Litter degradation and CN dynamics in reforested mangrove plantations at Gazi Bay, Kenya

Abstract

The main objective of this study was to assess how mangrove reforestation has influenced litter degradation and concomitant nutrient dynamics in previously deforested plantations. Dynamics of nutrients (carbon, nitrogen and C:N ratios) in decomposing leaves of conspecific species were investigated with litterbags in Sonneratia alba and Rhizophora mucronata reforested treatments using appropriate bare and natural less disturbed treatments as controls. Bare treatments had the lowest decay rates (Kd 1) and thus the highest t50 values (when 50% of the original weight had been decomposed) for both species. The contrary was true for natural treatments, while both parameters were intermediate in reforested treatments, suggesting that other than direct litter input, reforestation has modified site conditions which have enhanced organic matter decomposition. There were significant seasonal differences in decay rates for treatments within the R. mucronata species, with rates being higher during the wet season with accompanying lower t50 values. Decay rates were overall higher (P < 0.05) in the S. alba species and as a result no litter was retrieved from its natural treatment by the 5th week. Higher amphipod colonisation was observed in reforested and natural treatments than bare treatments, which may have contributed to higher decay rates in the former. There were significant differences (P < 0.05) in N concentration among treatments with natural and reforested treatments having similarly higher concentrations than bare treatments in both seasons. C:N ratios (an important determinant of nutritional leaf quality) were also similarly low in natural and reforested treatments and higher in bare treatments. Mangrove reforestation thus seems to have enhanced litter degradation and concomitant nutrient remineralisation, suggesting that other than species litter quality, tidal inundation and seasonal factors, specific stand management regimes play an important role in determining the efficiency of these ecological processes in mangrove ecosystems.