So, what makes soil carbon so important?

10 June 2021

Soil carbon is critical to soil health - it's the glue that helps hold the soil together.

Soil carbon is critical for two key reasons - the climate and soil health. Soil carbon is the basis of the soil food web (it's vital for all the earthworms, bugs and microbes) and it plays an important role in making nutrients available for plants.

Soils contain a very large amount of carbon and so even small changes (increases or decreases) affect CO₂ concentrations in the atmosphere.

So, how do we get carbon in the ground?

The carbon cycle - the image above shows how carbon enters and leaves the soil.

Carbon enters soil in various ways - it comes via plants through photosynthesis and enters via roots, decaying above-ground plant matter, as well as from animal dung and urine produced when plant material is eaten.

Soil organisms such as worms, beetles and microbes actively feed on these plant-derived carbon sources and respire much of it back into the atmosphere as carbon dioxide.

However, a small proportion of carbon becomes tightly bound to the surfaces of soil particles or encapsulated in clumps or aggregates of soil particles. When it's in this state, the carbon is physically or chemically protected and less accessible to microbes and the other organisms wanting to feast on it. When it's in this state, it's said to be ‘stabilised’ and can remain that way for tens to hundreds of years.

It's important we know what's happening to carbon levels in New Zealand, because changes in soil carbon could contribute to our national carbon footprint.

But New Zealand has good carbon stores in its soil, so what's the problem?

We need to find ways to maintain or increase the carbon in our soil for two reasons. It keeps the soil healthy, and because if we can take carbon out of the air and store it in the soil, that's a reduction of carbon in the atmosphere.

An international study has determined that an annual increase of just 0.4% in the soil carbon stocks in the top 30-40 cm of soil, would significantly reduce the carbon concentration in the atmosphere that humans cause.

Globally that's what we are interested in doing. In New Zealand, in particular, the National Soil Carbon Monitoring Project, a collaboration between Manaaki Whenua and the University of Waikato, and funded by the New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC) is assessing whether soil carbon stocks under New Zealand's agricultural land are increasing or decreasing, and how land use contributes to that change. The project will generate data that will help improve our estimates of carbon stocks and stock changes within a particular land use. Data will also improve our ability to predict how soil carbon stocks are likely to change when land use changes.

Dr Paul Mudge, Manaaki Whenua Soil Scientist

Looking after our soil is the first step to reducing greenhouse gases. Getting better data is the best way to understand how New Zealand’s agricultural soil carbon stocks are changing within different land uses.

It's really important that New Zealand not only hangs onto the carbon stocks it has, but also that it finds ways to increase the amount of carbon in its soil.

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The first phase of the study is to benchmark soil carbon stocks at 500 sites spread across five broad land use classes - cropland, perennial horticulture, dairy, flat-rolling drystock and hill-country drystock farms.

Between 2019 and 2030, each of the 500 sites will be sampled three times and soil samples sent to the laboratory for processing.

The samples need to be weighed, dried, and sieved so the organic carbon can be analysed and its carbon stocks calculated.

Strict site-selection, sampling, analysis, storage and data-management protocols will be followed to ensure results are robust, comparable and available.

To measure soil carbon stocks at a particular point in time in one particular location, researchers are careful to sample a known volume of soil, usually, a core of at least 30 cm.

This sampling intensity is designed to detect a minimum change of 2 tonnes of carbon per hectare.

As these sites are resampled over time, it will allow us to understand if and how our carbon stocks are changing under different agricultural land uses.

The jury is still out on whether we can actually increase our soil carbon significantly in New Zealand.

We know that some changes in land use (eg: conversion of cropping to pasture) can increase soil carbon, but within a given land use (eg pasture) there aren't yet any management practices that have been proven scientifically to increase soil carbon stocks consistently under New Zealand’s varied physical environments and land uses.

But research is underway to identify management practices that maintain or increase soil carbon, whilst at the same time reducing other greenhouse gases (particularly nitrous oxide).

If you have questions about how you can assess greenhouse gas elements on your farm, talk to the team from Toitū.

They are the experts of carbon certification and can advise you on how to develop a robust environmental management system on your farm.

Toitū will help you accurately estimate your greenhouse gas emissions, and put in place strategies to manage, reduce and offset your impacts.