Cycling and composition of organic matter in terrestrial and marine ecosystems

Decomposing natural organic matter found in terrestrial and marine environments consists of a heterogenous mixture of particles and molecules with variable physical and chemical properties. The amount of organic matter present in these systems is controlled by the relative rates of accumulation and loss. Accumulation is controlled principally by net primary productivity whilst losses are mainly a function of the biological stability of the biomolecules present. Processes of lateral transfer can also be significant to both rates of accumulation and loss in localized zones. In this paper, the processes and properties responsible for defining the biological stability of organic matter in terrestrial and marine ecosystems are examined. A conceptual model illustrating the implications that mechanisms of biological stabilisation can have on the molecular composition of natural organic matter in these ecosystems is presented. Molecular composition has typically been determined using a variety of selective degradative methodologies either individually or in combination. These methodologies often only identify 20-80% of the organic matter present with the extent of identification decreasing as the degree of biological processing increases. Solid-state C-13 nuclear magnetic resonance (NMR) can be used for routine assessment of the chemistry of organic materials; however, no direct measure of the molecular composition can be obtained. Previous work that examined the ability of using solid-state C-13 NMR data in a simple mixing model to predict molecular composition is extended in this study. The extended mixing model is then used to characterise changes in the molecular composition of decomposing organic material in soil and marine systems. The extended mixing model accounted for the distribution of C-13 NMR signal intensity of all samples examined and allowed the biomolecular and elemental composition to be estimated. Decomposition induced changes in molecular composition were very different in the terrestrial and marine systems examined, but the direction of change suggested a convergence towards the formation of similar decomposition products. (C) 2004 Elsevier B.V. All rights reserved.