Plant growth can mitigate some of this temporarily, given that photosynthesis converts carbon dioxide to organic carbon, forming part of the plant’s structure and thereby removing it from the atmosphere. The carbon remains in the biomass of growing plants for the length of time that the plant lives. However, once the plant dies, it is decomposed by soil microbes, during which most of the carbon fixed by plants is released again as carbon dioxide, with only a small fraction remaining stored as soil carbon.

But what if dead plant matter could be prevented from decomposing, therefore locking the carbon contained within it in place for much longer than usual? It is this concept on which soil chemist Professor Marta Camps of Massey’s School of Agriculture and Environment is focused. One option for locking up carbon in plant matter is to convert it to biochar, a stable form of carbon. Biochar is made by heating wood and other plant material slowly and in a low-oxygen environment, forming charcoal. ‘By converting wood to charcoal, the carbon forms stable chemical bonds that microbes are not able to break,’ says Professor Camps. ‘In this way, biochar can permanently sequestar carbon and so helps reduce CO2 emissions to atmosphere.’

However, implementing the production of biochar on a large scale is economically challenging; it requires labour and infrastructure. As a result, research in New Zealand and internationally has been focused on finding a use for biochar that would make its production economically feasible. ‘Scientists found evidence in Brazil that charcoal could increase soil fertility,’ says Professor Camps, ‘and so they had the idea of adding biochar into agricultural soils, increasing fertility while sequestering carbon. It was thought that this could be a solution for New Zealand dairy farmers, because they could just manage their emissions by thinking “my farm is emitting so much methane and nitrous oxide, I could add so much biochar and balance the emissions of my farm, in my farm.”’

Unfortunately, several years of research has shown that adding biochar to soil is too expensive for New Zealand farmers, particularly because it is difficult to increase the fertility of New Zealand soils with it. In Brazil, the soils are very poor, a consequence of millions of years of weathering. New Zealand soils are young and fertile, and so the benefit is not as great. There is enough benefit for investment in biochar in specific niches such as kiwifruit and vineyard growing and waste processing, but dairy farmers are in a different situation.

Professor Camps is now working on other ways to sequester carbon in soils. After the Paris Agreement, interest in increasing soil carbon grew. There are two aspects that contribute to this process — increasing the input of carbon into soil, and decreasing the output (i.e., the breakdown of soil carbon). In agricultural systems, the way to increase carbon input in addition to that associated with increasing plant growth is to add exogenous organic matter, such as compost, manure or biosolids. The output, which mostly occurs through decomposition by microbes, can be reduced by making sure that the organic carbon molecules in the added organic matter interact with clay minerals in the soil. This binds up the carbon-containing molecules, making it harder for microbes to decompose. ‘I’m working on understanding under what conditions these interactions occur or how we can increase these interactions and protect organic matter from decomposition,’ says Professor Camps.

Increasing soil carbon via the addition of animal and human waste to soil does come with some unexpected consequences. ‘For a long time, we have thought that increasing organic matter is a win-win situation, because it helps retain water, and contains plant nutrients such as nitrogen, phosphorus and sulphur, among others,’ Professor Camps explains. However, in some instances, the organic method has drawbacks. Because a large fraction of the organic carbon in manure will be decomposed to carbon dioxide while the phosphorus will be conserved, the calculated rate of manure application to soil should not exceed the crop or pasture phosphorus requirement, otherwise heavy applications will cause phosphorus to accumulate in soil over time, and a small fraction of it can then leach into freshwater systems, leading to algal blooms.

‘In Europe, scientists are examining ways to pretreat biosolids to extract phosphorus so that it can be used more efficiently, thus decreasing the phosphorus footprint while reducing their reliance on the supply from other countries,’ says Professor Camps. ‘In fact, biochar technologies are among the options considered for processing human waste and recycling phosphorus. It’s such a complex system, but it is very interesting to be involved in.’