Managing Phosphorus Fertilizer to Reduce Algae, Maintain Water Quality and Sustain Yields In Direct-Seeded, Flooded Rice.

Although the presence of nitrogen-fixing blue green algae (Cyanobacteria) is beneficial to crop productivity in particular rice ecosystems, in direct-seeded, California rice systems, the early-season growth of Nostoc spongiaeforme hinders crop productivity by forming mats on the surface of the floodwater that dislodge rice seedlings and block light.  Because the growth of N. spongiaeforme is known to be phosphorus (P) limited, the objective of this study was to determine whether changing the timing of P fertilizer application in a direct-seeded rice system: 1) reduces early-season (0-30 days after planting (DAP)), in-field water phosphate concentrations and N. spongiaeforme occurrence; 2) increases mid-season (20-60 DAP), in-field water phosphate concentrations to levels of concern for surface water quality; and 3) limits rice productivity. 

We measured water phosphate concentrations and N. spongiaeforme abundance in fields where phosphorus fertilizer had been applied either pre-plant or between 15 and 35 DAP.  Early-season, in-field water phosphate concentration was positively correlated with an increasing occurrence of N. spongiaeforme.  Additionally, where P was applied between 15 and 35 DAP, early-season water phosphate concentrations and N. spongiaeforme abundance were reduced compared to pre-plant applications.  While these mid-season applications of P fertilizer effectively reduced early-season interference from N. spongiaeforme, they also resulted in a spike in water phosphate concentration to levels above 0.1 ppm up to 30 days after the mid-season application. 

In a separate study we measured rice growth and P uptake in P-deficient fields for treatments where P fertilizer was applied the previous fall (FALL), immediately prior to planting (PRE), 35 DAP (35D), 49 DAP (49D), or not applied (control).  The effects of alternative P fertilizer applications on rice growth were mixed.  35D resulted in rice yield and P uptake at harvest equivalent to PRE with a higher harvest index and phosphorus harvest index (PHI), whereas, rice in both FALL and 49D treatments suffered a 6% yield penalty compared to PRE due, in part, to the fact that FALL took up less P than PRE, and 49D had a lower PHI than 35D. 

These results indicate that, if timed correctly, mid-season applications of P fertilizer can maximize rice yield while reducing early-season interference from N. spongiaeforme.  However, because mid-season applications of P also resulted in water phosphate concentration at levels that might compromise surface water quality, outlet water must be managed carefully post-application.