Spatial variability of soil nutrients in disturbed riverine mangrove forests at different stages of regeneration in the San Juan River estuary, Venezuela

Geostatistical analysis of selected soil properties were performed in two mangrove sites (B7 and B9) dominated by the mangrove species Rhizophora mangle along the San Juan River, Venezuela, to evaluate the effect of forest disturbance on nutrient spatial distributions. Plots within area B7 were clear-cut in 1972-1973 and in area B9 in 1982-1983. Four plots within each area were systematically selected and represented natural (2) and regenerated (2) forests. Individual plots were 1.5 ha in B7 and 1.25 ha. in B9 and sampled at a resolution of 25 m. Soil samples were obtained at the intersection of an aligned grid with 90 points in B7 and 78 points in B9. Measured soil properties included salinity, total nitrogen, total carbon, total phosphorus, osmolality, and pore water cation concentrations (potassium, calcium, magnesium, and sodium). Sites were sampled in May 1996 and 1997, although spatial analysis was performed only for the data obtained in May 1997. Geostatistical analysis showed that most of the variables tested were spatially auto-correlated within each area and that there were no differences between regenerated and natural plots. Structural variance as a proportion of sample variance ranged from 55-99% for most of the properties. The significant spatial dependence observed for most of the variables in natural and regenerated plots, despite management schedules, indicates that clear-cutting did not have an effect on modifying the distribution of nutrient concentrations. These results suggest that after 15 and 25 yr following forest disturbance nutrient distributions were reestablished either there was not a net effect on patterns of element loss or tidal input was stronger than plant modifications of soils.