Worldwide wetlands contribute to the global carbon cycle by emitting about a third of the global methane (CH 4 ) emissions. However, CH 4 and carbon dioxide (CO 2 ) dynamics remain poorly understood in the largest tropical wetland on Earth, the Pantanal. In this chapter, we aim to characterize the CH 4 and CO 2 biogeochemistry in the floodplain of the Paraguay River, near Corumbá, during the course of annual anoxia phenomena locally known as dequada. The strong anoxia is associated to the flooding of terrestrial habitats that enhances respiration, dissolved oxygen (DO) consumption, and methanogenesis. The extremely low DO also leads to high fish mortality in the region. CH 4 and CO 2 concentration in surface waters and diffusive water-air fluxes were measured in the oxbow Tuiuiú Lake and in the Paraguay River main stem in order to identify temporal and spatial patterns. The whole dataset shows that, for instance, dissolved CH 4 and diffusive CH 4 fluxes increased dramatically during the dequada. In the study area, CH 4 emissions can À2 h À1 during dequada climax. Riverine anoxic waters steadily penetrate the oxbow Tuiuiú Lake, indicating water inflow from the river main stem, whereas small reminiscent patches of oxbow waters not mixing with anoxic river waters may function as survival refuges to the aquatic wildlife. Clearly, the DO distribution during several dequadas in major rivers of the Pantanal highlights the importance of geomorphology on the biogeochemistry in the riverine floodplains of the Pantanal wetland.

Closed static chambers are frequently adopted for estimating gas fluxes across environmental interfaces. In this study we assessed the effects of three gas sampling rates and two methods of chamber placement in fertirrigated soils for estimating nitrous oxide (N 2 O) emissions, using chambers with similar design and on-site gas chromatography. The soils under analysis were fertirrigated with (liquid) digested swine manure at three different doses. The results indicate that N 2 O flux estimates are firmly determined by the chosen sampling rate, chamber placement, chamber design, and the emission magnitude itself. N 2 O fluxes were best estimated by faster sampling rates while gently placing the chamber at the soil surface due to conspicuous N 2 O emissions and relatively small chamber volume. A generalized chamber accumulation model developed by normalizing the dataset was used to illustrate effects on expected "low unforced-chamber", "high unforced-chamber" and "high forced-chamber" fluxes. We concluded that it is possible to adopt simple design and low-disturbing chambers with sufficient volume, height, and surface area for determining gaseous emissions across soil-air interfaces. Nevertheless, critical on-site gas sampling rate adjustment (by gas chromatography or other as precise real-time measuring device) is critical to avoid estimation inaccuracies in emission estimates.