Litter N and P dynamics in two secondary tropical dry forests after relaxation of nutrient availability constraints

Nitrogen (N) and phosphorus (P) cycling in litterfall were studied following N and/or P additions in two regenerating tropical dry forests (TDFs) growing on limestone in the Yucatán 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 replicate 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 3 consecutive years. Relaxation of nutrient limitation resulted in an increase of N and P fluxes to the soil at both the early-successional and late-successional forests. This effect was faster in the N flux (N masses increase in the 3-year period); while P flux increased after 2 years of repeated fertilization. However, changes in litterfall nutrient fluxes were not paralleled by nutrient accumulation (concentrations or pools) on the forest floor of both forests. The increase in the magnitude of N and P fluxes from litterfall combined with unchanged nutrient pools in the forest soil, suggests that fertilization accelerated the intrasystem cycling of both nutrients in early- and late-successional forests, by a rise the nutrient release.