Introduction
As in other ecosystems, the aquatic food web comprises detrital and grazing food chains (Lindeman 1942, Hairston and Hairston 1993). Terrigenous organic matters, such as leaf litter, are considered important energy and nutrient sources that sustain the food webs in ambient freshwater ecosystems. Organic carbon in the leaf litter can serve as a substrate for heterotrophic microbes in lakes and rivers (Carpenter et al. 2005, Cole et al. 2011, Hiltunen et al. 2017), thereby supporting the detrital food chains (Tanentzap et al. 2014, 2017, Hirama et al. 2022). When dissolved nitrogen and phosphorus are released from leaf litter and enter into aquatic ecosystems, they can be used for primary production by phytoplankton, which are the base of grazing food chains (Kissman et al. 2017, Hirama et al. 2022). Consequently, leaf litter can stimulate both the detrital and grazing food chains in aquatic ecosystems. However, the relative importance of the leaf litter in mass flow along the detrital and grazing chains may vary among the leaf litter species if the release rates of dissolved organic C (DOC), total dissolved N (TDN), and total dissolved P (TDP) differ among leaf litter species (Hirama et al. 2022).
The C, N, and P contents of leaf litter differ considerably among tree species (Schreeg et al. 2013, Yan et al. 2022). Furthermore, the release of nutrients from the leaf litter of coniferous species is lower and slower than from that of broadleaf species on soil (Usman 2013). Leaf litter from tree species with high lignin contents or high C to nutrient ratios generally decomposes more slowly and releases lower amounts of dissolved nutrients (Berg and McClaugherty 1989, Osono and Takeda 2004). Indeed, many studies have shown that the decomposition rate of the leaf litter has been shown to differ among tree species, suggesting that the release efficiencies of elements, i.e., fractions of DOC, TDN, and TDP in the C, N, and P contents of the leaf litter, respectively, differ among tree species.
P in leaf litter mainly exists as water-soluble orthophosphate and inositol phosphate (Chapin III et al. 1990, Yang et al. 2017). Therefore, the release efficiency of P is high and P is released rapidly during the early stages of litter leaching and decomposition (McComb et al. 2007, Schreeg et al. 2013, Pourhassan et al. 2016). In contrast, C and N in leaves exist mainly in structural components (Chapin III et al. 1990), such as lignin and cellulose, which are insoluble in water and difficult for most microbes to decompose (Johansson 1995, Kögel-Knabner 2002). Additionally, most of water soluble N in leaves are reabsorbed back to the trunk before defoliation in many species (Pate 1980). The fact suggests that the release efficiency of TDP from leaf litter is higher than that of DOC and TDN. However, few studies have examined the release efficiencies of these elements from the leaf litter soaked in water, which simulates the leaching and decomposition of leaf litter in freshwater ecosystems. Consequently, information regarding the tree species-specific release rates of DOC, TDN, and TDP of leaf litter in water is limited. If the release rates of these elements differ among the leaf litter of different tree species, it is likely that changes in the forest vegetation of a watershed can alter the relative importance of detrital and grazing food chains in aquatic ecosystems.
Therefore, in the present study, we investigated the C:N:P stoichiometry of leaf litter and the release efficiency of various tree species at the early stage of decomposition in a freshwater environment to examine whether changes in the watershed vegetation affect relative contributions of detrital and grazing food chains sustaining aquatic food webs. To this end, we soaked the leaf litter from 11 temperate tree species in aerated water and quantified the DOC, TDN, and TDP released over 28 days. Subsequently, we examined the following three uncertainties: does the release efficiency of leaf litter differ among DOC, TDN and TDP; are these release efficiencies species-specific; and what factors or elemental components in the leaf litter determine these release efficiencies?