Seasonal variations in bulk tissue, fatty acid and monosaccharide delta(13)C values of leaves from mesotrophic grassland plant communities under different grazing managements.

Leaves of 26 grass, herb, shrub and tree species were collected from mesotrophic grasslands to assess natural variability in bulk, fatty acid and monosaccharide delta(13)C values under different grazing management (cattle- or deer-grazed) on three sample dates (May, July and October) such that interspecific and spatiotemporal variations in whole leaf tissues and compound-specific delta(13)C values could be determined. The total mean leaf bulk delta(13)C value for plants was -28.9 per thousand with a range of values spanning 7.5 per thousand. Significant interspecific variation between bulk leaf delta(13)C values was only determined in October (P=<0.001) when delta(13)C values of the leaf tissues from both sites was on average 1.5 per thousand depleted compared to during July and May. Samples from May were significantly different between fields (P=0.03) indicating an effect from deer- or cattle-grazing in young leaves. The average individual monosaccharide delta(13)C value was 0.8 per thousand higher compared with whole leaf tissues. Monosaccharides were the most abundant components of leaf biomass, i.e. arabinose, xylose, mannose, galactose and glucose, and therefore, fluctuations in their individual delta(13)C values had a major influence on bulk delta(13)C values. An average depletion of ca. 1 per thousand in the bulk delta(13)C values of leaves from the deer-grazed field compared to the cattle-grazed field could be explained by a general depletion of 1.1 per thousand in glucose delta(13)C values, as glucose constituted >50% total leaf monosaccharides. In October, delta(13)C values of all monosaccharides varied between species, with significant variation in delta(13)C values of mannose and glucose in July, and mannose in May. This provided an explanation for the noted variability in the tissue bulk delta(13)C values observed in October 1999. The fatty acids C(16:0), C(18:2) and C(18:3) were highly abundant in all plant species. Fatty acid delta(13)C values were lower than those of bulk leaf tissues; average values of -37.4 per thousand (C(16:0)), -37.0 per thousand (C(18:2)) and -36.5 per thousand (C(18:3)) were determined. There was significant interspecific variation in the delta(13)C values of all individual fatty acids during October and July, but only for C(18:2) in May (P=<0.05). This indicated that seasonal trends observed in the delta(13)C values of individual fatty acids were inherited from the isotopic composition of primary photosynthate. However, although wide diversity in delta(13)C values of grassland plants ascribed to grazing management, interspecific and spatiotemporal influences was revealed, significant trends (P=<0.0001) for fatty acid and monosaccharide delta(13)C values: delta(13)C(16:0)<delta(13)C(18:2)<delta(13)C(18:3) and delta(13)C(arabinose)>delta(13)C(xylose)>delta(13)C(glucose)>delta(13)C(galactose), respectively, previously described, appear consistent across a wide range of species at different times of the year in fields under different grazing regimes.