Seedbed: Understanding the carbon to nitrogen connection
Thursday, June 10, 2010
Being aware of the relationship between these two elements can help you better manage your soil, whether you are using organic or mineral sources of nutrients.
by KEITH REID
With the widespread use of nitrogen fertilizers, it is easy to forget that carbon and nitrogen are inextricably linked in the soil and in the plants that grow on that soil. What affects one will inevitably affect the other. Understanding the relationship between these two elements can help you better manage your soil, whether you are using organic or mineral sources of nutrients.
The first thing to remember is that the bacteria, fungi and other micro-organisms in the soil have about eight parts carbon (C) to one part nitrogen (N). These are the critters that are feeding on crop residues, manure and compost, as well as on the organic matter already in the soil.
When a fresh source of food is added to the soil (for example, the carbon compounds in corn residue), the microbes immediately begin feeding. Some of the carbohydrates in the residue are respired as carbon dioxide and water, in the same way that we exhale the byproducts of respiration, and some is used to make more microbes. Unlike humans, the microbes don't get bigger when they overeat, but they do multiply, so the number of microbes rapidly increases but always with that eight parts of C to one part N.
What happens in the soil during this growth phase depends on the relative amounts of C and N in the residue (the C:N ratio). If there is a lot of carbon in the residue
(a high C:N ratio), the growth of the microbes is limited by the amount of nitrogen, unless they can find some in the soil. This is where the vast number of soil microbes works in their favour, since they can scavenge any mineral N from the soil much more efficiently than plant roots.
As the population of microbes expands, the concentration of ammonium or nitrate in the soil gets pulled down to very low levels, and any plant trying to grow in that soil can suffer from N deficiency. Eventually, the food for the microbes runs out and they begin to die off, to be eaten by the survivors until they too run out of food and are eaten in turn. During this collapse of the population, the nitrogen that was tied up by the microbes gets released back into the soil in forms that plants can use. At the same time, the husks of the microbial bodies, after repeated digestions, are eventually reduced to an indigestible blob that we refer to as humus. This is a complex mixture of carbon, nitrogen, oxygen, sulphur and other elements, with a C:N ratio of about 12:1.
If we add a material like poultry manure, which has a much narrower C:N ratio than corn stover, the microbes can get all the nitrogen they need from the added material, so they don't need to pull N from the soil. This is why the N from manure is available to crops much more quickly than from crop residue. The humus left at the end of the process, however, is almost indistinguishable from the decayed crop residue.
So where is the line between net release of nitrogen from an organic material, and tie-up by the microbial population? It may partly depend on the weather, since faster microbial growth under warm conditions will increase the nitrogen absorption as well. It may also depend on the particle size and nature of the carbonaceous material. Small pieces have more surface area, giving more places for the microbes to attack. Woody material with lots of lignin is slower to break down, so it won't tie up nitrogen as severely, but the effect will last longer.
Because of these variables, there is no single "critical value" for C:N ratio, but a range. Materials with C:N ratios less than 20:1 will almost always release nitrogen into the soil, while materials with ratios greater than 30:1 will almost always tie up nitrogen. C:N ratios between 20 and 30 can go either way. BF
Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, based in Stratford. Email: keith.reid@ontario.ca
