Quantifying soil ammonium and nitrite via soil gas emissions

Abstract

Monitoring the soil mineral nitrogen (SMN) status of cropland is challenging due to large spatiotemporal variability. Here, we present a novel approach that correlates NH ₃ , NO, and HONO emissions with SMN content, pH, and soil moisture, eliminating the need for destructive soil sampling. We conducted incubation experiments with five soils with different pH, texture, and organic matter content, using SMN and soil water gradients. In all soils, NH ₄ ⁺ soil content was positively correlated with NH ₃ emission as a function of soil pH and water content. For soil with pH 6.2 and 50% maximum water holding capacity (mWHC), the limit of detection (LOD) was comparable to established methods for total soil NH ₄ ⁺ analysis (LOD = 2.17 mg NH ₄ ⁺ ‐N kg ⁻¹ ) and was as low as 0.16 mg NH ₄ ⁺ ‐N kg ⁻¹ at pH 7.3 and 50% mWHC. In contrast, a significant correlation was found in only one of the soils between NO/NO ₂ ⁻ ( R ²  = 0.73, p  < 0.001) and HONO/NO ₂ ⁻ ( R ²  = 0.57, p  < 0.001). Furthermore, NO (LOD = 37.65 µg NO ₂ ⁻ ‐N kg ⁻¹ ) analysis was preferable to HONO (LOD = 56.14 µg NO ₂ ⁻ ‐N kg ⁻¹ ) due to its lower LOD and 14‐fold higher accuracy. However, the LOD was slightly higher than that of established photometric methods for NO ₂ ⁻ analysis.