Influence of applying calcium superphosphate on swine manure composting and phosphorus transformation

Objectives Calcium superphosphate is commonly used as an additive in animal manure composting, and can effectively reduce the nitrogen loss and increase the contents of nitrogen and phosphorus in the compost. However, its influence on availability and forms of phosphorus needs to be elucidated.Methods Swine manure (mixed with sawdust in 4:1 ratio in fresh weight) was mixed with calcium superphosphate under 0, 5%, 10% and 15% (in dry weight) before being aerobically composted for 42 days. Temperature, water content, pH, organic carbon, C/N ratio, total nitrogen, total phosphorus, available P and organic P were measured during the composting.Results The addition of calcium superphosphate significantly increased the temperature and water-holding capacity of swine compost, and extended the thermophilic phase for 2-10 days. The addition amounts of 5%-15% of calcium superphosphate significantly decreased the initial pH values of the compost by 0.33-0.68. The nitrogen losses after composting process, due to ammonium nitrogen volatilization, were 29.4% (CK), 26.6% (5% addition), 18.5% (10% addition) and 8.0% (15% addition), respectively. The final total nitrogen and total phosphorus concentrations in all treatments increased by 17.3%-34.2% and 37.0%-123.1%. Compared with the initial P content, the available P content in the treatment with no additive superphosphate was increased by 73.0% after 42 days of the composting, while those in the treatments with 5%, 10% and 15% superphosphate addition were decreased by 4.0%, 23.2% and 41.8%, respectively. During the process of composting, the ratios of available P over total phosphorous were continuously decreased in all treatments, demonstrating that the composting lowered the availability of phosphorous. The organic P was influenced by both the mineralization and phosphorus synthesis. In the final compost, the ratio of organic P to the total phosphorous was 22.1% in the control, which was higher than the treatments with 5%, 10% and 15% superphosphate addition, in which the ratios were 15.4%, 11.0% and 8.7%, respectively.Conclusions Calcium superphosphate addition in swine manure composting could effectively alleviate the nitrogen loss in the compost. The optimal addition of calcium superphosphate was 10% in dry weight. The composting process decreased the availability of phosphorus in the compost, which was not conducive to the P absorption and utilization by crops. Inorganic phosphorus was the main form of phosphorous in the final compost, and adding superphosphate decreased the content of organic P in the final compost.