Pushing the limits of fertilizer N recovery in a semiarid climate with improved N management

A common goal of nitrogen (N) fertility programs should be to provide for the highest recovery of applied fertilizer N in the targeted crop species consistent with maximum yield and economic returns. To improve fertilizer N recovery in semiarid climates, such as Montana, consideration should be given to the use of NBPT (urease inhibitor) with urea or inclusion of nitrate-N sources. Previous research has shown that surface applications of urea made during the over winter period (December to March) are susceptible to volatile ammonia losses. This study was conducted to contrast fertilizer N recovery, yield and grain protein (or % N) of wheat from different N sources including urea, urea augmented with NBPT, urea augmented with a nitrification inhibitor (NI = nitrapyrin) and NBPT, ammonium nitrate, and sodium nitrate NaNO₃. The goal was to assess fertilizer N management strategies that would improve fertilizer N recovery.

In 2017, a winter wheat field trial was established at a rain-fed field site at the MSU-Central Agricultural Research Center (CARC) near Moccasin, Montana. The N fertilizers were applied as a spring surface application soon after re-growth of the wheat after breaking winter dormancy. Fertilizer N recovery was estimated using ¹⁵N isotope enriched (~5%) fertilizer materials. Growing season conditions were characterized by below normal rainfall that gave lower than normal crop yields. The winter wheat yield and grain N concentration were responsive to N fertilization as pre-plant soil tests revealed low indigenous soil nitrate concentrations (< 40 lb available N/A). We found that ¹⁵N fertilizer recovery in grain and total biomass, or fraction of N in the crop derived from ¹⁵N-labelled fertilizer was generally higher for NaNO₃ compared to other N sources. We attributed this response to two factors: (1) volatilization of ammonia from the urea fertilizers; and (2) preferential microbial immobilization of N applied as ammonium compared to nitrate. The impact of volatility on ¹⁵N fertilizer recovery was evident from the addition of NBPT to urea. The addition of NBPT improved fertilizer ¹⁵N recovery in the above-ground biomass by almost 11%. The results of this trial were consistent with previous trials at N-responsive sites by finding that fertilizer N recovery from nitrate sources are often greater than from urea in Montana’s semiarid climate. A final project report will be prepared in 2018.