DIAGENESIS OF BELOWGROUND BIOMASS OF SPARTINA-ALTERNIFLORA IN SALT-MARSH SEDIMENTS

Belowground biomass of Spartina alterniflora lost 55% of its organic matter during 18 months of decomposition in salt-marsh sediments. Significant losses of lignin were observed under highly reducing conditions confirming previous reports of lignin degradation in the absence of molecular oxygen. The submolecular components of lignin varied in susceptibility to degradation and were ranked in the following descending order of resistance to decomposition: V is similar to P > S is similar to C. The preferential degradation of the cinnamyl and syringyl components constrains the quantitative usefulness of these compounds for estimating the contributions of herbaceous and angiosperm tissues to pools of dissolved and particulate organic matter. The relative concentration of lignin increased about 1.6-fold during decomposition owing to the more rapid loss of polysaccharides. Two phases in nitrogen dynamics were evident during the decomposition study. During the first 4 months of decomposition there was a net loss of N from Spartina tissues at a rate of about 10-mu-g N g-1 tissue d-1. After this initial phase of net N loss, a phase of N immobilization was evident and was tightly coupled with rates of microbial degradation of the tissues. Net rates of N immobilization were approximately 19-mu-g N g-1 tissue d-1; by the end of the 18-month study more N had accumulated in the tissues than was initially present. Stable N isotope compositions of decaying Spartina were also very dynamic and reflected the sources of N that were immobilized during decomposition and the N transformations that occurred on the detrital particles. The stable C isotope composition of the polysaccharide components of Spartina was approximately 6 parts per thousand heavier than that of the lignin component. The isotopic compositions of these components retained the 6 parts-per-thousand difference throughout the 18-month study. The stable C isotope composition of the bulk tissues, however, decreased gradually during decomposition as the isotopically heavier polysaccharides were preferentially lost. Residual material was relatively enriched in the isotopically lighter, lignin-derived C.