Relationship between UV-Vis spectrum of dissolved organic matter and enzyme activity under the effect of the natural nitrogen and phosphorus in sediment

The decomposition of organic matter(OM) into dissolved organic matter(DOM) isaccomplished by a series of bacterial extracellular enzymes, which can also be affected by nitrogen and phosphorus. There have been many studies on the molecular weight, source, humification characteristics, decomposition and transformation of OM, but fewer studies on the relationship between DOM with enzyme activity under the natural total nitrogen(TN) and total phosphorus(TP). So we investigated the natural TN and TP concentration in the offshore sediments of mangrove forest and oyster farm and explored their effects on the UV-Vis spectral characteristics of DOM in combination with the enzymatic activities of neutral phosphatase (Nep), lignin peroxidase (Lip) and polyphenol oxidase (PPO) in the profile sediment. The results showed that the mean TN concentration ranged from 16.3 to 19.9 g/kg and the mean TP concentration ranged from 0.41 to 0.47 g/kg at different depths of sediments. Local correlation analysis showed that the natural TN in the mangrove mid-tide zone could enhance the PPO activity, while TP in the oyster farm high-tide zone inhibited the PPO activity. Elevated TP concentration led to higher aromatic condensation and humification of OM, while enzyme activity did not correlate with any UV-Vis spectral parameter of the DOM within the overall area, indicating that the OM in the study area was mainly composed of terrestrial source that were not decomposed by enzymatic reactions but inorganic chemical reactions .