Soil CO₂concentrations and efflux dynamics of a tree island in the Pantanal wetland

Abstract

The Pantanal is the largest tropical wetland on the planet, and yet little information is available on the biome's carbon cycle. We used an automatic station to measure soil CO2 concentrations and oxidation‐reduction potential over the 2014 and 2015 flood cycles of a tree island in the Pantanal that is immune to inundation during the wetland's annual flooding. The soil CO2 concentration profile was then used to estimate soil CO2 efflux over the two periods. In 2014, subsurface soil saturation at 0.30 m depth created conditions in that layer that led to CO2 buildup close to 200,000 ppm and soil oxidation‐reduction potential below −300 mV, conditions that were not repeated in 2015 due to annual variability in soil saturation at the site. Mean CO2 efflux over the 2015 flood cycle was 0.023 ± 0.103 mg CO2‐C m−2 s−1 representing a total annual efflux of 593 ± 2690 mg CO2‐C m−2 y−1. Unlike a nearby tree island site that experiences full inundation during the wet season, here the soil dried quickly following repeated rain events throughout the year, which led to the release of CO2 pulses from the soil. This study highlights not only the complexity and heterogeneity in the Pantanal's carbon balance based on differences in topography, flood cycles, and vegetation but also the challenges of applying the gradient method in the Pantanal due to deviations from steady state conditions.

Loss of deep roots limits biogenic agents of soil development that are only partially restored by decades of forest regeneration

“…Though a weak acid, carbonic acid can be a potent contributor to soil acidification (Reuss and Johnson, 1986;Amundson and Davidson, 1990;Richter and Markewitz, 1995;Richter and Markewitz, 2001) as soil CO 2 concentrations can regularly exceed 5% in deep soil horizons (Richter and Markewitz, 2001;Johnson et al, 2013;Hasenmueller et al, 2015;Lathuillière et al, 2017), driving this reaction forward. Organic acids are also generated by microbes and roots and additionally contribute to soil acidification, as demonstrated by the commonly occurring OA oxalic acid: -+ 2-+ 2 4 2 2 4 2 4…”

Interactions between Vegetation, Hydrology, and Litter Inputs on Decomposition and Soil CO2 Efflux of Tropical Forests in the Brazilian Pantanal

“…However, it is possible that monthly measurements of Rsoil missed potentially important transient dynamics [60] that could affect our interpretation of spatial (upland vs. flooded forest) and treatment effects. Continuous measurements of soil CO 2 concentration in our upland forest indicate that seasonal variations in CO 2 production were relatively stable but within-season variations were more transient [42], and thus it is possible that Rsoil varied more within a given season than reported here.…”

“…The upland forest was situated on a slightly higher (ca. 5 m) flat plain than the gallery forest, and was never flooded during the study period [42]. This forest was dominated by a palm Scheelea phalerata (Mart.)…”

“…While we cannot rule out the possibility that the similarity in Rsoil between these forests might be an artifact of a small sample size and/or the short duration of our field study, the similarity in Rsoil between sites is striking given the large variations in hydrology. However, even though the upland forest was never flooded, occasional high water tables can cause intermittent soil anoxia [42,60]. Furthermore, C mineralization in soils that experience temporary anoxia can be higher than in soils that experience continuous aerobic conditions because iron reduction can release C that is protected in Fe-oxides, especially when aerobic conditions are established after soil becomes unsaturated [61].…”

“…However, temperature trends were almost identical to long-term averages, precipitation was only slightly higher (ca. 112 mm) than the long-term average, and the seasonal flood regime was typical for the flooded forest [33][34][35][36][37][38][39][40][41][42]60], so even though the experimental campaign was short, the environmental conditions were typical for these forests. A potentially larger question surrounds whether the initial responses in Rsoil and decomposition will hold over a longer period of time, and while we cannot answer this question with the data at hand, data collected from litter manipulation experiments in temperate and tropical forests suggest that the initial priming effect of litter holds over at least 3 years [53].…”