Importance of Carbon and Nitrogen to Life
These two elements are the building blocks of all life including the microbial biomass. As the micro-organisms grow they incorporate the carbon and nitrogen into the biomass and through respiration convert some into CO2. As conditions change and colonies die these elements are released back into the system for the next succession to utilise. A considerable amount of the total Carbon and Nitrogen in the finished product is held and recycled through this biomass. If the composting process is to succeed then there need only be sufficient Carbon and Nitrogen available to promote the respective stages.
Importance of Carbon and Nitrogen in Composting
Most literature places a reasonable emphasis on the ratio of the Total Carbon to the Total Nitrogen (C:N) in compost feed stocks with on average a recommendation of aiming for a balance of around 20-25/1. Sadly whilst there is technically good reasoning behind this it is in fact an over simplification. For it is the relative forms these two elements take and not the total content that is important. “Cellulose and lignin have similar percentages of carbon, and contain only carbon, hydrogen and oxygen, but the structures of the molecules that contain the carbon differ radically. Lignin decomposes far more slowly than does cellulose so that the quality of lignin as a carbon source is much lower than that of cellulose” (Sylvia, Fuhrmann et al. 1999).
Importance of Compost Nitrogen in Agriculture
The supply of nitrogen in soil systems is controlled by the rate of mobilisation and immobilisation of Nitrogen from the SOM and the microbial biomass. Nitrogen is mobilised when it becomes soluble (available to plants) and immobilised when it is not soluble. When mobilised Nitrogen may be utilised by a growing plant, the microbial biomass or leached and lost from the system. Nitrogen is immobilised when it is incorporated into living matter, principally the microbial biomass (but also the growing plant). When a soil contains a high proportion of Soil Organic Matter (plant and microbial remains) and a large active microbial biomass a considerable reserve of Nitrogen is held. As the organic matter is consumed and microbial communities die this nitrogen is released and becomes mobilised (diagram). The rate at which Nitrogen is mobilised and immobilised determines the availability. Along with the Carbon cycle the Nitrogen cycle is one of the most important processes for life.
A typical compost can contain 12.5g Nitrogen per Kilogram of compost. If all this Nitrogen was available then a mere 20 tons would be required to provide the Nitrogen requirements of a hectare of wheat (250kg N-1 ha-1 an-1). However as different feed-stocks, composting process and degree of maturity all influence the Nitrogen content, which could be as low as 2g or as high as 60g, it is necessary to know the Nitrogen potential to develop long term fertility strategies.
Compost Nitrogen: Forms and Availability
Total N content of compost is composed of available Nitrogen (NO3, NH4) and unavailable nitrogen held in a near infinite number of states and forms. Some of these forms are readily mineralised with the Nitrogen content becoming available in the first six months whilst others are resistant and can remain immobilised for decades or even centuries. Up to 20% of the total Nitrogen may become available in the short term (in the first year),a further 20% over the next ten years with the remainder mineralised in 50-100 years.
In the short-term annual dressing with compost to provide 250kg N-1 ha-1 an-1 (20 tons) could result in up 50kg Nitrogen being mineralised each year. With successive applications over the next ten years, contributions from the resistant pools could double this quantity, and in a further 50 years the contribution could match crop requirements resulting in a net Nitrogen balance (Gutser and Claassen 1994; Aichberger and Wimmer 1999; Aichberger, Wimmer et al. 2000).
Compost Nitrogen and mineral N fertiliser
In addition to providing up to 1/5 of the total N content compost has also been shown to have a positive effect of the efficiency of mineral fertilisers.
Mineral fertilisers are highly soluble and if not utilised by the growing crop are prone to being leached, both an economic loss and a possible environmental threat. The active microbial biomass in compost can act as a temporary reservoir immobilising surplus Nitrogen and releasing it continuously over the growing season. The resulting improvement in nitrogen efficiency can result in reducing Nitrogen fertiliser requirements by up to 50% (Maynard 1989; Maynard 1998; Mamo, Rosen et al. 1999; Stoppler-Zimmer, Gerke et al. 1999).
Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011