Blaming soil carbon for N2O emissions?

December 13, 2012 Louisa Kiely

There is a movement afoot to blame soil carbon for nitrous oxide emissions from soils. You might have read this somewhere recently: “The carbon (organic matter) content of a soil is a major driving factor in the amount of N2O it can emit. Farming systems that produce large amounts of carbon, either as pasture or crop residues, have the potential to emit higher levels of N2O. This is because the carbon provides energy to bacteria that carry out the denitrification process. Preliminary research from the Nitrous Oxide Research Program has found that in some regions retaining crop residues can lead to high N2O emissions.”

Are they recommending to farmers that they stop building carbon rich soils? Are those naughty N people trying to burst soil carbon’s balloon with yet another reason why increasing soil carbon is bad for you? Well, here is another perspective from a group of eminent scientists that includes Prof. Peter Grace from QUT: “To date the vast majority of evidence supports nitrogen input as the most robust and reliable default proxy for calculating N2O emissions.”* So it is the amount of N applied that determines how much N2O emitted.

It wasn’t an increase in soil carbon levels that caused the N2O curve to climb steadily for 40 years, obviously. Soil carbon levels have been falling ever since the first plough bit into the virgin soils of the Great South Land. It was inputs of N that caused the N2O spike, not soil carbon. The fact that reducing application of N is standard advice now in outreach and training programs. American farmers are even now still being encouraged to over-use N fertiliser “with the common practice of producers to apply N fertilizer rates based upon recommendations derived from yield goal calculations known to overestimate crop N needs.”

Yield goal estimates? What are they? “Since the 1970s it has been common practice… for producers to apply rates of N fertilizer based on recommendations derived from yield goal estimates. The agricultural departments of land grant universities and state agricultural organizations have typically endorsed yield-goal N fertilizer rate recommendations. These organizations are the most common source of external information and advice for producers” say Millar et al. The practice of over-prescribing N inputs by advisers was so widespread that the Methodology for reducing N fertilizer use on crops is accepting these levels as a business-as-usual scenario for proving additionality. 

* Millar, N, G.P. Robertson, A. Diamant, R.J. Gehl, P.R. Grace, and J.P. Hoben. 2012. Methodology for Quantifying Nitrous Oxide (N 2 O) Emissions Reductions by Reducing Nitrogen Fertilizer Use on Agricultural Crops. American Carbon Registry, Winrock International, Little Rock, Arkansas This methodology developed by Michigan State University (MSU) with support from the Electric Power Research Institute.

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