Sediments may act as a source or sink of ions from water, especially phosphorus (P). After the erosion process, the challenge is to retain suspended sediments inside the watershed area. We hypothesize that weir structures may mitigate P transfer from agricultural soils to aquatic systems. To test this, the present work aimed to evaluate P chemical fractions present in bottom sediments retained in the water reservoir of weirs built in two intensive agricultural watersheds and we discuss the sink/source behavior of these sediments. Samples of bottom sediments were collected from 12 reservoirs of weirs in two smalls Brazilian watersheds. Chemical P fractionation including adsorption and desorption kinetics was performed. Total P varied from 398 to 958 mg kg⁻¹. The easily and potentially desorbed phosphorus were correlated with the clay content and the concentrations of Fe, Mn, and Al, extracted by ammonium oxalate and dithionite-citrate-bicarbonate solutions, which are the carriers of highly reactive functional groups with phosphate. Bottom sediments have high maximum adsorption capacity under unrestricted phosphorus supply condition, while under natural condition (low anthropogenic pressure), the sediments were not saturated and therefore low desorption occurred. Only 1.34% and 7.75% of total P was in readily and potentially bioavailable P forms. The bottom sediments accumulated in reservoirs from anthropogenic areas acted as P source to water, while those from preserved areas (natural pasture or riparian forest) acted as sink of P from water. The storage of water and sediment contaminated with P in reservoir of weirs may be an efficient and temporary practice to mitigate the transfer of P to watercourses.

沉积物可能是水（尤其是磷（P））中离子的来源或汇。侵蚀过程之后，面临的挑战是将悬浮的沉积物保留在流域内。我们假设堰结构可以减轻磷从农业土壤向水生系统的转移。为了验证这一点，本工作旨在评估存在于两个集约化农业流域中的堰的水库中保留的底部沉积物中存在的P化学成分，并讨论这些沉积物的汇/源行为。从巴西两个小流域的12个堰堰水库收集了底部沉积物样品。进行了包括吸附和解吸动力学在内的化学P分馏。总磷在398至958 mg kg ^{-1之间变化}。草酸铵和连二亚柠檬酸盐-碳酸氢盐-碳酸氢盐溶液萃取的容易和潜在解吸的磷与粘土含量和铁，锰和铝的浓度有关，它们是与磷酸盐具有高反应性的官能团的载体。在不受限制的供磷条件下，底部沉积物具有最高的最大吸附能力，而在自然条件下（人为压力低），沉积物不饱和，因此解吸量较低。总磷中仅有1.34％和7.75％处于容易和潜在生物利用的磷形式。来自人为区域的水库中沉积的底部沉积物是水的磷源，而来自自然保护区（天然草场或河岸森林）的沉积物则是水的磷汇。