Effects of Long-Term Poultry Litter Application On Inorganic and Enzyme Hydrolyzable P of Texas Blackland Vertisol.

The effect of livestock manure on soil phosphorus (P) distribution has been studied for several decades; however, the majority of this work has focused on changes in inorganic P forms (P_i) following manure application. Up to 80% of the P in poultry manure is present as phytate or other organic forms; however, little work has been conducted to assess the impact of manure application on organic P in soils. Phytate is highly reactive with soil minerals, and over-application of poultry manure could lead to accumulation of soil P that is not available for plant uptake. Recently, an enzyme hydrolysis technique was developed that, coupled with sequential fractionation with H₂O, 0.5 M NaHCO₃, 0.1 M NaOH and 1.0 M HCl, has improved our understanding of how manure affects soil P distribution. We characterized P_i and enzyme hydrolysable organic P (P_e) in sequential extracts of surface soils from a long-term (2002 to 2012) experiment conducted on a Vertisol-dominated Texas Blackland Prairie. The soils were under different management systems,  including: (1) cultivated fields receiving inorganic fertilizer, (2) cultivated fields receiving poultry manure annually at rates of 4.5, 6.7, 9.0, 11.2, and 13.4 Mg ha^-1, (3) unfertilized native prairie, and (4) three improved pasture systems (two manured and one grazed). Concentrations of all P forms in the unfertilized remnant native prairie and grazed pasture remained constant over the study period. In general, P distribution was dependent upon fertilization strategy and land-use type. Cultivated fields receiving inorganic fertilizer had an increase in H₂O-extractable P_e, due largely to higher concentrations of soluble phytate. Inorganic fertilizer application increased Al/Fe-associated P_i (NaOH-P_i), and decreased all forms of NaOH-P_e (DNA, phosphate monoesters, phytate). The application of poultry manure increased concentrations of total P, soluble P_i, and Ca²⁺-associated P_i (HCl-P_i). Poultry manure also increased the amount of soil P_e that was sparingly soluble (NaHCO₃-P_e) and Al/Fe-associated (NaOH-P_e). Soils from non-grazed pastures that received poultry manure had an increase in most P forms over time, with the exception of soluble and Ca²⁺-associated P_e, which decreased over time. Overall, application of high rates of poultry manure to both cultivated and pasture soils decreased concentrations of hydrolysable phytate and increased total P_i; however, concentrations of mineral-associated organic P that was resistant to enzyme hydrolysis increased up to 3-fold. These results show that application of poultry manure influences soil P transformations and could potentially lead to over-accumulation of P in forms not available for plant uptake. Further work is warranted to evaluate how poultry manure application and other management practices affect microbial properties (e.g. activities of enzymes involved in hydrolysis of organic P) related to P cycling and fertility of soil.