A conceptual model of carbon stabilisation based on patterns observed in different soils

In principle, greenhouse gas emissions can be offset by increasing soil carbon stocks. Full utilisation of that potential, however, requires a good understanding of the controls on carbon stocks to identify factors that can be modified through management changes and distinguish those from factors that are inherent soil properties that cannot be modified. Here, we present a conceptual model of protected (or stabilised) carbon stocks in soils based on observations from two farms in New Zealand, and from a combined soils data set from observations from throughout New Zealand. These data showed that1) When other factors, such as climate, plant cover and pasture management, were identical, soil carbon stocks were highly, and linearly correlated with the soil's specific surface area estimated from soil water adsorption.2) The slopes of these relationships decreased with soil depth.3) Extrapolation of the relationships to zero specific surface area resulted in relatively small intercepts on the soil carbon axis. These intercepts decreased with soil depth.4) The intercepts were considered to correspond to unprotected labile carbon, with highest contents near the soils surface where most carbon inputs are received by soils.5) Together, these observations implied that virtually all protected carbon in the analysed soils was protected by the soil matrix rather than biochemically, and that mineral surface area was the functionally relevant key attribute that defined the soils' protective capacity.6) It implied that protected organic carbon, Cp, in a soil can be described as, where k is a simple constant, is the total carbon inflow rate into the soil, is specific surface area, and f(T, W, pH, Al, …) is a specific turn-over rate of protected carbon as a function of temperature, soil water, pH, aluminium concentration, or any other factors apart from soil texture that may affect soil-carbon turn-over rates.These observations improve our understanding of the important carbon-protection mechanisms in the soil, with significant implications for the optimal manipulation of carbon input rates into different soils to maximise overall soil carbon storage. They imply that overall carbon storage of soils could be enhanced by physically transferring any available carbon from soils with low to soils with high specific surface areas