12/6/2021 - By Dr. Jeff Atkinson

Each year topics of conversation for turfgrass management come and go. It seems conversations surrounding soil health and soil microbial activity have come to the forefront of many turf manager minds.

I recently received a question I could not immediately answer, "Do nitrogen stabilizers have a negative effect on soil microbial populations?" Fortunately, researchers previously dove into this question.

Defining Nitrogen Stabilizers

First, it is important to define nitrogen stabilizers. In a simple sense, nitrogen stabilizers are materials that prevent environmental nitrogen loss and enhance efficient plant nitrogen utilization. There are several nitrogen stabilizers available in the market, the two most common in the turf management world are NBPT (N-(n-butyl)-thiophosphoric triamide) and DCD (dicyandiamide). Generically speaking, NBPT slows conversion of urea to ammonium, thus reducing volatilization, and DCD slows conversion of ammonium to nitrate, thus reducing leaching. In addition to reducing nitrogen environmental losses, these materials have the effect of moderating nitrogen availability between applications and thus promote consistent growth, compared to non-stabilized or non-controlled nitrogen release sources.

Understanding Nitrogen Stabilizers

Second, it is useful to understand how nitrogen stabilizers work. For brevity, I will stick with NBPT and DCD. When urea is applied without a stabilizer it is converted to ammonium by a urease enzyme, which is ubiquitous in all soils. Without diving into the biochemistry, the urease enzyme has a specific spot which facilitates this conversion. When NBPT is applied with urea it competes for this spot on the urease enzyme, significantly slowing urease activity and thus the conversion of urea to ammonium. Since NBPT simply competes for urease active sites it does not have a pesticidal effect on soil microbiology.

DCD requires a bit more digging to determine off-target effects on soil microbiology. Under normal conditions ammonium is converted to nitrate by naturally occurring soil bacteria. DCD acts as a pesticide to kill these bacteria and thus slow the conversion of ammonium to nitrate, limit nitrogen losses through leaching, and prolong soil nitrogen availability. Considering the DCD mechanism of action, it is natural to question non-target effects of DCD on other soil microbiology. A paper published in Soil Biology and Chemistry titled, “Effect of the nitrification inhibitor dicyandiamide (DCD) on microbial communities in a pasture soil amended with bovie urine” explored this question. While fertilization of turf with bovine urine is certainly not a typical practice, the conclusions of the paper can be readily applied to turfgrass soil health.

The researchers applied 26.84 lb/A DCD to simulate a ‘worst case scenario’ and were unable to detect a change in the diversity of the soil bacterial community, with the exception of the ammonium-oxidizing bacteria, the bacteria responsible for the conversion of ammonium to nitrate. The authors went on to summarize, "The results suggest that application of DCD to pasture is a relatively benign intervention that has an important role to play in mitigating the environmental hazards imposed by ongoing land use intensification."

As turf managers we own the responsibility to protect the environment while providing high quality playing conditions. Based on the research, including nitrogen stabilizers in a foliar fertility program is an excellent way to decrease nitrogen environmental loss and enhance nitrogen use efficiency while limiting off target effects to soil microbiology.