Beyond caveat emptor: reducing nitrous oxide risk in the soil carbon offset market

Emily Stuchiner , Joseph C. von Fischer

Soil organic carbon (SOC) storage projects are an increasingly prominent natural climate solution, drawing investment from diverse climate-sensitive stakeholders. However, the OC additions that build SOC can alter emissions of the potent greenhouse gas (GHG) nitrous oxide (N2O). These emission changes can erode the intended GHG benefit when OC inputs fuel spatiotemporally episodic hot spots and hot moments (HSHM), which can dominate annual N2O emission budgets. Concerningly, HSHM are systematically under-quantified by the emission factors and process-based models that most SOC protocols rely on. This creates a poorly quantified risk to the demonstrable net climate benefit of SOC projects. To address this, we make two novel contributions. First, we introduce an assessment framework that scores the probability of project-induced N2O increases across distinct domains that are typically conflated in existing frameworks. One domain of the score characterizes N2O risk based on site and SOC intervention practices while the other evaluates the adequacy of N2O emission measurement, monitoring, reporting, and verification (MMRV) under the protocol being applied. Second, we propose that credit retirement be linked to the composite N2O risk score so that N2O emissions are explicitly factored into SOC accounting protocols. We close with a research-to-practice agenda that proposes two parallel sets of objectives. The first is protocol performance trials, wherein crediting workflows are run under intensive direct field measurements to empirically characterize N2O accounting accuracy in current protocols. The second is development of direct measurements that comprise “minimum viable MMRV” for robust N2O accounting in SOC projects right now. Our proposed activities will help prevent the establishment of crediting systems that retire GHG credits that are not representative of true CO2-equivalent offsets. Rigorous accounting of N2O in SOC projects must become a shared responsibility of the research community, project developers, buyers, and registries.