The soil carbon dilemma: Shall we hoard it or use it?

Rapidly rising concentrations of atmospheric CO 2 have prompted a flurry of studies on soils as potential carbon (C) ‘sinks’. Sequestering C in soils is often seen as a ‘win–win’ proposition; it not only removes excess CO 2 from the air, but also improves soils by augmenting organic matter, an energy and nutrient source for biota. But organic matter is most useful, biologically, when it decays. So we face a dilemma: can we both conserve organic matter and profit from its decay? Or must we choose one or the other? In this essay, I contemplate the merits, first of building soil C and then of decaying (losing) it, partly from a historical perspective. I then consider the apparent trade-off between accrual and decay, and reflect on how the dilemma might be resolved or assuaged. These fledgling thoughts, offered mostly to stir more fruitful debate, include: finding ways to increase C inputs to soil; seeking to optimize the timing of decay; and understanding better, from an ecosystem perspective, the flows of C, rather than only the stocks. Carbon sequestration is a sound and worthy goal. But soil organic matter is far more than a potential tank for impounding excess CO 2; it is a relentless flow of C atoms, through a myriad of streams—some fast, some slow—wending their way through the ecosystem, driving biotic processes along the way. Now, when we aim to regain some of the C lost, we may need new ways of thinking about soil C dynamics, and tuning them for the services expected of our ecosystems. This objective, perhaps demanding more biology along with other disciplines, is especially urgent when we contemplate the stresses soon to be imposed by coming global changes.