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).