Q+A - Soil Carbon

The Pastoral Greenhouse Gas Research Consortium is dedicated to mitigating greenhouse gas emissions from the agriculture sector by educating farmers and providing them with the tools to work at reducing their own carbon footprint. The following information is taken from their fact sheets on soil carbon, which show us why it is important and how it can be measured and stored for the benefit of both the agriculture sector and the environment.

A BRIEF INTRO ON SOIL CARBON

Soil carbon is an intrinsic part of everyday life on earth; it is the basis of soil fertility, releases nutrients for plant growth, promotes the structure, biological and physical health of soil, and is a buffer against harmful substances.

Soil carbon results from the interactions of ecological processes - namely the feeding of microbes on decomposing matter - and can be affected by human activities that disrupt these processes.

The important issue is how we humans limit our impacts on these processes and work to keep carbon stored in the soil, rather than let it be released into the atmosphere.

WHY DOES SOIL MATTER FOR CLIMATE CHANGE?

There is more carbon in soil than terrestrial plants and the atmosphere combined. Practices that increase the amount of carbon stored in soils cab offset some of the greenhouse gas emissions from agriculture. Conversely, practices that deplete soil carbon and release it back into the atmosphere can increase emissions from agriculture.

The total carbon content of New Zealand soils isn’t the issue, rather it’s how that total changes over time.

HOW IS CARBON STORED IN THE SOIL?

Carbon is constantly being moved between the atmosphere, plants, and soil.

During photosynthesis, plants algae and some micro-organisms use sunlight to convert CO2 and water into sugars that are incorporated into their cells – some of which are consumed by animals.

Decomposing animal material, plant matter, fungi, worms and an array of micro-organisms contribute carbon to the soil. Most of this matter quickly decomposes as microbes feed on it and release the CO2 back into the atmosphere as they respire.

However, a small proportion of it becomes tightly bound to the mineral surfaces of soil particles, or tightly trapped in soil clumps, where it is protected and less accessible to microbes. It can remain locked away in this ‘stabilised’ state for hundreds of years.

The question is how you increase the amount of carbon stored in soils, therefore reducing the amount released into the atmosphere.

WHAT ARE SOME WAYS OF INCREASING SOIL CARBON?

There aren’t yet any robust rules around reliably storing carbon in New Zealand’s pasture soils.

It is important, first, to maintain current stocks, as carbon can be lost quickly and recovered only slowly.

Beyond that, increasing soil carbon breaks down into two categories:

  •      Add more carbon and stabilise that carbon in the soil.
  •      Stabilise more of the existing input and reduce carbon turnover.

Research has shown that overgrazing reduces soil carbon, as it reduces overall plant cover and carbon inputs via roots. However, under grazing may equally contribute to soil carbon loss.

There are several options for increasing soil carbon currently in practice, though all of them need further research to determine their effectiveness under NZ conditions, and whether they are restricted to certain soil types, climatic conditions or management practices.

The First:

Add Nitrogen - The addition of nitrogen fertilizer or clover fixation might increase soil carbon in the short term, but in the longer term, it reaches a plateau. This option needs further testing, and it must be noted that it may create undesirable side-effects like the production of nitrous oxide.

The Second:

Optimise Irrigation - This should increase soil carbon due to increased plant growth and greater inputs of carbon into the soil, and hence remove carbon dioxide from the atmosphere. However, irrigation also encourages greater soil microbial activity, which in turn would convert this soil carbon into C02 and release it back into the atmosphere. Again, more research is needed.

The Third:

Increase Root Inputs of Carbon - Increasing the amount and turnover of roots should deposit more carbon into the soil, where some of it would be incorporated into stable soil organic matter. This method is currently being researched with tests on different pastures to see which is most effective. Mixed swards and plant species have previously shown to have the greatest root biomass and turnover, but more research is needed, particularly into how different pastures affect milk and meat production.

The Fourth:

Add Biochar - There is strong evidence that Biochar represents a very stable form of carbon, so it could be used to store more carbon in soils. The main challenge at present is the cost of the material, and the wide areas it would need to cover, which make Biochar an economically unfeasible solution for NZ.

WHAT ARE THE BENEFITS OF INCREASING SOIL CARBON?

Scientists estimate that an increase in soil organic carbon stocks of 0.4% per annum would compensate human-induced greenhouse gas emissions on a global basis. Soil carbon also provides a source of nutrients, helps particle aggregation, increases water storage and protects from soil erosion and compaction. These factors can significantly improve food production.

HOW DO WE MEASURE IT?

Measuring soil carbon and how its levels change over time, is a costly and labour-intensive exercise. This is because soil carbon can vary significantly from paddock to paddock and year to year.

Traditionally it is done by extracting soil cores and analysing their carbon content in a lab. However, this method limits the ability to make a credible farm-scale measurement.

If data points are too sparse, they could give a misleading picture of the average or total soil carbon stocks across an entire farm.

If we are to increase our ability to characterise and monitor these changes in soil organic carbon stocks, we need to develop rapid, practical, accurate, and cost-effective methods that combine spot measurements with robust tools to interpolate between data points across the diverse landscapes spanned by typical New Zealand farms. Recent research is opening exciting and cost-effective opportunities to make farm- and paddock-scale sampling and estimates of soil carbon more affordable and accurate.

For example:

  • Soil Spectroscopy: Soil Spectroscopy is a sensing technology developed to accelerate the prediction of soil properties. It relies on the fact that the reflectance of light from a soil surface is related to the bonding and stretching vibrations of molecules in the soil. The issue with this method is that it’s time and cost-effective. However, Internationally, soil spectroscopy is acknowledged as a major advance in the estimation of soil carbon, and other soil properties, allowing many more values to be estimated for the same time and cost as traditional analytical methods.
  • Digital Soil Mapping: Digital Soil Mapping uses environmental datasets along with advanced modeling methods (e.g. geostatistics, data mining) to develop spatial models of soil properties such as soil carbon. This method has been employed successfully in the Hawkes Bay Region and is being evaluated by regional councils throughout the North Island.

CONCLUSIONS

  • Soil carbon supports healthy and productive farm systems, and greater understanding of the distribution and changes in soil carbon across farms can help farmers adjust and improve management of this essential resource.
  • Soil carbon sequestration could offset some greenhouse gas emissions that are currently difficult to reduce otherwise
  • New soil spectroscopy and digital soil mapping technologies help reduce the uncertainty of soil carbon stock and stock change predictions, enabling soil carbon stock changes to be monitored at farm scales

As global warming continues to intensify, it is essential that emission-reducing strategies like these are studied, tested and employed.

For more information on soil carbon, visit www.nzagrc.org.nz