Changes in Chinese fir plantations root exudation strategies seasonally and as tree age

Root exudates play an important role in nutrient acquisition and defense of trees. However, there are few studies on the mechanism of productivity decline in late stage from the perspective of root exudates. Therefore, we collect root exudates in the summer and winter from 8-year-old (young), 18-year-old (middle-aged), and 49-year-old (overmature) trees in Chinese fir plantations to assess how their exudation rates and metabolite composition changed across these different life stages. Results showed that the rates and metabolite composition of root exudation varied significantly across plantation ages and between seasons. Specifically, the young Chinese fir trees increased exudation rates and produced more carbohydrates and quercetins, and overmature trees secreted more lipids and salicylic acids. This indicates that young Chinese fir plantations adopted root metabolic strategies which increased nutrient acquisition to satisfy the demands of rapid growth, but overmature plantations might produce more defensive metabolites to improve their defense capabilities. Additionally, in the winter, trees had slower root exudation rates and released a smaller proportion of organic acid, carbohydrate, and quercetin metabolites than in the summer, but a greater proportion of lipid and salicylic acid metabolites. We also found that soil microbial factors (microbial community composition and diversity) had a significant effect on the composition of primary and secondary metabolites in the winter. Overall, as plantation age increased, Chinese fir root metabolic strategies related to nutrient acquisition decrease, while those related to defense increase. These findings provide a new perspective for our understanding of the mechanisms that lead to productivity declines of Chinese fir plantations in later growth stages.