Potential use of reclaimed struvite as a phosphorus fertilizer source and effects of low molecular weight organic acids (LMWOAs) in the rhizosphere on its performance in three contrasting soils

Abstract

Phosphorus (P) is a crucial macronutrient for crop growth, often sourced from rock phosphate-based fertilizers. Closing the P cycle through recycling and reusing is vital. Struvite, a reclaimed P product from wastewater treatment plants, offers promise as an alternative P fertilizer, promoting sustainability and reducing reliance on nonrenewable resources. The role of low molecular weight organic acids (LMWOAs) exuded in the rhizosphere is recognized as potentially aiding struvite dissolution in crop fields. This thesis research was conducted to address the knowledge gap regarding struvite's effectiveness as a phosphorus fertilizer across diverse soil pH, especially when compared to conventional phosphorus fertilizers, and to examine how adding low molecular weight organic acids influences struvite. The objectives of the first study were to investigate dissolution, P mobility, and potential plant availability of reclaimed struvite in comparison to conventional P fertilizers: monoammonium phosphate (MAP), diammonium phosphate (DAP), and ammonium polyphosphate (APP) in three different soils (mildly calcareous, acidic, and neutral) using short-term laboratory incubation studies combined with wet chemical analysis and synchrotron-based X-ray techniques. Soils were incubated for 1, 2, 5, 9, and 12 weeks in Petri dishes with four fertilizer treatments, reclaimed struvite (Str), MAP, DAP, APP, and a control. The objectives of the second study were to assess the impact of LMWOAs on struvite P dissolution, mobility, and potential plant bioavailability and the fate of LMWOAs in the same three soils using short-term laboratory incubation studies combined with wet chemical analysis and synchrotron-based X-ray techniques. Soils were incubated with three different soils for 1, 5, 9, and 12 weeks in Petri dishes with struvite (Str), with and without LMWOAs addition with a control. The citric acid (CA), oxalic acid (OA), malic acid (MA), and acetic acid (AA) were used as LMWOAs and were added as single organic acids at two rates (41mM-Low and 82mM-High) or as an acid mixture (CA + OA or CA + OA + MA + AA) at one rate (82mM- High). At the end of the incubation, dishes were sectioned into three concentric rings at different distances from the point of application (POA). The pH, resin extractable P to assess potential plant available P, and total P to determine P diffusion from the POA were analyzed. Percent P added (PPA) and Percent resin P (PRP) values were calculated based on total P and resin extractable P. To determine the reaction products of P X-ray absorption near edge structure (XANES) analysis was performed. Based on the results of the first study, reclaimed struvite showed comparative performance to conventional fertilizers after 9 weeks of incubation in neutral and acidic soils, but not in mildly calcareous soils. The formation of insoluble secondary P minerals via precipitation and sorption into CaCO₃ were the culprits of lower P mobility and availability in P fertilizers in calcareous soils. Modification or blending of struvite with conventional P fertilizers may enhance struvite performance under calcareous soil conditions. The results of the second study showed that in general, the efficacy of struvite in terms of mobility, and/or solubility was enhanced in the presence of LMWOAs in neutral and acid soils, while most of the LMWOAs added into mildly calcareous soil were degraded or sorbed. Compared to struvite alone (Str) treatment (20 and 40%), Str + CA + OA + MA + AA High treatment (40% and 50%) showed superiority in P diffusion into further sections, respectively, in neutral and acid soils after 12 weeks. Phosphorus availability (PRP per Petri dish) in neutral (39% and 44% of PRP) and acid soils (66 and 78% of PRP) after 12 weeks of incubation in struvite alone (Str) and Str + CA + OA + MA + AA High treatments, respectively. This is likely because the mixture of ligands is more successful in chelation and ligand exchange reactions. Moreover, relative to struvite alone (Str) treatment, P availability of Str + CA High treatment was higher in acid soil (PRP per dish 66% vs 71%, respectively), and Str + OA High treatment showed higher P availability in neutral soil (PRP per dish 40% vs 57%, respectively) after 12 weeks of incubation. This is likely because CA promotes P release through the chelation of Al and Fe in acid soils while OA promotes P release in less acidic/ neutral soils due to the high acidic strength of oxalic acid. Future studies with plants will give better insights into how these treatments impact crops.