Extractable Soil Phosphorus and Inorganic Nitrogen following Application of Raw and Anaerobically Digested Swine Manure

Received for publication October 1, 2004. Processing of swine (Sus scrofa domestica) manure in an anaerobic digester for biogas production is not a complete waste treatment process. Therefore, digested manure must be utilized in some manner, most likely as a source of plant nutrients. The objective of this study was to compare the effect of raw and digested liquid swine manure application on soil test P (STP) and inorganic N. A laboratory incubation study was conducted for 112 d, with a factorial combination of raw manure, digested manure, and inorganic fertilizer at five nutrient rates (0, 12.5, 25, 37.5, and 50 mg total P kg^–1 and 0, 50, 100, 150, and 200 mg total N kg^–1). Raw and digested swine manure produced the same NH4–N disappearance, NO3–N formation, net inorganic N, and increase in STP. Routine STP methods estimated similar P recovery with both manure sources, averaging 21% at the end of incubation. For the first 28 d of incubation, the STP levels were higher for fertilizer than manure; STP levels were similar for all P sources after 28 d. Nitrification of manure NH4 was rapid, reaching background concentrations by 14 d, with conversion rate similar to fertilizer NH4–N. By the end of incubation, maximum net extractable inorganic N, predominantly NO3–N, averaged 20% less than total applied N for both raw and digested manure. Anaerobic digestion did not substantially affect manure nutrient supply, and we conclude that anaerobically digested liquid swine manure can provide similar plant-available N and P as expected from raw swine manure.