Soil microbial biomass and activity in tropical riparian forests

A large body of work has demonstrated that riparian ecosystems play a critical role regulating interactions between terrestrial and aquatic components of temperate zone landscapes. However, there have been relatively few studies of the role of riparian forests in tropical landscapes. There is a strong need to evaluate the effects of conversion of these forests to pasture and residential land uses on their ability to prevent the movement of nutrients and other pollutants to coastal water bodies. The objectives of this study were (1) to measure a suite of microbial properties that are indicators of the ability of soils to attenuate pollutants and cycle nitrogen (N) in tropical riparian forests under different land use management and (2) to evaluate the effects of conversion of two types of tropical forested wetlands (cativo, Prioria copaifera and yolillo, Raphia taedigera) on this same suite of soil microbial variables. To accomplish objective #1, we measured these variables in surface soils in intact forest, disturbed forest, pasture and residential land use sites in two locations in the Atlantic Zone of Costa Rica. For objective #2 we measured these variables in surface soils of three replicate intact and converted cativo and yolillo forests. Results suggest that land use conversion does not decrease levels of microbial biomass and activity in these soils. Intact forest sites did not have higher levels of microbial biomass and activity than ‘disturbed sites.’ In several cases, levels of these variables were consistently lower in intact forest than disturbed sites. There were no statistically significant differences between intact and converted cativo and yolillo sites. The relatively low level of activity in the intact sites is likely a ‘succession effect’, where the disturbed sites have younger, more actively growing vegetation that supplies more readily available organic matter to the soil microbial community. High correlations between soil organic matter and microbial biomass and activity strongly support this ‘successional’ explanation of our results.