Nitrogen turnover in drying sediments of an Amazon floodplain lake.

In the Amazon floodplain large areas are subject to annual cycles of drying and rewetting. The turnover of nitrogen in the periodically drying sediments is an important regulator of floodplain fertility. In the present study the transition of a lake sediment from flooded to dry conditions was studied with respect to microbial nitrogen turnover. Soil nitrogen pools, as well as the activity and abundance of denitrifying and nitrifying bacteria, were investigated during one dry season. During the first weeks after drying, most of the inorganic nitrogen vanished from the sediment. The process was inhibited by a nitrification inhibitor, showing that coupled nitrification-denitrification was responsible for the nitrogen loss. Assimilation by plants or microbes, as well as leaching, were not important mechanisms of nitrogen loss. During a period of only 10 days, 59% of the total denitrification and 94% of the total N2O emission during the dry period occurred. Cell numbers of denitrifiers were not correlated with activities. Denitrification was not correlated with other sediment variables but was regulated by the patchy distribution of reduced and oxidized zones in the uppermost centimeters of the sediment. This heterogeneity was probably introduced by the bioturbation of small insects, which was restricted to a rather short time period shortly after drying.