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 has 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 (N2O) 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.
