Managing phosphorus fertilizer to reduce algae, maintain water quality, and sustain yields in water-seeded rice

In water-seeded rice systems, cyanobacteria (Nostoc spongiaeforme) hinder early-season crop growth by dislodging and reducing light to seedlings. Since algae are often phosphorus (P) limited, we investigated whether changing the timing of P fertilizer application could reduce algal growth without reducing crop yields or increasing mid-season water P concentrations to levels of concern for water quality. Water P and algae were monitored in 10 and 12 (respectively) side-by-side fields (16–60ha in size) where P fertilizer was applied pre-plant or where P application was delayed until after rice plants had emerged above the surface of the floodwater (2–5 weeks after seeding). Early-season water P concentration and algal occurrence were higher (P<0.001 and P=0.018, respectively) when P fertilizer was applied pre-plant as opposed to delayed. In fields receiving a delayed P application, water P increased to as high as 1.68mgL⁻¹ immediately following application and subsequently declined by 0.054mgL⁻¹day⁻¹ (P=0.029). A separate study evaluated the effect of P fertilizer timing on crop productivity and P uptake. Triple-super-phosphate was either not applied or was applied to the soil surface in the fall prior to the cropping season, immediately prior to planting, 35 days after seeding (DAS) and 49 DAS at a rate of 25kgha⁻¹P. P uptake and agronomic P use efficiency (APUE) were similar when P was applied at seeding or 35 DAS. However, relative to P application at seeding, yields were reduced by 6% and there was lower APUE when P was applied after harvesting the previous crop or at 49 DAS (P<0.05). These results indicate that correctly timed, delayed fertilizer P applications can maximize rice yield while reducing early-season interference from algae. However, because delayed applications of P fertilizer also increased water P concentrations, drainage water must be managed carefully following application.