| 英文摘要: | Rewarding smallholders for sequestering carbon in agricultural land can improve food security while mitigating climate change. Verification of carbon offsets in food-insecure regions is possible and achievable through rigorously controlled monitoring. Global food demand is projected to double by the middle of this century, but greenhouse gas emissions from food production must be reduced. Mitigation and adaptation are often regarded as separate, though complementary, objectives in climate policy. Possible trade-offs can, in some cases, be reversed for synergy1, 2, 3 with the potential to make smallholder farmers in food-insecure regions the main beneficiaries. Providing incentives for the adoption of carbon-sequestering agricultural practices to increase crop productivity in the developing world could enhance food security and contribute to climate equity while mitigating climate change4. Such a productivity increase would also reduce the pressure to expand agricultural land, thus further reducing emissions3. The target to reduce global warming below critical limits, while achieving equitable per-capita rights to emit among countries, pushes the required speed of technology shift in industrialized countries beyond reach5. However, emissions trading between the industrial and developing worlds could make the mission possible; indeed, this approach may be the only way forward. Exchange schemes — charging for emissions and rewarding emission off-sets — seem likely to be one of the few options for incentivizing the transformation to low-carbon societies worldwide. Such trading could reward the bypassing of high-emission production systems in the transition economies and the developing world. The sink capacity of carbon in agricultural soil has been estimated as having the potential to offset 5–15% of fossil-fuel-based greenhouse gas emissions6. To be sufficiently extensive, a reward scheme for farmers to store soil carbon should entail low transaction costs and access by farmers and payers ranging from citizens to enterprises and public actors. The loss of carbon and associated nutrients from soils has been the main cause of declining crop productivity in sub-Saharan Africa (SSA)6, which is projected to remain the most food-insecure region in the future. The reversal of carbon stock losses, or prevention of their decline, could induce greater and more resilient yields7 through improved water and nutrient supplies8. Carbon stocking in soil organic matter does not require only carbon but also presumes increased stocks of nitrogen, phosphorus and sulphur9, a fact that is critical to the effectiveness of sequestration measures. Agroforestry can integrate above- and below-ground carbon sequestration with biological nitrogen fixation and efficient nutrient uptake through mycorrhizae, while also providing food and fodder security, and fuel wood. Replacing the practice of burning manure for fuel with anaerobic digestion, other sources of bioenergy, or solar stoves would allow nutrients and carbon to be recycled to the soil. However, efficient uptake and recycling do not help to improve carbon storage if key nutrients are deficient in the system9; placing small amounts of fertilizer close to roots, or incorporating it in feed to avoid sorption in unavailable forms in soil, can prevent carbon-rich but nutrient-poor organic residues being lost from the soil9. Carbon in soil and vegetation can also be restored through exclusion of free-grazing livestock in areas where only selective harvesting is allowed. Furthermore, residue-based biochar carbon has a very long persistence in soil, while also contributing to yield-enhancing soil functions10. Increased food productivity resulting from increased soil carbon stocks would be a primary gain for African smallholder communities2 and carbon payments could act as a trigger to overcome the reluctance of farmers to change practices2, 11. A core requirement for the carbon market to function is verification of carbon sequestration; so far, this has been a barrier for smallholder farmers because the transaction costs of the present vehicles such as the Clean Development Mechanism and National Appropriate Mitigation Actions are high, and the time lag in payments undermines any incentive2. To involve smallholders, the verification of carbon offsets cannot be carried out on a case-by-case basis but rather must be 'practice-based', that is, based on knowledge of the impact of specific agricultural management practices under various agro-ecological conditions2, 12. At present, empirical knowledge of the carbon effects of agricultural practices in food-insecure regions is scarce. For SSA, the few empirical estimates are based primarily on descriptive data with no controls or replication. Model-based simulations raise expectations, but have substantial uncertainty13. Thus, measurements are necessary to estimate the carbon budget of agricultural land even in temperate conditions14, where many process-based models were developed. In developing countries the soil carbon dynamics and land uses may differ and there are limited data even for model parameterization15, which may potentially bias estimates of carbon stock changes16. Securing financial support for agricultural carbon enhancement requires empirical verification for each type of management practice, agro-ecological zone and soil (Fig. 1). As long-term experiments would not satisfy the immediate demands for documented effects of management practices, other approaches must be applied. We propose that programmes for monitoring land managed by farmers with rigorous controls and replication should be employed to acquire this urgently needed knowledge. A useful approach would be to employ replicated matched pairs of geo-referenced field plots, including the potentially carbon-sequestering management and, because of the notable spatial variation in carbon stocks, an adjacent field plot in which the previous traditional management continues, to serve as the control. Such approaches have been used, without replication, in comparisons of cropping systems17. The soil-forming properties must be equal on the matched plots and sufficient replication reduces the significance of potential differences; the plots with the 'improved' management may even form a chronosequence. The fact that traditional long-term management in, for example, SSA is relatively homogeneous per region, agro-ecological zone and management system ensures availability of valid controls.
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