Comparisons of the effects of different drying methods on soil nitrogen fractions: Insights into emissions of reactive nitrogen gases (HONO and NO)

Reactive nitrogen (N_r) emission from soils, e.g., nitrous acid (HONO) and nitric oxide (NO), is a key process of the global nitrogen (N) cycle and has significant implications for atmospheric chemistry. To understand the underlying mechanisms of soil N_r emissions, air-dried or oven-dried soils are commonly used in the laboratory. To date, few studies have compared the effects of different drying methods on soil N_r gas fluxes and N fractions. Here, the authors studied soil water content, pH, (in)organic N content, and N_r gas fluxes of air-dried, freeze-dried, oven-dried, and fresh soils from different land-use types. The results showed that the soil pH of air-dried and oven-dried samples was significantly lower compared with fresh soil from farmland and grassland, but higher compared with forest soil. The difference in soil pH between freeze-dried and fresh soil (mean ± standard deviation: 0.52 ± 0.31) was the lowest. In general, all drying methods increased the soil NH₄ ⁺-N, NO₃ ⁻-N, and dissolved organic N contents compared with fresh soil (P < 0.05). The maximum HONO and NO flux and total emissions during a full wetting–drying cycle of fresh soil were also increased by air-drying and oven-drying (P < 0.001), but comparable with freeze-dried soil (P > 0.2). In conclusion, all drying methods should be considered for use in studies on the land–atmosphere interface and biogeochemical N cycling, whereas the freeze-drying method might be better for studies involving the measurement of soil N_r gas fluxes.