Larger floods reduce soil CO2 efflux during the post-flooding phase in seasonally-flooded forests of Western Amazonia

Seasonally-flooded várzea forests of Western Amazonia are one of the most productive and biodiverse wetland forests in the world. However, data on their soil CO₂ emissions, soil organic matter decomposition rates, and soil C stocks are scarce. This is a concern because hydrological changes are predicted to lead to increases in the height, extent, and duration of seasonal floods, which are likely to have a significant effect on soil C stocks and fluxes. However, with no empirical data, the impact of altered flood regimes on várzea soil C cycles remains uncertain. This study quantified the effects of maximum annual flood height and soil moisture on soil CO₂ efflux rate (R_s) and soil organic matter decomposition rate (k) in the várzea forests of Pacaya Samiria National Reserve, Peru. The study was conducted between May and August 2017. The results showed that R_s (10.6-182.7 mg C m^-2 h^-1) and k (0.016-0.078) varied between and within sites, and were considerably lower than the values reported for other tropical forests. In addition, R_s was negatively affected by flood height (P < 0.01) and soil moisture (P < 0.001), and it decreased with decreasing river levels post flooding (P < 0.001). In contrast, k was not affected by any of the above-mentioned factors. Soil moisture was the dominant factor influencing R_s, and it was significantly affected by maximum flood height, even after the floods had subsided (P < 0.001). Consequently, we concluded that larger floods will likely lead to reducedR_s, whilst k could remain unchanged but with decomposition processes becoming more anaerobic.