Guiding soil carbon sequestration with a novel organic material returning index globally. A meta-analysis

Straw returning and manure returning are key strategies for forming new soil organic carbon in croplands, contributing to agricultural sustainability and climate change mitigation. However, their effectiveness and cost-efficiency depend on environmental conditions, and there remains a lack of practical assessment methods. To address this gap, using data from 347 cropland sites, we developed a novel Organic Material Returning Index, which consists of separate indices for manure and straw. These indices score each amendment based on new soil organic carbon formation efficiency (the ratio of new soil organic carbon formation to organic carbon input) by integrating key drivers with relative importance weights. A global threshold (k=0.20) was applied to identify sites suitable for straw returning. Our analysis reveals that temperature primarily regulates manure efficiency, while soil pH drives straw efficiency. The index identifies 63.0% of global croplands as suitable for cost-effective straw returning. The index offers a transparent, mechanistically grounded framework, enabling site-specific strategies to maximize soil organic carbon sequestration and support sustainable agricultural policies worldwide.