Predicting phosphorus loss to water from animal manures

Animal manures are applied to cropland based on rough estimates of nitrogen (N) availability over the growing season. Nitrogen-based application of manure often has led to soil phosphorus (P) overloading where P is susceptible to environmental loss. Concerns related to non-point nutrient pollution from croplands have generated state and federal initiatives to decrease P inputs to soils, including those from animal manures. Our ability to predict bioavailable P release from animal manures is currently hampered by a poor understanding of the factors controlling P mineralization and solubilization.

The objective of this research is to develop a mechanistic understanding of phosphorus bioavailability from animal manures applied to soils.

Specifically we propose to: 1) investigate the biological, chemical and physical factors controlling P release from dairy and poultry manure applied to a variety of soil types at different soil test P levels; 2) identify the P pools in manure that are most important to P mineralization and solubilization using physical fractionation, wet chemistry and spectroscopy techniques and 3) evaluate the use of near infrared (NIR) spectroscopy as a tool for predicting P release from animal manures.

In year 1, we will investigate P release from manure applied to a soil at two different soil test P levels. Also, we will characterize the initial P pools in dairy and poultry manures. We will also test the appropriateness of near infrared reflectance (NIR) and x-ray absorption (XAS) spectroscopies as tools for identifying P chemical constituents and predicting P release from manure and other organic amendments.

In the second year, we will expand the research to investigate bioavailable P release from a number of manures in a range of soil types.

Investigation 1: Manure incubations in a Plano silt loam soil at two different soil test levels: We have collected and are characterizing dairy manure from the UW-Madison dairy research barns and poultry litter from a commercial broiler operation in west-central Wisconsin. We will use a modification of the chloroform fumigation extraction method for measuring microbial biomass P. We collected soil from the University of Wisconsin’s Arlington Agricultural Research Station. We set up incubations by mixing field moist soils with dairy manure at a rate equivalent to 25 wet tons per acre and broiler litter at an equivalent total elemental P rate. Unamended soil serves as a control. The pans will be incubated at constant gravimetric water content (22-24%) for 12 hr. at 25^oC and 12 hr. at 18^oC for 36 wks. Soluble P release will be measured weekly using anion exchange membranes. At 2, 4, 8, 12, 20, and 28 weeks we will remove 50 grams to measure TOP, DIW-extractable P, DOC, pH and electrical conductivity, and microbial biomass P. At the end of 36 weeks, we will repeat the entire set of initial chemical analyses. We will determine relationships between bioavailable P (AEM P) release and the measured parameters using either multiple regression or multivariate statistical approaches like principal components analysis or canonical discriminant analysis. Once this incubation is complete, we plan to do a series of similar incubations with one type of manure in different soils and a number of types of manure in the same soil to identify effects of soil type and manure composition on soluble P release.

Investigation 2: Characterizing manure by its P forms: We will obtain at least 20 different dairy manures and 10 different commercial broiler litters from farms around WI. We will analyze these samples using the methods for manure characterization used in Investigation 1. We will also test the use of X-ray absorption spectroscopy (XAS) to identify chemical P constituents in selected manure samples and manure-soil mixtures. We will also investigate readily available P pools using a modified DIW extraction method.

Investigation 3: Use of Near Infrared Reflectance (NIR) Spectroscopy to Predict P Release from Manure: We will conduct NIR spectroscopic analysis on the manure samples used in Investigations 1 and 2. We will then relate the spectra to data on total P, organic P, and or AEM P that are available for these samples using NIR Systems software.

Water Availability of Phosphorus from Animal Manures and Fertilizers Applied to Soils - a powerpoint presentation