Effects of feed phosphorus supplement and manure phosphorus application on phosphorus leaching

Phosphorus (P) is a key element in agricultural production. However, the rapid expansion of livestock production, coupled with inefficient manure recycling, has resulted in significant losses of manure P to water bodies. In addition to fueling the growth of toxic algae, this pattern has led to significant financial losses. China has updated the recommended standards for the use of feed P and introduced measures to increase livestock manure recycling. These strategies greatly reduced P excretion and loss to water bodies. However, the effect of integrating optimal feed P management with appropriate field manure application to reduce manure P leaching remains unclear. Water bodies contain various forms of P, such as dissolved inorganic P (DIP) and dissolved organic P (DOP). These P fractions were converted to themselves and were affected by the application rate of manure P. In general, livestock manure is applied based on its nitrogen content or without regard to its nutrient content, a practice that increases the risk of manure P loss. Therefore, appropriate application of livestock manure is crucial for mitigating the risk of P leaching. To promote manure recycling in China, a leaching simulation experiment was conducted using a two-factor, completely randomized block design. The two factors were the rate of total manure P application and manure derived from livestock fed different P supplements. Two rates of total manure P application were 50 mg(P)·kg⁻¹ equivalent to the crop requirement at 80 kg(P)·hm⁻² and 165 mg(P)·kg⁻¹ equivalent to farmer practice at 260 kg(P)·hm⁻² in North China Plain. The dietary P supplements used were dicalcium phosphate (DCP) and mono-dicalcium phosphate (MDCP). DCP is the most widely used phosphate supplement, while MDCP is a new, highly water-soluble phosphate with 18.5 times greater water-soluble P content than DCP. Each treatment was replicated three times, with the control group receiving no fertilization. We analyzed the effects of optimizing feed P supplements and manure application rates on leaching in terms of various P forms. Compared with DCP, MDCP reduced the leachate DIP and DOP contents of pig, broiler, duck, and layer manures, with decreases ranging from 18.2%–31.0% for DIP and 19.4%–55.9% for DOP. The loss of active P in the form of DIP and DOP was reduced by MDCP. For example, the cumulative amounts of DIP in the leachate were 53.1, 7.4, 18.0, and 12.9 mg in pig, broiler, duck, and layer, respectively. These values accounted for 19.7%, 6.9%, 16.7%, and 6.3% of the active P in the applied manure, respectively. However, the manure derived from MDCP, the cumulative amounts of DIP leaching were 39.9, 5.6, 12.4, and 8.3 mg for pig, broiler, duck, and layer, respectively. These accounted for 18.3%, 6.4%, 12.8%, and 4.7% of the P in the applied manure, respectively. To reduce the risk of P leaching in the field, application of animal manure derived from highly water-soluble feed P supplements and optimizing the total P application rate is recommended. Additionally, factors, such as crop type, soil type, rainfall, and manure composition, along with the principles of the 4R (right source, right rate, right time, and right place) should be considered to ensure sustainable P management.