Soil carbon saturation: concept, evidence and evaluation

Abstract

Current estimates of soil C storage potential are based on models or factors that assume linearity between C input levels and C stocks at steady-state, implying that SOC stocks could increase without limit as C input levels increase. However, some soils show little or no increase in steady-state SOC stock with increasing C input levels suggesting that SOC can become saturated with respect to C input. We used long-term field experiment data to assess alternative hypotheses of soil carbon storage by three simple models: a linear model (no saturation), a one-pool whole-soil C saturation model, and a two-pool mixed model with C saturation of a single C pool, but not the whole soil. The one-pool C saturation model best fit the combined data from 14 sites, four individual sites were best-fit with the linear model, and no sites were best fit by the mixed model. These results indicate that existing agricultural field experiments generally have too small a range in C input levels to show saturation behavior, and verify the accepted linear relationship between soil C and C input used to model SOM dynamics. However, all sites combined and the site with the widest range in C input levels were best fit with the C-saturation model. Nevertheless, the same site produced distinct effective stabilization capacity curves rather than an absolute C saturation level. We conclude that the saturation of soil C does occur and therefore the greatest efficiency in soil C sequestration will be in soils further from C saturation.

Organic Farming and Soil Carbon Sequestration: What Do We Really Know About the Benefits?

“…However, there is strong indication that even in the case of SOM-rich permanent grasslands or pastures manure application increases SOC (Jones et al 2006;Olson and Papworth 2006;Dijkstra et al 2006) and that SOC saturation levels, as proposed by Stewart et al (2007), do not exist, or have not been reached yet. Together, there is evidence that the overall C balance of soil, provided similar amounts of manure are available, is not systematically different between, on the one hand, mixed (organic) systems where manure is applied within single farms or, on the other hand, specialized conventional farming that selectively spreads high rates of manure but only on a fraction of the conventional land.…”

Stability and stabilisation of biochar and green manure in soil with different organic carbon contents

“…In the recently developed C saturation concept, Six et al (2002), Stewart et al (2007Stewart et al ( , 2008, and Gulde et al (2008) indicate that the physicochemical characteristics of soils put a ceiling on the protective capacity of SOC pools by soil aggregates and clay minerals, which limits increases in SOC even with the addition of organic inputs (e.g. Campbell et al 1991;Paustian et al 1997;Solberg et al 1997;Kimetu et al 2008).…”

Testing the modified Rothamsted Carbon Model for paddy soils against the results from long-term experiments in southern China

“…This may be due to the combination of small values for both the SOC increase and the C input. This relationship suggests that SOC was approaching saturation at the sites with double rice crops (Stewart et al 2007). However, at higher levels of C input, the model's predictions diverged greatly from the observed values.…”