Leaf Tissue Degradation and Impact on Isotopic Signals during Natural Decay of Metasequoia Glyptostroboides

We examined leaves at different developmental stages from a single tree of Metasequoia glyptostroboides Hu et Cheng representing a series of changes during natural senescence and decay from green leaf to lake catchments. Observations using light microscope and SEM on transverse cuttings of these leaf samples indicate noticeable anatomical decay of soft tissues on leaves reached to the lake bottom and a year of deposition. These morphological changes are accompanied by the reduction of bulk carbon and nitrogen concentrations. A slight negative shift of bulk carbon isotope composition started to occur when leaves reach to the ground but prior to entering the lake water, and this δ¹³C shift is accompanied by a reduction of half of nitrogen concentration. A 1‰ negative change of δ¹³C was observed after a year of decay.

Different from the alteration of leaf internal soft tissues, the cuticle membrane covering the epidermal cell layer is morphologically intact through the whole duration of decay, which is consistent with chemical analysis results. Both carbon and hydrogen isotope compositions of n-alkanes of these leaves change little during the decay process with an average fluctuation of 0.8‰ for δ¹³C and 8‰ for δD respectively.

Our data indicate that: (1) although not obvious in morphological and anatomical observation, major N-containing biomolecules experience decay through senescence before leaves reach to the ground. (2) The negative shift of bulk carbon isotope value was due to the loss of liable biomolecules such as polysaccharides. (3) When reach to the lake, lake water facilitates decay process, resulting in the destruction of internal tissue probably through the loss of polysaccharide compounds. (4) The stability of decay resistant components such as cuticle and associated waxes render molecular carbon and hydrogen isotope signals that are affected little during early diagenesis.