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Soil Carbon

There has been an increase in the debate and information around soil carbon. As we see it soil carbon can be broken down into three discussion groups:

A key concept to remember is that relative change is important. So if the carbon in your soil or grass is the same now as it was in 1990 then it is already accounted for by the Kyoto protocol.

Soil Carbon for $

Nicole Masters, of Integrity Soils said at a recent soil carbon conference that she finds, ” the principles of carbon farming incredibly exciting, this really is farming for the future! I would like to see farmers be rewarded for their practices as opposed to being taxed on emissions. There are many good examples around the world of farmers being rewarded credits through improving their soil management techniques.”

Excitement and interest in New Zealand has been stimulated by reports of sales of soil carbon overseas. However, while there is scope and benefit from further build-up of soil carbon, potentially including direct financial rewards, this is not unlimited or without risk (see Info sheet on soil carbon). Scientists in Australia and New Zealand suggest caution. Dr Mark Peoples, an Australian scientist with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) states, “while there are many good reasons for farmers to increase soil carbon, carbon trading for profit might not be one of them”.

In an interview with Radio New Zealand, Jacqueline Rowarth Professor of Agriculture at Massey University stated that there are farmers doing a good job of building up soil carbon at the moment but the Kyoto protocol is set up to reward changes in behaviour, so those already building up carbon will have to do so at a faster rate. She went on to state that to bring all of New Zealand’s soils into the ETS under the Kyoto Protocol, we would have to know what the base level was, for all soils, at 1990 to a depth of 30cm. Jacqueline also said that if soil was brought into the ETS farmers could be able to claim additional carbon gains but on the other hand, they would be liable for any carbon losses which could occur with temperature fluctuations, floods and droughts.

Once a set of rules are brought in for a particular source or sink of carbon, then these rules are required to be applied over the whole country.

At present the only market to sell any soil carbon from NZ soils is on the voluntary market (see info sheet 6). Experience with forestry has shown that significant verification and validation is required to sell carbon credits on the voluntary market overseas. The methodology for assessing the stocks and therefore value of carbon in the soil are currently under review in New Zealand . For sales to occur there needs to be a independently verified method of proving that the soil carbon increases are auditable, traceable and durable. It is expected practical systems will evolve as a result of private and government initiatives. The problem of measurement is characterised by the scale of changes. The soil carbon pool under pasture is approximately 300 to 400 tons CO2 e, documented annual changes are in the order of 3-4 tonnes CO2 e. That equates to a 1% change which is impossible to measure accurately. It is likely that changes in the order of 5 to 10% are required before those changes are measureable on a practical scale.

Soil Carbon for Soil Function

Soil is complex and hosts a vast pool of living organisms and while it is not always clearly understood, the benefits of increasing organic matter (carbon) to soil function have long been known. These include preservation of soil structure, improved aeration, water infiltration and water holding capacity, encouragement of earthworms and other soil fauna. Soil biology remains, it seems, undiscovered although some practitioners appear to be moving forward by taking more care and applying biological principles and preparations to enhance soil organic matter and carbon levels. Ongoing studies such as Agriculture Research Group on Sustainability (ARGOS) (click here for their website) are gathering on-farm data to document relative productive efficiency, effectiveness and sustainability (economic and environmental) of conventional and organic farming methods. This will provide good information at a systems level to guide progress in this fundamental area of agriculture.

Organic farming relies on practices such as composting, mulching and fallowing, all of which maintain or enhance soil organic matter, to balance soil nutrients. The devastation of cropping soils in the US Mid-West manifested as wind erosion was a direct result of soil organic matter loss and poor crop residue management. Soil conservation by retaining crop residues and organic matter has become enshrined in US constitution (US Farm Bill, 1985).


Thousands of years ago, South Americans of the Amazon Basin began using charred animal waste and wood to make what the Portuguese called terra preta (black earth). The terra preta soil they created remains fertile for thousands of years without the use of any fertiliser.

Today, we call this “biochar” – a highly porous charcoal made from any form of organic waste – ranging from forest to manure – through a process known as pyrolysis, which is when biomass is burned at 400-550 degrees in the absence of oxygen. This process converts organic matter, which can decay and rot into a very stable form which largely resistant to decomposition and may be permanently stored.

Various techniques can be used to produce biochar including slow pyrolysis, flash pyrolysis, fast pyrolysis, hydrothermal carbonisation and microwave conversion. There is renewed interest in biochar because it is a way of converting carbon sequestered in organic matter such as trees, into a form which can be permanently stored. This allows forests to regrow and reabsorb more carbon dioxide from the atmosphere. Under current Kyoto rules once a tree is harvested it is assumed that the carbon dioxide is immediately emitted back into the atmosphere. This current rule limits a forest to sequestering carbon only for its first rotation. If the timber is converted into biochar then the forest will be able to sequester more carbon dioxide in its next rotation. It is suggested that if biochar is added to soils it can improve the water holding capacity of the soil, and improve general soil performance. A centre of excellence has been established in New Zealand to research these issues (click here more info).

An alternative view is that using organic waste such as wood residues or straw for direct substitution of coal for electricity generation, results in a greater net reduction in carbon discharge, and therefore increased overall net benefit in reducing carbon in the atmosphere. However, it is still early days with the development of policy and science around the process of creating and storing biochar, but there is sufficient interest to investigate this aspect further.