Abiotic nitrate loss and nitrogenous trace gas emission from Chinese acidic forest soils

There are an increasing number of studies, which have shown the potential importance of abiotic denitrification in nitrogen biogeochemistry through pure chemical coupling between nitrate/nitrite reduction and Fe(II) oxidation. However, there is little direct evidence showing the environmental significance of abiotic nitrate (NO₃^-) reduction in acidic soils. We assessed the magnitude and gaseous product stoichiometry of abiotic nitrate reduction in acidic forest soils based on sterilized anoxic soil incubations at different soil pHs and nitrate loadings. The results showed that 24.9, 53.4, and 88.7% of added nitrate (70 mg N kg^-1) were lost during 15 days incubation at pHs 3.9, 4.8, and 5.6, respectively. Nitrous oxide (N₂O) was found as the dominant gaseous product of abiotic nitrate reduction, accounting for 5.0, 28.9, and 47.9% of nitrate losses at three pH levels, respectively. Minor but clear NO accumulations were observed for all nitrate-amended treatments, with the maxima at intermediate pH 4.8. The percentage of NO increased significantly with soil pH decline, leading to a negative correlation between NO/N₂O ratio and soil pH. Though saturations were found under excessive nitrogen loading (i.e., 140 mg N kg^-1), we still pose that abiotic nitrate reduction may represent a potentially important pathway for nitrate loss from acidic forest soils receiving nitrogen deposition. Our results here highlight the importance of abiotic nitrate reduction in the soil nitrogen cycle, with special relevance to nitrate removal and nitrogenous trace gas (NO and N₂O) emissions from acidic soils.