Effective soil spectral band selection and modeling methods can improve modeling accuracy. To establish a hyperspectral prediction model of soil organic matter (SOM) content, this study investigated a forested Eucalyptus plantation in Huangmian Forest Farm, Guangxi, China. The Ranger and Lasso algorithms were used to screen spectral bands. Subsequently, models were established using four algorithms: partial least squares regression, random forest (RF), a support vector machine, and an artificial neural network (ANN). The optimal model was then selected. The results showed that the modeling accuracy was higher when band selection was based on the Ranger algorithm than when it was based on the Lasso algorithm. ANN modeling had the best goodness of fit, and the model established by RF had the most stable modeling results. Based on the above results, a new method is proposed in this study for band selection in the early phase of soil hyperspectral modeling. The Ranger algorithm can be applied to screen the spectral bands, and ANN or RF can then be selected to construct the prediction model based on different datasets, which is applicable to establish the prediction model of SOM content in red soil plantations. This study provides a reference for the remote sensing of soil fertility in forests of different soil types and a theoretical basis for developing portable equipment for the hyperspectral measurement of SOM content in forest habitats.

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 modelling (SEM) was used to explore influences of soil fertility, enzyme activity and organic fraction on stand biomass. FTIR analysis showed that 11 infrared absorption peaks existed in the soils of this study area, attributed to silicates, aromatics, carbonate ions, sugars, esters, polysaccharides, aliphatic hydrocarbons and phenolic alcohols. Combined with the results of peak area integration, the content of esters, aromatics and phenolic alcohols was significantly higher in 17- and 21-year-old stand soils than in control soils. The results of SEM showed that organic components were negatively related ( p < 0.01) to enzyme activity and biomass, with standardized coefficients of 0.53 and 0.49, respectively. In summary, 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. Such changes can directly inhibit the increase in eucalyptus biomass and indirectly negatively affect biomass by inhibiting enzyme activity.

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.