Investigation of the inorganic and organic phosphorus forms in animal manure

The most viable way to beneficially use animal manure on most farms is land application. Over the past few decades, repeated manure application has shown adverse effects on environmental quality due to phosphorus (P) runoff with rainwater, leading to eutrophication of aquatic ecosystems. Improved understanding of manure P chemistry may reduce this risk. In this research, 42 manure samples from seven animal species (beef and dairy cattle, swine, chicken, turkey, dairy goat, horse, and sheep) were sequentially fractionated with water, NaHCO₃, NaOH, and HCl. Inorganic (P_i), organic (P_o), enzymatic hydrolyzable (P_e; monoester-, DNA-, and phytate-like P), and nonhydrolyzable P were measured in each fraction. Total dry ash P (P_t) was measured in all manures. Total fractionated P (Pft) and total Pi (P_it) showed a strong linear relationship with Pt. However, the ratios between P_ft /P_t and P_it/Pt varied from 59 to 117% and from 28 to 96%, respectively. Water and NaHCO3 extracted most of the Pi in manure from ruminant+horse, whereas in nonruminant species a large fraction of manure P was extracted in the HCl fraction. Manure Pe summed over all fractions (P_et) accounted for 41 to 69% of total Po and 4 to 29% of Pt. The hydrolyzable pool in the majority of the manures was dominated by phytate- and DNA-like P in water, monoester- and DNA-like P in NaHCO3, and monoesterand phytate-like P in NaOH and HCl fractions. In conclusion, if one assumes that the Pet and Pit from the fractionation can become bioavailable, then from 34 to 100% of Pt in animal manure would be bioavailable. This suggests the need for frequent monitoring of manure P for better manure management practices.