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

Abstract

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 (NH 4 +), nitrate (NO 3 −), labile (N lab) and recalcitrant (N rec) soil organic matter and adsorbed NH 4 +), 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 N lab, while immobilization of NH 4 + predominantly entered the N rec pool. A fast exchange between the NH 4 + and the adsorbed NH 4 + pool was observed, possibly via physical adsorption on and release from clay lattices, providing an effective buffer for NH 4 +. Moreover, high NH 4 + immobilization rates into the N rec 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 NH 4 + oxidation and organic N oxidation. More than 86% of the NO 3 − 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 NO 3 − consumption. DNRA rather than NO 3 − immobilization ensures that NO 3 − 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.