Six years after initial input of manure: Stabilized carbon gains and soil structure in a temperate syntropic agroforestry system

<title>Abstract</title> <p> Agroforestry is increasingly being adopted for its environmental benefits, particularly their capacity to sequester carbon in both woody biomass and soil. Especially within sandy soils, trees are established with additional organic matter to improve soil conditions. This study investigates an organic syntropic alley cropping system, established in 2019 on sandy loam soil in Brandenburg, Germany. The system comprises 1 m-wide, north–south-oriented tree strips initially amended with 15 t ha ^− 1 of manure. The tree strips are bordered by a mulch layer and alternated with 10 m-wide forage crop alleys. An adjacent forage crop plot served as a treeless control. Mechanical soil disturbance was avoided after establishment. To evaluate soil organic carbon (SOC) sequestration and soil structural stability, we assessed the proportion of water-stable macroaggregates (WSA) and their associated soil organic matter (SOM) content, determined SOM–mineral binding strength, and measured the slaking index. These parameters were related to microbial biomass carbon (C _mic ) as well as fungal and bacterial abundance. We hypothesized that SOM would be stabilized through the formation of stronger bonds to the mineral matrix, leading to an increased proportion of WSA and reduced susceptibility to slaking. The tree strips exhibited increased intra-aggregate SOM, stronger SOM–mineral associations, and reduced slaking potential, but no changes in WSA compared to both the forage crop alley and the control. These changes coincided primarily with greater C _mic and increased fungal abundance. In contrast, no effects were detected in the adjacent forage crop alleys, which show no systematic differences compared to the control. </p>