IMPACT OF MICROCLIMATE AND MINERAL STATUS ON NITROGEN FIXATION IN THE MOIST GULLY FORESTS OF BARBADOS, WEST INDIES

Plant community structure, the related intra-gully climate, soil and fine litter nitrogen fixation, and nutrient dynamics were studied. Applewaite Gully, Barbados, supports a sunken forest with a temperature/precipitation ratio of 0.017 placing it on the moist end of the dry forest spectrum (25°C/1475 mm annual rainfall). The abrupt topographic change characteristic of the gullies results in a reduced vapor pressure deficit which maintains adequate litter and soil moisture to support nitrogen fixation. Mean acetylene reduction activity (ARA) value for fine litter was 25.1 mM C₂H₄m⁻² year⁻¹ at 191% moisture content and the mean soil ARA value was 14.3 mM C₂H₄m⁻²year⁻¹ at 59.5% moisture content. The potential nitrogen contribution by litter and surface soil nitrogen fixation ranged between 0.78 and 0.95 kgNha⁻¹ year⁻¹. Fine litter and soil nitrogen fixation potentially amends the low nitrogen content of infertile gully soils. The N values for fine litter and soil were low and characteristic of infertile soils, whereas P values were high. ARA exhibited a strong negative correlation with both soil and litter %N. Soil ARA may have been reduced due to high soil clay content limiting the availability of organic material.