Adaptation of NO₂⁻ Extraction Methods to Different Agricultural Soils: Fine-Tuning Based on Existing Techniques

Soil nitrite (NO₂ ⁻) is an important reactive intermediate in many nitrogen transformation processes, but it is unstable under acidic conditions and may be lost as gaseous N. The canonical extraction method of soil NO₂ ⁻ using a potassium chloride (KCl) solution greatly underestimates its concentration. To reflect the concentration more accurately, we optimized the extraction method of soil NO₂ ⁻ for three agricultural soils differing in soil texture and pH, an alkalic fluvo-aquic soil and acidic Mollisol and Ultisol soils, respectively. Both extractable soil ammonium (NH₄ ⁺) and nitrate (NO₃ ⁻) were systematically investigated to optimize the simultaneous extraction of soil inorganic nitrogen. The effects of different extractants (deionized water (DIW), un-buffered 2 mol L⁻¹ KCl, and pH-buffered 2 mol L⁻¹ KCl), shaking time (10 and 30 min), and storage duration of the extracts (stored at −20 °C for 1 day, and at 4 °C for 1, 3, and 6 days) on the determination of soil inorganic nitrogen were investigated. The results showed that the un-buffered KCl extractant significantly underestimated soil NO₂ ⁻ concentration compared to DIW. The highest recovery of NO₂ ⁻ was obtained by extracting with DIW at 10 min of shaking for all three soils. Compared with DIW, the concentration of NH₄ ⁺ and NO₃ ⁻ in soil extracted from the KCl solution increased significantly. Furthermore, the soil inorganic nitrogen content of extracts stored at 4 °C for one day was closer to the direct measurements of fresh samples than with the other storage methods. Overall, the recommended analysis method for soil NO₂ ⁻ was extraction by DIW, shaking for 10 min, and filtering with a 0.45 µm filter, while soil NH₄ ⁺ and NO₃ ⁻ were extracted with a KCl solution and shaken for 30 min. The extract should be stored at 4 °C and analyzed within 24 h. Our study provides an efficient extraction method for soil NO₂ ⁻ and supports studies on the biogeochemical nitrogen cycle, e.g., in the investigation of soil nitrous acid (HONO) and nitric oxide (NO) emissions.