Optical properties of dissolved organic matter in throughfall and stemflow vary across tree species and season in a temperate headwater forest

Abstract

Tree-derived dissolved organic matter (DOM) comprises a significant carbon flux within forested watersheds. Few studies have assessed the optical properties of tree-derived DOM. To increase understanding of the factors controlling tree-derived DOM quality, we measured DOM optical properties, dissolved organic carbon (DOC) and calcium concentrations in throughfall and stemflow for 17 individual rain events during summer and fall in a temperate deciduous forest in Vermont, United States. DOC and calcium fluxes in throughfall and stemflow were enriched on average 4 to 70 times incident fluxes in rain. A multiway model was developed using absorbance and fluorescence spectroscopy to further characterize DOM optical properties. Throughfall contained a higher percentage of protein-like DOM fluorescence than stemflow while stemflow was characterized by a higher percentage of humic-like DOM fluorescence. DOM absorbance spectral slopes in yellow birch (Betula alleghaniensis) stemflow were significantly higher than in sugar maple (Acer saccharum) stemflow. DOM optical metrics were not influenced by rainfall volume, but percent protein-like fluorescence increased in throughfall during autumn when leaves senesced. Given the potential influence of tree-derived DOM fluxes on receiving soils and downstream ecosystems, future modeling of DOM transport and soil biogeochemistry should represent the influence of differing DOM quality in throughfall and stemflow across tree species and seasons.

Disentangling the responses of dissolved organic carbon and nitrogen concentrations to overlapping drivers in a northeastern United States forested watershed

“…In this scenario, DOC can accumulate despite warm summer temperatures and nitrate would be reduced to gaseous forms (Hungat et al, 2003;McNeill and Unkovich, 2007). Indeed, summer DOC concentrations were elevated at high flow, which is in agreement with previous studies (Ryan et al, 2021(Ryan et al, , 2022. Nitrate concentrations were also high, albeit more variable, which might signal that denitrification was spatially and/or temporally more variable.…”

“…Nitrate concentrations were also high, albeit more variable, which might signal that denitrification was spatially and/or temporally more variable. Additionally, recent research emphasized the role of throughfall as an important source of soil and stream dissolved organic matter during summer months when leaves are out (Ryan et al, 2021(Ryan et al, , 2022. The influx of throughfall is relatively enriched in C and might contribute to observed patterns.…”

Canopy Leaching Rather than Desorption of PM2.5 From Leaves Is the Dominant Source of Throughfall Dissolved Organic Carbon in Forest

“…Dissolved organic matter (DOM) in throughfall exhibits diverse structural and functional characteristics within forest ecosystems. Structurally, throughfall DOM encompasses a mixture of organic compounds including carbohydrates, aliphatic, and aromatic compounds, displaying both humic‐ and protein‐like fluorescence spectra (Asaoka et al., 2022; Bischoff et al., 2015; Ryan et al., 2022). In contrast, DOM in rainwater primarily consists of saturated aliphatic compounds (Kieber et al., 2006; Santos et al., 2009).…”

“…The optical properties of throughfall DOM indicate that trees can release aromatic substances (Ryan et al., 2022; Van Stan et al., 2017). Considering the presence of humic‐like substances in fine particulate matter with diameters that are 2.5 μm or smaller (PM 2.5 ) (Asaoka et al., 2022; Zheng et al., 2013), the wash‐off of PM 2.5 from tree leaves and branches has the potential to contribute to throughfall DOC.…”

“…carbohydrates, aliphatic, and aromatic compounds, displaying both humic-and protein-like fluorescence spectra (Asaoka et al, 2022;Bischoff et al, 2015;Ryan et al, 2022). In contrast, DOM in rainwater primarily consists of saturated aliphatic compounds (Kieber et al, 2006;Santos et al, 2009).…”