Effects of reclamation and regeneration processes on organic matter from estuarine soils and sediments

Three adjacent areas in the Urdaibai Estuary (NW Spain) comprising a reclaimed salt marsh, a natural salt marsh and a regenerated salt marsh were sampled in order to study edaphic processes and changes in organic matter (OM) composition related to reclamation and regeneration of soils and sediments in estuarine environments. For this, a general analytical description of the soil and sediment profiles was carried out and the OM was characterized using solid state 13C nuclear magnetic resonance (NMR) spectroscopy and analytical pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). The reclaimed salt marsh displayed very different characteristics from the natural salt marsh throughout the whole profile, indicating an important degree of change in soil quality and, particularly, in OM composition caused by the reclamation. At this reclaimed site, characterized by oxic conditions and a lack of tidal influence, transformation of the former salt marsh to grassland resulted in input of OM richer in carbohydrates and with a lower contribution of aromatic compounds than the OM in the salt marsh soils. On the other hand, the edaphic characteristics of the regenerated marsh were similar to those of the natural marsh and the vegetation cover was the same. However, the natural marsh accumulated twice the amount of organic carbon at the surface as the regenerated marsh. Furthermore, the natural marsh contained more aromatic material, as detected with NMR and the Py–GC/MS results indicated a greater presence of microbial material. These differences indicate that the regenerated marsh has not completely recovered its natural characteristics, despite the long period (more than 40y) since the beginning of the regeneration process. We propose the composition of soil OM as a suitable long term indicator of reclamation and regeneration processes in the salt marsh environments.