Leaf-litter leachate is distinct in optical properties and bioavailability to stream heterotrophs

Abstract

Dissolved organic C (DOC) leached from leaf litter contributes to the C pool of stream ecosystems and affects C cycling in streams. We studied how differences in leaf-litter chemistry affect the optical properties and decomposition of DOC. We used 2 species of cottonwoods (Populus) and their naturally occurring hybrids that differ in leaf-litter phytochemistry and decomposition rate. We measured DOC and nutrient concentration in leaf leachates and determined the effect of DOC quality on heterotrophic respiration in 24-h incubations with stream sediments. Differences in DOC composition and quality were characterized with fluorescence spectroscopy. Rapidly decomposing leaves with lower tannin and lignin concentrations leached ∼40 to 50% more DOC and total dissolved N than did slowly decomposing leaves. Rates of heterotrophic respiration were 25 to 50% higher on leachate from rapidly decomposing leaf types. Rates of heterotrophic respiration were related to metrics of aromaticity. Specifically, rates of respiration were correlated negatively with the Fluorescence Index and positively with Specific Ultraviolet Absorbance (SUVA 254) and T280 tryptophan-like fluorescence peak. These results reveal that leaf-litter DOC is distinctly different from ambient streamwater DOC. The relationships between optical characteristics of leaf leachate and bioavailability are opposite those found in streamwater DOC. Differences in phytochemistry among leaf types can influence stream ecosystems with respect to DOC quantity, composition, and rates of stream respiration. These patterns suggest that the relationship between the chemical structure of DOC and its biogeochemistry is more complex than previously recognized. These unique properties of leaflitter DOC will be important when assessing the effects of terrestrial C on aquatic ecosystems, especially during leaf fall.

Litter identity affects assimilation of carbon and nitrogen by a shredding caddisfly

“…, Kaplan and Bott , Meyer , Wymore et al. ). In contrast, in slowly decomposing litter, more C and N are bound in complex compounds that are not water‐soluble (Rahman et al.…”

“…(1) Rapidly decomposing litter has high leaching rates, resulting in compounds largely unavailable to invertebrate shredders. Dissolved organic carbon leached from litter can be respired by microbes within 24 h, with little energy transfer to the macroscopic food chain (Cummins et al 1972, Kaplan and Bott 1983, Meyer 1994, Wymore et al 2015. In contrast, in slowly decomposing litter, more C and N are bound in complex compounds that are not water-soluble (Rahman et al 2013).…”

“…We determined the mass of C and N lost to leaching by placing 0.5 g of each litter type in 200 mL of deionized water in acid-washed glass beakers (Wymore et al 2015(Wymore et al , 2018. After 24 h, leachate was filtered through precombusted glass fiber filters (Whatman GF/F) and frozen until analysis.…”

“…Invertebrates assimilated more N from slowly decomposing Populus angustifolia litter compared with rapidly decomposing Populus fremontii litter , and insect emergence was higher on slowly decomposing P. angustifolia . In contrast, high decomposition rates of P. fremontii were correlated with higher microbial biomass and leaching of dissolved compounds (Pastor et al 2014, Wymore et al 2015. Although microbes living on litter can be an important food source for leaf-shredding insects, some compounds that retard microbial decomposition might increase element transfer to invertebrates (Compson et al 2018).…”

“…, Wymore et al. ). Although microbes living on litter can be an important food source for leaf‐shredding insects, some compounds that retard microbial decomposition might increase element transfer to invertebrates (Compson et al.…”