Variability in the quality and potential decomposability of Pinus sylvestris litter from sites with different soil characteristics: acid detergent fibre (ADF) and carbohydrate signatures
Introduction
The potential rates of plant litter decomposition are mainly determined by the availability of carbon and nutrients to saprotrophs and the presence of modifiers, such as plant secondary compounds, which limit their activities (Swift et al., 1979; Anderson, 1991). In agricultural systems decomposition rates of crop residues are generally high and can be predicted from their C-to-N ratio because most of the C and N they contain are in compounds susceptible to microbial attack (Taylor et al., 1989). In temperate forest ecosystems there is a clearer and more complex “two phase” pattern of litter decomposition described by exponential mass loss. This can be summarised as the rapid decomposition of a labile fraction followed by the slow decomposition of a more recalcitrant less biodegradable fraction related to lignin concentration (Taylor and Parkinson, 1988; Harmon et al., 1990; Gallardo and Merino, 1993). An inverse relationship between litter mass loss and initial lignin concentration often exists, not only because lignins are recalcitrant as a consequence of their aromatic composition and structure (e.g. Harbourne, 1988; Stafford and Ibrahim, 1992; Waterman and Mole, 1994), but also because they can physically inhibit the activity of carbohydrases by masking substrate surfaces of hemicellulose and cellulose (Melillo et al., 1982). The quality, quantity and spatial configuration of the ligno–cellulose complex in the plant cell wall is therefore a key rate determinant of litter mass loss, while concentrations of labile carbon and nutrients (particularly N and P) influence the development of microbial biomass exploiting the litter resource.
The biochemical composition of forest litter is primarily a function of the tree species (e.g. Berg and Wessen, 1984; Johansson, 1994) but intraspecific responses to soil conditions and physical environmental factors also result in considerable variation in foliage and litter quality. Trees growing on soils of low inherent fertility, particularly even-aged plantations with high nutrient demands, generally have lower foliar nutrient concentrations, higher reabsorption of N and P before leaf abscission (Boerner, 1984; Melillo et al., 1982) and lower nutrient concentration in litter (Boerner, 1984; Johansson, 1994) than those growing on more fertile soils. Higher concentrations of polyphenols are also found in foliage of trees under nutrient stress (Davies et al., 1964; Bryant et al., 1983; Lambers, 1993). Flanagan and van Cleve (1983) showed for black spruce that nitrogen availability affected lignin and nitrogen concentrations in the litter and consequently litter decomposition rates. Under extreme conditions the negative feed-backs between soil nutrient availability, litter quality and nutrient immobilization during decomposition rates can result in arrested stand development (Miller, 1979; Flanagan and van Cleve, 1983). These studies indicate that there is considerable variation in litter constituents within the same tree species growing on different soils which can affect decomposition rates. There have been few comprehensive studies on intraspecific variation in a suite of litter quality constituents other than Johansson (1994) in Sweden; and none using analytical methods which provide insight into the qualitative characteristics of cell wall constituents in relation to microbial activity.
We have investigated the variation in the biochemical composition of Scots pine needles and litter on four soils with different ion exchange characteristics. Results reported in Sanger et al. (1996) showed that the amount, composition and degree of polymerization of lignin varied between soils of different base status. Here we report the results for analyses of the carbohydrate fractions in the same sets of litter. The relative proportions of selected hexose and pentose sugars are also investigated to assess the baseline for determining the proportions of microbial and plant-derived polysaccharides in needles and decomposing litter (Oades, 1984; Murayama, 1984; Moers et al., 1990).
Section snippets
Sample preparation
Needles from four Pinus sylvestris stands were collected in May 1994, in mid-Devon, U.K. Each stand was growing on a soil with different cation exchange characteristics listed in Table 1. All of the sampling sites (within a 10 km radius) had similar precipitation, aspect, hydrology and climate. Approximately 400 g second year class needles (needles that had remained on the tree over winter) and litter from the soil surface (Oi or L layer) were collected from 10 randomly selected trees >10 m apart
Soil chemistry
Soil ion exchange characteristics for each site, listed in Table 1, have been discussed in detail by Sanger et al. (1996) in an associated paper; only a brief account will be given here. All of the soils had organic surface horizons (<5 cm) above mineral soil with pH values of 3.45 (Haldon), 3.67 (Yarner), 4.92 (Parke) and 5.10 (University). Exchangeable Ca2+, Mg2+, K+ and CEC showed the same rank order as pH. Percentage base saturation varied widely between the sites: Haldon, 7%; Yarner, 23%;
Discussion
Our results indicate that within species variation in the chemical composition of P. sylvestris needles and litters is significant, and certain biochemical characteristics may be related to the cation exchange characteristics of the mineral soil. As found by Florence and Chuong (1974) and Johansson (1994), no clear trend was observed between the total element chemistry of needles, litter and soil chemistry. Leaching of labile N constituents from needles upon litter fall may explain the high
Acknowledgements
The authors are indebted to the European Community for funding this project (Contract No. EV5CT920141).
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