Temporal analysis
Since our study area is located in the inter-tropical zone, with mild
winter, the bulk precipitations not show seasonal variation for DOC
concentrations caused by pollution emissions from domestic heating, as
reported in the studies in higher latitudes as by Inagaki, Sakai, and
Ohnuki (1995) in Japan, Pan et al. (2003) in China, and Siudek et al.
(2015) in Poland. However, one of two rainfall events with higher DOC
concentrations occurred in winter (EV3). The tendency of less
precipitation - drier vegetation and litter - coupled with the practice
of balloon release typical of the São João festivities that occur at
this time in Brazil (June to August), makes winter the highest
occurrence time of forest fires in the Atlantic Forest, in particular in
the Tijuca Forest. It seems that DOC concentrations in BP tend to be
close to 3 mg.L-1 in our study area, however, they may
present higher values linked to stochastic causes.
Storms formed over the ocean usually have lower DOC concentrations than
those originated over the continent (Kieber et al., 2002). However,
according to reports from the Rio de Janeiro State Meteorological
Monitoring System, only EV5 was not caused by cold fronts. This
indicates similar origins between events in relation to rainfall
formation including the DOC-peak events (EV3 and EV7), considering that
these cold fronts that usually reach Rio de Janeiro by sea winds from
the southwest (Dereczynski et al., 2009), this is not an explanatory
factor for our DOC peaks
There was a dilution effect, but not associated with rainfall volume as
reported by Li et al. (2016), being such an effect linked to the
rainfall intensity. Iavorivska et al. (2017) reported the trend of
decreasing DOC concentrations during rainfall events which seems to be
an important factor for the occurrence of a dilution effect linked to
rainfall volume. These authors also mention an interactive set of
factors that can contain the downward trend or even generate increases
in DOC concentrations over the course of rainfall events that may help
explain our weak correlation between DOC concentrations and rainfall
volumes. In particular, the low-temperature reduction from the rainfall
beginning (maintenance of the dilution capacity) and the maintenance of
atmospheric OM sources (urban spatial context) are factors, among those
mentioned by the authors, which may be key elements in this matter.
The TF DOC showed a bigger dilution effect relative to the rainfall
intensity and the maintenance of the absence of correlation with the
total rainfall volumes, in comparison to occurs in BP. The rainfall
intensity protagonism for the dilution effect in detriment of the total
rainfall volume infers the idea of some temporal continuity about the OM
sources to be considered for both wet and dry deposition.
The two events with the highest wet deposition (BP data) lost prominence
in comparison to the dry deposition (CL data), demonstrating that the
factors behind the wet deposition “peaks” did not conduce the same
increase in dry deposition. Similarly to that observed by Kieber et al.
(2002), this difference can be explained by the constant action of sea
breezes pushing the air laden with organic components into inland areas,
which adds to the fact that rainfall is often caused by cold fronts that
reach the sea (Dereczynski, 2009).
The arise of the pre-wash effect in TF, not observed in BP, is evidenced
by the statistical correlation of concentrations and some antecedent
humidity parameters. That is, the pre-wash effect occurs similarly to
the reported by Turgeon and Courchesne (2008), which found greater DOC
flows in a dry climatic context. However, here, this effect was
manifested statistically related to the previous rainfall volume and not
to the number of previous drought days, indicating that small rainfall
volumes may not be effective for washing. In this context, the previous
time range that stood out as the most relevant for this rainfall
accumulated volume was 15-days.
About litter leaching, authors such as Dawson et al. (2008) and Jiang et
al. (2014) found higher DOC concentrations from the temperate forest
floor in the summer, when litter decomposition rates and DOC solubility
are highest. The low thermal amplitude typical of the area reduces the
temperature effect (freezing and solubility) relevance on litter
leaching described by Zhou et al. (2015).
The evergreen character of the forest in question results in a strong
temporal correlation between litterfall and rainfall since stormy winds
mechanically act for the fall of senile leaves (constantly produced).
Therefore, the occurrence of rainfall tends to be accompanied by fresh
leaves with high leaching potential, which helps to explain the pre-wash
absence in this compartment. That is, the small variance in temperature,
with the absence of litter freezing at the winter, and the poor
seasonality of litterfall with constant new leaves replacement
throughout the year result in no seasonal-tendency for litter DOC
leaching in our study area.
The absence of a dilution effect on litter leachate indicates that the
DOC available on the forest floor does not deplete easily (Schmidt,
Wang, Chang, & Matzner, 2010). Michalzik, Kalbitz, Park, Solinger, and
Matzner (2001) also found no dilution effect in litter leachate in a
temperate forest. In this situation, higher amounts of rainfall result
in more DOC infiltration into the soil or export through the drainage
channel (dependent on the soil DOC retention). The continuity and
efficiency of biochemical action on litter can be an important factor
for the maintenance of leaching rates. No dilution or prewash effects
were observed for soil solution.
The stream water DOC concentration is largely given by the balance
between atmospheric deposition, biomass leaching, soil sorption, and
biodegradation processes on the OM in the terrestrial environment
(Nosrati, Govers, & Smolders, 2012; Jiang et al., 2014; Oni et al.,
2014). In the post-event baseflow condition, the SW DOC concentrations
were similar between the analyzed events, which highlights the soil
capacity to dampen the oscillation in DOC flows from the other
compartments.
DOC modulations correlated to variations in canal discharge, as observed
by Turgeon and Courchesne (2008) and Roig-Planasdemunt et al. (2016),
were not contemplated in the present study since water in stormflow
condition was not collected. However, hortonians runoffs, the main
sources of DOC to the canal during rainfall events according to these
authors, are uncommon in our area due to the high water infiltration
capacity in the soil (Coelho Netto, 1987).
A study of variations of inorganic compounds chemistry of stream water
in association with stream water discharge modulation was carried out in
the same (our) area by Ovalle (1985). This author reported the tendency
of stream water, in the first moment of increased discharge, to be
chemically close to the soil solution on the influence of more intense
rainfalls (“piston” effect pushing edaphic water), and that the water
started to return to the chemical condition of the baseflow even before
the water discharge reduction. When the rainfall was intense enough to
generate overland flows, either Hortonian or saturation flows, a large
amount of OM was transported to the stream in addition to typical
inorganic solubilization of elements of rocky outcrops.