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 (Rs) 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 Rs (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, Rs 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 Rs, 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 reduced Rs, whilst k could remain unchanged but with decomposition processes becoming more anaerobic.