Tree species mixing effects on root exudation rate and exudate metabolome: variations across forest stand age.

Root exudates play a crucial role in soil carbon sequestration and nutrient cycling within forest ecosystems. However, limited attention has been given to how forest management strategies, such as tree species mixing, influence the quantity and quality of root exudates, particularly across different stand ages. In this study, we collected root exudates from Pinus massoniana Lamb. trees in pure and mixed stands (with Castanopsis hystrix Hook. f. & Thomson ex A. DC.) at four stand ages (25, 36, 46 and 63 years) to examine the root exudation rate of carbon (REC) and the metabolomic profile of exudates. We also assessed stand characteristics, root traits and soil properties to explore their interactions with root exudation. Results indicated that species mixing had minimal effects on REC, except in the 36-year-old stand. However, tree species mixing significantly influenced the metabolome of root exudates, with the primary differentially accumulated metabolites (DAMs) being amino acids and peptides, fatty acids and shikimates and phenylpropanoids. The mixing effects on all metabolites significantly varied with stand age, with the maximum (26.92-46.75%) occurring at the 46- or 63-year-old stands and the minimum (-17.64 to 6.04%) occurring at the 25- or 36-year-old stands. Root traits were the dominant drivers regulating mixing effects on REC across stand ages, while stand characteristics and soil properties primarily regulated the variation in mixing effects on metabolites with stand age. Overall, our findings demonstrate that the effects of tree species mixing on root exudates are stand age-dependent and highlight the potential functions of DAMs. Determining the exact role of DAMs under tree species mixing requires further research into the relationship among DAMs, rhizosphere microbial communities and ecological processes, thus providing more comprehensive proposition for sustainable forest management.