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.