Dinâmica de fósforo em solos de manguezais replantados no estuário do rio Pacoti (Ceará, NE-Brasil)

Abstract

Mangroves and salt flats (apicuns) are coastal ecosystems essential for carbon sequestration and nutrient cycling, being sensitive to climatic variations and anthropogenic activities. This study evaluated how the colonization of salt flats by mangroves alters phosphorus (P) dynamics in the soil, quantifying its total content and biogeochemical fractions. The research was conducted in the Pacoti River estuary, Ceará, comparing three conditions: salt flat (AP), recovering mangrove (REC), and established mangrove (MG), during the dry and rainy periods. The methodology involved collecting soil samples up to 50 cm deep to determine pH, redox potential (Eh), and sequential P fractionation into six distinct fractions (PEx, PFe, PAl, PAH, PCa, and PHm). The results demonstrated that the area factor was the main source of variation, with pseudo-total P levels significantly higher in the established mangrove (123.3 ± 69.4 mg.kg⁻¹), followed by the recovery area (64.1 ± 40.4 mg.kg⁻¹) and the salt flat (30.7 ± 23.3 mg.kg⁻¹). A marked seasonality was observed, in which the dry period favored the accumulation of organic and inorganic fractions associated with metals, while the rainy period, marked by a reduction in redox potential and an increase in pH, promoted the mobilization of phosphorus into more bioavailable forms. The fraction associated with apatite (PCa) showed greater stability against seasonal variations. It is concluded that the colonization of salt flats by mangroves increases the soil's nutrient retention capacity. The progressive similarity in speciation dynamics between the recovery areas and the preserved mangrove indicates that the environmental restoration process in the Pacoti River estuary is succeeding in reestablishing the ecosystem's biogeochemical functions.