Enhanced efficiency fertilizers had little impact on yield

In this 2-year study, there was no significant benefit of a urease inhibitor, with or without a nitrification inhibitor, on canola and wheat N removal, N uptake, nitrogen use efficiency or yield. These inhibitors also did not significantly reduce the amount of leached nitrate in the soil.

Enhanced efficiency fertilizers are used to reduce nitrogen (N) losses from ammonia volatilization, nitrate leaching, nitrous oxide emission,  and dinitrogen gas emission.

Urease inhibitors (N-(n-butyl) thiophosphoric triamide, NBPT) have been used to prevent volatilization losses of N. Nitrification inhibitors (NIs) inhibit the activities of ammonia-oxidizing organisms that convert ammonium to nitrite, to reduce N loss due to nitrate leaching and nitrous oxide emissions. NIs include dicyandiamide (DCD), 2-chloro-6 (trichloromethyl) pyridine (commonly known as nitrapyrin, NPN), 3,4-dimethyl pyrazole phosphate (DMPP), and 2-amino-4-chloro-6-methyl pyrimidine (AM).

A 2-year research study was conducted to determine which of the four common nitrification inhibitors (DCD, NPN, DMPP, and AM), when combined with NBPT, maximizes yield, N removal, and N uptake, as well as reduces nitrate leaching from surface-applied urea fertilizer.

The Manitoba research was conducted at Carman on a sandy loam soil and Portage la Prairie  on a clay loam soil in 2019 and 2020. The Carman site had an annual cereal-canola rotation, and a perennial grass system. In the fall of 2018, soil nitrate-N tests found an average 26 lbs/ac (29 kg/ha) residual N at Carman. The Portage site had a canola-wheat-soybean rotation, and soil residual N averaged 84 lbs/ac (94 kg/ha) nitrate-N.

Nine fertilizer were surface applied at 71 lbs/ac (80 kg/N ha) on plots seeded to canola in 2019 and wheat in 2020. Nitrogen treatments were broadcast in the spring following seeding:

Control – no fertilizer
Untreated urea,
NBPT-treated urea (Agrotain Advanced),
NBPT + DCD
NBPT + NPN
NBPT + DMPP
NBPT + AM
Super U (NBPT + DCD)
Urea + ARM U Advanced (NBPT + DMPP).
Plots at Carman had lysimeters installed to measure water and nitrate leaching.

Carman received 32% less precipitation than normal during May to August of 2019 and 38% less in 2020.  Portage received 40% less precipitation than normal during May to August of 2019 and 38% less in 2020.

EEFs ineffective
Averaged over all fertilizer treatments, addition of N fertilizer significantly increased yield over the control without added N at the 4 site-years. Yields, though, in both years were impacted by the lower precipitation.  In 2019, canola yield was 99% higher at Carman and 18% higher at Portage with added N fertilizer than without added N fertilizer.  In 2020, wheat yield on N fertilized plots was 72% higher at Carman and 60% higher at Portage than the unfertilized control.