Variability in the quality and potential decomposability of Pinus sylvestris litter from sites with different soil characteristics: acid detergent fibre (ADF) and carbohydrate signatures

Abstract

We investigated variations in the biochemical composition and biodegradation of Scots pine ( Pinus sylvestris) needles and litter on four soils with different cation exchange characteristics, ranging from 7 to 92% base saturation. Trends in cellulose, non-crystalline holocellulose concentrations (as measured by a trifluoracetic acid (TFA) hydrolysis technique) and ratios of lignin-to-cellulose and lignin-to-N in needles and litters varied significantly between sampling sites and reflected trends in soil base status under P. sylvestris stands. Although these relationships need to be tested rigorously on a more extensive sample set, clearly the biochemical quality of needles and litter within the same tree species is not uniform. Detailed analysis of the carbohydrate composition of TFA extracts from plant tissues showed that the ratios of selected hexoses (predominantly of microbial origin) to pentoses (mainly from plant cell wall constituents) were between 1 and 15 times lower in decomposing litter than needles. The hexose-to-pentose ratios also increased from 0.117:1 to 0.136:1 (as rhamnose+fucose-to-arabinose+xylose) and from 1.32:1 to 1.67:1 (as galactose+mannose-to-arabinose+xylose) with increasing base saturation of soils. It is concluded that the former ratio most clearly differentiated between the contrasting sites, whereas the latter ratio was more diagnostic of biotransformations during litter decomposition and associated development of microbial biomass. The study highlights a need to further clarify intraspecific differences in tree leaf and litter biochemistry in relation to soil characteristics and challenges the conventional definition of litter quality in terms of C-to-N or lignin-to-N ratio without consideration for the biochemical quality and spatial patterns of a “holocellulose” fraction. The determination of constituent carbohydrates in TFA extracts may provide a useful diagnostic for the potential decomposition of litter and the depolymerisation of structural carbohydrates.