Effects of interplanting Sarcandra glabra under Cunninghamia lanceolata plantation on volatile organic compounds in soil leachate, and on soil nutrients and enzyme activities

The research focused on comparing the changes in volatile organic compounds (VOCs) in soil leachate, soil nutrients and enzyme activities, as well as their relationships between pure plantations of Cunninghamia lanceolata and interplanted plantations of Sarcandra glaba under C. lanceolata. This study provides a scientific basis for understanding the impact of understory planting on the soil chemical environment. The study examined differences in VOCs in extracts from rhizosphere and non-rhizosphere soil (denoted as CS and FS, respectively) of pure plantations of C. lanceolata, and the rhizosphere soil of C. lanceolata and S. glaba (denoted as CS1 and GS, respectively), and the non-rhizosphere soil (denoted as FS1) shared by these two species in the interplanting stands. Furthermor, soil nutrient content and enzyme activities were also determined. Differences in soil VOCs between pure plantations and interplanted stands were analyzed, and key VOCs causing the differences were screened to reveal their correlation with soil nutrient content and enzyme activities. Results showed that (1) A total of 63 VOCs were detected in the soil extracts of pure stands and interplanted stands, with only 2 VOCs (2,4−Di-tert-butylphenol and pentadecane) being common components. The soil extracts from CS, CS1, GS, and FS1 mainly contained hydrocarbons and phenols as VOCs, while esters and organosilicones were predominant in the FS soil. (2) Cluster analysis showed that CS, GS and CS1 were clustered into one class, and FS1 and FS were each in one class; there were six shared components of VOCs in the leachate of rhizosphere and non rhizosphere from pure and interplanted stands of C. lanceolata. (3) The contents of total nitrogen, total phosphorus, ammonium nitrogen, available phosphorus in both rhizosphere and non-rhizosphere soils showed significantly higher levels in interplanted plantations compared to pure C. lanceolata plantations (P < 0.05). These levels were also positively correlated with specific VOC components (2,4−Di-tert-butylphenol and octacosane), along with increased activities of sucrase and urease enzymes. The types of VOCs in soil extracts from C. lanceolata plantations after interplanting with S. glaba were higher compared to pure C. lanceolata plantations, predominantly hydrocarbons and phenols. Among these compounds, specifically VOCs, mainly 2,4−Di-tert-butylphenol, exhibited a discernible promotion effect on soil nutrients and soil enzyme activity. The soil fertility of forest land was improved to some extent after interplanting S. glaba under C. lanceolata forest.