Soil nitrogen conservation mechanisms in a pristine south Chilean Nothofagus forest ecosystem

A 15N tracing study was carried out to identify microbial and abiotic nitrogen (N) transformations in a south Chilean Nothofagus betuloides forest soil which is characterized by low N inputs and absence of human disturbance. Gross N transformation rates were quantified with a 15N tracing model in combination with a Markov chain Monte Carlo sampling algorithm for parameter estimation. The 15N tracing model included five different N pools (ammonium (NH4+), nitrate (NO3−), labile (Nlab) and recalcitrant (Nrec) soil organic matter and adsorbed NH4+), and ten gross N transformation rates. The N dynamics in the N. betuloides ecosystem are characterized by low net but high gross mineralization rates. Mineralization in this soil was dominated by turnover of Nlab, while immobilization of NH4+ predominantly entered the Nrec pool. A fast exchange between the NH4+ and the adsorbed NH4+ pool was observed, possibly via physical adsorption on and release from clay lattices, providing an effective buffer for NH4+. Moreover, high NH4+ immobilization rates into the Nrec pool ensure a sustained ecosystem productivity. Nitrate, the most mobile form of N in the system, is characterized by a slow turnover and was produced in roughly equal amounts from NH4+ oxidation and organic N oxidation. More than 86% of the NO3− produced was immediately consumed again. This study showed for the first time that dissimilatory nitrate reduction to ammonium (DNRA) was almost exclusively (>99%) responsible for NO3− consumption. DNRA rather than NO3− immobilization ensures that NO3− is transformed into another available N form. DNRA may therefore be a widespread N retention mechanism in ecosystems that are N-limited and receive high rainfalls.