Long-Term Effects of Agroforestry Systems on Soil Health in Subtropical Agricultural Landscapes

Abstract:

Soil health is the foundation of sustainable agricultural production, and its degradation under intensive monoculture has become a major constraint in subtropical agricultural landscapes. Agroforestry systems (AFS), characterized by the integration of trees with crops and/or livestock, have been proposed as a promising approach to improve soil health. However, most existing studies on AFS and soil health are short-term, and the long-term (≥10 years) effects of different AFS types on soil health indicators remain unclear. This study evaluated the long-term effects of three typical AFS (alley cropping, silvopasture, and forest garden) on soil health in subtropical agricultural landscapes across three countries (China, Spain, and Japan) with a 15-year monitoring period. Soil health indicators included physical properties (soil bulk density, aggregate stability, water-holding capacity), chemical properties (soil organic carbon, total nitrogen, available phosphorus, available potassium, pH), and biological properties (soil microbial biomass carbon, microbial diversity, enzyme activities). Results showed that compared to monoculture, long-term AFS significantly improved soil health across all indicators. Specifically, AFS reduced soil bulk density by 18-25%, increased aggregate stability by 22-30%, and enhanced water-holding capacity by 20-28%. In terms of chemical properties, AFS increased soil organic carbon by 35-48%, total nitrogen by 30-42%, and available nutrients by 25-38%, while maintaining soil pH at a neutral level. For biological properties, AFS increased soil microbial biomass carbon by 40-55%, enhanced microbial diversity (Shannon-Wiener index) by 28-36%, and improved enzyme activities (urease, phosphatase, sucrase) by 32-45%. Among the three AFS types, alley cropping had the most significant effects on soil organic carbon sequestration and total nitrogen accumulation, while silvopasture performed best in improving soil physical properties, and forest garden had the highest positive impact on soil microbial diversity. The long-term benefits of AFS on soil health were closely related to tree species selection, litter input, and management practices. Structural equation modeling revealed that the improvement of soil health by AFS was mainly mediated through increased organic matter input and enhanced microbial activity. These findings highlight the long-term sustainability of AFS in improving soil health in subtropical agricultural landscapes and provide evidence-based recommendations for the design and management of AFS to promote soil health conservation.