Decomposition dynamics of six salt marsh halophytes as determined by cupric oxide oxidation and direct temperature-resolved mass spectrometry

This paper presents the results of a comparative study on the aerobic decom position of six salt marsh plant species over a period of 2 yr. In addition to ash-free dry weight (AFDW) determination and elemental analysis (C and N), two analytic methods have been applied to obtain insight into the decom position dynamics of lignin in the various plant tissues. The analytic meth ods an (1)cupric oxide (CuO) oxidation followed by gas chromatography-mass spectrometry (GC-MS) and(2) direct temperature-resolved mass spectrometry ( DT-MS). AFDW losses could generally be well described by double exponential relatio ns with time. Carbon-to-nitrogen ratios increased during the initial stages of decomposition and decreased again afterward. For five of the six plant species, a correlation between initial lignin content and AFDW loss was obs erved. Decay dynamics of lignin denoted a rapid relative increase during th e first weeks of field exposure, followed by stabilizing contents over the next 2 yr. CuO oxidation data indicate the establishment of a stable "ligni n endmember" within 1-2 months. DT-MS data, on the contrary, showed continu ous modification of the lignin polymer throughout the duration of the exper iment. Evidence was found for the incorporation of (presumably) microbial N-acetyl glucosamine in the complex residue produced upon decomposition. Combination of CuO oxidation and DT-MS data suggested that lignin degradation products became attached to the original macromolecular material and could still be identified as lignin-derived material. The data support a humification mec hanism via condensation of small degradation products instead of the select ive preservation of certain biomacromolecules (like lignin).