Direct Observation of Alginate-Promoted Soil Phosphorus Availability

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 concentration-dependent 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.