Clarifying confusions over carbon conclusions

<p>Incremental gains in soil organic carbon (SOC) at scale has enormous potential to mitigate global warming, yet confusion over contexts that elicit SOC accumulation abound. Here, we examine how adaptation (through irrigation, fertiliser, crop type and rotations), status quo SOC levels and soil type impact on long-term SOC sequestration and net greenhouse gas (GHG) emissions. We discover an inverse relationship between antecedent SOC stocks and relative change in SOC stocks, with greater initial SOC levels resulting in lower ex poste change in SOC. Sequestration was greatest for clays and least for sands, although changes in SOC in sandy loam soils were low. Diversified whole farm adaptations – through inclusion of grain legumes within wheat/canola crop rotations – were more conducive to improvement in SOC stocks, followed by Intensified systems (use of greater rates of irrigation, farm areas under irrigation, more nitrogen fertiliser and inclusion of rice and maize in rotations). Adaptations that Simplified farm systems by reducing irrigation and fertiliser use resulted in the lowest SOC sequestration. In most cases, long-term SOC stocks fell when SOC at the outset was greater than 4-5%, regardless of soil or crop type, rotation, production system or climate. We contend that that (1) management interventions primarily impacted SOC in the soil surface (0-30 cm) and had de minimus impact on deep SOC stocks (30-100 cm), (2) crop rotations including wheat and canola were more conducive to improvement in SOC stocks, (3) scenarios with high SOC had little impact on crop productivity and not necessarily the lowest GHG emissions intensity, (4) productivity and GHG emissions intensity were more related to nitrogenous fertiliser, rather than SOC stocks, and (5) aspirations for improving SOC are likely to be futile if antecedent SOC stocks are high (4-5%). We conclude that potential for improving SOC stocks exists in contexts where antecedent stocks are low (<1%), which may include regions with degradation, chronic erosion and/or other constraints to vegetative ground cover that could be sustainably and consistently alleviated.</p>