Mechanisms Involved in Soil Ammonium Production in a Mauritia flexuosa Palm Swamp Community

Abstract

When soils are water saturated, the reduced forms of soil nitrogen such as ammonium are favored. Under anaerobiosis, ammonium production has been explained by several mechanisms such as organic nitrogen mineralization (ONM), dissimilatory nitrate reduction to ammonium (DNRA) and the release of adsorbed ammonium during microbial reduction of iron oxides (MRFeO). Nevertheless, in anaerobic soils the dominance of one mechanism over the others is still debated. Nitrogen appears to influence the reproductive dynamics of Mauritia flexuosa, the dominant canopy species of the “morichal” ecosystem. Even though M. flexuosa has been classified as “vulnerable”, ammonium production in morichal soils is not well understood. Considering this, we evaluated in a morichal soil from SE Venezuela, the mechanisms associated with ammonium production under anaerobic conditions. To this end, we designed an anaerobic incubation experiment to follow over time the concentrations of nitrate, extractable ferrous iron and ammonium together with the activity of β-glucosaminidase as an index of N mineralization. To account for the relative importance of ONM, DNRA and MRFeO on ammonium production, we used multiple linear regression models. The results showed that the three mechanisms are involved in ammonium production. However, ONM is the main source of ammonium in the initial 15 days of soil incubation, while MRFeO explains ammonium produced after 15 days of soil incubation. However, under climate change scenarios these mechanisms may be altered, thereby increasing the N limitation of morichales and consequently its vulnerability. Since morichales are threatened ecosystems, that play a key ecological, economic and cultural role, studies on the soil nitrogen dynamics are important to design and evaluate effective management, conservation and protection strategies. This study can be a relevant contribution in this area.