Consequences of Plant Diversity on the Quality of Carbon Input

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The stability of soil carbon and hence the amount of stored carbon depends on the chemical structure (i.e.,the intrinsic stability) of molecules,their interaction with mineral surfaces (i.e., their "storage capacity"), and the amount of carbon submitted to decomposition (Lichtfouse et al. 1998; Kaiser and Guggenberger 2003). We will not discuss the latter two factors, as the carbon storage capacity is mainly controlled by parent soil material and it is not influenced by biodiversity. The effect of biodiversity on the productivity of forests and consequently on the litter production has already been discussed (Pretzsch, Chap. 3, this Vol.). The main focus of this chapter will be the chemical and isotopic composition of organic matter.

Most plant-derived carbon belongs to a small number of chemical structures. These are mainly carbohydrates, organic acids, lipids, lignin, and proteins. Some of them, such as carbohydrates, organic acids, and proteins, are preferred energy sources for soil organisms and thus less recalcitrant in soils than lignin or lipids (Gleixner et al. 2001a). In consequence, the decomposition rate of plant litter will change with litter quality (see also Hattenschwiler, Chap. 8, this Vol.) and stable plant-derived structures may accumulate in soil. Wood for example, as the most abundant plant biomass, mainly consists of cellulose and lignin (Fig. 9.2a, b). Cellulose is chemically less stable than lignin and lignin accumulates, i.e., is selectively preserved, in wood decomposition. This is well known for example for brown rot fungi (Gleixner et al. 1993).

Lignin itself is a complex polymer made from three different lignin monomers, coumaryl, coniferyl, and sinapyl alcohols, differing in their methoxyl substitution in the ortho position of the phenolic ring (Fig. 9.2 c). The contribution of the three monomers characterizes the lignin and indicates its origin. Monocotyledons, like grasses, are rich in coumaryl alcohol, whereas dicotyledons are rich in coniferyl and sinapyl alcohol. In conifers coniferyl alcohol is the main lignin monomer, whereas in broadleaf trees sinapyl alcohol dominates. Depending on the biodiversity of the plant community, the composition of lignin biomarkers might differ and the selective preservation of remaining lignin molecules might determine the quality of stored carbon.

The selective preservation of chemically resistant molecules is also known to occur in lipids (Lichtfouse et al. 1998), e.g., alkanes (Fig. 9.2d). The composition of alkanes, which are part of the epicuticular waxes (Eglinton et al. 1962), is characteristic for different plant types and enables reconstruction of the paleoenvironment (Brassell et al. 1986; Eglinton and Hamilton 1967). Other constituents of the epicuticular waxes, such as alkanoic acids, hydrox-yalkanoic acids, alcohols, alkanediols, alkanals, or alkyl esters are less stable than alkanes (and have inspired less taxonomic information; Riederer 1989). Green algae, often present in soils, synthesize alkanes mainly with a chain length of 17 carbon atoms, whereas higher plants synthesize alkanes with chain lengths of mainly 27, 29, and 31 carbon atoms (Rieley et al. 1991). The relative composition of each different alkane depends on its origin (Schwark et al. 2002). Grasses of the understory vegetation are dominated by the C31 alkane (Cranwell et al. 1987; van Bergen et al. 1997), whereas deciduous trees consist of mainly C27 and C29 alkanes (Almendros et al. 1996; Spooner et al. 1994). Using a ternary mixing diagram for the C27, C29, and C31 alkanes, the different alkane composition of various trees from southern Italy can be

Fig. 9.2. Chemical structure of main biochemical elements of plants. a Cellulose, b lignin, c lignin monomers, d alkanes; n indicates the number of repeated structures to reach the corresponding total chain length

noted (Fig. 9.3). Fagus sylvatica synthesizes mainly the C29 alkane, Quercus cerris mainly C31. The relative abundance of these three alkanes is also reflected in lipid extracts from soil. In beech forests the upper horizons of the soil are clearly dominated by the C29 alkane (Fig. 9.3).

These two examples, lignin and alkanes, suggest that plant-derived differences in chemical composition (i.e., the quality) of plant biomass depend on plant species composition, and the occurrence of such biomarkers in soil car-

Fig. 9.3. Relative composition of alkanes C27, C29, and C31 in various plant species and in depth horizons of soil under beech vegetation

bon differs accordingly. However, not only the quality of biomass produced depends on the species composition. The amount and composition of carbon flow to roots for exudation are also species-dependent (Grayston et al. 1997). The relative amount of root exudate as fraction of plant carbon assimilation varies thusly: 40 % for Liriodendron tulipifera, 60 % for Pinus sylverstris, and even 78 % for Pseudotsuga menziesii. In general, the exudates consist of various carbohydrates, amino acids, aliphatic and aromatic fatty acids, sterols, and enzyme- and hormone-like substances (Grayston et al. 1997). The composition of the exudates varies greatly between different species. For example, deciduous trees exude preferentially the amino acids cysteine and homoser-ine, whereas evergreens have no preferential amino acid exudation pattern. Exudation patterns of carbohydrates, like glucose, fructose and sucrose, and organic acids, such as acetic, succinic, and oxalic acid, all of which may be major components in tree root exudates, also differ with tree species. However, no clear pattern is obvious. Even different species of the same genus Pinus have different exudate compositions. Moreover, age and developmental stage of the trees and environmental conditions, such as nutrient status, pH, water availability, temperature, light intensity, carbon dioxide concentrations, and presence of microorganisms, affect the quality and quantity of root exudation (Grayston et al. 1997).

Root exudates are the major carbon source for soil microorganisms living in association or symbiosis with tree roots. Soil microorganisms oxidize most root-derived carbon, and microbe-derived compounds, like fucose or rham-nose, are dominant in the dissolved carbon. In consequence, the direct impact of root exudates on carbon storage is small. However, strong feedback can be expected between the tree species composition as expressed in root exudation and soil microbial composition. Evidence exists that changing artificial root exudations can affect the species composition of soil microorganisms (Baudoin et al. 2003). Spore germination (mainly), hyphal elongation, and branching as well as chemotaxis are effected by root exudates. Some feedbacks are rather specific, such as is the symbiosis between N-fixing Frankia and Alnus; or they may be nonspecific, such as are the ectomycorrhizal fungi Laccaria laccata or Boletus edulis, which have a broad range of host plants. However, the impact of differences in the below-ground biodiversity on carbon storage is not well understood and controversial (Hooper et al. 2000).

In addition to the importance of chemical structure for the quality of carbon input to soils, all organic compounds have a unique isotopic "fingerprint" characteristic for its origin (O'Leary 1981; Schmidt and Gleixner 1998). This fingerprint can be used to estimate the importance of biomass from different trees for carbon storage. Well known are interspecies differences, as with the greater enrichment of 13C in C4 versus C3 plants of ~12-15 %o or the isotopic enrichment of wood and litter from conifers versus broadleaf trees of ~5 %. Moreover, N-fixing plants like Fabaceae or Alnus species have a unique 15N signal; and, independently of the transpiration rate and leaf anatomy, the D content of trees also varies. Intermolecular isotopic differences are known as well. Lignin, for example, is depleted in 13C relative to cellulose by up to 6 %. This isotope information is widely used to trace the origin and turnover of soil carbon (Boutton and Yamasaki 1996). However, most investigations are made using only bulk soil or plant material. Differing decomposition rates of chemical structures introduce isotopic shifts of bulk soil organic matter that mimic isotope effects or source differences. For example, the relative increase of the lignin content in remaining wood will cause an isotopic shift of the remaining wood to more-depleted S13C values. Using the isotopic information of individual molecules overcomes this problem; molecules isolated from soil found to have the same isotope content as their plant precursors indicate their selective preservation (Kracht and Gleixner 2000).

1 The Ô13C value in "per mille" [%o] is the relative difference of the isotopic ratio R of the heavy isotope (13C) to the light isotope (12C) of a sample to a reference material times 1,000. Thus, Ô13C value [%]=(RSample-Rstandard/Rstandard) ¥ 1,000. International standard for carbon is V-PDB, a carbonate (Coplen 1996).

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