Functional role of DNRA and nitrite reduction in a pristine south Chilean Nothofagus forest

Abstract

Nitrite (NO2 - ) is an intermediate in a variety of soil N cycling processes. However, NO2 - dynamics are often not included in studies that explore the N cycle in soil. Within the presented study, nitrite dynamics were investigated in a Nothofagus betuloides forest on an Andisol in southern Chile. We carried out a 15 N tracing study with six 15 N labeling treatments, including combinations of NO3 - ,N H 4 ? and NO2 - . Gross N transformation rates were quantified with a 15 N tracing model in combination with a Markov chain Monte Carlo optimization routine. Our results indi- cate the occurrence of functional links between (1) NH4 ? oxidation, the main process for NO2 - produc- tion (nitritation), and NO2 - reduction, and (2) oxidation of soil organic N, the dominant NO3 - production process in this soil, and dissimilatory NO3 - reduction to NH4 ? (DNRA). The production of NH4 ? via DNRA was approximately ten times higher than direct mineralization from recalcitrant soil organic matter. Moreover, the rate of DNRA was several magnitudes higher than the rate of other NO3 - reducing processes, indicating that DNRA is able to outcompete denitrification, which is most likely not an important process in this ecosystem. These functional links are most likely adaptations of the microbial community to the prevailing pedo- climatic conditions of this Nothofagus ecosystem.