Effects of thinning treatments on nutrient release from decomposing needle litter of Sugi (Cryptomeria japonica D. Don) in northeastern Taiwan.

Nutrient release processes associated with needle litter decomposition were monitored for 2 yr after thinning in a 50-yr-old Sugi (Cryptomeria japonica D. Don) plantation in northeastern Taiwan. Three thinning intensities [unthinned (T0), low (T28), and moderately low (T36)] were applied. Litterbags were used to measure the Sugi needle litter decomposition. Results of decomposition for 2 yr illustrated that the current thinning intensities in this study gave clear effects on nutrient release and return of Sugi needle litter. C release from the litter was the fastest. In addition to C, this study categorized nutrient dynamics of the decomposition litter into 3 types based on the quantity correlation of nutrient loss and changes in the weight remaining and concentration of nutrients: (1) N and P, (2) Ca and Mg, and (3) K. The overall relative mobilities of the nutrients examined after 2 yr were as follows: C ＞ (P, K) ＞ Ca ＞ Mg ＞ N. In this study, only the relative mobility of K and P changed with the different thinning intensities. The annual return of all nutrients showed no significant difference between treatments T0 and T28. Significantly lower nutrient return rates existed with treatment T36, and these may have contributed to the significantly lower input of Sugi leaf litterfall. The quantities of C and other nutrients returned through litterfall decreased with an increasing thinning intensity. Amounts of bioelements of Sugi needle litter that were returned to the soil were as follows: C ＞ Ca ＞ N ＞ K ＞ P ＞ Mg for the 3 thinning intensities in this study. It is possible that the current thinning level was not high enough to produce very significant changes in needle litter decomposition, nutrient release, or nutrient availability. For future studies, we suggested that both higher thinning intensity treatment and long-term decomposition studies of changes in temperate and moist environmental conditions are necessary.