Predicting Economic Optimal Nitrogen Rate with the Anaerobic Potentially Mineralizable Nitrogen Test

Estimates of mineralizable N with the anaerobic potentially mineralizable N (PMNan) test could improve predictions of corn (Zea maysL.) economic optimal N rate (EONR). A study across eight US midwestern states was conducted to quantify the predictability of EONR for single and split N applications by PMNan. Treatment factors included different soil sample timings (pre‐plant and V5 development stage), planting N rates (0 and 180 kg N ha−1), and incubation lengths (7, 14, and 28 d) with and without initial soil NH4–N included with PMNan. Soil was sampled (0–30 cm depth) before planting and N application and at V5 where 0 or 180 kg N ha−1were applied at planting. Evaluating across all soils, PMNanwas a weak predictor of EONR (R2≤ 0.08; RMSE, ≥67 kg N ha−1), but the predictability improved (15%) when soils were grouped by texture. Using PMNanand initial soil NH4–N as separate explanatory variables improved EONR predictability (11–20%) in fine‐textured soils only. Delaying PMNansampling from pre‐plant to V5 regardless of N fertilization improved EONR predictability by 25% in only coarse‐textured soils. Increasing PMNanincubations beyond 7 d modestly improved EONR predictability (R2increased ≤0.18, and RMSE was reduced ≤7 kg N ha−1). Alone, PMNanpredicts EONR poorly, and the improvements from partitioning soils by texture and including initial soil NH4–N were relatively low (R2≤ 0.33; RMSE ≥ 68 kg N ha−1) compared with other tools for N fertilizer recommendations. Core IdeasAnaerobic potentially mineralizable N (PMNan) is a weak predictor of economic optimal N rate (EONR).Predictability of EONR by PMNanimproves when accounting for soil texture.For coarse‐textured soils, PMNanat V5 improves EONR predictability.Increasing incubation length does not substantially improve EONR predictability.PMNanalone is not a reliable management tool for N rate determination. Anaerobic potentially mineralizable N (PMNan) is a weak predictor of economic optimal N rate (EONR). Predictability of EONR by PMNanimproves when accounting for soil texture. For coarse‐textured soils, PMNanat V5 improves EONR predictability. Increasing incubation length does not substantially improve EONR predictability. PMNanalone is not a reliable management tool for N rate determination.