Plant-soil-microbial interactions as mechanisms of nitrogen retention in a tropical wet forest

In humid tropical forests, the potential for N leaching losses are high due to abundant rainfall throughout the year, however plant-soil-microbial interactions have not been well investigated as mechanisms of N retention. Microbial N retention in tropical forests may be an important function which contributes to the maintenance of high forest productivity. This study examines seasonal patterns in the dominant overstory (including long-term monthly litterfall production and sap flow velocity), microbial biomass-C and N, extractable soil N, and potential N mineralization to determine if the soil microbial community retains N during periods of reduced plant uptake. The field experiment utilizes long-term forest fertilization and aboveground litterfall exclusion plots to investigate if nutrient availability or aboveground C input stimulates seasonal patterns in microbial-C and N. Due to concern of soil acidification resulting from long-term inputs of acidifying fertilizers, the effects of fertilization on soil acidity and fertility were also studied. Applying acid forming fertilizers for thirteen years has resulted in a significant reduction in pH from 4.40 to 3.70 and base cation saturation from 66% to 34%. Aluminum and Fe concentrations have also significantly increased. Soil acidification in fertilized plots can be attributed to an increase in acid dissociation rather than acid strength or quantity. Seasonal variation was observed in all variables measured with the exception of potential N mineralization rate. Litterfall production and sap flow velocity increased significantly with temperature during the summer months, and were lowest during the cooler and drier months of February and March. During this period of reduced plant productivity, microbial biomass-C and N were higher, suggesting nutrient retention. Fertilization and litter exclusion had significant effects on extractable soil nitrate, ammonium, and total inorganic N concentrations. However microbial biomass-C and N were stimulated by nutrient availability, rather than aboveground litter input. Microbial biomass-C and N were significantly higher in fertilized plots, whereas lifter exclusion had no significant effect on microbial biomass-C or N. Belowground processes such as lifter input and root exudation appear to be more important than aboveground lifter input for the regulation microbial biomass-C and N.