Soil organic matter recovery drives soil multifunctionality during secondary forest succession in abandoned pastures of the Colombian Amazon

Abandoned pastures with degraded soil quality in the Amazon region often undergo succession to secondary forests. While aboveground responses to natural regeneration have been well investigated, the impacts on soil organic matter and soil functioning are still poorly understood. Here, we evaluated the temporal dynamics of SOM-related soil properties and their contribution to soil multifunctionality along a 45-year chronosequence of secondary forest regeneration following pasture abandonment in northwestern Colombia. We measured key properties of the soil as proxy indicators of six soil functions: i) support for plant growth, ii) C sequestration, iii) nutrient storage and availability, iv) resistance to degradation, v) water and air flux regulation, and vi) biological activity and integrated them into a soil multifunctionality index. Forest regeneration led to marked increases in soil multifunctionality, with the strongest responses associated with SOM-related indicators. Soil carbon sequestration showed the largest recovery, with sub-index values nearly doubling after 15 years of regeneration, highlighting SOM accumulation as a primary driver of functional restoration. Functions closely linked to SOM, including biological activity and resistance to degradation, also increased by 1.4- and 1.3-fold, respectively, over the same period. Overall soil multifunctionality reached approximately 76% of its reference capacity after 15 years of natural regeneration. These findings demonstrate that SOM recovery plays a central role in restoring soil multifunctionality in degraded Amazonian pastures and underpins the recovery of soil-mediated ecosystem services during ecosystem restoration.