Buckwheat Atmospheric Nitrogen Fixation

Legumes fix nitrogen from the atmosphere, some are far more efficient than others.  Some legumes sequester a tiny percentage of the nitrogen they consume, others sequester significantly more than they consume.  Buckwheat is not a legume.  

There are a few non-legumes that are known to sequester significant amounts of atmospheric nitrogen.  

I have heard that buckwheat can sequester atmospheric nitrogen.  I never thought much of it because there are a lot of garden myths and lots of garden nonsense around, and everywhere I read about this claimed it as a fact but provided no evidence whatsoever. 

I started searching and found many studies that demonstrated the ability of buckwheat to enrich soil with major nutrients, in particular, phosphorus and nitrogen, after being grown as a cover crop.  From those papers I was convinced that buckwheat can and does enrich soil, but I was still not convinced if any fixation of atmospheric nitrogen was happening.  

While the mechanism behind increasing soil nitrogen probably doesn't matter a great deal to the average home gardener, I had a lot of questions regarding how buckwheat increases soil nitrogen.  

From my understanding there are three possibilities with buckwheat regarding how soil nitrogen is increasing after it is grown.  Perhaps one or even a mix of all three is occurring.

1. Deep roots can gather resources from lower in the soil/subsoil.  This is transported into its leaves and stems that mulch down later and become available in the surface layers of soil.  
2. Chemically changing the form of resources already present in the soil so that they are now available.  I believe this is probably what buckwheat does with soil phosphorus.  
3. Sequestering nitrogen from the atmosphere through bacterial action on and/or near its roots.  The bacteria may colonise and live on the roots, or the roots may secrete something that feeds the free living bacteria living near by in the soil.  

Then I read a study that demonstrated significant increases in soil nitrogen after growing buckwheat as a cover crop.  Page 32 states: 

“Despite the fact that buckwheat is not a nitrogen scavenger, concentration of nitrogen in soil significantly increased in both soil layers indicating stimulation of biological nitrogen fixation by bacteria in the rhizosphere”.  

The study mentioned above seems to indicate that there is an increase in available nitrogen in the soil after using buckwheat as a cover crop.  This demonstrated significant increases in soil nitrogen after growing buckwheat.  

The paper went on to mention bacteria in the rhizosphere as being one possible reason for the increased soil nitrogen.  There are several possible reasons behind the increased soil nitrogen, these possible reasons were not explored in any detail.  

My buckwheat flowering

After this I also read a few really old papers, one was titled Nonsymbiotic Nitrogen Fixation in Soils of a Semi-Arid Region of North China.  This paper suggested that active non-symbiotic fixation of nitrogen might be an important factor strangely high yields obtained in poor semi-arid soils in Northern China.  

I then found and read a few published peer reviewed papers that make similar claims of increased soil nitrogen after planting buckwheat.  Some of these papers mention which bacteria are likely responsible for this phenomenon.  

Interestingly enough, buckwheat is often considered to be non-mycorrhizal, yet I was able to find several reports of arbuscular mycorrhizal colonisation of buckwheat roots (such as https://link.springer.com/article/10.1007/s00572-008-0181-6), and vesicular–arbuscular mycorrhiza in buckwheat (such as https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=ea93a54578db5ab2a1a813f7697e895992bea0d9).  Mycorrhizal action could account for some of the increased phosphorus that is found in soils after buckwheat is grown as a cover crop.  

Then I stumbled across a thesis written in 1997 published on the FAO website.  This was titled: Nitrogen fixing microorganisms of the buckwheat rhizosphere and their influence on the plant productivity.  

It was a fascinating read.  From this thesis it sounds as though if buckwheat is properly inoculated, it fixes a significant amount of nitrogen from the atmosphere.  It also states that the grain yield is also increased by 15-20% when inoculated.  This is intriguing as it is the first time I had read anything reliable that claims buckwheat fixes atmospheric nitrogen.  

Buckwheat does not form root nodules, but that does not exclude it from fixing atmospheric nitrogen as there are a number of non-nodulating legumes that also fix nitrogen.  This thesis indicates that there may be significant bacterial nitrogen fixation occurring in the rhizosphere, and demonstrated a significant yield increase as a result of that nitrogen fixation.  

Buckwheat setting seed

Based on the above, it is safe to say that buckwheat does play a role in fixing a significant amount of atmospheric nitrogen.  Sadly, the amount of atmospheric nitrogen that is fixed by buckwheat during its short life appears to be unknown due to a lack of study in this area.  

Given the massive economic significance of buckwheat, combined with how incredibly nutritious buckwheat is, along with it being a major crop that feeds a massive number of people who eat it as a staple food, I find it odd that this phenomenon would not have been studied a whole lot more.  I wish I knew more.