Comparison to other environments
Some DOC content can be observed in BP due to the dissolution/carriage
of volatile organic compounds found in the atmosphere. There is a wide
variety of organic molecules in the atmosphere such as light
hydrocarbons, aldehydes, and ketones, as these molecules can vaporize
under natural environmental conditions. In addition, a large amount of
organic soot is released into the atmosphere by industrial activities
and the burning of fossil fuels (Siudek, Frankowski, & Siepak, 2015).
DOC concentrations up to 2 mg.L-1 may only have
biogenic origin due to volatile exudates from natural environments, so
values above this level usually represent some anthropogenic influence
(Liu & Sheu, 2003). In the case of the present study, the forest
fragment is totally inserted in the second-largest metropolis of Brazil,
the Rio de Janeiro, which explains the fact that only one of the
rainfall events analyzed presented value below the mentioned level.
The average BP DOC found in the present study is close to that reported
by Siudek et al. (2015) in an urban environment in Poland and by Coelho
et al. (2008) in an urban-rural transition area in Brazil, occasions
where some anthropogenic influence was cited as an important explanatory
factor. Lower values were reported by Costa et al. (2016) in an area
away from urban centers in Brazil, by Kieber, Peake, Willey, & Brooks
(2002) under the influence of sea breezes in the coastal zone of New
Zealand and by Roig-Planasdemunt, Llorens, and Latron (2016) in a
forested area in Spain, locations with lower anthropogenic influence.
Higher values were reported under more extreme conditions of pollutant
emissions, as by Godoy-Silva, Nogueira, and Campos (2017) in an
agricultural area influenced by sugarcane burning in southeastern
Brazil.
Our results of TF DOC seem slightly higher than those reported by
Bäumler and Zech (1997) in temperate forests in Germany, Liu & Sheu
(2003) in Taiwan, and Chen, Yoshitake, Iimura, Asai, and Ohtsuka (2017)
in Japan, as well as in the intertropical zone (Brazil) by Costa et al.
(2016). The DOC concentrations in this compartment correspond to the
bulk precipitation DOC contents increased by canopies leaching, in turn,
controlled by dry deposition de OM from the atmosphere. Thus, the
proximity relationship to urban centers tends to be the most important
factor controlling the increases in through-canopy water, so that, like
as for bulk precipitation, the urban context must answer for the
slightly higher values in the forest fragment studied.
The under-litter DOC concentration in our area was similar to the
reported in coniferous forests by Park and Matzner (2003) in the Czech
Republic, and by Fröberg, Berggren, Bergkvist, Bryant, and Mulder
(2006), and Fröberg et al. (2011) in coníferous and silver birch forests
in Sweden. The higher litterfall mass combined with the efficiency of
microbial activity, both typical features of tropical forests, should
explain the maintenance of similar values to those reports in
higher-latitudes, where litter accumulation above ground tends to be as
higher as the organic carbon source-stock (Pregitzer & Euskirchen,
2004). The fast chemical transformations on the litter by microbiota
constantly making new available solubles should play a key role.
The soil solution DOC concentrations have similar values (surface) and
tendency to decrease with increasing depth as observed in studies in
different areas and ecosystems, as by Richter and Markewitz (1996) in
the United States (conifers), by Liu and Sheu (2003) in China (Conifers
and Hardwoods), by Leinemann et al. (2016) in Germany (hardwoods) and by
Roig-Planasdemunt et al. (2016) in Spain (grasses). The sandy texture of
the A and B soil horizons should be the main reason for the decrease
rates to be lower than that reported in most cited studies, considering
the clay performance as a favorer the OM adsorption (Kaiser et al.,
1996; Kaiser & Guggenberger, 2007; Singh, Sarkar, Biswas, Churchman, &
Boland, 2016; Singh et al., 2017).
The Archer River, under baseflow conditions, has low DOC concentrations,
closely to the reported by Hale and Godsey (2019) in the United States
(coniferous and grasses) and by Figueiredo et al. (2014) in Spain
(agricultural and forest). Slightly higher values, but close to those
exposed here, were reported by Liu and Sheu (2003) (leafy and coniferous
forests). The efficiency of OM mineralization and the rare occurrence of
hortonian surface flows (high water soil infiltration capacity) should
be the most relevant explanatory factors for the occurrence of DOC
concentrations below those reported in some temperate (Chaplot &
Ribolzi, 2014) and boreal (Ledesma et al., 2016) environments. There are
no wetlands or peatlands in our study area, which are usually, along
with overland flows, significant sources of DOC in streams or lakes in
steppe vegetation and boreal or temperate forests (Hongve, 1998; Chaplot
& Ribolzi, 2014; Oni, Futter, Molot, & Dillon, 2014).