A recent paper in Waste Management & Research opens up a discussion that may become increasingly relevant in mitigating climate change. As the authors point out:
- More than twice as much carbon is held in soils as in vegetation or in the atmosphere, so understanding the build-up of organic carbon in soil is important.
- The loss of organic carbon in soils has been one of the major consequences of industrial agriculture.
- Composted organic fertilizers offer a huge opportunity to lock up considerable amounts of carbon while restoring soil fertility and health.
Application of organic matter on a regular basis increases carbon content in soils, but at some point the system reaches a steady-state where the mineralization of organic carbon into CO2 offsets the annual accumulation of organic matter. The authors argue that there is considerable potential for a time-limited carbon sequestration in soils in the short/medium term as the system moves toward a stead state. This is important because a short-term reduction in net GHG emissions buys us time and can be very valuable while we move toward long-term emission reductions elsewhere in the economy. In addition, as organic soil amendments replace synthetic fertilizers, the slower release of nitrogen make it less prone to producing N2O. (According to the IPCC, about 1.25% of readily-available nitrogen from fertilizers turns into N2O through denitrification/nitrification. N2O has a global warming potential of 310 relative to CO2 -- so any reduction in N2O is a big deal.)
Unfortunately, we lack a good methodology to take into account these dynamics and the time profile of emissions (see my earlier post on dynamic carbon footprinting). Current life-cycle assessment methodologies perform a static analysis, ignoring the dimension of time. This may be fine for modeling thermal processes which lead to CO2 emissions immediately, but not for biological processes that occur over decades or even centuries.