Defining carbon in soil
From the March 2008 edition of Agriculture Today.
There is an argument going around - going something like this: Australian soils are very low in organic carbon and so will benefit from more organic carbon inputs.
While this is generally true for many of our cropping soils, we still have to know just what kind of carbon (C) will give the most benefits.
For this we need to have a better understanding of soil organic carbon and what it does in soils.
Soil organic carbon (SOC) is not all the same - it is not a uniform material but is made up of very heterogeneous mixtures of both simple and complex substances containing C.
Basically, these include organic materials of plant and animal origin at all stages of decomposition as well as the microorganisms in soils.
One has been under no-tillage and stubble retention (conservation agriculture) growing wheat and lupins in rotation and the other conventionally tilled (three passes) and stubble burnt (traditional tillage) growing wheat continuously.
Total SOC is indicated by the size of the circles. SOC under traditional tillage was only 1.5 per cent (compared to 2.5pc of that under conservation agriculture).
Comparing the size of the different C pools (as indicated by the size of the individual slices), all of the reduction in C has occurred in the labile and slow C pools and there was no change in the inert C pool.
Not only is there a reduction in total SOC, more important is the fact all losses have occurred in younger C pools.
Corresponding to these changes in soil carbon, the soil under traditional tillage has lower structural stability, lower nutrient availability and lower biological activities - showing all the signs of a degraded soil.
Averaged wheat yield (last five years) of the degraded (lower SOC) soil was only 38pc of the soil under conservation agriculture (2.9 tonnes per hectare compared to 1.1 t/ha).
The latter is usually a small proportion, less than five per cent of SOC.
For convenience, SOC has been divided into different pools based on composition and ease of decomposition.
Firstly, the labile pool - includes easily decomposable organic materials which stay in the soil for fairly short periods, from a few days to months.
They are important as food/energy sources for soil organisms, as source of plant nutrients, such as nitrogen and phosphorus and for promoting stability of large soil aggregates.
This pool also includes microorganisms, many of which are involved in the actual decomposition and recycling processes.
Secondly, the slow pool - includes the well decomposed and stabilised organic materials, often referred to as humus.
They stay in the soil for many years and are important for stabilising soil structure (microaggregates), improving water holding capacity and retaining plant nutrients, eg. cations.
Thirdly, the inert pool - includes biologically very resistant organic materials which are thousands of years old in soils.
Chemically they are similar to charcoal and because of their charge properties and porous nature can retain cations and improve soil physical properties.
For a healthy soil, the three pools of organic carbon are all present and are needed to serve different functions of the soil ecosystem. Therefore SOC is a good indicator of soil health.
Not only total SOC but the proportion of the different pools is also indicative of the "health" status of the soil.
Organic carbon inputs can restore and increase soil health. Many degraded soils lack labile and slow pools of organic carbon.
To restore soil health, we need to add organic C materials that can replenish the labile pool, to increase food source for soil microbes and provide available nutrients for plants and slow pool, to improve soil structure and soil physical properties.
Manure and composts which are rich in these forms of organic materials have been traditionally used to maintain soil fertility and crop yield increases are often obtained as a result of their application.
Another time-honoured practice of soil organic carbon replenishment is green manuring, the practice of growing a crop which is then turned under the soil.
On the other hand, high carbon materials referred to as biochar or agrichar have been promoted as soil amendments.
As the main constituent of these materials is charcoal, addition of these materials will only increase the inert C pool and therefore on its own may not have any positive effect on soil health and crop yield in the shortterm.
Nevertheless, inert carbon potentially has C sequestration value and might have other benefits to soils eg. liming value, but further research is needed to confirm this.
The author, Principal Research Scientist, Dr Yin Chan, is a world authority on soil carbon, based at NSW Department of Primary Industries’ Richmond office, (02) 4588 2108, yin.chan@dpi.nsw.gov.au
