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?