Phosphorus is one of the key nutrients that can cause algal blooms and related water quality problems in lakes, rivers and estuaries worldwide, according to a report in Science Daily. Phosphorus entering waters originates from a variety of sources.

Agricultural land receiving long-term applications of organic by-products such as animal manure is one of the major contributors. Such soils often become enriched with phosphorus, leading to elevated phosphorus, loss through erosion and run-off. Information on the chemical characteristics of phosphorus in these soils is essential to improving our understanding of how phosphorus behaves in soils and how it is transported in run-off to devise better management practices that protect water quality.

A group of scientists in the USA and Australia have identified the chemical forms of phosphorus, using 31P nuclear magnetic resonance (NMR) spectroscopy, in soils receiving organic by-products for at least eight years (treated) as compared with soils not receiving phosphorus application (untreated).

Regardless of the type of organic materials applied (dairy, swine, poultry or spent mushroom compost), orthophosphate (inorganic phosphorus) was the single dominant phosphorus form – more so in treated soils (79 to 93 per cent of total phosphorus) than in untreated soils (33 to 71 per cent).

Orthophosphate was also the only phosphorus form that differed dramatically between paired soils – three to five times greater in treated than untreated soils. Other phosphorus forms included condensed inorganic phosphorus and various organically bound phosphorus groups. However, their amounts were relatively small and differences between each paired soils were insignificant.

Surprisingly, the study revealed no evidence of phytate-phosphorus accumulation in any of the soils receiving organic wastes. Phytate is an organic storage form of phosphorus that is known to be present in animal manures, in particularly large proportion (up to 80 per cent of total phosphorus) in poultry manure. Phytate-phosphorus is generally considered to be recalcitrant in the agro-ecosystem because of its chemical structure. However, the lack of phytate-phosphorus accumulation in several soils receiving poultry manure in this study indicates that manure-derived phytate-phosphorus may not be biologically and environmentally benign.

Zhengxia Dou, the lead author, stated, "In terms of potential phosphorus loss in the long run, organic materials containing large amounts of phytate-phosphorus such as poultry manure may not differ from other material containing mainly inorganic phosphorus."

Andrew Sharpley, a collaborating scientist, further explained to Science Daily, "When the soils' phosphorus sorption capacity was nearly saturated after years of manure application, phytate or other organic phosphorus forms could be exposed to breakdown and potential loss. Therefore, 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."

Reference

Dou Z. et al., 2009. Phosphorus speciation and sorption-desorption characteristics in heavily manured soils. Soil Science Society of America Journal, 73 (1): 93 DOI: 10.2136/sssaj2007.0416.