How can adding organic matter trigger nitrogen deficiency?
Plants depend on microbes to break down organic matter and make the nutrients available to them. Most microbes get energy from carbon compounds such as sugars, carbohydrates, fats, and other substances. Mixing organic material into the soil triggers a feeding frenzy and a burst in microbial growth. To grow, microbes need carbon for energy and nitrogen to build proteins. For every twenty to thirty carbon atoms they consume, they use about one nitrogen atom. If that nitrogen is not available from the newly-added organic material, microbes will take it from the soil, and deprive growing plants of nitrogen.
As a rule-of-thumb, materials with C:N ratios less than 30:1 will not trigger temporary nitrogen deficiency.
The nitrogen is not lost from the soil - it is still present in the cells of microbes - but plants cannot use it. During this initial decay process, microbes are giving off large amounts of CO2 to the atmosphere and the carbon-to-nitrogen ratio of the remaining organic material declines. Microbial activity slows because the remaining compounds are more recalcitrant (difficult to decompose). At this point, nitrogen from the dying microbes becomes available to plants.
