Assad Darwich

Nitrogen dynamics in seasonally flooded soils in the Amazon floodplain

Large areas of the Amazon are subject to seasonal flooding due to water level changes of the river. This ‘flood pulse’ causes rapidly changing conditions for microorganisms living in the soils which affects the cycling of nitrogen in the ecosystem. An understanding of the nitrogen dynamics in the seasonally flooded soils is essential for the development of productive and sustainable management concepts. We measured nitrogen concentrations, denitrifier enzyme activity (DEA), cell numbers of nitrifying and denitrifying bacteria, respiration, pH and total carbon in the seasonally flooded soils over one entire annual hydrological cycle. By comparing three sites with different vegetation (forest, aquatic macrophyte stand and bare sediment with annual herbs) we assessed the effect of vegetation on soil nitrogen dynamics. Inorganic nitrogen was always dominated by ammonium indicating reduced conditions in the soil even during the terrestrial phase. Although conditions were generally poor for nitrification we observed high numbers of nitrifying bacteria between 104 and 107cells g−1. Pulses of ammonium as well as high DEA were observed during the transition periods between aquatic and terrestrial phase. Thus the alternation between aquatic and terrestrial phase promotes nitrogen mineralization and denitrification in the soils. There were no plausible correlations between microbial activities and numbers with soil physical or chemical parameters except a relation between the numbers of nitrate reducing bacteria and soil moisture (R2 = 0.81) and ammonium (R2 = 0.92) at one site. This shows the complex regulation patterns in this habitat. Different vegetation did not alter the general patterns of nitrogen dynamics but the absolute extend of fluctuations. We conclude that both the soil physical and chemical changes directly caused by the flood pulse and the vegetation have a great impact on microbial nitrogen turnover in the soils. The effects of the flood pulse can be buffered by a fine soil texture or a litter layer which prevents desiccation of the soil during the terrestrial phase.

Nitrogen Turnover in the Várzea

Nitrogen is one of the most important macronutrients for all organisms. It reaches the floodplain in the dissolved and particulate load of the Amazon River during floods and from the atmosphere as dry and wet deposition and via nitrogen fixation. Anthropogenic sources are negligible in most parts of the varzea. From a few studies on nitrogen fluxes in Amazonian environments there is some evidence that nitrogen can be a limiting factor for primary production in the varzea, at least in the aquatic phase (Forsberg 1984; Setaro and Melack 1984; Furch and Junk 1993). For more insight into the pathways of nitrogen input and output, nitrogen fluxes were measured in various ecotopes at Lago Camaleao. This lake belongs to the class of varzea lakes that are not hydrochemically affected by the terra firme, in contrast to Lago Calado, which is a dendritic varzea lake influenced by the runoff from the terra firme. Both lakes have been examined intensively during the last two decades for the nitrogen exchange between river and lake, the input by precipitation and nitrogen fixation, and the output by denitrification (Furch 1982, 1984b; Melack and Fisher 1988; Junk and Piedade 1993c; Lesack 1993; Lesack and Melack 1995; Kern et al. 1996). The study at Lago Camaleao was focused on the gaseous nitrogen exchange between atmosphere, hydrosphere, and pedosphere, and provided new results for nitrogen fixation and denitrification in the Amazon floodplain.

Nitrogen Balance of a Floodplain Forest of the Amazon River: The Role of Nitrogen Fixation

The high biomass production in the varzea depends on a high supply of nitrogen, one of the most important macronutrients. There are three main paths for nitrogen to reach the floodplain. Nitrogen derives firstly from the water of the Amazon River when it inundates the floodplain during rising water, secondly from atmospheric deposition, and thirdly from biological N2 fixation (Kern and Darwich 1997). Atmospheric N2 is fixed in various ecotopes, primarily on high elevational ranges of the floodplain. At an elevational range of 22–25 m a.s.l. the forest under study is located on a ridge on Marchantaria Island. It is influenced by the water level of the Camaleao Lake, leading to an average inundation period between 4.7 and 7.6 months per year. In this most advanced successional stage of phytocoenoses, pathways of nitrogen input and output were studied next to Lake Camaleao on Marchantaria Island (Kreibich et al. 2006). This island is not affected hydrochemically by non-inundated upland (terra firme). Interpretation of the results are therefore restricted to exclusive white-water habitats of the central Amazon floodplain.