Fate of plant detritus in a European salt marsh dominated by Atriplex portulacoides (L.) Aellen

Abstract

This paper deals with the organic matter dynamics of a dwarf shrub species, Atriplex portulacoides, which characterizes the majority of ungrazed European salt marshes. These dynamics were investigated by the simultaneous estimation of primary production, litter production, decomposition processes and accumulation of organic matter in sediment. We used harvested biomass techniques and the litter-bag method, as well as more recent techniques such as determination of natural isotopic compositions. The results of this study have been compared with those of other studies of salt marshes of the Eastern Coast of America dominated by Spartina alterniflora. Studies of the fate of halophytic organic matter in salt marshes have shown that processes such as consumption, decomposition, or flooding rapidly could distribute organic material to different compartments of the marsh system.Aboveground productivity of A. portulacides was 1700 g m-2 yr-1. Approximately 80% of this production was transformed rapidly in necromass. A large part of this dead material (86%) fell as litter directly onto the sediment at the sampling site. The remainder was washed away by tides, either out of the marsh, or into the upper marsh where it accumulated in drift lines. Decomposition of A. portulacoides on the sediment was followed using litter-bags. Decay constants, based on the first order decay function, ranged between 0.0404 d-1 and 0.0113 d-1 and appeared high when compare with data obtained in other American and European salt marshes. Nitrogen immobilisation was not detectable in this decomposition experiment. During decay, nitrogen concentrations increased, whereas absolute amount of nitrogen decreased. After microbial decomposition, plant detritus was rapidly incorporated into sediment. The carbon isotopic composition of organic matter in sediment showed that organic matter in the middle marsh came from both aboveground production of A. portulacoides and microalgae. The patterns for these two kinds of organic matter in different granulometric fractions depended on tidal submersion frequency. During the summer, when the salt marsh was not disturbed by tides, the clay fraction got richer in plant organic matter, whereas, during the other periods, this fraction was characterized by microalgal organic matter. This study provides new data about the dynamic of organic matter. It remains still difficult to establish comparisons between North American and European salt marsh functioning according to the variability of primary production, microbial decomposition and tidal range in these ecosystems.