, our findings agree with other studies suggesting that DOM produced in forest biomass or the soil profile can be shunted downstream to the mainstem relatively unaltered (Raymond et al, 2016). In our previous research, ∼90% of source-specific marker formulae disappeared during a 14-day bioincubation, demonstrating high bioavailability under nutrient replete conditions (Behnke et al, 2022). Those 14-day incubations likely exceeded the residence time of water and DOM in sub-catchment in Aangooxa Yé, which are thought to be on the order of hours to a couple days (though exact residence times for the system are not known; Fellman et al, 2020).…”

“…As noted above, a recent study described 252 source‐specific marker formulae leached from specific soil horizons and plant types in the study watershed (Table S2 in the Supporting Information ; Behnke et al., 2022). These marker formulae, the majority of which (176) originated from spruce stemflow, occurred in all samples of a given source and in no samples of any other sources (Behnke et al., 2022). All 252 source‐specific marker formulae were found in this watershed data set.…”

“…A recent study in the Aangooxa Yé watershed defined a list of source‐specific marker formulae (ranging from 1 to 176 formulae for each of 10 sources identified; Table S2 in the Supporting Information ) that were here used to pinpoint source marker formulae in this sample set (Behnke et al., 2022). The number of marker formulae in each sample was assessed.…”

“…Our examination of the relationship between hydrology and four molecular indicators of fresh terrestrial DOM input (source-specific marker formulae, spruce stemflow-specific marker formulae, soil layer marker formulae (Behnke et al, 2022), and AI mod weighted average; Figure 5) together demonstrate the complexity of characterizing a system with pronounced seasonal changes using simplistic hydrologic variables. The best model of AI mod (AI mod ∼ daily precipitation * site) showed an increase of ∼0.03 (0.00-0.05, 95%CI; p = 0.07) units of AI mod per cm of precipitation with an interaction term for site (Table S7 in the Supporting Information S1), suggesting that though the slope of the increase was different in different sub-catchments, precipitation unsurprisingly appears to modestly enhance the connectivity of headwater streams to terrestrial (e.g., high aromaticity) DOM source pools (DiDonato et al, 2016;Holt et al, 2021).…”

“…We measured DOM composition throughout the watershed, sampling the mainstem river above tidal influence and soil pore waters and tributary streams from sub‐catchments draining three main landscape units in the NPCTR (poor fen, forested wetland and upland forest; D'Amore et al., 2015; McNicol et al., 2019). We applied previously identified source‐specific marker formulae from the Aangooxa Yé watershed (Behnke et al., 2022) to our fluvial data set to trace DOM from specific sources in the stream network. We used the presence/absence of source‐specific marker formulae along the fluvial network as an indicator of DOM uptake or transformation based on the behavior of source‐specific DOM in our previous bioincubations.…”