Nitrogen fixing in leguminous plants.

Atmospheric nitrogen is notoriously unreactive having a very strong triple bond between the two atoms in the molecule yet it is vital for all living organisms.

Rhizobium bacterioids in nodules on pea roots

The bacteria and the archaea are probably the only organisms that can ‘fix’ nitrogen from the atmosphere.  Some of these bacteria are always free living like the Azotobactor.  However, it is suggested that free living nitrogen fixing bacteria do not contribute significant amounts of nitrogen to the soil.

Rhizobia are free living heterotrophic bacteria that can form symbiotic relationships with leguminous plants.  There are many different species of Rhizobium; some which can form symbiotic relationships with many plants and some which are specific to certain plants.  They are nitrogen fixers but only when in a symbiotic relationship with a leguminous plant.

Some non leguminous plants can form symbiotic relationships with other genera of nitrogen fixing bacteria; such as alder with Frankia alni.

Rhizobia are able to fix nitrogen from the air using the enzyme nitrogenase.

Rhizobium bacterioids in nodules on the roots of Vicia Faba 
broad beans.

These  bacteria are able to produce ammonia from atmospheric nitrogen and plants can use this to make amino acids and eventually proteins.  Nitrogen fixed by bacteria can also be used to make plant nucleotides.

Nitrogen is used to make amino acids that are the building blocks of proteins.  The majority of cell components in bacteria, fungi, plants and animals are made from proteins.  Contrary to the common misconception, lettuce does contain protein.  And protein contains nitrogen.

As the nitrogen fixed in this way is essential to the plant and bacteria, none of this initially fixed nitrogen is lost to the rhizosphere or the bulk soil. However, nitrogen is lost as proteins when cells are sheared off during root elongation and  by protein rich root exudates.  There is a great turnover of roots and when they die they add nitrogen to the soil in the form of proteins and nucleotides.  There is some suggestion that nitrogen fixed in this way can be transferred to other plants through the hyphae of micorrhizal fungi.  I'll go along with this.

Broad bean (Vicia faba) root with nodules.

The suggestion that a lot of the nitrogen in the plant is transported to the seeds when they are formed is more or less true but it does not mean that the plant is denuded of protein. That's why we eat peas and bean seeds because they contain lots of protein.  And protein contains nitrogen.   Even in the most emaciated plant there is still some valuable nitrogen and more than most in legumes and other nitrogen fixing plants.  There is also noticeable amounts of nitrogen, in the form of proteins, in the most woody parts of trees.  Every drop helps as the vicar said as he peed into the sea.

So the tops of legumes are particularly valuable and worth digging into the soil.  The tops of any plants are worth digging into the soil or composting for their nitrogen and other nutrients.  What would you do with them anyway?  I've been doing this for over fifty years now and it is second nature and perfectly clear to me that recycling nutrients is really self evidently worth while.

Don't burn allotment waste because all your nutrients are going up into the air as nitrous oxide, nitrogen oxide, sulphur dioxide and carbon dioxide.  You are probably loosing lots of phosphorus and potassium in the smoke.  Burning is a very quick way of reducing the fertility of  your allotment soil.

The Phaseolus species of beans originated in South America but the Rhyzobium bacteria that is associated with it has come to Europe and is probably ubiquitous now.  Similarly with lots of the other specialist Rhyzobium bacteria.  After the crop of seeds has been harvested it leaves the old plants which should be dug into the soil to add nitrogen in the form of proteins because proteins contain nitrogen.

The greatest amount of nitrogen is added to the soil when the plant dies or is dug into the soil. The proteins and nucleotides in leaves, stem and roots are decomposed by bacteria in the soil releasing the nitrogen originally fixed by rhizobia.  The releasing of nitrates by decomposition is called mineralisation.  This nitrogen enters the soil as soluble nitrate compounds and is available for other plants to take up.

A proportion of the Rhizobia bacterioids  in the root nodule escape back into the soil when the host plant dies. Rhizobia return to a heterotopic lifestyle in the soil and cannot fix nitrogen when they are free living.

The root nodules are not little bags of nitrogen.  They are 'bags' of rhizobium bacteroids and most of the atmospheric nitrogen that they have fixed has been passed to the plant as ammonia and converted into amino acids and these are converted into proteins. And proteins contain nitrogen.

So, this is why you need to dig in the tops as well as the roots of your peas and beans.  Do not burn the tops or you will loose all your hard earned nitrogen in the form of nitrogen oxide gases.

If nitrogen fixation through leguminous plants is used and fixation process is working efficiently then much less nitrogen fertiliser is needed for replacement due to crop removal.

Soil fertility can be raised by residual nitrogen being retained in the soil after harvesting.

Crops grown in rotation with or as a mixture with legumes can utilize nitrogen.

There is much less loss of nitrogen due to leaching when nitrogen fixers are used to increase soil nitrogen.