How do replanted mangroves affect phosphorus dynamics in their soils?

ABSTRACT

Phosphorus is a limiting element for the productivity of mangroves, which in turn are important ecosystems in regulating nutrients cycle and climate change by sequestering carbon (C). Despite this, there is an intense process of degradation in these environments. In addition to providing socio-environmental services, mangrove replanting can also alter the dynamics of nutrients in soils. Therefore, this study aims to understand the changes in soil phosphorus (P) fractions after a mangrove restoration. Soil samples from an unvegetated area (NV), a mature mangrove (R) and 7 and 9 year old replanted mangroves at SE-Brazil (APA Guapi-mirim, Rio de Janeiro state) were collected and analyzed to characterize the redox conditions (Eh), pH, and iron (Fe) fractionation, Total Organic Carbon (TOC) contents and P fractionation (exchangeable P; P associated with reducible Fe and Mn oxyhydroxides; associated with Al silicates and hydroxides; associated with humic acids; associated with Ca and Mg; associated with humin). The results indicate an increase in TOC as the age of the mangrove restoration increases (from 8.6 to 17.9%). The pH values were significantly lower, reaching very acidic values, associated with an increase in Eh. Both parameters also showed strong seasonal variation, with a drop in Eh during the wet period (from 165% to -46%) and an increase in pH in the same period (from 6.0 to 6.7). Regarding P fractionation, the main P pool was organic P forms, which showed the highest concentrations in all studied sites. Unvegetated areas showed higher organic P forms (NV 108.8 µg g-1) than vegetated areas (M7 55.7 µg g-1, M9 83.6 µg g-1, R 87.3 µg g-1). Vegetated sites also showed lower levels of the PEx, PFeMn and Papatite fractions (total forest mean 2.4 µg g-1, 5.8 µg g-1, 3.0 µg g-1, respectively). Besides no clear trend on P fractionation through seasons and forest age, pseudo-total P increased following the forest recovery (e.g. M7nutrient retention capacity with the replanting of the mangrove. Mangroves replanting affected P dynamics, promoting an accumulation of P in soils, also indicated the reestablishment of ecosystem services, such as nutrient cycling. Seasonal variations influenced the ability of P retention and release for adjacent areas. Our findings indicate that mangrove restoration affected the key parameters (pH, Eh, TOC) which controlled soil processes (redox reactions, organic matter degradation), impacting the cycles of Fe, C, and P cycles. Therefore, crucial soil ecosystem services (e.g., nutrient regulation and C storage) were improved by replanting mangroves. However, seasonal trends on nutrient dynamics still need further studies to better understand soil processes, especially in replanting environments.