Navel orange fine root nutrient content and rhizosphere effects varied with tree ages and soil depths in a hilly red soil region of China

Root nutrients are required for the growth and development of roots and shoots, which are closely related to soil quality and fruit productivity. In this study, trees of contrasting stand ages, including young, middle-aged, and old trees, were selected in field sites with common management practices in the navel orange (Citrus sinensis) orchards of Southern China. The fine root N decreased with soil depth and the fine root P was higher of old trees (1.82 g kg(-1)) than young (1.57 g.kg(-1)) and middle-aged trees (1.37 g. kg(-1)). Soil organic C, total N, and P generally increased with plant age in the rhizosphere and bulk soils, while reverse trend was found for sucrase and urease activities. Interestingly, acid phosphatase activity was much higher at a lower depth (202 and 82 g.kg(-1) OC d(-1) for rhizosphere and bulk soils, respectively) than at an upper depth (46 and 43 g.kg(-1) OC d(-1) for rhizosphere and bulk soils, respectively). In conclusion, root and soil nutrients generally increased with plant growth. Fine root N decreased while fine root P remained unchanged with soil depth due to the content and rhizosphere effect of index at different soil layers related to N and P, respectively.