Anaerobically Mineralized Nitrogen: Sample Processing Alternatives to Simplify Its Determination

Anaerobically mineralized nitrogen (AMN) is the ammonium nitrogen (NH4+-N) released during a short anaerobic incubation. Its determination requires subtracting the initial NH4+-N, making the procedure labor-intensive. Since initial NH4+-N is generally negligible, its measurement might be omitted. However, NH4+-N can accumulate during sample processing. We hypothesized that: i) finer disaggregation, higher temperature, and/or thinner layers during the drying process reduce initial NH4+-N; ii) initial NH4+-N increases with soil organic carbon (SOC) content. Eight agricultural soil samples (0–5 and 5–20 cm) were processed under two levels of aggregate disruption (homogenization and 8-mm-sieving), two drying temperatures (30 and 50 °C), and two thicknesses (1 and 4 cm). Moist samples initial NH4+-N (Ammonium-field) was negligible (1–3 mg kg−1), but processing and drying increased initial NH4+-N (Ammonium-dry) by over 500%. The results did not support the hypotheses: i) homogenization did not differ from 8-mm-sieved in Ammonium-dry; ii) higher drying temperature increased Ammonium-dry; iii) thinner soil layers reduced Ammonium-dry only at 50°C and 0–5 cm; and iv) Ammonium-dry was not associated with SOC. Processing had minimal impact on AMN and Ammonium-dry plus AMN (Ammonium-dry+AMN). Nonetheless, across all processing conditions, Ammonium-dry+AMN was strongly correlated with AMN, suggesting its potential as a robust soil health indicator regardless of processing.