Root Traits and Soil Bacterial Composition Explain the Rhizosphere Effects along a Chronosequence of Rubber Plantations

Rubber tree plantations (Hevea brasiliensis) are expanding into the tropical regions of southwest China to ensure production to meet the growing demand for latex. The effects of long-term plantations on soil carbon processes are still unclear. Also, the effects of the plant’s rhizosphere on the decomposition of soil organic matter (SOM) play a crucial role in predicting soil carbon dynamics. The rhizosphere and soils corresponding to a chronosequence of ages (4, 15 and 30 years) of rubber plantations were collected and incubated to determine the effect of the rhizosphere (RE) on SOM decomposition. We also examined the soil physicochemical properties; bacterial community structure; and root morphological, chemical, and physiological traits to further explore the underlying mechanisms of the RE on SOM decomposition. The REs on SOM decomposition varied significantly in the different age classes of the rubber plantations, and the higher the REs on SOM decomposition in an older plantation might limit the accumulation of organic carbon in the soil. Root traits, including the specific root length, root nitrogen content, and root carbon/nitrogen ratio, varied significantly in response to the plantation age and explained more of the variance in the RE on SOM decomposition than the soil and microbial properties. Due to the changing root morphological and chemical traits along the age chronosequence, the rhizosphere bacterial community composition tended to shift the carbon utilisation strategy and the bulk soil nitrogen content decreased. These variations also affected the RE on SOM decomposition. Our results indicate that the development of rubber plantations would prevent soil carbon accumulation, especially in the rhizosphere, by increasing the RE on SOM decomposition, which would be predicated by root morphological and chemical traits.