Beyond carbon sequestration: Soil as conduit of solar energy.
The prospect, so alluring, of sequestering carbon (C) to mitigate CO2 build-up in air has prompted a flurry of soil studies, but questions still linger about whether early optimism was fully justified. My objectives are to review briefly the mechanisms of carbon accrual, consider constraints on soil C sequestration for mitigating climate, and contemplate questions for further collective conversation. Carbon in soil is ever in flux; not a stagnant reservoir, but a stream of atoms flowing through. Its instantaneous stock can be increased by adding more atoms to the stream or by slowing their rate of flow to CO2. The latter (decay) is influenced by intrinsic recalcitrance of the substrate, by protective interactions of carbon with the mineral matrix, and by the favourability of localized conditions for biological activity. Together, these mechanisms create a continuum of susceptibility to decay. Although many practices can abet carbon accrual, their effectiveness is constrained by the finitude of carbon gain, possible effects on other greenhouse gas emissions, susceptibility to future loss, the difficulty of measuring gains precisely, and the complexity of landscape-scale dynamics. In the light of these problems and prospects, I propose, as a working hypothesis, that we focus less on carbon, and more on using wisely the energy it carries; in other words, maximizing carbon ‘stocks’ is less critical than maintaining ‘flows’ to sustain the manifold functions performed by ecosystems. Among its implications, this hypothesis (i) enforces an ecosystem perspective, considering all inevitable trade-offs in the use of finite energy, (ii) emphasizes the importance of sustaining energy capture via photosynthesis, (iii) promotes a long view, evaluating carbon flows over several decades or longer and (iv) enfolds human aspirations, which ultimately shape carbon flows by demands on lands. Far from deflating the urgency of soil carbon research, this reoriented perspective may move soil carbon beyond a segregated stock to its central place as a dynamic hub in the energy cycles of our ecosystems.
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