INTRODUCTION
Spatial flows of dissolved organic carbon (COD) are an important component of the carbon (C) balance in ecosystems and the global cycle of this element (Cole et al., 2007). Rainwater permeates terrestrial environments, transporting considerable amounts of organic carbon (OC) to waterbodies, which can affect limnic ecosystems and water resources quality (Graeber et al., 2018).
The development of models that can synthesize the dynamics of DOC in river basins, aimed to assist the management of water resources and meet the growing demand for information about the C cycle, demands an increasing knowledge about the production and migration dynamics of OC in the terrestrial environment and the rainfall action on these process (Neff & Asner, 2001). The watershed is the spatial matrix of capitation, production, storage, decomposition, and export of OC.
In line with the relevance of this theme, some researchers have directed their efforts towards the study of the balance of DOC in hydrographic basins, as well as towards the flows between forest compartments and to aquatic ecosystems (Hongve, 1999; Jiang et al., 2014; Meyer et al., 1988). The modulation of DOC concentrations in streams has been evaluated in their capacity to indicate the rates of surface runoff in the basin area using the hydrogeochemical signature idea (Chaplot e Ribolzi, 2014; Figueiredo et al., 2014).
In forest environments, litter above the ground is the main source of OC for deep soil horizons and aquatic environments (Camino-Serrano et al., 2014; Hafner, Groffman, & Mitchell, 2005; Moyer, Powell, Gordon, Long & Bliss, 2015). In the soil, complex chemical interactions between water, microorganisms and organic matter (OM) are manifested (Kaiser, Peake, Willey, & Brooks, 1996; Singh et al., 2017;). Leaching of litter and sorption to mineral particles are the main processes in the DOC dynamics of terrestrial environments and their flow into aquatic environments. These processes have equally important local features, such as primary productivity, decomposition rates, pluviometric dynamics, and soil water infiltration capacity.
The pluviometric dynamics given by the conditions of antecedent humidity and characteristics of the rainfall events are variables that act directly on the effects of DOC dilution and environments pre-washing (Li et al., 2016; Michalzik, Kalbitz, Park, Solinger, & Matzner, 2001). In addition, it acts in conjunction with the soil water infiltration capacity to define the water flow pathways in terrestrial environments and, consequently, regulate the sorption opportunities of DOC to soil particles and arrival rates to waterbodies (Graeber et al., 2018).
Concentrated in temperate and boreal forests, studies on DOC leaching has given attention to the control exercised by litter decomposition rates. Several studies have compared coniferous and hardwood forests about DOC flows including leaching of their leaves, discussing aspects such as material lability (Fröberg, Hansson, Kleja, & Alavi, 2011; Lee et al., 2018; Liu & Sheu, 2003). In tropical environments, DOC studies that enter the universe of litter leachate and soil solution are still scarce (e.g. Zhou et al., 2015), and studies on the balance of DOC in hydrographic basins are a little more common, addressing DOC inputs by rainfall (e.g. Godoy-Silva, Nogueira, & Campos, 2017) and/or outputs through the drainage channel (e.g. Costa et al., 2017).
Our study area is a tropical rainforest environment with primary productivity and litter decomposition rates different from temperate and boreal forests. Previous studies confirmed that the rugged relief of the area differentiated litter decomposition rates on the hillslope-scale (Miranda & Avelar, 2019), a representative situation of the forested mountains of the Brazilian coast and with a singular potential for research about DOC litter leaching.
In this way, our work aimed to analyze and discuss the water DOC concentrations from different forest compartments, in a hillslope-scale, to contribute with a general panorama about the leaching and balance of DOC in tropical rain forests, coupling an approach about the decomposition rates influence for DOC leaching from litter provided by a favorable laboratory-situation (decomposition rates heterogeneity). Analysis of the rainfall characteristics allowed us to infer its influence on the DOC concentrations in the different forest compartments. Thus, the comparison between hillslope positions as environmental spatial heterogeneity and the comparison between rainfall events based on the potential control exercised by the precipitation dynamics conform, respectively, the spatio-temporal scope of the present paper.