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 decomposition 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 decomposition dynamics of lignin in the various plant tissues. The analytic methods are (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 relations 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 observed. Decay dynamics of lignin denoted a rapid relative increase during the first weeks of field exposure, followed by stabilizing contents over the next 2 yr. CuO oxidation data indicate the establishment of a stable "lignin endmember" within 1-2 months. DT-MS data, on the contrary, showed continuous modification of the lignin polymer throughout the duration of the experiment. Evidence was found for the incorporation of (presumably) microbial N-acetylglucosamine 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 mechanism via condensation of small degradation products instead of the selective preservation of certain biomacromolecules (like lignin).