Comparison of humus horizons from two ecosystem phases on northern Vancouver Island using 13C CPMAS NMR spectroscopy and CuO oxidation

Abstract

Much forested land in the wetter zones of northern Vancouver Island is characterized by thick humus layers, with two distinct ecosystem phases: the younger "HA" phase arising from disturbance is productive after clearcutting, but in the old-growth "CH" phase, seedlings suffer growth check after 5–8 yr, with reinvasion of the ericaceous shrub salal (Gaultheria shallon Pursh.). We used solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and CuO oxidation to examine whether chemical differences in the humus might be associated with difference in forest productivity after clearcutting. NMR spectra of woody horizons, which were similar for CH and HA sites, were dominated by signals from lignin of decomposed wood. Non-woody humus types were typical of forest litter layers, and were dominated by signals in the O-alkyl region. The differences between CH and HA sites were: (i) higher tannin content in the CH sites, most likely from salal inputs and (ii) higher ratio of carbohydrate to lignin C, indicating less effective decomposition in CH sites. Oxidation with CuO also showed more advanced decomposition in the non-woody horizons of HA than of CH sites. Less effective decomposition possibly due in part to tannin accumulation could contribute to the lower forest productivity on salal-dominated CH sites in this region. Key words: 13C NMR, CuO oxidation, decomposition, humus, tannin, salal