Sometimes referred to as ‘the key to soil fertility and stability’, soil carbon is an oft-forgotten hero of agriculture. Recently, though, it has gained a lot more publicity, largely because we have realised that capturing carbon - in the soil or biomass - is a valid way to reduce the increase of carbon dioxide in the atmosphere, and so may help to offset some of the effects of global warming.
When scientists talk about soil carbon, they often break it down to four categories: small (<2 mm) crop residues that are found in (and on) the soil; smaller particulate organic carbon found in the soil; humus found in the soil; and biologically stable organic carbon. Each of these fractions has unique characteristics that are of interest to scientists. From a practical farming point of view, though, we can view soil carbon as organic matter that can be further broken down by soil organisms, and stable organic matter that is essentially unavailable for use by the soil biota. This stable matter is normally in the form of charcoal, and today is frequently referred to by the term ‘biochar’.
As well as keeping excess or unwanted carbon dioxide out of the atmosphere, soil carbon – or organic matter – has some significant benefits to farming. Organic matter doesn’t just contain carbon; it also has a range of other nutrients that are released in the soil and can be used to support pasture or crop growth. Organic matter also provides energy that supports the growth and activity of the soil life – worms and bacteria, for instance – that help keep the soil healthy.
What’s more, organic matter improves soil structure, which in turn increases the water-holding capacity of the soil, reduces erosion and reduces the risk of soil compaction. Organic matter in soil also helps to retain soil nutrients (by providing sites for nutrients to adsorb to), so reducing loss to the environment by leaching or run-off.
Depending on future developments with the ETS scheme, soil carbon may also prove to be a source of income – or cost - to farmers (see box).
Compared to countries such as America and Australia, New Zealand soils are relatively rich in soil carbon. This is good for farming and for the environment, but may not favour us in future ETS arrangements. New Zealand pastoral soils typically have around 11% organic matter, whereas much poorer soils may have less than 2%.
Pastoral grazing lands store 51% of the soil carbon in New Zealand, but research has shown that the actual amount of carbon in our pastoral soils is decreasing.
Work comparing recent soil samples with equivalent samples from the same soils taken 20 years ago has shown that the average loss of soil carbon over that time was 1 tonne/ha/year. Why this should be the case is not altogether known, although factors such as cultivation practices and farming intensity may have contributed to the outcome. Certainly increased grazing intensity changes the balance within the carbon cycle, as more carbon is lost as carbon dioxide, and less is returned to the soil.
One sure way to increase the loss of soil carbon is to practice conventional cultivation. Turning the soil over stimulates microbial activity and the breakdown of organic matter, allowing some of the carbon to be released as carbon dioxide gas. Conservation tillage techniques – no-tillage, reduced-tillage, strip-tillage – promote the accumulation or preservation of soil carbon.
Other practices that promote the loss of soil carbon include fallowing, overgrazing and removal of stubble, e.g. by burning. Avoiding these actions will help to conserve soil carbon. Maintaining soil fertility, and so encouraging good pasture species and cover, helps to increase the amount of carbon that is cycled into soil. Conversely, high stocking rates appear to reduce the amount of carbon entering soil. Farm management practices that promote long-term economic and environmental sustainability will also protect soil carbon levels.
