Long-term application of straw and biochar improves soil carbon fractions, enzyme activity, and tomato yield under continuous greenhouse cultivation

In recent years, there has been increasing emphasis on the sustainable organic material reutilization, including straw and biochar. This study investigates the effects of prolonged applications of straw (R), biochar (B), and their combination (RB) on soil organic carbon fractions, enzyme activities, and tomato yield under continuous greenhouse cultivation, compared to a control (CK). The results showed that organic amendments reduced soil bulk density. Compared to the CK treatment, the R treatment caused a slight decrease in soil pH, whereas the B and RB treatments significantly increased soil pH by 4.10% and 6.10%, respectively. Relative to the CK treatment, the R, B, and RB treatments significantly elevated the levels of dissolved organic carbon (DOC), easily oxidizable organic carbon (ROC), microbial biomass carbon (MBC), and particulate organic carbon (POC) by ranges of 10.28%-24.34%, 38.94%-49.69%, 109.73%-121.21%, and 102.56%-196.93%, respectively. The activities of catalase (CAT), α-glucosidase (AG), β-glucosidase (BG), β-xylosidase (XYL) and cellulase (CL) were significantly enhanced after organic material addition. Additionally, the R, B, and RB treatments increased yields by 22.61%, 18.25%, and 23.74%, respectively. By integrating long-term field observations with structural equation modeling, this study offers novel mechanistic evidence that particulate organic carbon (POC) serves as a central mediator linking organic amendments to yield improvement-a regulatory pathway that has not been systematically elucidated in previous research, offering a mechanistic basis for sustainable soil management in continuous greenhouse production.