Abstract
The response of organic carbon (C) concentrations in ecosystem solutions to environmental change affects the release of dissolved organic matter (DOM) from forests to surface and groundwaters. We determined the total organic C (TOC) concentrations (filtered <1-7µm) and the ratios of TOC/dissolved organic nitrogen (DON) concentrations, electrical conductivity (EC), and pH in all major ecosystem solutions of a tropical montane forest from 1998 to 2013. The forest was located on the rim of the Amazon basin in Ecuador and experienced increasing numbers of days with >25°C, decreasing soil moisture, and rising nitrogen (N) deposition from the atmosphere during the study period. In rainfall, throughfall, mineral soil solutions (at the 0.15- and 0.30-m depths), and streamflow, TOC concentrations and fluxes decreased significantly from 1998 to 2013, while they increased in stemflow. TOC/DON ratios decreased significantly in rainfall, throughfall, soil solution at the 0.15-m depth, and streamflow. Based on Δ14 C values, the TOC in rainfall and mineral soil solutions was 1year old and that of litter leachate was 10years old. The pH in litter leachate decreased with time, that in mineral soil solutions increased, while those in the other ecosystem solutions did not change. Thus, reduced TOC solubility because of lower pH values cannot explain the negative trends in TOC concentrations in most ecosystem solutions. The increasing TOC concentrations and EC in stemflow pointed at an increased leaching of TOC and other ions from the bark. Our results suggest an accelerated degradation of DOM, particularly of young DOM, associated with the production of N-rich compounds simultaneously to changing climatic conditions and increasing N availability. Thus, environmental change increased the CO2 release to the atmosphere but reduced DOM export to surface and groundwater.
Highlights
Dissolved organic matter (DOM) pouring out of forest ecosystems into surface and groundwaters contributes about 20% to the C input via rivers to the ocean and links the terrestrial and aquatic carbon (C) cycles (Camino-Serrano et al, 2014; Dai et al, 2012; Regnier et al, 2013)
We hypothesized that (a) total organic carbon (TOC) concentrations and fluxes decreased during the past 15 years, because of enhanced degradation favored by reduced soil waterlogging and increasing N availability. (b) Increasing degradation is reflected by increasing TOC/dissolved organic nitrogen (DON) ratios, because the more polar N-rich compounds are preferentially degraded as reflected by the reported decreasing DON concentrations in ecosystem solutions. (c) We expected that the DOM was mainly derived from fresh C sources in the dense forest canopy and the thick soil organic layer, and that the young age of the DOM allowed for its fast response to environmental change
The TOC fluxes with rainfall (τ = −0.301, p < .001), throughfall (τ = −0.252, p < .001), and streamflow (τ = −0.113, p = .051) decreased significantly, albeit with streamflow only marginally, while the TOC fluxes with stemflow increased significantly (τ = 0.394, p < .001) and those with litter leachate did not show a temporal trend
Summary
Dissolved organic matter (DOM) pouring out of forest ecosystems into surface and groundwaters contributes about 20% to the C input via rivers to the ocean and links the terrestrial and aquatic carbon (C) cycles (Camino-Serrano et al, 2014; Dai et al, 2012; Regnier et al, 2013). DOM concentrations are about 75% lower than that in corresponding organic layers across various climate zones and soil types, mainly because of DOM sorption to the minerals rendering the soil the first DOM sink along the water flow path through a forest ecosystem (Camino-Serrano et al, 2014; Möller et al, 2005). We only found a few reports on temporal trends in DOM concentrations in forest ecosystem solutions covering at least a decade (Borken et al, 2011; Wu et al, 2010) In these studies, which were restricted to temperate ecosystems, DOM concentrations in mineral soil solutions mostly decreased. This was mainly explained by increased DOM sorption to the soil solid phase in response to decreasing acid rain. We hypothesized that (a) TOC concentrations and fluxes decreased during the past 15 years, because of enhanced degradation favored by reduced soil waterlogging and increasing N availability. (b) Increasing degradation is reflected by increasing TOC/DON ratios, because the more polar N-rich compounds are preferentially degraded as reflected by the reported decreasing DON concentrations in ecosystem solutions. (c) We expected that the DOM was mainly derived from fresh C sources in the dense forest canopy and the thick soil organic layer, and that the young age of the DOM allowed for its fast response to environmental change
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