Discussion
When the leaf litter of various tree species, including broadleaf and coniferous species, was soaked in water, we found that P had the highest release efficiency (>50 %), followed by C (20−30 %), and N release is the lowest (<10%). This was consistent with the results of previous studies examining the decomposition rate of the leaf litter of various tree species, showing that P was rapidly lost at the early stage of decomposition (McComb et al. 2007, Schreeg et al. 2013). The high release efficiency of P likely stems from the fact that P exists mainly as inorganic orthophosphate or inositol phosphate in plant tissues (Chapin III et al. 1990, Yang et al. 2017), which are highly water soluble and thus are leached from litter easily.
In this study, we successfully fit the time course changes inVE to the Michaelis−Menten equation, except for N in some species. However, the estimated V max for P exceeded 100% in several leaf litter species including Erman’s birch, oak (young), Japanese larch, and hinoki cypress (Table S1). In these species, the P release efficiency from the leaf litter increased almost linearly within our observation period (Figure. S3), indicating that 28 days was too short for TDP release to reach the saturation level ofV P in these species. Indeed, k , the half saturation time, of P seemed to be considerably larger than that of C and N in all species. This trend suggests that the soluble parts of C and N in leaf litter were released more rapidly into the water than that of P, although the fractions of these parts in leaf litter were limited compared to that of P. In the leaf litter, although some of the organic C species, such as sugar, small molecular carbohydrates and organic acids are water soluble, most of the supporting structural carbon, such as lignin and cellulose, are water-insoluble (Kögel-Knabner 2002) and require more time to decompose (Nykvist 1963). Similarly, N biomass in leaf litter is mainly structural proteins. Although there are highly water-soluble nitrogen such as free amino acids, these are relatively minor in mass (Schneider et al. 1998, Franklin et al. 2020) and mostly recycled during leaf senescence before shedding (Chapin III et al. 1990, Nahm et al. 2006). Accordingly, regardless of species, the maximal release efficiency of P was considerably higher than those of C and N in the 28-day leaching experiment.
Studies based on water immersion of litter and litterbags suggest that the release efficiencies of DOC, TDN and TDP from deciduous broadleaf leaf litter are higher than those from evergreen coniferous leaf litter (Kiikkilä et al. 2011, Usman 2013, Pourhassan et al. 2016). Although we found higher DOC and TDN V max in broadleaf leaf litter, we did not find a significant difference in the TDPV max between the broadleaf and coniferous leaf litter because of the large variations within each of the two taxonomic groups. Interestingly, DOC, TDN, and TDP V max (or Max-V ) were not positively correlated with the C, N, and P contents, respectively, of the leaf litter (Figure S4a, e, i), suggesting that the soluble fraction of an element does not change proportionally with the elemental content in leaf litter. However, we found that DOC V max increased with P content in the leaf litter (Figure 3a). These results are consistent with previous findings that more nutritious leaf litter contains more soluble organic C (Poorter and Bergkotte 1992). Although the TDPV max was not related to the leaf litter P content itself, TRA C and TRA P were correlated positively with leaf litter P content. These results are consistent with those of previous studies showing that the leaf litter with relatively high P content generally has a high decomposition rate (Schlesinger and Hasey 1981, Osono and Takeda 2004). However, neither TDN V max nor TRA N were associated with the C, N, or P contents in the leaf litter mass and were consistently low relative to those of C and P. As mentioned above, these results may have been affected by the resorption of the water-soluble N in leaves by trees prior to shedding, regardless of the elemental contents in the leaves.
The C:P and N:P ratios of the maximal total release amounts in leachate, i.e., TRA C:P and TRA N:P, varied considerably among the leaf litter and were substantially lower than the C:P and N:P ratios of the leaf litter biomass. In contrast,TRA C:N was considerably higher than the C:N ratio for the leaf litter biomass. The result implies that the rates and ratios of organic carbon and nutrients released into water cannot be predicted solely through the elemental ratios of leaf litter mass. WhileTRA C:P increased with increasing C:P ratio of leaf litter mass, no such increase was observed forTRA C:N and TRA N:P. Furthermore, as TRA is estimated from V maxand mass contents of leaf litter, and as V max, Pwas not related to P contents, our results indicate that the P release efficiency is relatively less varied among the leaf litter species compared to the C and N release efficiencies. The variations ofTRA C:P with C:P ratio of leaf litter mass is thus associated with the P content and TRA C but not necessarily with V max,P.
Traditionally, leaf litter is viewed as the substrate and nutrient sources for bacterial production in ambient aquatic systems (e.g., Cole et al., 2011; Lennon and Pfaff, 2005; Lindeman, 1942). However, the high P release efficiency supports the idea that leaf litter can also be a supplemental nutrient source for autotrophic organisms, such as algae, whose growth rates are often limited by P supply (Guildford and Hecky 2000, Sterner and Elser 2002). Among the leaf litter species,TRA C:P was high in some species, such as Japanese maple and zelkova. Bacteria and algae compete for P in aquatic systems, and an increase in DOC supply often favors bacteria in the competition (Gurung et al. 1999, Thingstad et al. 2008, Hitchcock et al. 2010). Thus, an increased input of these leaf litter into ambient aquatic ecosystems may stimulate the heterotrophic microbial production more than the primary production in these ecosystems. However,TRA C:P for the rest of leaf litter species, such as oak, Siebold’s beech, Japanese larch and hinoki cypress, was considerably lower than 100, although the C:P ratios of these leaf litter mass were higher than 300 in most cases, and, on average, 660. Thus, these leaf litter species may stimulate the grazing food chains much more than the detrital chains by supporting the primary production in aquatic ecosystems. This possibility implies that the stoichiometric impact of leaf litter on aquatic ecosystems would be better understood if the water-soluble fractions of elements in the leaf litter species are considered.
Note that in this study, we incubated the leaf litter under a non-axenic condition, yet we used fresh distilled water to initiate the experiment. Thus, some fraction of the nutrients released from the leaf litter may have been fixed by microbes. However, if these bacteria were suspended in the experimental water, the fractions were included in the measured nutrients (TDN and TDP) in this study. The exception is some C fractions in leaf litter and DOC released may have been respired by microbes. This also implies that the V C quantified in this study might be an underestimation. However, assuming this underestimation, it is likely that V C would decrease with time. As such a trend was not observed, the effect of microbial respiration onV C, if present, would be minimal.
Finally, the stoichiometry of organic and inorganic nutrient release from leaf litter is likely to change depending on both leaf litter species and leaf litter senescence. Observations of oak leaf litter showed that the V max of DOC and TDP were lower but that of TDN was higher in the aged leaf litter, which had lower N and P contents than young leaf litter (Figure 2). This trend may have been caused by C and P mineralization loss and N immobilization through microbial activity on leaf litter during the aging process on the soils (Gallardo and Merino 1992, Fellman et al. 2013). The immobilized N in aged leaf litter forms N-rich humus. A fraction of these compounds may have been released later following the immersion into water and decomposition of leaf litter (Gallardo and Merino 1992). To better understand these processes, it is necessary to examine how the senescence of leaf litter on soil affects the nutrient release efficiencies when it is soaked in water.