Spatial analysis
Because it is a multi-diverse forest with low species dominance, the
small differences in vegetation structure and composition observed
between hillslope positions were not sufficient to configure significant
variations for the TF DOC concentrations, without effects arising from
allometric variations between tree-species as observed between conifers
and hardwoods (Liu & Sheu, 2003; Inagaki et al., 1995). The spatial
heterogeneity of canopy leaching is high, but it occurs on a small
spatial scale (punctual) and the means obtained from 300
m2 area were very similar when comparing to the
hillslope positions. The variance of the TF DOC data within each
hillslope position reflects that the local minutiae in the dry
deposition, canopy washing, and rainwater interception control the DOC
enrichment process in this compartment.
The slightly larger canopy opening in the SMT did not materialize lower
DOC values as expected. Probably, the canopy opening variation between
the SMT and the other hillside positions is not sufficient to generate
differences as observed by Costa et al (2016) between native forests and
cacao agroforestry, since the largest native forest biomass would
manifest in less canopy opening (unmeasured).
An apparent duality regarding the relationship between litter
decomposition rates and DOC leaching seems to manifest itself
prominently on the studied hillslope. Lower DOC concentrations from
birch forest floor compared to conifers were reported by Froberg et al
(2011) and argued for the higher biological activity on birch litter
(lower C / N ratio), resulting in thin soil litter stocks above ground.
Similarly, lower DOC concentrations have also been reported for Mull
humus form compared to the adjacent area with Moder humus (the
difference between humus forms is due to the greater accumulation of the
sub-horizon H for Moder type) by McDowell and Wood (1984). The key issue
is the maintenance or not of a MO source stock above ground and the
greater leaching probability when percolating thicker litter stocks.
However, it is noteworthy that the non-observation of the litterfall
rates can be a large error source factor when relating the litter
accumulation above ground and DOC leaching in the comparison between
areas.
On the other hand, some studies showed the positive effect of
decomposition for the DOC leaching in a comparison between broadleaved
forest areas (Camino-Serrano et al., 2014; Guggenberger & Zech, 1993;
Zhou et al., 2015), as well as by experiments with controlled leaching
and/or decomposition. (Don & Kalbitz, 2005; Lee et al., 2018).
According to Zhou et al. (2015), at the first moment, the DOC leaching
is high due to the great solubles availability in fresh leaves, with
subsequent rate decreases. Continuity of DOC leaching depends on the
availability of new solubles, which is greater in situations where
biochemical activity in material degradation is more intense. The key
issue here is the availability of new solubles by chemical rework on the
material, mobilizing C for leaching. However, laboratory-controlled
experiments may not consider the effect of constant accumulation of
semi-degraded material in topsoil that occurs under natural field
conditions. Additionally, the positive relation between decomposition
and leaching of DOC may be affected by considering losing massive C by
microbiota respiration (mineralization) in environments where
decomposition is more intense (e.g., tropical forests).
Camino-Serrano et al. (2014) comment about this duality from the idea of
concurrent pathways to C: being breathed by the microbiota or being
leached by water. Efforts to modeling the DOC leaching behavior from the
coníferous forest floor also face this duality, represented by the
challenge to combine the leaching of semi-decomposed material
accumulated in topsoil to the large contribute by fresh leaves (Ǻgren,
Kleja, & Bosatta, 2018).
In this way, by analyzing our data, it not surprising to us that the
lower DOC concentrations in the VBT litter leachate, where decomposition
is faster and there is little accumulation of OM material as DOC source,
with a high loss of C by microbiota respiration. It is also not
surprising that the largest litter accumulation in the SMT did not
materialize in higher DOC concentrations, as the availability of new
solubles is slower. Both mid-hillslope positions, with similar litter
stock between themselves and intermediates compared to the summit and
bottom valley, showed very close DOC concentrations between themselves
and higher compared to geomorphological extremes.
Our inference, considering the fact that hillslope positions have
similar litterfall, is that the mid-hillslope conditions, with
intermediate decomposition rate, presents a balance between source
material accumulation and availability of new solubles that favors DOC
leaching. The leaching is lower at the valley bottom due to the little
material accumulation (less C available) but is also limited at the
summit by the lower biochemical work over the material (fewer solubles
available).
The second-degree polynomial tendency presented in figure 9 does not
represent a proposed mathematical model of this relation, being only a
tool to emphasize the nonlinear character of the behavior for the
studied area. Given the studied area representativeness to the Atlantic
forest domains of the mountainous environment (Miranda & Avelar, 2019),
we believe such nonlinear behavior may be a standard to be considered
for the large extension of the forested domains of the Serra do Mar and
Mantiqueira, main actual Atlantic Forest strongholds.
Higher soil solution DOC concentrations at 50 cm depth in summit
compared to the VBT result from the joint action of higher inputs (UL
DOC) and lower adsorption rates once the higher MO content incorporated
into the soil results in higher occupation of sorption sites (kaiser et
al., 1996; Singh et al., 2016). At higher depth (100 cm) the difference
between hillslope positions reduces its significance due to the soil
acting as a sorption environment, a similar observation as reported by
Park and Matzner (2003). In both cases (50 and 100 cm) the reported
differences (SMT vs VBT) are further influenced by occasional increases
in-depth concentration that has been treated as an interaction of
spatio-temporal factors.