Measuring, modelling and managing soil carbon for future farming

Increasing the quantity and stability of carbon stored in agricultural soils has real potential to offset greenhouse gas (GHG) emissions to the atmosphere. However, realising this potential is not as straightforward as it might first appear.

Firstly, soil carbon stocks are notoriously difficult to measure. Agricultural soil carbon stocks vary according to a number of factors including soil type, climate, geographical location and current and previous management practices, there is a lot of spatial variability and changes with time are slow. It just isn't possible to measure the carbon stored in every tiny bit of soil in every hectare of pasture. So measurements of soil carbon are taken at representative point locations, then estimates of soil carbon need to be made for all of the space in between those points. Consider that there are 11 million hectares of grassland in NZ and you can see why understanding of soil carbon crucially relies not only on measurements but also models that help interpolate between measurements and understand changes over time.

Given the challenges in accurately determining current soil carbon stocks, working out the impacts a certain farm management practice has on soil carbon stocks is also incredibly challenging. The complicated dynamics of soil carbon, that is how long the carbon actually stays in the soil is a specific form, adds another layer of complexity.

Taking into account these challenges, previous NZAGRC-funded work has used data mining and modelling to quantify current levels of soil carbon and determine the maximum amount that could be stored. In many places NZ grassland soil carbon levels were shown to be relatively high, due to the short time since forested sites were converted to pasture. Whilst this is good news, it means that the potential to increase stocks may be limited and it is very important to ensure that farming practices do not deplete the stocks that we already have over time.

NZAGRC Principal Investigators Dr David Whitehead (Landcare Research) and Prof Frank Kelliher (AgResearch/Lincoln University) have taken all of this into account whilst planning the NZAGRC soil carbon programme out to 2017. "The focus of the programme to date has been to use experimental measurements and modelling to test the ability of potential management practices to manipulate the rates of input, incorporation and stabilisation of carbon in soils", says David. "We need to continue to develop opportunities that will provide farmers with practices that can minimise soil carbon losses and increase gains".

The team note that strategies need to be both practical and cost effective to be adopted. With this in mind, a key focus of the updated programme in on two experimental farm sites in the Waikato and Canterbury. "We are really excited to bring our expertise to work on these farms", says David. "The sites are set up to be able to continuously measure a wide range of factors. We can carefully manipulate the system by, for example, planting different pasture species or irrigating or adding nitrogen fertiliser and then see how the change affects the whole farm carbon balance." The large amount of data generated can then be used to estimate conversion of carbon inputs into soil carbon stocks. Additionally, modelling can be used to forecast what the combined effects of different management strategies are likely to be on soil carbon. The end goal is to be able to provide farmers with practical, meaningful advice on how to best manage their farms to maximise soil carbon stocks.

"We can't forget the measurement problem though", adds Frank. "In order to really understand how soil properties and management practices affect long-term storage of soil carbon, we need the best tools possible, and this means next-generation computer models. Changing practices can have long lasting impacts on soil. With the best will in the world, it's difficult to take experimental measurements on the same site over decades. Even if we could, understanding the sheer volume of data generated in light of all of changes over that time period would be daunting". The goal of Frank and his team's work on developing new tools is to identify the soil properties and grassland management practices that most affect soil carbon stabilisation and vulnerability to loss over the short and long term. The overall aim of the NZAGRC programme is to increase stabilised soil carbon stocks as our farmers head into the future.

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