Conservation agriculture (CA) is an agronomic system based on minimum soil disturbance (no-tillage, NT), permanent soil cover, and species diversification. The effects of NT on soil organic carbon (SOC) changes have been widely studied, showing somewhat inconsistent conclusions, especially in relation to the Mediterranean and humid subtropical climates. These areas are highly vulnerable and predicted climate change is expected to accentuate desertification and, for these reasons, there is a need for clear agricultural guidelines to preserve or increment SOC. We quantitively summarized the results of 47 studies all around the world in these climates investigating the sources of variation in SOC responses to CA, such as soil characteristics, agricultural management, climate, and geography. Within the climatic area considered, the overall effect of CA on SOC accumulation in the plough layer (0–0.3 m) was 12% greater in comparison to conventional agriculture. On average, this result corresponds to a carbon increase of 0.48 Mg C ha⁻¹ year⁻¹. However, the effect was variable depending on the SOC content under conventional agriculture: it was 20% in soils which had ≤ 40 Mg C ha⁻¹, while it was only 7% in soils that had > 40 Mg C ha⁻¹. We proved that 10 years of CA impact the most on soil with SOC ≤ 40 Mg C ha⁻¹. For soils with less than 40 Mg C ha⁻¹, increasing the proportion of crops with bigger residue biomasses in a CA rotation was a solution to increase SOC. The effect of CA on SOC depended on clay content only in soils with more than 40 Mg C ha⁻¹ and become null with a SOC/clay index of 3.2. Annual rainfall (that ranged between 331–1850 mm y⁻¹) and geography had specific effects on SOC depending on its content under conventional agriculture. In conclusion, SOC increments due to CA application can be achieved especially in agricultural soils with less than 40 Mg C ha⁻¹ and located in the middle latitudes or in the dry conditions of Mediterranean and humid subtropical climates.

中文翻译：

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地中海和亚热带湿润气候保护性农业下的土壤有机碳：全球荟萃分析

保护性农业 (CA) 是一种基于最小土壤干扰（免耕，NT）、永久性土壤覆盖和物种多样化的农艺系统。NT 对土壤有机碳 (SOC) 变化的影响已得到广泛研究，得出的结论有些不一致，尤其是与地中海和潮湿的亚热带气候有关。这些地区非常脆弱，预测的气候变化预计会加剧荒漠化，出于这些原因，需要明确的农业指南来保持或增加 SOC。我们定量总结了全球 47 项关于这些气候的研究结果，这些研究调查了 SOC 对 CA 响应的变化来源，例如土壤特征、农业管理、气候和地理。在所考虑的气候区内，与传统农业相比，CA 对耕层（0-0.3 m）中 SOC 积累的总体影响要高 12%。平均而言，该结果对应于碳增加 0.48 Mg C ha⁻¹ 年⁻¹。然而，在常规农业下，效果因 SOC 含量而异：在≤ 40 Mg C ha ^{-1的土壤中为 20%，而在 > 40 Mg C ha -1}的土壤中仅为 7% 。我们证明，10 年的 CA 对 SOC ≤ 40 Mg C ha ⁻¹的土壤影响最大。对于小于 40 Mg C ha ^-1的土壤，在 CA 轮作中增加具有较大残留生物量的作物比例是增加 SOC 的一种解决方案。CA 对 SOC 的影响仅在超过 40 Mg C ha ^-1的土壤中取决于粘土含量，并且在 SOC/粘土指数为 3.2 时变得无效。年降雨量（范围在 331–1850 mm y ⁻¹) 和地理对 SOC 的特定影响取决于其在常规农业中的含量。总之，特别是在低于 40 Mg C ha ^-1且位于中纬度地区或地中海和潮湿亚热带气候的干燥条件下的农业土壤中，可以实现因 CA 应用而导致的 SOC 增量。