Soil N and P dynamics in two secondary tropical dry forests after fertilization

Soil nitrogen (extractable, microbial, potential N-transformations, and total) and phosphorus (available, microbial, and total) cycling were studied following N and/or P additions in two regenerating secondary tropical dry forests (TDFs) growing on limestone in the Yucatan Peninsula (Mexico) differing in successional stage and nutrient status. Our sites included forests in early-successional stage (10-year-old), where primary production is limited by N and by P, and in late-successional stage (~60-year-old), where primary production is limited by P. Four independent plots (12 m×12 m) at each forest were either left intact (controls) or fertilized with N, with P, or with N plus P for three consecutive years. In both forest soils, N availability was high: potential N mineralization and nitrification were consistently high and positive, and the ratio nitrification/N mineralization was always greater than one. In the youngest forest, the repeated P fertilization significantly increased total pool sizes of N and its potential mineralization in the soil. In this forest, both N and P additions were actively incorporated into the microbial biomass (between 15 and 30% of N and P added were immobilized). Microbial biomass is thus an important factor contributing to nutrient immobilization in this N and P-limited site. Similar to younger forest, additions of nutrients to the late-successional forest increased the potential N-transformations and P fertilization increased the total N in the soil. However, in contrast to the younger counterpart, fertilization did not significantly alter the large standing pools of soil microbial biomass N and P. In summary, these results suggest that N and P inputs during the recovery of tropical dry forest on limestone soils in Yucatan follow different pathways depending of the successional stage of the vegetation.