Physical properties of soils in Swiss agroforestry systems

Agroforestry is a promising nature-based solution to counteract climate change. In addition, agroforestry has the potential to increase the soil’s chemical, biological and physical quality. The latter is affected by the tree roots and the increase of organic matter input in soil, which stabilize the soil structure and increase the porosity of the soil. This thesis aims to assess the soil physical properties in silvoarable alley cropping systems in Switzerland. As part of this thesis, 33 alley cropping agroforestry systems were sampled to assess bulk density, gravimetric water content, and measure penetration resistance. All samples and measurements were performed in the tree row, the arable plot in the alleys between the tree rows and in an adjacent arable control plot without trees, respectively. The analyses were performed using response ratios between the plots (tree – alley, alley – control and agroforestry – control) to assess differences between the plots. Correlations with internal system variables (density, diversity, age and average tree volume of each agroforestry system) were determined. The results showed that there were no significant differences between the plots for neither bulk density, the gravimetric water content nor the penetration resistance. For the bulk density and the gravimetric water content, differences were visible between the soil depths: the topsoil bulk density was lower than in the subsoil for the bulk density and viceversa for the gravimetric water content. No significant correlation was found between the internal variables and the bulk density. For the gravimetric water content, there were positive effects for tree diversity, and negative effects for tree density and age. The more tree species were present in the agroforestry, the lower was the penetration resistance in the alley and agroforestry plots than in the control plots at depths between 31 and 50 cm. In addition, other significant results were found for the tree-alley and the agroforestry-control ratios of the penetration resistance at different depths and variables. The positive influences of the diversity variable are likely due to the different root traits of the various tree species. The age of the trees and therefore the growth of the root system could have an influence on the soil properties. Most of the agroforestry systems within this study are younger than ten years. Therefore, a repetition of the measurements in the future seems reasonable, as differences between the plots could become visible. Additionally, management practices and further soil properties of each site should be considered, to refine the outcome and confirm the results of this study.