The impact of Pentaclethra macroloba on soil microbial nitrogen fixing communities and nutrients within developing secondary forests in the Northern Zone of Costa Rica

In the current study, we showed that Pentaclethra macroloba appears to influence the microbial communities within the soil of secondary forests in the Northern Zone of Costa Rica. The soil closest to the Pentaclethra macroloba trees (tree soil) had a lower abundance of the N-fixing bacteria Frankia, but greater amounts of both nifH gene and Rhizobium than soils furthest away (forest soils). These soils also had greater amounts of microbial biomass and were more efficient at organic carbon use as indicated by lower metabolic quotient values (qCO2). This suggests that the use of Pentaclethra macroloba in restoration of soils from harvested forests may provide a good strategy for recuperating nitrogen in the soils due to its association with N-fixing bacteria near the roots of the trees and in the nearby soils. As well, these trees were associated with enhanced microbial biomass, microbial activity, and microbial mediated enhanced efficiency of C use into the biomass, suggesting their value in improving soil richness. A tangential discovery during this study was that it was the first, to our knowledge to show the presence of Frankia in association with Pentaclethra macroloba. It is interesting to note that significant amounts of Frankia were found throughout the soils in this study, and it was present in all sites at levels far greater than Rhizobium. It is likely that Frankia is more critical in recuperating soil N within these developing secondary forests. Resumen: En este estudio mostramos que Pentaclethra macroloba parece influir sobre las comunidades microbianas en los suelos de bosque secundario en la zona norte de Costa Rica. El suelo mas cercano a los arboles de Pentaclethra macroloba (suelo de arbol) tuvo una abundancia menor de la bacteria fijadora de N Frankia, pero cantidades mayores tanto del gen nifH como de Rhizobium que los mas alejados (suelos de bosque). Estos suelos tambien tuvieron cantidades mayores de biomasa microbiana y fueron mas eficientes en el uso del carbono organico, segun lo indicaron los valores mas bajos del cociente metabolico (qCO2). Esto sugiere que el uso de Pentaclethra macroloba en la restauracion de suelos de bosques cosechados puede brindar una buena estrategia para recuperar el nitrogeno del suelo debido a su asociacion con bacterias fijadoras de N cerca de sus raices y en los suelos aledanos. Asi mismo, estos arboles estuvieron asociados con valores mas altos de biomasa microbiana y de