Title: Controls and dynamics of canopy-derived dissolved organic matter from co-dominant broadleaved deciduous canopies to the soil of a temperate catchment in the northeastern United States
Description: Terrestrially-derived dissolved organic matter (DOM) (composed of dissolved organic carbon (DOC) and nitrogen (DON) flux and quality exert significant influence over carbon (C) and nitrogen (N) cycling, and related stream and soil ecologies. Yet, no study known to the author has simultaneously investigated all forest canopy-derived DOM inputs (throughfall, stemflow, and litter leachate) from trees of contrasting canopy structure to the soils of wooded ecosystems across temporal scales (annual, seasonal, and intra-storm DOM fluxes) characterizing their relative aromaticity (SUVA₂₅₄) and molecular weight (E₂:E₃ and S[superscript R] via UV-vis spectroscopic metrics, and estimating their contributions to soil solution 1 meter from the stemflow infiltration pathway using end member mixing analysis (EMMA) for two trees of contrasting canopy structure (Liriodendron tulipifera L., tulip poplar, and Fagus grandifolia Ehrh., American beech). DOC concentrations and fluxes produced markedly stronger seasonal patterns than DON seasonal differences for both species, especially in stemflow. Since seasonal DON fluxes were nearly negligible for both species in our study, diminished DOC:DON ratios were almost exclusively a result of seasonal DOC dynamics. Aromaticity (SUVA₂₅₄) and relative molecular weight (E₂:E₃ and S[subscript R] of canopy-derived fluxes generally increased under leafless conditions, likely due to enrichment of lignin-degradation byproducts from greater bark contact. Litter leachate DOM fluxes were the greatest of all monitored canopy-derived DOM fluxes from either species. But, the thinner, rougher barked, less-inclined branch angle (plagiophile) L. tulipifera canopy resulted in greater throughfall and less stemflow flux; whereas, the oppositely-structured F. grandifolia canopy produced greater stemflow fluxes across all time scales from L. tulipifera and F. grandifolia canopies, respectively. The F. grandifolia canopy structure also enhanced precipitation capture and washoff of dry deposited materials, resulting in lower aromaticity, lighter weight throughfall and stemflow DOM fluxes. Conversely, L. tulipifera canopy morphology provided greater canopy surface area for chemical exchange and increased hydrologic residence time for entrained rain droplets, producing more aromatic, heavier weight canopy-derived DOM fluxes. Thus, the lower molecular weight, less aromatic, more spatially-concentrated DOM inputs to forest soils beneath the smooth-barked, more steeply-inclined branch angle (erectophile), denser F. grandifolia canopy may engender hot spots and/or moments of C- and N-cycling as it quality indicates greater lability. EMMA results examing canopy water source contributions to near-stem soil solution within the stemflow infiltration pathway suggest that canopy structure can also alter the way in which canopy-derived DOM fluxes enter the soil (in conjunction with catchment and meteorological conditions). The stemflow-favoring F. Grandifolia canopy resulted in less mixed, stemflow-dominated near-stem soil solutions where litter leachate was a major contributor. Event comparisons indicate that drier catchment conditions and increased rainfall intensity may enhance the contributions of these dominant canopy water sources. EMMA results also found throughall signatures in near-stem soil solutions, indicating this diffuse canopoy-derived flux may move through soils relatively unaltered. Most past studies examing the hydrological and biogeochemical cycling of forested catchments have treated the canopy as a "black box" where fluxes enter dilute and exit enriched. The results of this study clearly show the structure of above-ground plant biomass and precipitation within canopies can drastically influence DOM enrichment, flux and quality to the soils and (potentially) down-gradient watershed compartments.
Subjects: Soils, Organic compound content, Water, Throughfall, Rain and rainfall, Forest hydrology, Forest ecology, Forest canopy ecology, Environmental aspects
Places: United States