Carbon sequestration in soils: sources, sinks and pitfalls

When we talk about “sequestering” carbon in the soil to mitigate anthropogenic climate change, what are we really saying as scientists, and what is the public getting from it?  And perhaps even more importantly, what implicit assumptions are we as scientists making that may affect the validity of the widely held belief that increased C storage in soils is a good thing, and can offset our CO2 emissions?

In a recent soil carbon series in the European Journal of Soil Science, one review paper attempts to address these issues. “Carbon sequestration” has become a popular idea among policy makers and scientists alike. As anyone who keeps up on ecosystem science literature will know, C sequestration has also become an explicit motivation for numerous studies of soil and ecosystem properties.

Current best estimates claim that worldwide, soils store 684-720 Pg of C within the upper 30 cm, and 1462-1548 Pg to a depth of 1m. (For anyone not familiar with these terms, 1Pg = 1×10ˆ15 gram). The upper 30% of the soil profile to 1m clearly stores a disproportionate amount of C; this ecologists know to be the zone where continuous inputs from roots and organic matter replenish C. To put these numbers in an ecosystem context, the amount of C stored globally in soils to 30cm is twice the amount  of C stored as CO2 in the atmosphere and three times the amount of C stored in above ground vegetation. Clearly, soils represent a huge pool in global C budgets and should be managed with the knowledge that changes may represent a powerful feedback on the global C cycle. This vast storage potential is what has popularized the idea of sequestering even more C in soils and has spurred a myriad of different research approaches, from ecosystem-scale reforestation efforts to molecular studies of C-mineral binding interactions.

Given the amount of hope that has already been invested in this powerful idea, it is important for scientists interested in soil C to keep several caveats in mind when planning and conducting their research, and when communicating that research to a broader audience at a science or policy conference.

Caveat #1) Carbon sequestration does not necessarily mean climate change mitigation.

Increasing the amount of C stored in a particular ecosystem’s soil does not by default decrease the amount of CO2 released to the atmosphere. This is partially an issue of scale. If policy makers choose to set aside a certain amount of pasture land for reforestation (with the assumption that this will increase the net C storage in these systems), it is often the case that new land will have to be cleared somewhere else to compensate for the lost agricultural production.  The clearest way around this problem is to be selective in land use changes: land that is more fertile should be exploited for farming, and land that is less fertile should be reforested to boost C storage. However, an increasing global population poses obvious constraints to this line of reasoning. Ultimately, the fact remains that in the upcoming decades, we will need to boost our agricultural yields to meet global demand, and furthermore, climate change is predicted to decrease the fertility of some of the world’s most productive regions.

Moreover, allowing land to go back to its “natural” state does not necessarily lead to a net accumulation of C. A forest respires far more CO2 per acre than a wheat field. Due to the high water demands of forests relative to crops, it can sometimes be the case that trees dry up otherwise inundated subsurface soils, and in doing so create a soil environment that accelerates the decomposition of C.  One must have a refined understanding of the sources and sinks of CO2 in any particular ecosystem to claim that it will or will not “sequester” C.

Finally, management changes leading to increased C storage may increase or decrease the flux of other greenhouse gases, most notably N2O and methane. Given that these gases have 298 and 25 times the global warming potential of CO2, respectively, they are not a trivial consideration. Forests often hold less nitrogen than pastures (due to the different C:N demands of woody vs. non-woody tissue), and consequently release more N2O through the microbial process known as denitrification.

Caveat #2) The amount of carbon that can be locked up in soil in finite

Our understanding of how C is actually “stabilized” in soil on a molecular scale is still evolving. Scientists are using powerful technologies such as x-ray crystallography to understand in detail the chemical interactions between soil minerals and carbon-rich compounds that cause C to be tightly bound and inaccessible to microbes. As a general rule, the more mineral surface area available for C binding, the more tightly C is bound. In subsurface soils lower levels of C mean higher (mineral surface area: C) ratios and thus tighter binding of the C that is present.

However, mineral stabilization has its limits. Laboratory experiments are finding that as C is added to soils, “steady states” are sequentially reached. Careful manipulation of the  chemical and thermodynamic parameters of a soil may bump that soil from a lower steady state to a higher one, but whether such techniques can be applied on a whole-ecosystem scale remains to be seen.

To repeat, caveat #2 is that the amount of carbon that can be locked up in soil is finite, which leads to caveat #3.
At present, the amount of CO2 we put in the atmosphere does not seem to be.

Powlson et al. 2011. Soil carbon sequestration to mitigate climate change: a critical re-examination to identify the true and the false. European Journal of Soil Science 62: 42-55.

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