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

Estimates of mineralizable N with the anaerobic potentially mineralizable N (PMN an ) test could improve predictions of corn ( Zea mays L.) 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 PMN an . 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 NH 4 –N included with PMN an . Soil was sampled (0–30 cm depth) before planting and N application and at V5 where 0 or 180 kg N ha −1 were applied at planting. Evaluating across all soils, PMN an was a weak predictor of EONR ( R 2 ≤ 0.08; RMSE, ≥67 kg N ha −1 ), but the predictability improved (15%) when soils were grouped by texture. Using PMN an and initial soil NH 4 –N as separate explanatory variables improved EONR predictability (11–20%) in fine‐textured soils only. Delaying PMN an sampling from pre‐plant to V5 regardless of N fertilization improved EONR predictability by 25% in only coarse‐textured soils. Increasing PMN an incubations beyond 7 d modestly improved EONR predictability ( R 2 increased ≤0.18, and RMSE was reduced ≤7 kg N ha −1 ). Alone, PMN an predicts EONR poorly, and the improvements from partitioning soils by texture and including initial soil NH 4 –N were relatively low ( R 2 ≤ 0.33; RMSE ≥ 68 kg N ha −1 ) compared with other tools for N fertilizer recommendations. Core Ideas Anaerobic potentially mineralizable N (PMN an ) is a weak predictor of economic optimal N rate (EONR). Predictability of EONR by PMN an improves when accounting for soil texture. For coarse‐textured soils, PMN an at V5 improves EONR predictability. Increasing incubation length does not substantially improve EONR predictability. PMN an alone is not a reliable management tool for N rate determination.