The Availability of Legacy Phosphorus for Rice Growth in Phosphorus-Rich Paddy Soils: A Two-Season Case Study

Understanding the short-term availability of legacy phosphorus (P) in soils is important for improving fertilizer management and reducing unnecessary P inputs. This study evaluated whether soil legacy P could temporarily support rice growth under continuous cultivation and assessed the short-term potential for reducing P fertilizer applications. Field experiments were conducted over two consecutive rice cropping seasons in paddy soils located in Taoyuan (TY) and Changhua (CH), Taiwan. Rice grain yield, grain P concentration, and soil P dynamics were compared between plots receiving chemical fertilizers (CF) and those without chemical P fertilization (NCF). Results showed no significant differences in grain yield or grain P concentration between CF and NCF treatments over the two cropping seasons. Sequential P extraction and P K-edge X-ray absorption near-edge structure (XANES) analyses of soils collected before planting and after harvest revealed a redistribution of soil P from moderately and slowly labile pools to more labile fractions during rice cultivation. These changes suggest that legacy P contributed to maintaining plant-available P during the short experimental period. Overall, this study provides short-term (two-season) field evidence that, in P-enriched paddy soils, legacy P can partially supply rice P demand without additional P fertilization. However, the long-term sustainability of this strategy and its applicability under different soil and management conditions require further investigation.