Assessment of the effects of multigenerational succession on soil chemical environment, fertility and enzyme activity in Eucalyptus plantations using structural equation modeling

Abstract Eucalyptus is a short-rotation tree species widely used in tropical and subtropical regions around the world. However, the high-intensity management mode of multi-generational succession has also made eucalyptus planting controversial due to ecological and environmental problems, including soil degradation and declines in stand biomass. This paper aims to reveal the effects of multi-generational succession of eucalyptus on soil fertility, organic structure, and biological properties. Soil samples were collected from eucalyptus plantations of different stand ages (5, 11, 17, and 21 years old) in a typical area in south Asia; soil organic fraction structure and content characteristics were investigated using Fourier transform infrared (FTIR); and structural equation modeling was used to explore the direct and indirect influences of soil fertility, enzyme activity, and organic fraction on stand biomass. The results showed that the soil fertility and enzymatic activity indexes were significantly different (p < 0.05) in different stand age soils, with the exception of total potassium and cellulase. FTIR analysis showed that 11 infrared absorption peaks existed in the soils of this study area, attributed to silicates, aromatics, carbonate ions, sugars, esters and polysaccharides, aliphatic hydrocarbons, and phenolic alcohols. Combined with the results of peak area integration, the content of esters (1088, 1165 cm1), aromatics (1631 cm1), and phenolic alcohols (3423 cm1) was significantly higher in 17- and 21-year-old stand soils than in control soils. The results of the structural equation model showed that organic components were negatively related (p < 0.01) to enzyme activity and biomass fertility, with standardized coefficients of 0.53 and 0.49, respectively. In summary, we concluded that multi-generation succession of eucalyptus trees can change the structure of soil organic functional group composition and promote the enrichment of aromatic and phenolic alcohol functional groups. The results of the structural equation modeling established the relationships between stand age and soil fertility, organic fraction, enzyme activity, and biomass. The increase in aromatic and phenolic alcohol functional groups was the main change in the organic fraction of the soil. Such changes can directly inhibit the increase in eucalyptus biomass and indirectly negatively affect biomass by inhibiting enzyme activity, and the combination of these two inhibitory pathways could be the main reason for the decline in stand biomass after successive plantings of eucalyptus.