Dairy manure treatment effects on manure phosphorus fractionation and changes in soil test phosphorus

In this research, a sequential fractionation procedure coupled with enzyme hydrolysis was used to categorize the phosphorus (P) forms of 18 manure samples collected from in-barn composted bedded pack (beef manure), anaerobic digestion with liquid-solid separation (dairy manure), and liquid-solid separation systems (dairy manure). This research also determined the effects of those P forms on the increase in soil test P (STP) of five soil series. The soils used had initial Bray-1 P ranging from 16 to 43 mg P kg^-1. Total dry-ash P (P_t) of the manures ranged from 1.4 to 15.0 g P kg^-1; total inorganic P (P_it) accounted for 20 to 81 % of P_t; and enzymatically hydrolysable P (P_et) accounted for 5 to 26 % of P_t. Liquid-solid separation tended to concentrate the manure P in the liquid fractions. In contrast, anaerobic digestion did not affect the manure P distribution compared with the undigested raw manure from the same system. No differences in P distribution were found for the compost bedded pack manure. In the soil incubation study, manure and fertilizer were applied at 40 mg total P kg^-1. Separated liquid manure from two systems tended to increase STP more than the separated solid manures from the same systems. Although anaerobic digestion modified some of the physical and chemical properties of the treated manures, it did not clearly impact how digested manure increased STP compared with the raw manures. Overall, the increase in STP after treated manure application was found to be a function of soil clay content and manure P_it + P_et applied.