Lower nitrogen leaching from struvite than monoam monium phosphate is more size-dependent than phosphorus leaching

The use of wastewater-derived struvite (MgNH₄PO₄·6H₂O) as a lower water solubility alternative to highly water-soluble phosphorus (P) fertilizers such as monoammonium phosphate (MAP) has been widely hypothesized to reduce fertilizer nitrogen (N) and phosphorus (P) losses from soil, which stands to improve water quality. To test this hypothesis, inorganic N and P leaching were quantified using soil columns (Ap horizon of a smectitic, mesic Aquic Argiudoll) over 3.67 pore volumes (v_p). We additionally compared estimates of fertilizer N leaching by direct ¹⁵N tracing of ¹⁵N-labeled struvite and MAP, and to account for potential particle size effects also evaluated N leaching for commercial granular (unlabeled; 2.8 mm diameter) versus synthesized (¹⁵N-labeled; 0.3 mm diameter) fertilizer forms. Relative to MAP, struvite exhibited 4-fold lower N leaching and >300-fold lower P leaching from granular fertilizers, but similar N leaching and 10.7-fold lower P leaching from fine-sized fertilizers. Dissolution (%) was 4-fold lower for struvite than MAP regardless of fertilizer particle size. Though normalizing N and P leached to fertilizer dissolution did not alter estimated MAP losses, leaching of P and to a much greater extent N was markedly higher for struvite, particularly fine-sized. Overall, these findings provide strong support for the hypothesized advantage of struvite over highly water-soluble ammonium phosphate fertilizers such as MAP to mitigate leaching of applied N and P when used in granule form. Additionally, we find indirect evidence for incongruent dissolution of struvite that merits further mechanistic evaluation given implications for struvite N versus P fate following application to soil.