Leaf breakdown in a subtropical stream: nutrient release patterns

Nutrients released from leaf litter during breakdown are of importance to the nutrient budget of loworder forest streams. The leaf litter of three species (Machilus thunbergii, Schefflera octophylla, and Ficus erecta) was collected soon after natural abscission to examine the breakdown rates and to determine nutrient (N P. Na, K. Ca, and Mg) release patterns in a subtropical forest stream in northern Taiwan. Breakdown rates showed a strong, negative correlation (r(2) = 0.996) with initial C:N ratios. S. octophylla and F erccta had similar C:N ratios and decomposed at similar rates (k = 0.0362 day(-1), 0.0376 day(-1), respectively). M. thunbergii had the highest C:N ratio and the slowest breakdown rate (k = 0.0115 day(-1)), Increased in nitrogen concentrations during the breakdown suggested the immobilization of this nutrient. The increase amounted to 0.30, 0.76, and 0.59 mmol/g for M. thunbergii, S. octophvlla, and F. erecta leaf litter, respectively, and it was correlated with the breakdown rates (r(2) = 0.832). Phosphorus immobilization was correlated with initial N:P ratios. It was detected in M. thumbergii and F. erecta showing lower initial P concentrations and higher N:P ratios than S. octophylla. Similar N:P ratios of the three leaf species indicated a correlation between N and P concentrations during the breakdown. This suggested that low initial N concentrations of the leaves rather than initial P concentrations limited the breakdown. Nutrient release patterns were associated with initial nutrient concentrations. Leaves with higher initial nutrient concentrations exhibited higher release rates. Nutrient release patterns differed among leaf species, but in general release rates for different nutrients were in the order K > Mg > Na > Ca > C > P > N. Exceptions were Na and Ca for M. thunbergii and S. octophylla, respectively, that exhibited the lowest release rates.