Decomposition rates and nutrients dynamics in small-diameter woody litter in four forest ecosystems in Washington, USA

Decomposition rates and nutrient dynamics in small-diameter woody litter (twigs, cones, and branches) were studied in four ecosystems in western Washington: high elevation Pacific silver fir (Abiesamabilis (Dougl.) Forbes) and low elevation Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco), western hemlock (Tsugaheterophylla (Raf.) Sarg.), and red alder (Alnusrubra Bong.). Conifer twigs decomposed faster (k = 0.14–0.24 year−1) than cones (k = 0.09–0.12 year−1) and branches (k = 0.03–0.11 year−1). Decomposition constants were related better to initial lignin/initial N ratios (r = −0.64) than initial lignin concentrations. N was generally the least mobile nutrient while K was the most mobile. Many nutrients were strongly immobilized in conifer fine woody litter, including N, Mg, Mn, and Ca. There was little immobilization of N in red alder branches. N release from decomposing woody litter appears to be controlled by a critical C/N ratio. This critical C/N ratio, however, was not constant and increased as the substrate decomposition rate increased.