Soil Organic Matter Processes in Old-Growth Redwood Forest Canopies

Organic soils up to I in thick cover tree surfaces within the canopies of old-growth redwood [Sequoia sempervirens (D. Don) Endl] forests 50 in or more above the ground. Very little is known about litter quality and litter decomposition processes within these canopies. A combination of solid-state cross-polarization magic-angle spinning C-13 nuclear magnetic resonance (NMR) spectroscopy, natural C-13 and N-15 isotopic analyses, and proximate analyses were used to gain insight into soil organic matter accumulation and decay processes within two arboreal soils. Arboreal soil, redwood, and Fern (Polypodium scouleri Hook. & Grev) litter as well as epiphyte roots were analyzed. Proximate analyses showed that the arboreal soil parent materials contained a high concentration of the acid residue friction (i.e., Klason lignin), from 379 g kg(-1) for fern rhizomes tip to 652 g kg(-1) for redwood bark litter. This high concentration of acid residue fraction in the soil parent materials probably promotes organic matter accumulation within the tree crown. Results from C-13 NMR spectroscopy analyses indicated a preferential decomposition of carbohydrates and an accumulation of alkyl structures from the redwood and fern litter to the <2-mm soil materials and from the Oi to the Oa horizons in both soils. On the other hand, there was no consistent variation in terms of C/N ratios and stable isotope composition along the depth gradient, suggesting, that organic matter in these soils has undergone limited microbial processing but instead reflects the selective preservation of recalcitrant litter moieties.