33P-isotope labelling ammonium phosphate fertilizers reveals majority of early growth maize phosphorus is soil-derived

In soils managed to have adequate to high Mehlich-3 phosphorus (P) concentrations throughout the US Maize Belt, the majority of crop P is soil-derived. Struvite, a low water solubility ammonium phosphate fertilizer, may be therefore substituted for relatively high water-soluble monoammonium phosphate (MAP) without adversely impacting maize (Zea mays L.) P uptake and growth, while minimizing fertilizer P loss risk. We determined the relative contribution of struvite and MAP to maize P uptake and soil solution P in soils representative of the US Maize Belt by radiolabelling fertilizers with ³³P. We found 8% (struvite) to 22% (MAP) of early-to-mid vegetative growth stage (V7) maize P was fertilizer-derived, and thus, 78%–92% was soil-derived. Despite similar aboveground P uptake and maize growth, maize P use efficiency (PUE) determined directly by ³³P was <5% for MAP (4.9%) and struvite (1.9%) indicating that in soils with adequate to high crop-available P, early season fertilizer PUE is relatively low. If prorated to harvest stage, in-season PUE was estimated to be 8% for struvite and 20% for MAP. MAP and struvite did not differ in relative contributions to water-extractable P, a proxy for P loss risk, potentially reflecting lag effects in struvite P dissolution and/or the relatively fine particle size of synthesized fertilizers (<0.1 mm diameter). Since maize aboveground biomass and P uptake were similar for both struvite and MAP, struvite could be an effective P fertilizer for soils with adequate to high Mehlich-3 P concentrations common across the US Maize Belt.