Effects of gap size on nutrient release from plant litter decomposition in a natural forest ecosystem

The rates of nutrient release were investigated during the Castanopsiskawakamii (Hayata) leaf, branch, and bark litter decomposition processes over a 360-day period within five different size classes of forest gaps that comprise a gap size gradient. Five different size classes of gaps were gap 1, under the closed canopy of pure C. kawakamii vegetation; gap 2, small gaps with a diameter of less than 5 m; gap 3, small to intermediate gaps with a diameter of 5–15 m; gap 4, intermediate to large gaps with a diameter of 15–30 m; gap 5, large gaps with a diameter of bigger than 30 m. After 360 days, decomposing plant litter under canopy condition or small gaps lost weight more rapidly than those in large gaps. Loss of K was highest among all nutrients measured for all three types of litter in all five size classes of gaps. Net immobilization of N and P occurred for all three types of litter in the first 2 or 4 months. Litter residue concentrations of N and P then declined until the end of this study, decreasing to 47% and 70%, respectively, of initial amounts. Loss of Ca and Mg averaged 60 and 40%, respectively, during the period of this study; loss of S averaged 50% and was continuous. Loss of nutrients measured were highest in leaves, intermediate in bark, and lowest in branches under the same size class of gaps, and were highest in gaps 1 and 2, intermediate in gap 3, and lowest in gaps 4 and 5 for the same type of litter. Cellulose and lignin components showed the similar patterns of mass loss as nutrients with highest in gaps 1 and 2, intermediate in gap 3, lowest in gaps 4 and 5 (P ≤ 0.001). Annual decomposition rate was correlated to the microclimatic factors for all types of litter within five different gap size conditions. Soil moisture content was the best predictor of annual decay rate (R2 = 0.922, P ≤ 0.001) among the microclimatic factors. The results indicated that small scale of natural disturbance do not influence the nutrient dynamics during plant litter decomposition; however, rates of nutrient release are strongly inhibited with the increase of scales of disturbance. In this study, gap size of 15 m in diameter is critical in determining the rates of nutrient release from plant litter decomposition processes.