Abstract: Taking three navel orange orchards varied in stand age but similar in fertilization mode in south Jiangxi as test objects, this paper studied the seasonal changes of their soil nutrient supply and enzyme activities. The soil organic carbon (OC), total nitrogen (TN), and total phosphorus (TP) contents all increased with stand age. Soil sucrase activity was higher in the 20-year-old orchard than in the 9- and 31-year-old orchards, soil urease activity was in reverse, and soil acid phosphatase activity had no significant difference among the three orchards. The increase of soil OC, TN, and TP contents with stand age was not consistent with their transformation rates based on the determinations of soil enzyme activities, e.g., in 31-year-old orchard, the transformation rates of soil C and N had a decreasing trend. In addition, the seasonal changes of soil sucrase, urease, and acid phosphatase activities did not meet the seasonal shifts of the orange tree’s nutrient demands, i.e., C accumulation in early winter, N need in spring and summer, and P transfer in late autumn and early winter. It was deduced that the lower transformation rate of soil nutrients, imbalance of soil N and P supply, and inconsistency between the seasonal changes of soil nutrient supply and plant nutrient demand would be the main causes inducing the decrease of productivity and the decline of fruit quality in elder navel orange orchards of south Jiangxi.

Key words: Western Erdos, Soil, Tetraena mongolica, Micro-elements contents