Characterization of organic matter in a forest soil of coastal British Columbia by NMR and pyrolysis-field ionization mass spectrometry

Organic matter in the soil profile under a young Douglas-fir stand in coastal British Columbia was characterized by examining intact samples of fresh litterfall and organic horizons (LF, H), and fractions (floatables, humic acid [HA], fulvic acid [FA], humin [HU]) from the three mineral horizons (Ae, Bm, BC). Some 30–40% of the carbon in the mineral horizons was found in poorly-decomposed plant material floatable in water, a fraction whose characteristics changed little with depth, and which contained over 1% Fe. The proportion of soil C in HA plus FA was approximately 8%, but the ratio of C in FA/HA increased with depth. Solid-state ¹³C NMR spectra of litterfall, LH and H samples showed effects of decomposition, in particular a decrease in 0-alkyl C from litterfall to LH to H, and degradation of resolution from LF to H. For the mineral soil fractions, both floatables and de-ashed HU (HU_d prepared by HCl/HF treatment) indicated high levels of the original plant biopolymers, including a large alkyl component. Solution ¹³C spectra of the HAs from mineral horizons showed little difference with depth, except that peaks due to lignin were more pronounced for the Bm HA. The NMR spectra of FAs were high in 0-alkyl and carboxyl C. Pyrolysis-field ionization mass spectrometry confirmed and extended the results from NMR and chemical analyses, in particular demonstrating the accumulation of suberin in some fractions and the leaching and decomposition of lignin components with increasing depth in the mineral horizons. The general features of the HA, FA and HU_d from this forest soil, and the effects of decomposition and pedogenesis were similar to those widely found for agricultural and forest soils. However, the accumulation of suberin, and the leaching and decomposition of lignin are particularly associated with forest soils. The low proportion of soil C in HA and FA, and the high proportion in poorly decomposed, iron-rich plant fragments suggest that decomposition is somewhat limited at this site, which is classified as having low fertility. The high accumulations of alkyl C from suberin may also indicate, or contribute to inhibition of decomposition.