Soil organic matter and soil acidity in Mangrove areas in the river Paraiba Estuary, Cabedelo, Paraiba, Brazil

Mangrove ecosystems are of great environmental significance, because of their fragility and role in feeding and breeding various animal species. In northeastern Brazil, the disorderly occupation of estuarine areas and the urban sprawl have led to a considerable loss of the original area occupied by mangroves. In the municipality of Cabedelo, State of Paraiba, there are about 4,900 ha of remnant mangrove areas in the estuarine complex of the Paraiba River. However, information about the attributes of mangrove soils at this location is quite scarce. The aim of this study was to quantify the soil organic matter and soil acidity in mangroves located in the estuary of the Paraiba River, State of Paraiba, Brazil, in order to increase the database of soil attributes in this region. The study area is in local influence of the Restinga de Cabedelo National Forest (Flona), an environmental conservation unit of the Chico Mendes Institute for Biodiversity Conservation. For the choice of sampling points, we considered an area that receives direct influence of the eviction of domestic and industrial effluents. The soil of the study area is an “Organossolo Haplico” in Brazilian Soil Classification (Histosol), and was sampled at four point sites: one upstream of the effluent discharge (P1), one in the watercourse receiving effluent water (P2), one downstream of the effluent discharge (P3) and another near Flona (P4), at 0-20 and 20-40 cm, in four replications in time (28/08/2012 in the morning and afternoon, and 21/01/2013 in the morning and afternoon). Potential acidity, pH and soil organic matter (SOM) were determined. No significant differences were detected in the potential acidity of the four collection sites, which ranged from 0.38 to 0.45 cmol c dm -3 . Soil pH was greatest at point P4 (7.0) and lowest at point P1 (5.8). The SOM was highest at point P1 (86.4 %) and lowest at P2 (77.9 %). The attributes related to soil acidity were not sensitive to indicate differences in the environments we evaluated, and seem not to indicate the contamination of mangroves by discharge of sewage into water bodies that border these environments. The high organic matter content of soils in the mangrove may have helped avoid significant fluctuations in soil acidity, because of the large buffering capacity.