Changes in coupled Iron and organic Carbon geochemistry following mangrove conversion to shrimp ponds and pasture in the Brazilian Amazon

Abstract

Mangroves play a crucial role in nutrient cycling, habitat provision, and carbon storage, yet they are under threat from human activities such as deforestation and land use change. The loss of over 180,000 hectares of mangrove forests in recent decades raises concerns about the dynamics of soil organic carbon (SOC). Although the impact of land use change on SOC stocks is documented, the biogeochemical processes governing SOC stabilization in mangroves are not well understood. The significance of reactive iron (FeR), especially poorly crystalline Fe oxides, in SOC cycling challenges traditional perspectives. This study aimed to investigate the influence of land use change, specifically the conversion of mangroves to shrimp ponds and pasture, on Fe-organic carbon (OC) associations to enhance our understanding of SOC stabilization processes in mangrove soils. The study focused on two pristine mangroves, three shrimp ponds, and one pasture along the Curuçá-river estuary in the Amazonian forest (North Brazil). Field measurements included pH and Eh assessments, while soil samples up to 100 cm were collected and transported to the laboratory. Physical fractionation of soil organic matter (SOM) isolated mineral-associated organic matter (MAOM), and wet-chemical extraction methods quantified different Fe operational fractions. These included Fe complexed with organic matter (Fec), Fe in poorly crystalline Fe oxides (Feca), and Fe in crystalline oxides (Fecbd), with their sum referred to as reactive Fe (FeR). OC concentrations were determined before and after the sequential wet-chemical extraction to quantify OC bound to Fe. Mangroves exhibited suboxic conditions with equilibria between pyrite and Fe oxides, while shrimp ponds displayed stability of iron oxides and pasture between Fe2+/Fe iron oxides. The MAOM in mangroves ranged from 17 to 25 g C kg-1, with concentrations generally higher in the subsurface. The reduction in MAOM due to conversion to shrimp pond and pasture was accompanied by a sharp decrease in Fe-bound OC in all depths as the Fe-bound OC represented 10.5 ± 0.9% of the relative OC content in MAOM in mangroves, 6.5 ± 2.9% in shrimp ponds, and only 1.9% in pasture. Concerning Fe fractions, the dominant fraction in mangroves was Fec (3.6 ± 0.8 g kg-1), followed by Feca (0.9 ± 0.04 g kg-1), and then Fecbd (0.5 ± 0.1 g kg-1). Fec experienced the most substantial loss due to land use change, representing a loss of 85% in shrimp ponds and 97% in pasture. There was an increase in Fecbd in detriment of Fec and Feca in shrimp ponds (2.8 ± 2.4 g kg-1), leading to nearly equivalent FeR content of mangroves. In the other hand, there was a substantial loss of FeR in pasture (more than 85%). Land use change from mangrove to shrimp pond and pasture resulted in oxidation of pyrite, leading to the precipitation of more crystalline Fe oxides or Fe loss as soluble Fe2+. This process decreased Fe species prone to SOC accumulation (Fec and Feca), releasing OC to the soil solution and highlighting the susceptibility of Fe bound-OC as a significant SOC pool in mangroves.