AQUATIC AND RIPARIAN WEED MANAGEMENT TO PROTECT U.S. WATER RESOURCES IN THE FAR WEST UNITED STATES
Location: Exotic and Invasive Weeds Research
Title: Managing phosphorus fertilizer to reduce algae, maintain water quality, and sustain yields in water-seeded rice
Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 8, 2012
Publication Date: May 13, 2012
Citation: Lundy, M.E., Spencer, D.F., Van Kessel, C., Hill, J.E., Linquist, B.A. 2012. Managing phosphorus fertilizer to reduce algae, maintain water quality, and sustain yields in water-seeded rice. Agriculture, Ecosystems and Environment. 131(2012):81-87 doi:10.1016/j.fcr.2012.03.005.
Interpretive Summary: In California rice fields, algae can cause significant reduction in yield. One approach to reducing algal growth is to reduce phosphate levels in the water. However, phosphate is applied to rice fields to enhance rice yield. In this study conducted in several active rice fields, we measure the effects of altering the timing of phosphate fertilizer applications on the levels of phosphate in the water and the abundance of algae. The results show that when timed correctly delayed application of phosphate fertilizer reduces growth of algae but does not reduce rice yields. Delayed phosphate fertilizer applications temporarily increase mid-season levels of phosphate in field water samples. These findings provide an alternative method for managing algae in rice fields and may lead to a reduction in the amount of copper sulfate applied as an algaecide.
In water-seeded rice systems blue-green algae (cyanobacteria) hinder early-season crop growth by dislodging rice seedlings and reducing light. Since algae are often phosphorus (P) limited, we investigated whether changing the timing of P fertilizer application could reduce algae without reducing crop yields or increasing mid-season water P concentrations to levels of concern for water quality. Water PO4-P and algae were monitored in fields receiving pre-plant applications of P fertilizer as well as in fields where P application was delayed until after rice plants had emerged from the surface of the floodwater (2 to 5 weeks after seeding). Early-season water PO4-P concentration and algal occurrence were higher (P<0.001 and P=0.018, respectively) in pre-plant fields than delayed fields. In fields receiving a delayed application of P fertilizer, water PO4-P increased to as high as 1.68mg L-1 immediately following the application but declined rapidly after the application (P=0.029). A separate study measured how the application timing of fertilizer P affected crop yields. Triple-superphosphate was either not applied or was applied to the soil surface the fall prior to the cropping season, immediately prior to planting, 35 days after seeding (DAS) or 49 DAS at a rate of 25 kg ha-1 P. Yields were similar when P was applied at seeding or 35 DAS; however yields were reduced by 6% when P was applied either the previous fall or 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, outlet water must be managed carefully following P application.