Root dipping application of wastewater-derived struvite to rice seedlings for a safe and efficient supply of N and P in paddy cropping systems

Background and Aims Root dipping (i.e. attaching fertilizer-soil slurry to the seedling roots) with N and P fertilizers is a method to enhance nutrient uptake efficiency (NUE) in paddy rice (Oryza Sativa L.), yet NH4+ toxicity may be the limiting factor. This study was set up to test waste-derived struvite (ST) for root dipping relative to a soluble N-P source (MAP). In addition, root dipping with ST was compared to using a non-toxic P source (TSP). Methods Short-term NH4+ toxicity effects on transplanted seedlings were studied for MAP and ST applied at equal N dose by root dipping in two soils with contrasting CEC, i.e. different risk for toxicity due to NH4+. In a 44 DAT greenhouse trial, long-term P availability from ST was compared to TSP at equal P dose in a P deficient soil. Results A diffusion test showed slow NH4+ release from ST compared to MAP in both soils. Root dipping with ST showed 2 times higher biomass than MAP at equal N dose and no signs of NH4+ toxicity, the high CEC soil buffered better against that toxicity. Shoot biomass at 44 DAT was two times higher for ST than for TSP, likely because of a pH increase with ST. Conclusion Root dipping with ST is a safe method to enhance NUE in paddy rice. With its liming effect and its lower solubility, ST offers a safer, more efficient and sustainable alternative to MAP and TSP.