Discoveries that Changed our Understanding of the Nitrogen Cycle
Since the start of the 21st century, scientists have made several discoveries that will substantially improve farmers’ capacity to recapture applied nitrogen. First, recent studies have shown that nitrogen rich soil organic matter is largely microbial, not plant, residue and is associated with soil minerals. Fertilizer nitrogen cycles into and out of solution throughout the growing season, residing for much of the time in mineral-associated organic matter (MAOM). As soil conditions fluctuate, plant roots respond to and take up nitrogen released from MAOM to support new crop growth and microbial N cycling continues.
Second, our knowledge of the variety of soil microbes involved in nitrogen cycling has changed significantly. For instance, we now know that nearly all nitrification (i.e. turning ammonia into nitrate) in soils is caused by soil Archaea, a distinct and relatively abundant group of soil microbes that was largely unknown until the start of the 21st century. And, we now know that all nitrogen cycling reactions are catalyzed by diverse microbes that respond to their local environmental conditions.
Lastly, the nitrogen cycle is much more complex and dynamic than we previously thought. For example, recent studies have shown that sugars commonly found in root exudates can switch the direction of the nitrogen cycle in soils AND lead to the release of organic nitrogen into the soil solution in a matter of hours. Other studies have shown that the vigor of plant root growth is substantially dependent upon the soil’s organic nitrogen pools, not just the soluble NH4+ and NO3- ions supplied by inorganic fertilizers and in the waste products of soil organisms as once thought.
Together, these recent results of basic science research affirm the importance of creating soil and cropping conditions that promote root growth. Doing so with the tools of the Maximum Farming System®, will improve yields through improved nitrogen use efficiencies.
Optimally Responding to New Discoveries
Farmers can use these new insights to meaningfully change their practices. They need to understand that soil carbon and nitrogen cycles are intimately connected by soil microbes that feed on root exudates. Because plant roots respond to and take up organic as well as inorganic nitrogen sources, farmers will also benefit from more actively managing the carbon cycle in their soils. This will allow them to rely more on soil nitrogen stocks and apply less fertilizer N to optimize crop yields.
“Soil carbon and nitrogen cycles are intimately connected through the actions of soil microbes that feed on root exudates.”
To improve fertilizer NUE, farmers must optimize the air-water balance of their soils and promote root growth year-round. They need to place fertilizer N as efficiently as possible into the root zone when it is most likely to be taken up. They also need to use effective rates and formulations of balanced fertilizers at multiple time points to ensure optimal N uptake by plants. Together these insights form the basis for Maximum Farming fertilizer recommendations.
The discovery of ammonia-oxidizing Archaea helped to clarify the inconsistent results of field trials using nitrification inhibitors. It is now clear why such chemicals were generally valuable only in soils with disrupted soil biology (e.g. receiving full tillage and high rates of ammonia) or where fertilizers were inefficiently applied (e.g. to the surface in the spring or incorporated in the fall). However, our current understanding suggests that commercially available nitrification inhibitors will be less useful when banded UAN is applied below the surface at planting and side dress, especially when mixed with certain other nutrients, like K and S, whose presence can improve N uptake efficiency.
The Path to Better Fertilizer Use
From new discoveries, we gain new perspectives and useful ideas. The ongoing research conducted by Ag Spectrum and its network of University and ARS research collaborators provides Maximum Farming System Associates with cutting edge insights that can help them to increase their farm profitability. Such insights about the nitrogen cycle challenge farmers to think differently, but, in the end, they will profit by acting on them.
Contact a Maximum Farming System Associate today to further learn how Ag Spectrum truly adheres to its motto of “Research, Respect, Results.”
-submitted by Dr. Brian Gardener, Technical Director