Enhancement of stable carbon pools and slaking resistance in a temperate organic syntropic alley-cropping system on sandy loam

Agroforestry systems are increasingly implemented in temperate regions due to their wide range of environmental benefits, including their potential to sequester carbon in both woody biomass and soil. This study examines an organic syntropic alley-cropping system for fruit and nut production, established in 2019 on sandy loam in Brandenburg, Germany. The site consists of 1 m-wide north-south-oriented tree rows amended with 15 t ha^‑1 of organic manure at establishment, bordered by a mulch layer, and alternating with 10 m-wide rows of forage crops (a grass-alfalfa mixture). The control side represents the previous managment regime.

To assess the stability of soil organic carbon (SOC), we measured the amount of SOC in the fine particulate fraction (<63 µm), the percentage of water-stable aggregates and their concentration of soil organic matter (SOM), the binding strength of SOM to the mineral matrix, and the slaking index (SI). We further compared these data with microbial biomass and community composition.

Our results show an increase in intra-aggregate SOM with stronger binding to the mineral matrix and reduced slaking potential in the tree strip compared to the crop alley and cropland control. This corresponds with a significantly higher microbial biomass and an increase of total fungi as well as in the bacterial genus Streptomyces, both of which are assumed to play a role in soil aggregate stabilization. In contrast, the tree row had no influence on carbon storage or soil structure within the crop alley.