Improving Nitrogen Management With The Anaerobic Potentially Mineralizable Nitrogen Test

Abstract

The anaerobic potentially mineralizable N (PMNan) test alone or combined with the pre-plant (PPNT) and pre-sidedress (PSNT) nitrate tests may improve predictability of corn (Zea may L.) N needs. Forty-nine N response studies were conducted from 2014-2016 across various soil and weather conditions in the US Midwest to 1), evaluate the influence of soil and weather conditions on PMNan from different soil sampling timings (pre-plant vs. V5 corn development stage), N rates (0- and 180-kg-N ha-1 applied at planting) and incubation lengths (7-, 14-, and 28-d); and 2), evaluate the predictability of grain yield, relative yield, N uptake, and economic optimal N rate (EONR) of corn with PMNan, PPNT, and PSNT alone and combined. The top 30-cm of soil was sampled for PMNan analysis before planting and N fertilization and at V5 in the 0- and 180-kg-N ha-1 treatments. Soil was sampled for the PPNT before planting (0-90 cm) and for the PSNT at V5 (0-60 cm) in 30-cm increments. Soil parameters measuring or associated with greater organic matter increased PMNan along with evenly distributed rainfall while increasing temperatures decreased PMNan. Soil parameters followed by precipitation and air temperature had the best relationships with PMNan. Differences in PMNan due to sampling time, N rate, and incubation length were best explained by soil parameters. Soil and weather variables alone (R2 ≤ 0.41) or combined (R2 ≤ 0.60) did not predict PMNan well enough to estimate PMNan reliably. Including PMNan with PPNT or PSNT as separate variables with the initial amount of NH4+ from the PMNan test improved predictability of grain yield and EONR. The PMNan timing and N rate used varied by soil texture or growing degree-day (GDD) category evaluated. Regardless of the improvements in predictability of crop responses from including PMNan with PPNT or PSNT in addition to other modifiers (separating sites by texture or GDDs, using deeper soil NO3--N sampling depths, delaying soil NO3--N sampling from pre-plant to V5, or including initial NH4+), the predictability of crop responses after N fertilization (a common practice in commercial fields) was not substantial enough (R2 ≤ 0.41) for using these soil tests alone to predict corn N needs.