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

Abstract

Estimates of mineralizable N with the anaerobic potentially mineralizable N (PMNan) 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 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−1 were applied at planting. Evaluating across all soils, PMNan was 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 PMNan and initial soil NH4–N as separate explanatory variables improved EONR predictability (11–20%) in fine‐textured soils only. Delaying PMNan sampling from pre‐plant to V5 regardless of N fertilization improved EONR predictability by 25% in only coarse‐textured soils. Increasing PMNan incubations beyond 7 d modestly improved EONR predictability (R2 increased ≤0.18, and RMSE was reduced ≤7 kg N ha−1). Alone, PMNan predicts 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 Ideas Anaerobic potentially mineralizable N (PMNan) is a weak predictor of economic optimal N rate (EONR). Predictability of EONR by PMNan improves when accounting for soil texture. For coarse‐textured soils, PMNan at V5 improves EONR predictability. Increasing incubation length does not substantially improve EONR predictability. PMNan alone is not a reliable management tool for N rate determination.

Corn nitrogen rate recommendation tools’ performance across eight US midwest corn belt states

“…The estimation was as follows: where N Fert was the treatment N fertilizer rate (by plot); N Irr was the inorganic N applied through irrigation (site level); N min was the potential N mineralization measured (by replication block) (Clark, ); PPNT was the preplant soil NO 3 –N in the profile (0–0.90 m; by replication block); N uptake was the measured above‐ground grain and biomass total N at plant maturity (by plot); and N roots was an estimated N content in the roots at plant maturity (by plot). Nitrogen mineralization was measured using the surface (0–0.30 m) PPNT soil samples with a 7‐d anaerobic incubation procedure (Bundy & Meisinger, ; Clark, ; Keeney & Bremner, ). This procedure provides a potential mineralization rate under optimal conditions.…”

Toward Solving the Nitrogen Mineralization Puzzle

“…Laboratory analyses like potentially mineralizable N (PMN) have been proposed to mirror N mineralization in soil health studies, and there are correlations among the proposed laboratory methods (Cappellazzi & Morgan, 2021). However, no combination of laboratory methods predicts actual N mineralization, and studies have found little correlation between these laboratory tests and crop yield or commercial N fertilizer recommendations (Clark et al, 2019;Sullivan et al, 2020). Moebius-Clune et al (2016) wrote "Although inseason soil N testing is available …, employing models that account for the impact of weather on fertilizer needs …, along with soil testing, is likely the future of nitrogen management."…”

United States Midwest Soil and Weather Conditions Influence Anaerobic Potentially Mineralizable Nitrogen

“…The relationship between soil and weather parameters and mineralization values have typically been examined on soil samples taken in the spring before fertilization. However, it has recently been shown that soil sample collection timing, N fertilization, and the incubation period of the PMN an test can affect PMN an by increasing or decreasing it, depending on soil properties and the weather conditions before sampling (Clark, 2018; Culman et al, 2013). The relationship between soil properties and weather conditions and PMN an from different sample timings, N fertilization rates, and different incubation lengths has not been investigated.…”