Nowadays, the continuous organic carbon (OC) assessment in soil and its various particle-size fractions is needed to correctly estimate the soil organic matter (OM) contents and dynamics. However, the existence of several widely used analytical methods for OC and OM determination hinders the comparison of OC and OM data taken by different laboratories, in different times, soil classes, horizons and particle-size fractions. Although these methods are usually related by means of empirical soil-dependent factors, how these coefficients vary among soils is seldom addressed. In the present work 67 samples from the A horizons (0 to 20–40 cm depth) of 58 forest and agricultural calcareous soils from the Valencia province (Eastern Spain) were taken, the silt-plus-clay separated, and both the fine earth and the silt-plus-clay analysed through the wet dichromate self-heated and externally heated oxidations, the dry combustion with CO₂ measurement and the loss-on-ignition. As a consequence, the readily oxidizable carbon (RXC), the total oxidizable carbon (TXC), OC and OM were obtained, respectively. Furthermore, the coefficients to convert among these properties, namely, the Walkley-Black factor (f_WB), the oxidation recovery factor (f_XR), the carbon valence in the organic matter (v_C,OM), and the van Bemmelen (f_VB) factor were assessed. The RXC in both fine earth and silt-plus-clay was 75% of the OC thus supporting a common f_WB of 1.33. However, the OM in the fine earth presented less f_XR than the silt-plus-clay. This apparent inconsistency between f_WB and f_XR was caused by the different v_C,OM in the fine earth (3.45) and the silt-plus-clay (4.56) and hence, the different oxidation state of the OC in each fraction. This was revealed by how f_WB depends on f_XR and v_C,OM through f_WB = 4 f_XR/v_C,OM. Therefore, the different v_C,OM exactly compensated for the different f_XR in each fraction to give the same f_WB for both. Besides, the different v_C,OM in each fraction was consistent with the fact that carbon accounted for 61% and 71% of the OM mass in, respectively, the fine earth and the silt-plus-clay, thus supporting the use of a van Bemmelen factor below the standard of 1.72 and different, i.e., 1.64 and 1.41, for each particle-size fraction. This fact can be understood taking into account that v_C,OM linearly depends on f_VB through v_C,OM = 4 – α – β f_VB, where α and β are two empirical coefficients. Therefore, it has been shown how the conversion coefficients among the RXC, TXC, OC and OM depend on two independent chemical characteristics, one OM-stoichiometry-related, the carbon mass fraction, and another OM-reactivity-related, the oxidation recovery. As a consequence, it will be better understood that the determinations of OC and OM in soils, remarkably when the Walkley-Black method is used, vary not only because of changes in OC and OM magnitude, but also because of changes in OM stoichiometry and reactivity. The differences in the soil OC and OM data obtained by using different laboratory methods, and in different times, particle-size fractions, soil depths, classes, etc., will be thus better understood.

如今，需要对土壤中的连续有机碳 (OC) 及其各种粒度分数进行评估，以正确估计土壤有机质 (OM) 含量和动态。然而，几种广泛使用的 OC 和 OM 测定分析方法的存在阻碍了不同实验室在不同时间、土壤类别、层位和粒度分数所获取的 OC 和 OM 数据的比较。尽管这些方法通常通过经验土壤相关因素相关联，但很少涉及这些系数如何在土壤之间变化。在目前的工作中，从瓦伦西亚省（西班牙东部）的 58 个森林和农业钙质土壤的 A 层（0 到 20-40 厘米深度）采集了 67 个样本，将淤泥加粘土分离，₂测量和烧失量。结果，分别获得了易氧化碳（RXC）、总可氧化碳（TXC）、OC和OM。此外，这些特性之间的转换系数，即沃克利-布莱克因子 ( f _WB )、氧化恢复因子 ( f _XR )、有机质中的碳价 ( v _C,OM ) 和 van Bemmelen ( f _VB ) 因素进行了评估。细土和粉砂加粘土中的 RXC 均为 OC 的 75%，因此支持1.33的常见f _WB。然而，细土中的 OM 呈现出较少的f _XR比淤泥加粘土。f _WB和f _XR之间的这种明显不一致是由细土 (3.45) 和粉砂加粘土 (4.56) 中不同的v _C,OM引起的，因此，每个部分中 OC 的氧化态不同。通过f _WB  = 4 f _XR / v _{C,OM ，}f _WB如何依赖于f _XR和v _C,OM揭示了这一点。因此，不同的v _C,OM正好补偿了不同的f _XR在每个分数中，为两者提供相同的f _WB。此外，每个部分中不同的v _C,OM与碳分别占 OM 质量的 61% 和 71% 的事实一致，细土和粉砂加粘土，因此支持使用van Bemmelen 因子低于 1.72 的标准，并且对于每个粒度级分不同，即 1.64 和 1.41。考虑到v _C,OM通过v _C,OM  = 4 – α – β f _VB线性依赖于f _{VB ，}可以理解这一事实，其中 α 和 β 是两个经验系数。因此，已经表明 RXC、TXC、OC 和 OM 之间的转化系数如何取决于两个独立的化学特征，一个与 OM 化学计量相关，碳质量分数，另一个与 OM 反应性相关，氧化恢复。因此，将更好地理解土壤中 OC 和 OM 的测定，尤其是当使用 Walkley-Black 方法时，不仅因为 OC 和 OM 量级的变化而变化，而且因为 OM 化学计量和反应性。从而更好地理解使用不同实验室方法获得的土壤 OC 和 OM 数据在不同时间、粒度分数、土壤深度、类别等的差异。