The contribution of different adsorption processes to soil organic matter (SOM) stabilization and the consequences of the intensification of land use on the adsorption of SOM are not yet fully understood. Therefore, this study aimed to explore the adsorption of dissolved organic carbon (DOC) in soils as well as desorption of organic carbon (OC) caused by phosphate addition. We conducted desorption and sorption experiments with DOC, phosphate (Na₂HPO₄), and chloride (KCl) in topsoil and subsoil samples from a Ferralsol, an Andosol, and a Podzol. Furthermore, we quantified the size of DOC by size-exclusion chromatography.

We found that phosphate addition caused a strong increase in the DOC concentration in all soils. The DOC concentration was elevated by up to a factor of 4.5 compared to a control (water only) within five minutes after phosphate addition and kept increasing further with time, particularly in the Ferralsol. After 10 days, the DOC concentrations in the P addition treatment were between 5.5 and 18.0 times higher than in the control treatment. In contrast, chloride addition did not lead to increased DOC concentrations compared to the control (water only). Phosphate addition led mostly to desorption of organic matter of medium molecular size (10–100 and 100–1000 kDa) in the Ferralsol and the Andosol and of large molecular size (>1000 kDa) in the Podzol. In contrast, potassium chloride addition shifted the size distribution of DOC in the soil solution towards small compounds (<10 kDa), likely because KCl addition affected the aggregation of DOC compounds, in contrast to Na₂HPO₄ addition. Furthermore, phosphate addition also decreased subsequent sorption of DOC in the Ferralsol and the Andosol by a factor of up to 2.9 and 2.1, respectively.

Our results have far-reaching implications because they show that phosphate addition can lead to desorption, and thus to destabilization of SOM, particularly in Ferralsols, and can also prevent further sorption of organic matter in soils. Our study provides a mechanistic explanation of why phosphate addition can decrease soil organic carbon sequestration and stabilization. This mechanism should be considered in the analysis of land-use and tillage effects on SOM sequestration in the future. Furthermore, our study indicates that also medium and large molecular size organic matter is stabilized through adsorption in soils.

不同吸附过程对土壤有机质 (SOM) 稳定的贡献以及土地利用集约化对 SOM 吸附的影响尚不完全清楚。因此，本研究旨在探讨土壤中溶解有机碳（DOC）的吸附以及磷酸盐添加引起的有机碳（OC）的解吸。我们对来自 Ferralsol、Andosol 和 Podzol 的表土和底土样品中的DOC、磷酸盐 (Na ₂ HPO ₄ ) 和氯化物 (KCl)进行了解吸和吸附实验。此外，我们通过尺寸排阻色谱法量化了 DOC 的尺寸。

我们发现磷酸盐的添加导致所有土壤中 DOC 浓度的强烈增加。在添加磷酸盐后 5 分钟内，与对照（仅水）相比，DOC 浓度升高了 4.5 倍，并且随时间进一步增加，尤其是在 Ferralsol 中。10 天后，磷添加处理中的 DOC 浓度比对照处理高 5.5 至 18.0 倍。相比之下，与对照（仅水）相比，添加氯化物不会导致 DOC 浓度增加。添加磷酸盐主要导致 Ferralsol 和 Andosol 中中等分子大小（10-100 和 100-1000 kDa）和 Podzol 中大分子大小（>1000 kDa）的有机物解吸。相比之下，₂ HPO ₄添加。此外，磷酸盐的添加还降低了 Feralsol 和 Andosol 中 DOC 的后续吸附，分别高达 2.9 和 2.1 倍。

我们的结果具有深远的意义，因为它们表明添加磷酸盐会导致解吸，从而导致 SOM 的不稳定，特别是在铁盐中，并且还可以防止土壤中有机物质的进一步吸附。我们的研究提供了为什么添加磷酸盐可以减少土壤有机碳固存和稳定的机制解释。未来在分析土地利用和耕作对 SOM 封存的影响时应考虑这种机制。此外，我们的研究表明，中等和大分子大小的有机物质也通过土壤中的吸附而稳定。