Monday, 13 June 2011

Carbon sequestration in soil

The Carbon Farming Initiative, currently the subject of hot debate in Australia, aims to include agriculture in a developing carbon trading scheme.  The scheme, intended for introduction this year, aims to help farmers and others earn additional income from reducing agricultural emissions of greenhouse gases (GHG) such as nitrous oxide and methane through modifying farm management.  

A recent report, however, suggests that one of the main ways envisaged for farmers to reduce emissions, carbon sequestration through improved soil management, is not as straightforward as was once thought.

The report, in the Weekly Times Now, suggests that storing and building significant or saleable amounts of soil carbon may not be practical in all soils, climates and crop and pasture systems.

According to farm consultant Graham Lean, "soil carbon is probably not going to be a great source of carbon storage.  And if we lock-up resources for it, we might be locking up even more valuable nutrients than carbon, such as phosphate and others."

However, other carbon sequestration measures and practices that reduce emissions could help.  Carbon sequestration by trees could generate increased agroforestry incomes, and decreasing methane emissions by livestock could decrease carbon outputs.

But carbon and nutrient cycles are strongly linked and farmers need to think about how they balance inputs, from crop production and residue management, with outputs released during soil decomposition.  CSIRO's land and water expert Dr Lynne Macdonald said that while maximising inputs would maximise carbon stored in soil, farmers needed to understand the whole system and realise carbon and nutrient cycling were tightly tied. 

She did say that soil carbon had a big part to play in sustainable agriculture and a place in soil function in structure and chemical aspects, but "we have to think of the bigger picture – carbon and nutrient cycling – not just carbon alone."

"Carbon is bound to nitrogen and phosphorus and other elements, and has come from plant material," Lynne said.  "It comes as a package, because carbon is bound to nitrogen and phosphorus during plant growth, then when it comes back into the soil they are bound and if you turnover (decompose) the organic matter you get mineralisation of the nutrients."  This mineralisation leads to the loss of these nutrients.

"Whereas if it doesn't turn over and you keep it there then those nutrients remain bound.  As a consequence of holding carbon there, [it] will [also] be holding nutrients."