Can the Anaerobic Potentially Mineralizable Nitrogen Test Improve Predictions of Fertilizer Nitrogen Rates in the Cornbelt?.

Correctly estimating the amount of mineralizable nitrogen (N) can enhance nitrogen use efficiency.  The anaerobic potentially mineralizable nitrogen (PMN_An) test is a tool that may help improve predictions of  N uptake, grain yield, and the economical optimum nitrogen rate (EONR) of corn (Zea mays L.).  A 30 site-year study across eight US Midwestern states was conducted to quantify PMN_An across various soils and regions and to determine the influence of soil sampling timing, N fertilizer, and laboratory incubation lengths (7, 14, and 28 days) on improving the PMN_An correlation with N uptake, grain yield, and EONR.  Soil was sampled from 0-30 cm before planting and N fertilizer application (PPNT) or at the V5 leaf stage where 0 (V5₀) or 179 kg-N ha^-1 (V5₁₇₉) were applied at planting.  Overall, PMN_An ranged from 0.19 to 136 mg kg^-1 with a mean of 36 mg kg^-1.  Mean PMN_An for coarse, medium and fine textured soils were 22-, 34-, and 45- mg kg^-1, respectively.  This indicates greater potential for mineralization with finer textured soils, which in general had greater organic matter content.  There was no consistent pattern in PMN_An with time of sampling.  The PMN_An was larger in ten site-years at PPNT and nine site-years at V5_0, with no difference in the other eleven site-years.  There was also no consistent pattern in response to the addition of N fertilizer.  The V5₁₇₉ PMN_An was lower than the V5₀ in ten site-years, greater in one site-year, and no different in 19 site-years.  All sites increased in PMN_An as laboratory incubation length increased. Preliminary results for the PPNT 7 day incubation indicate that for each one mg kg^-1 increase in PMN_An the EONR was reduced 1 kg-N ha^-1.  Additional results of the correlations between PMN_An and N uptake, grain yield, and EONR will also be discussed.