Abstract

Understanding nitrogen (N) retention mechanisms in pristine humid temperate rainforest soils is critical for effective ecosystem management and nutrient conservation. The potential abiotic transformation of nitrite (NO2-) into organic N forms in the absence of microbial activity in these ecosystems remains largely unexplored, despite its role in mitigating N leaching. This study focuses on the abiotic incorporation of nitrite (NO2-) into dissolved organic nitrogen (DON) under anoxic conditions, a mechanistic step not directly evaluated in previous research, which employed 15N-labelled nitrate (NO3-). To address this gap, we used 15N-labelled NO2- at 5 and 15 mg L-1 in a lab incubation study under anoxic conditions to trace the contribution of abiotic nitrite transformation to organic N formation in organic matter-rich soils from temperate rainforests developed on both volcanic and non-volcanic parent materials. The added 15N declined rapidly after 15 min by 52% and 60% in both soil solutions, while it started to form labelled DON, increasing by 11% and 34%, after five days of incubation, with the highest accumulation at 15 mg L-1 of 15N-NO2-. These results show that up to 77% of the added 15N-NO2- can be abiotically incorporated into the DON of unpolluted old-growth temperate rainforest, whether developed on volcanic or non-volcanic soils. Nitrogen input has a stronger effect than soil parent material from which the soils originate. This reveals the natural resilience of unpolluted temperate rainforests to N loss, with implications for long-term ecosystem stability and nutrient cycling.