Mass balance assessment of phosphorus dynamics in a fertilizer trial with 57 years of superphosphate application under irrigated grazed pasture

Improving the efficiency of phosphorus (P) use is a major challenge for agricultural production and sustainability. Using a combination of new and historic data, a mass balance approach was employed to construct and discuss a comprehensive P budget under temperate irrigated grazed pasture that had received different inputs of superphosphate fertilizer for 57 years [nil (Control), 188 kg ha(-1 )(188PA) and 376 kg ha(-1) (376PA)]. Most (97-99%) of the applied P was accounted for in soil storage, plant residues, removal in animal products, excretal transfers, losses via irrigation outwash, rainfall runoff and leaching in the soil-plant-animal system. Management of soil available P that exceed the critical level 17-22 mg L-1) for optimal pasture production can result in low P balance efficiency and excessive soil legacy P in the soil profile (0-1 m). Results of this study revealed that accumulation of P in soil and plants (68-80%), P losses by irrigation outwash (8-11%), and excretal transfers to stock camps (6-12%) were important factors that determined applied P use efficiency. These findings highlight the need to apply appropriate quantities of P fertilizer to maintain optimal soil P fertility, plant growth, and animal production, together with enhanced utilization of accumulated soil P and reduced P transfer in drainage.