Nitrite transformations under acidic conditions in temperate and subtropical forest ecosystems

Nitrite (NO2−) is an intermediate in a number of soil N transformations, and is a precursor for NO and N2O emissions. Due to the transient nature of NO2− it is often neglected in N cycling research. Thus, the full suite of production and consumption pathways of NO2− in terrestrial ecosystems is poorly understood. The primary objectives of this study were to clarify the production and consumption pathways of NO2− in the acidic forest soils and compare the NO2− dynamics between temperate and subtropical forest soils. A series of 15N tracing studies were performed, where 15N was added as NH4+, NO3−, or NO2− to temperate and subtropical forest soils, and NO2− transformations were investigated in these acidic forest soils. The results showed that the average NO2− production rates ranged from 4.42 to 5.90 mg kg− 1d− 1 and from 1.29 to 2.77 mg kg− 1d− 1 in the temperate and subtropical forest soils, respectively, within the 1-6 h incubations. Oxidation of soil organic N was the dominant NO2− production pathway in the acidic forest soils, which was negatively related to soil pH (p < 0.01). The average consumption rates of NO2− varied during the incubation period from 12.46 to 14.10 mg kg− 1d− 1 and from 5.84 to 6.74 mg kg− 1d− 1 in the temperate and subtropical forest soils, respectively. The NO3− pool recovered 6-44% of added 15N-NO2− in the studied forest soils, which was positively related to soil pH (p < 0.05). The 32-36% of added 15N-NO2− was incorporated into insoluble soil organic N pool (SON) associated with soil organic C concentration. Nitrite incorporation into DON was also important for soil N retention in the subtropical strongly acidic forest soils (pH 4.7), however, in the temperate acidic forest soils, 15N was not detected in the DON pool. Self-decomposition of NO2− to NO and NO2 was a main pathway under subtropical strongly acidic conditions. The NO2− turnover was most likely the key driving force for N transformations in the acidic forest soils. Further work regarding the dynamics of NO2− and their mechanisms in soils of the different terrestrial ecosystems is important to gain an improved understanding of the global N cycle.