Direct Observation of Alginate-Promoted Soil Phosphorus Availability

Abstract

Phosphorus (P) is a nonrenewable resource with low availability in soils and thus can be a yield-limiting factor for food production. Alginate from brown algae has been proved to be a promising fertilizer additive to promote P utilization efficiency so as to achieve sustainable P management. However, there has been a lack of direct observation of how alginate promotes P availability due to the complexity of the soil system. Here, by combining in situ atomic force microscopy (AFM) and Raman spectroscopy, we in real time observed the nanoscale phase transformation kinetics of amorphous P-bearing minerals (APM) (Fe−P and Ca−P) prepared with the addition of alginate before (treatment II) and after (treatment I) the synthesis of APM. The surface of crystalline P-bearing minerals (CPM) derived from the phase transformation of APM without alginate addition showed obvious nanoscale etch pits after exposure to alginate-bearing solutions at mineral−water interfaces, indicating the solubilization of coprecipitated P, which was then quantified through batch dissolution experiments. Overall, the results revealed that alginate delayed the phase transformation of APM and enhanced the dissolution of CPM in a concentrationdependent or polymerization degree-dependent manner to retain plant-available forms of P. In addition, treatment II could more significantly delay the phase transformation than treatment I. AFM-based dynamic force spectroscopy (DFS) suggested that, consistent with the formation of molecular organomineral bonding, alginate with a higher polymerization degree had a higher binding energy to P-bearing minerals, which would contribute to APM stabilization and CPM dissolution through stronger alginate− mineral interactions. These findings provide direct evidence for the P availability-promoting effect of alginate as an additive as well as some guidance for the better design of P fertilizer additives to achieve sustainable P management in agriculture.

Direct Nanoscale Imaging Reveals the Mechanism by Which Organic Acids Dissolve Vivianite through Proton and Ligand Effects

“…An efficient dissolution ability of Fe and P by organic acids may be attributed to the fact that the dissociation of protons from carboxyl groups promotes bond breaking, thus leading to mineral dissolution. , The carboxyl group has the ability to chelate metals as ligands, and its metal chelation ability increases with the increasing number of carboxyl groups. Studies have indicated that proton-promoted mineral dissolution results from the exchange action between H + and cations on mineral surfaces and the hydrolysis action of metal–O bonds under acidic conditions. − Organic ligands can form surface complexes with metal ions, thus decreasing the energy barrier of mineral hydrolysis reactions, eventually promoting mineral dissolution. − It is through this mechanism that citric acid dissolves inorganic P minerals. ,, Additionally, alginate acid dissolved inorganic Fe–P precipitates by different mechanisms at various solution-pH-induced H + concentrations and −COOH dissociation states . Under acidic conditions, the dissolution of Fe–P minerals is driven by both H + and −COO – in the solution, while under neutral or alkaline conditions, the dissolution of Fe–P precipitates is promoted only by −COO – …”

“…5,31,32 Additionally, alginate acid dissolved inorganic Fe−P precipitates by different mechanisms at various solution-pH-induced H + concentrations and −COOH dissociation states. 33 Under acidic conditions, the dissolution of Fe−P minerals is driven by both H + and −COO − in the solution, while under neutral or alkaline conditions, the dissolution of Fe−P precipitates is promoted only by −COO − . 34 It has been reported that organic acids such as oxalic acid and citric acid can release P by dissolving inorganic Fe−P precipitates in a pH-dependent manner, 22,35,36 but there exist some controversies about the role of H + in mineral dissolution.…”