Phosphorus NMR can help distinguish between the nature of organic and non-organic farming and provide clues about how phosphorus from both sources affects waterways and coasts.

Key to the generation of algal blooms is an abundance of the nutrient, phosphorus. High phosphorus contamination caused by run-off from agricultural land contributes to such water-quality problems in lakes, rivers, and estuaries across the globe.

Now, Zhengxia Dou, Charles Ramberg, John Toth, and Yu Wang, of the University of Pennsylvania, Andrew Sharpley of the University of Arkansas, Fayetteville, Sue Boyd, Chengrong Chen, D. Williams, and Zhihong Xu of Griffith University, Nathan Campus, in Brisbane, Australia, have turned to ³¹P NMR to investigate. Agricultural land receiving long-term applications of organic by-products such as animal manure is one of the major contributors to phosphorus pollution, the team says.

Soils rich in phosphorus are often associated with elevated phosphorus through erosion and runoff, which means that obtaining a clearer view of the chemical characteristics of phosphorus in soils is necessary if better management practices are to be devised with the aim of protecting water quality.

The team has looked at soils receiving organic by-products for at least eight years (treated) as compared with soils receiving no phosphorus application (untreated). They have found that the type of organic material applied, whether dairy, swine, poultry, or spent mushroom compost, dramatically increases inorganic but not the organic phosphorus.

Orthophosphate (inorganic P) is found to be the single dominant form of phosphorus present in treated soils. This inorganic form represents between 79 and 93 percent of the total phosphorus content, the researchers say. This is significantly higher than the inorganic phosphorus content of untreated soils, which contain from 33 to 71 percent.

Moreover, Dou and colleagues demonstrated that orthophosphate was the only form of phosphorus that differed dramatically between treated and untreated soils, on average levels were some three to five times higher in the treated soil.

They also found various other forms of phosphorus, including so-called condensed inorganic phosphorus, also known as polyphosphate, and various chemically organic phosphorus compounds and ions. The quantities of any one of these materials, however, was relatively small and any differences between concentrations in treated and untreated soils was negligible.

Intriguingly, the team found no evidence of the accumulation of "phytate-P" in any of the soil samples treated with manure and other nutritious waste. The researchers point out that phytate-P, more formally inositol hexakisphosphate (IP6), is a chemically organic form of phosphorus that is present in most plant tissues and represents the main store of phosphorus in plants. It is found in high concentrations in seeds and bran but is indigestible to animals other than ruminants. It is thus commonly found in animal manures, often representing almost four-fifths of the total phosphorus content of poultry manure, for instance.

Because of its indigestibility phytate-P has previously been considered a recalcitrant form of phosphorus, largely unavailable to the agro-ecosystem. The discovery by Dou and her colleagues that it is all-but missing from treated soils suggests that phytate-P is not as inactive as was once thought. The lack of phytate-P accumulation in several soils receiving poultry manure indicates that manure-derived phytate-P may represent another source of phosphorus pollution after all.

"In terms of potential phosphorus loss in the long run, organic materials containing large amounts of phytate-P such as poultry manure may not differ from other material containing mainly inorganic phosphorus," explains Dou. Team member Andrew Sharpley adds that, when the capacity of a soil to adsorb phosphorus is almost saturated after many years of manure application, phytate or other chemically organic phosphorus forms could be exposed to breakdown and potential loss.

"It is important to strive towards balancing phosphorus inputs with outputs and to prevent phosphorus from building up in soils to which manure is applied," he says. The results would suggest that this applies equally to modern farm practices as it does to so-called organic farming that uses manure rather than inorganic phosphorus fertilisers.

Related links:

• Soil Sci Soc Amer J, 2009, 73, 93-101
• Dou Page

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.