Sequential phosphorus (P) extraction (SPE) is a well-established and widely applied method for quantitatively and qualitatively determining the critical nutrient P in freshwater sediments. It allows the estimation of P bioavailability and facilitates the evaluation of the long-term effects of eutrophication mitigation measures. Most current protocols do not differentiate between redox-sensitive Fe(III)-P and more stable reduced Fe(II)-P minerals, such as vivianite. In this study, we tested a modified SPE protocol designed to quantify Fe(II)-P (vivianite-P) as a separate phase through the complexation of Fe(II) with 2,2′-bipyridine (Bipy). Seven lakes were selected as study sites with different sedimentary Fe and P contents and restoration histories. We validated the Bipy extraction step through direct comparison with results from the conventional protocol and the application of direct mineral detection methods, including x-ray absorption near-edge structure at the Fe and P K-edges, x-ray diffraction, optical microscopy, and scanning electron microscopy with energy dispersive x-ray spectroscopy. The Bipy fraction was primarily extracting P that was conventionally extracted in the bicarbonate-dithionite (redox-sensitive Fe/Mn-bound) and NaOH (metal-[Fe/Al-]bound) fractions. The results from the direct detection methods indicated that the extracted Fe(II)-P was predominantly vivianite. The efficiency of the Bipy extraction was decreased in samples with high crystallinity, but excessive Fe(II) or high organic content had minimal impact. Hence, it is highly recommended to use x-ray diffraction and x-ray absorption near edge structure in combination with the modified extraction protocol. Overall, the method tested with different freshwater sediments provides robust results when quantification of Fe(II)-P including vivianite is an important objective.

Brazil is an agricultural giant that plays a crucial role in the Global Phosphorus Challenge (GPC), and whose highly weathered soils are currently dependent on phosphorus (P) fertilizers derived from phosphate rock, a dwindling and critical resource. Brazil imports > 50% of its P fertilizers and P recovery from waste is not yet explored in the country, making it vulnerable to market instabilities, phosphate rock availability, and geopolitical conflicts. To make matters worse, Brazilian research budget has been shrinking for 7 years straight, hindering scientific efforts and causing significant ‘brain drain’, further undermining the country’s capacity to tackle this critical problem. However, an opportunity comes with the new Brazilian Federal government (starting January 2023), which promises to make significant investments in science and higher education. We call for all stakeholders to seize this important moment and timely collaborate in creating multidisciplinary P-related projects, taking advantage of the soon-to-be available resources to develop knowledge, technologies, and training networks to shape a new generation of experts in P management in the tropics. We are confident that through agriculture intensification, intelligent use of resources, new legislation and governance, Brazil will stride towards sustainable food production, bringing immediate value to Brazil and the world by protecting the Amazon forest and advancing to overcome the GPC.