Biogeochemistry 53: 161–179, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Phosphorus cycling in a Mexican tropical dry forest ecosystem JULIO CAMPO 1∗ , MANUEL MAASS 2 , VÍCTOR J. JARAMILLO 2 , ANGELINA MARTÍNEZ-YRÍZAR 3 & JOSÉ SARUKHÁN 1 1 Departamento de Ecología Funcional y Aplicada, Instituto de Ecología, Universidad Nacional Autónoma de México. A. P. 70-275, México D. F. 04510; 2 Departamento de Ecología de los Recursos Naturales, Instituto de Ecología, Universidad Nacional Autónoma de México. A. P. 27-3 Xangari, C. P. 58089 Morelia Mich., México; 3 Instituto de Ecología, Unidad Regional Hermosillo, Universidad Nacional Autónoma de México. A. P. 1354, Hermosillo, Sonora, México ( ∗ author for correspondence, e-mail: jcampo@miranda.ecologia.unam.mx) Key words: atmospheric P input, P losses, P fluxes, P input-output, phosphorus cycle, tropical dry forest Abstract. The study was conducted in five contiguous small watersheds (12–28 ha) gauged for long-term ecosystem research. Five 80 × 30 m plots were used for the study. We quantified inputs from the atmosphere, dissolved and particulate-bound losses, throughfall and litterfall fluxes, standing crop litter and soil available P pools. Mean P input and output for a six-year period was 0.16 and 0.06 kg·ha −1 ·yr −1 , respectively. Phosphorus concentration increased as rainfall moved through the canopy. Annual P returns in litterfall (3.88 kg/ha) represented more than 90% of the total aboveground nutrient return to the forest floor. Phosphorus concentration in standing litter (0.08%) was lower than that in litterfall (0.11%). Phosphorus content in the litterfall was higher at Chamela than at other tropical dry forests. Mean residence time on the forest floor was 1.2 yr for P and 1.3 yr for organic matter. Together these results suggest that the forest at Chamela may not be limited by P availability and suggest a balance between P immobilization and uptake. Comparison of P losses in stream water with input rates from the atmosphere for the six-year period showed that inputs were higher than outputs. Balances calculated for a wet and a dry year indicated a small P accumulation in both years. Introduction Phosphorus is typically the most conserved nutrient in tropical forest ecosys- tems.Dissolved P output generally does not exceed 0.3 kg·ha −1 ·yr −1 , while nitrogen, calcium, magnesium and potassium losses are between 2 to 20 kg·ha −1 ·yr −1 (Bruijnzeel 1991).The coupled P conservation by leaves (Jordan et al. 1980), roots and mycorrhizas (Stark & Jordan 1978; Cuevas & Medina 1988), microorganisms and geochemical sorption (Salcedo et al.