Nutrient Distribution and Biogeochemical Cycling in Middle-Aged Acacia crassicarpa Plantation

The standard plot method was used to study the distribution and biological cycling of 10 nutrient elements（N，P，K，Ca，Mg，Fe，Mn，Zn，Cu and B） in the 7-year-old（middle-age）Acacia crassicarpa plantation．The results indicated that concentrations of the 10 nutrient elements in different organs of A．crassicarpa plantation were in the order of leaves>bark>branch>root>stem. The contents of N were the highest，K and Ca the second，Mg and P the lowest for macro-elements，whereas Mn and Fe were the highest，B and Zn the second，and Cu the lowest for micro-elements．The nutrient storage of the plantation was 1 121.08 kg·hm^-2，of which overstorey of A. crassicarpa trees stored 848.94 kg·hm^-2 accounting for 75.72%，forest floor 116.05 kg·hm^-2 account for 10.35%，and standing litter 156.08 kg·hm^-2 account for 13.92%．The amounts of nutrient elements in different parts of trees was in the order of stem＞leaves＞branch＞bark＞root．The amounts of annual uptake，retention and return of 10 elements in the plantation were 261.87，140.59 and 121.28 kg·hm^-2，respectively．The cycling coefficient and recycling period were 0.54 and 6.04. As was observed，the nutrient utilization efficiency in middle-aged A. crassicarpa plantation was lower，but the return rate was higher and the turnover period was shorter，resulting in the recovery and maintenance of forestland fertilization.