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One greenhouse pot experiment was used to investigate the availability of phosphorus in struvite derived from urine affected by soil pH (cinnamon soil, pH 7.3; paddy soil, pH 5.3) and irrigation water (pH 6.0 and 7.5) with bird rapeseed (Brassica campestris L.). The biomass of applied struvite in paddy soil was significantly greater than that of applied calcium superphosphate. However, statistically significant differences were not observed in cinnamon soil. Soil-applied struvite had a higher Olsen P compared to soil-applied calcium superphosphate irrespective of soil type. The biomass of applied struvite and irrigation with pH 6.0 water was greater compared to that with irrigation with pH 7.3 water irrespective of soil type, accompanied with significantly higher leaf chlorophyll concentration. Therefore, struvite has the potential to be an effective P fertilizer, and acidic irrigation water has greater influence on the availability of phosphorus in struvite than does acidic soil. KEYWORDS: struvite, phosphorus, plant availability, soil, irrigation water ■ INTRODUCTIONPhosphorus (P), as an indispensable resource, is an essential nutrient for crop growth. The depletion timeline of P rock reserves ranges from 30 to 300 years.1 Although this estimation provides substantial uncertainty and more accurate data are lacking, there is no doubt regarding the irreversible depletion of high-concentration rock P reserves and the decline of the quality of the reserves (such as % P and presence of impurities) over time, 1 resulting in higher economic and energy cost for P exploitation. If P can be recovered in an economical way, it can re-enter the agricultural supply chain, displacing the consumption of traditional P fertilizer. It helps to create an ecofriendly environment by reducing the need for P rock 2 and to alleviate the P load emissions to water bodies. Human urine accounts for approximately 50% of the total P load in municipal wastewater, with <1% of the total wastewater volume.3 Direct application of urine in agriculture is feasible in practice. 4 However, huge quantities of separated urine are often difficult to transport and to store due to its unpleasant odor. Furthermore, health risk due to micropollutants and nitrogen (N) loss due to ammonia volatilization will be unavoidable during the application of urine to soil. Therefore, P recovery from urine in the form of struvite crystals will be a promising option with easy transportation, hygiene safety, and less nutrient loss.Struvite crystal is formed with an equal molar concentrations of magnesium (Mg), ammonium, and phosphate combined with six water molecules (MgNH 4 PO 4 ·6H 2 O), which contains 5.7% N, 12.6% P, and 9.9% Mg by weight. The solubility of struvite is highly dependent on pH, 5 and it is sparingly soluble in neutral and alkaline solutions, but readily soluble in acid solution. The P in the urine is mainly present as inorganic phosphate, and struvite obtained from urine has the advantages of higher P 2 O 5 content (28.9%) compared to common ...

Phosphorus loss from surface runoff contributes to eutrophication of surface water, a problem that is often severe from polders with organic soils where agricultural production is intensive. A soil P test is essential to predict the potential for P losses to precisely conduct environmental risk assessment and to efficiently develop and evaluate beneficial management practices. This study evaluated the possibility of using the environmental and agronomic soil P tests, soil P sorption index (PSI), and degree of soil P saturation (DPS), which are used for mineral soils, to predict surface runoff dissolved reactive P (DRP) from organic soils. Forty‐four soils from eight subgroups representative of organic lands across Ontario were selected to provide a wide range of soil test P (STP) within each category. A surface runoff study was conducted following the U.S. National Phosphorus Research Project protocol. Flow‐weighted mean runoff DRP concentration (DRP30) was linearly related to soil water‐ and CaCl2–extractable P concentrations but with data distribution patterns that inefficiently represented the soil variability in P release potentials. The runoff DRP30 was significantly related to Bray‐1 P and FeO‐extractable P concentrations in split‐line models, each with a change point, but not to Mehlich‐3 P and Olsen P. All DPS values calculated based on STP and their derived PSIs were closely related to runoff DRP30 in either a linear or a split‐line model. The DPS values expressed as Bray‐1 P/(PSI + Bray‐1 P) and FeO P/(PSI + FeO P) showed the highest correlation with runoff DRP30 and thus can be recommended as environmental risk indicators of surface runoff DRP from organic soils.

Batch experiments were conducted to examine the influence of various process parameters on phosphorus (P) recovery by struvite formation from urine. The results showed that the Mg/P molar ratio is one of the most important parameters affecting P recovery. The Mg/P molar ratio of 1.3 was found the most reasonable for struvite formation, and the P removal efficiency reached more than 96.6%. An increase in pH of urine solutions from 8.7 to 10.0 did not significantly affect P removal, but the quality of crystal formed at pH 10.0 was poor based on scanning electron microscopy analysis. A longer mixing time positively affected struvite formation, and compared to without mixing, the P removal efficiency increased from 72.7 to 97.3% after 5 min of mixing. The addition of seed material had no influence on the P removal efficiency, but contributed to the formation of struvite clusters.