Crop replacement by secondary forests increases soil organic carbon (OC) stocks. Quantifying this increase is subject to biases: the changes in soil compaction and, in calcareous zones, the changes in soil inorganic carbon (IC) content, which affect the carbon balance. How relevant are both biases, though? We studied this problem in NE Spain, subhumid Mediterranean, in a set of current crops, old forests, and new forests grown on croplands abandoned after 1956. We compared soil OC stocks on a fixed-depth basis (30 cm), and on an equivalent-mass basis: uppermost 425 kg m⁻² of fine mineral matter (amount in crop soils). Down to 30 cm, OC stocks were lowest in crops (5.45 kg m⁻²) and highest in old forests (7.50 kg m⁻²), which gives a potential OC sequestration in the mineral soil of 2.05 kg m⁻². On an equivalent-mass basis, the potential is 2.50 kg m⁻², meaning that the fixed-depth criterion underestimates OC sequestration by more than 20%. These figures are surpassed by those of IC: crops store 17.73 kg IC m⁻² down to 30 cm, old forests 12.28, suggesting a potential loss upon afforestation of 5.45 kg IC m⁻². Nevertheless, the lack of negative relationships between IC losses and OC gains within a given land use suggests that the contrasting IC stocks result from past management, mainly tillage. The decarbonation is only apparent. Our results stress the need of accounting for changes in carbonate content and soil compaction in future studies about carbon sequestration in soils.

中文翻译：

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废弃作物植树造林后地中海土壤中的碳固存：由于土壤压实和碳酸盐储量变化引起的偏差

次生林作物替代增加了土壤有机碳 (OC) 储量。量化这种增加会受到偏差：土壤压实的变化，以及在钙质区，土壤无机碳 (IC) 含量的变化，这会影响碳平衡。但是，这两种偏见的相关性如何？我们在 1956 年后废弃的农田上生长的一组现有作物、旧森林和新森林中研究了这个问题。 我们在固定深度（30 厘米）和等效质量基础：最上层 425 kg m ^-2的细矿物质（作物土壤中的量）。低至 30 cm，OC 库在作物中最低（5.45 kg m ^-2），在老林中最高（7.50 kg m ^{- 2})，这在 2.05 kg m ^-2的矿质土壤中提供了潜在的 OC 封存。在等效质量基础上，潜力为 2.50 kg m ^-2，这意味着固定深度标准低估了 OC 封存超过 20%。这些数字被 IC 超越：作物储存 17.73 kg IC m ^-2到 30 cm，老林 12.28，表明植树造林后的潜在损失为 5.45 kg IC m ^-2. 然而，在给定的土地利用中，IC 损失和 OC 收益之间缺乏负相关关系，这表明形成对比的 IC 存量来自过去的管理，主要是耕作。脱碳只是明显的。我们的结果强调在未来关于土壤碳固存的研究中需要考虑碳酸盐含量和土壤压实的变化。