Soil carbon stocks and bulk density: spatial or cumulative mass coordinates as a basis of expression?

Abstract

Accounting for CO2 fluxes by determining changes in stocks of soil carbon (C) as a result of land use change is an option for complying nations under the Kyoto Protocol. The 1996 IPCC guidelines for C accounting recommend that soil C stocks to a depth of 30 cm be used in such accounting. However, the soil bulk density often changes with land use and the soil C per unit ground area to a fixed depth will also change even without any change in the mass fraction of C in dry soil. This problem will generally arise when soil C accounting is taken to a fixed depth (i.e. uses ‘spatial coordinates’). For accuracy in determining the land use change effects on soil C, soil sampling should be referred to a fixed dry soil mass per unit ground area (i.e. use ‘cumulative mass coordinates’). There has been intermittent literature‐discussion about this issue over several decades. Methods to accomplish C accounting on a mass coordinate basis, none of them accurate or efficient, have been suggested. Here, we propose a simple, accurate methodology for determining soil C stocks using cumulative mass coordinates, which does not involve repeat sampling trips, nominal specification of the location of boundaries between soil horizons, or independent sampling for determining soil bulk densities. Each core is taken a little (say 10 cm) below the nominal mass/depth required and the retrieved core is sliced into two at a point a little above the nominal mass/depth (say 10 cm above). An accurate determina…