Submitted to: International Journal of Food, Agriculture, and the Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 10, 2005
Publication Date: March 22, 2005
Citation: Jayasundera, S., Schmidt, W.F., Reeves Iii, J.B., Dao, T.H. 2005. Direct 31p nmr spectroscopic measurement of phosphorous forms in dairy manures. International Journal of Food, Agriculture, and the Environment. 3(2):335-340.
Interpretive Summary: Phosphorous (P), nitrogen (N), and potassium (K) are the major mineral nutrients required for plants to grow. Excess nutrients added to soils either as synthetic fertilizers or manures can be transported to streams, rivers, and ground waters. The P in manures can be chemically bound to organic matter (the organic form) or it can be in a mineral form (also known as the inorganic form). The organic and inorganic forms of P have very different properties. The organic form releases P at a much slower rate than the inorganic form. Commercially, aluminum (Al), iron (Fe), and calcium (Ca) cations are added to P to slow the rate of release of P from the organic matter. Knowing the rate and amount of P added to the environment from different manures depends first upon identifying the chemical forms of P present. Molecular spectra of intact manure samples provided evidence that the more abundant form of P in most of the samples is organic P. The same procedure can be used to determine if the organic and inorganic forms exist in the sample as free anions or as stable complexes with cations, e.g., Fe, Al and/or Ca. Chemical tests for P can incorrectly confuse insoluble organic P with insoluble inorganic P. This information will be useful in the development of new tools to measure the type and quantity of P present in manure and in predicting the amount of P that can be be transported to surface waters.
Quantitative assessments of the distribution of phosphorous (P) from agricultural sources in environmental matrices require the identification of the different forms of P over time. Inorganic P applied to agro systems adds to the naturally occurring P in plants, animals, soils, and water and can be biochemically converted into organic P. Organic P in bovine feeds can convert back into inorganic forms of P via microbial and/or enzymatic degradation within bovine digest systems. The extent of conversion depends on the bioavailablity of the plant species P and on the digestibility within the bovines. Organic and inorganic P in animal diets can complex with the Fe, Al, and Ca ions. Solid-state 31P MAS NMR analysis with high power decoupling and cross polarization transfer techniques was used to investigate the P composition in a diverse array of authentic bovine manures that were also chemically analyzed. Water soluble and insoluble inorganic and organic P forms were identified in the manure samples. Organic P (i.e. phytate) was detected in a significant amount of the samples analyzed and was the major fraction of P suggesting that dairy manures can contain significant amounts of undigested phytates. Chemical tests alone can incorrectly interpret insoluble organic P as inorganic P. Solid state 31P MAS NMR enabled the qualitative and quantitative analysis of the organic P in intact samples.