Phosphorus (P) is a nonrenewable resource and a critical element for plant growth that plays an important role in improving crop yield.  Excessive P fertilizer application is widespread in agricultural production, which not only wastes phosphate resources but also causes P accumulation and groundwater pollution.  Here, we hypothesized that the apparent P balance of a crop system could be used as an indicator for identifying the critical P input in order to obtain a high yield with high phosphorus use efficiency (PUE).  A 12-year field experiment with P fertilization rates of 0, 45, 90, 135, 180, and 225 kg P₂O₅ ha^–1 was conducted to determine the crop yield, PUE, and soil Olsen-P value response to P balance, and to optimize the P input.  Annual yield stagnation occurred when the P fertilizer application exceeded a certain level, and high yield and PUE levels were achieved with annual P fertilizer application rates of 90–135 kg P₂O₅ ha^–1.  A critical P balance range of 2.15–4.45 kg P ha^–1 was recommended to achieve optimum yield with minimal environmental risk.  The critical P input range estimated from the P balance was 95.7–101 kg P₂O₅ ha^–1, which improved relative yield (>90%) and PUE (90.0–94.9%).  In addition, the P input–output balance helps in assessing future changes in Olsen-P values, which increased by 4.07 mg kg^–1 of P for every 100 kg of P surplus.  Overall, the P balance can be used as a critical indicator for P management in agriculture, providing a robust reference for limiting P excess and developing a more productive, efficient and environmentally friendly P fertilizer management strategy.

Phosphorus (P) leaching is a major problem in greenhouse vegetable production with excessive P fertilizer application.  Substitution of inorganic P fertilizer with organic fertilizer is considered a potential strategy to reduce leaching, but the effect of organic material addition on soil P transformation and leaching loss remains unclear.  The X-ray absorption near-edge structure (XANES) spectroscopy technique can determine P speciation at the molecular level.  Here, we integrated XANES and chemical methods to explore P speciation and transformation in a 10-year field experiment with four treatments: 100% chemical fertilizer (4CN), 50% chemical N and 50% manure N (2CN+2MN), 50% chemical N and 50% straw N (2CN+2SN), and 50% chemical N and 25% manure N plus 25% straw N (2CN+2MSN).  Compared with the 4CN treatment, the organic substitution treatments increased the content of labile P by 13.7–54.2% in the 0–40 cm soil layers, with newberyite and brushite being the main constituents of the labile P.  Organic substitution treatments decreased the stable P content; hydroxyapatite was the main species and showed an increasing trend with increasing soil depth.  Straw addition (2CN+2SN and 2CN+2MSN) resulted in a higher moderately labile P content and a lower labile P content in the subsoil (60–100 cm).  Moreover, straw addition significantly reduced the concentrations and amounts of total P, dissolved inorganic P (DIP), and particulate P in leachate.  DIP was the main form transferred by leaching and co-migrated with dissolved organic carbon.  Partial least squares path modeling revealed that straw addition decreased P leaching by decreasing labile P and increasing moderately labile P in the subsoil.  Overall, straw addition is beneficial for developing sustainable P management strategies due to increasing labile P in the upper soil layer for the utilization of plants, and decreasing P migration and leaching.

Partial substitution of chemical fertilizers by organic amendments is adopted widely for promoting the availability of soil phosphorus (P) in agricultural production.  However, few studies have comprehensively evaluated the effects of long-term organic substitution on soil P availability and microbial activity in greenhouse vegetable fields.  A 10-year (2009–2019) field experiment was carried out to investigate the impacts of organic fertilizer substitution on soil P pools, phosphatase activities and the microbial community, and identify factors that regulate these soil P transformation characteristics.  Four treatments included 100% chemical N fertilizer (4CN), 50% substitution of chemical N by manure (2CN+2MN), straw (2CN+2SN), and combined manure with straw (2CN+1MN+1SN).  Compared with the 4CN treatment, organic substitution treatments increased celery and tomato yields by 6.9−13.8% and 8.6−18.1%, respectively, with the highest yields being in the 2CN+1MN+1SN treatment.  After 10 years of fertilization, organic substitution treatments reduced total P and inorganic P accumulation, increased the concentrations of available P, organic P, and microbial biomass P, and promoted phosphatase activities (alkaline and acid phosphomonoesterase, phosphodiesterase, and phytase) and microbial growth in comparison with the 4CN treatment.  Further, organic substitution treatments significantly increased soil C/P, and the partial least squares path model (PLS-PM) revealed that the soil C/P ratio directly and significantly affected phosphatase activities and the microbial biomass and positively influenced soil P pools and vegetable yield.  Partial least squares (PLS) regression demonstrated that arbuscular mycorrhizal fungi positively affected phosphatase activities.  Our results suggest that organic fertilizer substitution can promote soil P transformation and availability.  Combining manure with straw was more effective than applying these materials separately for developing sustainable P management practices.