Scientists are to begin a long-term nationwide study that will assess, for the first time, whether soil carbon stocks on New Zealand agricultural land are increasing or decreasing.

The data generated will help New Zealand more accurately meet its greenhouse gas emission reporting obligations under international climate change agreements and indicate to the primary sector where it should focus its efforts if it wants to maintain and/or increase soil carbon stocks, according to the NZ Agricultural Greenhouse Gas Research Centre.

NZAGRC director Dr Harry Clark says soil carbon is critical for soil health and function as it influences soil structure, nutrient cycling and water retention. It’s also an important factor in climate change.

“Globally, there’s more carbon stored below ground in soils than above ground in plants and in the atmosphere combined.

“For this reason, practices that remove even a small percentage of carbon dioxide from the atmosphere and lock it up as carbon in the soil could have a very beneficial effect on atmospheric greenhouse gas concentrations.

“Conversely, practices that deplete soil carbon and release carbon dioxide back into the atmosphere add to greenhouse gas emissions from other sources.”

Benchmark stocks

The NZAGRC has allocated funding for the first phase of the study, which will benchmark soil carbon stocks at representative sampling locations on agricultural land throughout the country. Subsequent phases will benchmark at additional locations and monitor how stocks are changing over time.

Harry says monitoring change over time is key from a climate change perspective. “We want to know whether soil carbon stocks are rising and potentially reducing concentrations of atmospheric carbon dioxide or falling and adding to those concentrations.

“This study will provide that information in much more detail than we’ve ever had before and provide greater insight into the conditions and practices that lead to increases or decreases in soil carbon stocks.

 Broadly, for grazed pastures on flat land in NZ, current evidence suggests soil carbon content has not changed in the last two to three decades, except for drained peat soils where carbon continues to be lost, says Harry.

“There is some evidence that hill country grassland soils gained soil carbon between about 1980 and 2010, but it isn’t clear how widely spread these gains are and whether they’re ongoing. The study will give us a much clearer picture of what’s happening across the country.”

Soil carbon levels vary significantly over time and from place to place, even within a single paddock. Weather, climate, soil types, land use and farm-management practices are all important influences.

Current data suggest carbon stocks are already generally high under NZ pastoral soils, compared to soils in many other countries, for several reasons.

High starting point

These include NZ having a temperate climate resulting in continuous inputs of carbon into our soils. The chemical and physical properties our soils meaning they generally have a large capacity to protect carbon from being released back into the atmosphere. And our soils have generally been well managed with little continuous intensive tillage and cropping, a practice that’s decreased soil carbon in many other countries.

“From this high starting point, it’s considerably harder to add to NZ’s soil carbon stocks than it is in many other parts of the world, where more challenging environmental conditions and/or long-term intensive cropping have resulted in a low baseline of soil carbon levels,” says Harry.

Study lead, Dr Paul Mudge of Manaaki Whenua – Landcare Research, says in total 500 sites will be sampled covering short rotation cropland, perennial horticulture, dairy, flat-rolling drystock and hill-country drystock. About 100 monitoring sites will be established within each land-use type, and strict site-selection, sampling, analysis, storage and data-management protocols will be followed to ensure results are robust and comparable.