Ferrous wheel hypothesis II: Abiotic incorporation of mineral nitrogen into organic pools in volcanic soils of temperate forest ecosystems

Understanding the pathways of nitrogen (N) retention in pristine forest soils is essential for effective ecosystem management and nutrient conservation. The incorporation of nitrate (NO₃⁻) and nitrite (NO₂⁻) into organic N in soils without microbiological contribution remains a very intriguing question. This study explores the abiotic incorporation of nitrate (NO₃⁻) and nitrite (NO₂⁻) into organic N of volcanic soil under sterilized and anoxic conditions, providing insights into mineral N losses occurring as dissolved organic N (DON) rather than the commonly accepted nitrate leaching. We evaluated the hypothesis that nitrate (NO₃⁻) can be reduced to nitrite (NO₂⁻), which subsequently reacts with organic matter through nitration and nitrosation, leading to the formation of organic nitrogen. This mechanism, which is of great ecological significance, supports the Ferrous Wheel Hypothesis (FWH). The FWH proposes that ferrous iron, Fe(II), reduces NO₃⁻ to NO₂⁻ within anaerobic microsites, and that Fe(II) is then re-oxidised to ferric iron, Fe(III), contributing to the formation of dissolved organic N (DON). Both NO₃⁻ and NO₂⁻ declined rapidly by 51 and 94 %, while labelled organic N increased by 20–38 % for NO₃⁻ and 42–44 % for NO₂⁻ within seconds. The incorporation of ¹⁵N into organic forms was confirmed using ATR-FTIR and benzene:isopropanol extraction, with the lowest and highest accumulation observed at 5 and 15 mg NO_₃⁻ kg⁻¹, respectively. These results demonstrate that NO₃⁻ incorporation into organic N can occur primarily through abiotic processes, supporting the FWH, as both DON and solid-phase organic N were measured. These findings highlight the natural resilience of volcanic soils in unpolluted old-growth temperate rainforests to N loss and provide new insights into long-term ecosystem stability and nutrient cycling. Further research should investigate the interplay between abiotic and biotic N transformations under field conditions and across diverse forest ecosystems.