Poultry-litter biochars (PLBs), which were prepared at two pyrolytic temperatures, were applied to the soils of croplands with four consecutive harvests of water spinach to assess the effects of PLBs on the soil properties and the growth of water spinach. The results show that PLB amendment resulted in an increase of soil pH. The electrical conductivity values, and the concentrations of extractable inorganic nitrogen, exchangeable potassium, and available phosphorus in the soils drastically increased in the 0.5% and 1% biochar-amended soils. However, most of the significant changes due to PLB amendment disappeared after four consecutive harvests of water spinach. The growth of water spinach was enhanced in the soils amended with PLBs, especially the one prepared at 350 °C. Nonetheless, the application of 1% PLBs to the soil resulted in an imbalance between calcium and magnesium in water spinach.

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.