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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.