SLU news

Reducing phosphorus leaching from arable soils

Published: 20 November 2019

Elevated phosphorus concentration in surface waters cause problems for biodiversity and humans. In a new doctoral thesis, Frank Schmieder has investigated what kind of phosphorus forms are present in certain arable soils known for high phosphorus leaching losses and how these forms may contribute to leaching.

Algal blooms in Swedish lakes and in the Baltic Sea are now occurring more often and with increasing severity. One reason for this is artificially elevated nutrient concentrations in surface waters. Elevated nutrient concentrations, or eutrophication, include loss of habitat for many animal and plant species, negative impacts on tourism and fishery industries, and problems for drinking water supply.

From a clear lake to a slimy mess

One of the most problematic nutrients for eutrophication is phosphorus. When the phosphorus concentration in surface water increases strongly, algal growth can become rampant and turn a clear lake into a green slimy mess. Today, most of the phosphorus that ends up in Sweden’s lakes and rivers and in the Baltic Sea comes from fertilisers applied to agricultural fields.

Difficult to reduce phosphorus loads

To reduce phosphorus loads to surface waters is difficult, partly because phosphorus in soil is present in a great number of different forms, for example phosphate minerals, phosphate adsorbed to the surface of other soil minerals or organic phosphorus.

– All these phosphorus forms show complex behaviour in soils. In addition, there are a number of transport pathways by which agricultural phosphorus can leave a field. When attached to small soil particles, phosphorus can be transported with water flow off the soil surface after heavy rainfall. In dissolved form, it can be transported with infiltrating water downwards to drainage pipes or to the groundwater, something that is referred to as leaching, explains Frank Schmieder.

Sophisticated analytical techniques to analyse different forms of phosphorus

Today there are many sophisticated analytical techniques such as nuclear magnetic resonance spectroscopy (NMR) and X-ray absorption spectroscopy (XAS), which can be used today to examine phosphorus in soil.

– In my thesis, I used a combination of these new techniques and as well as conventional extraction techniques to analyse the phosphorus forms present in agricultural soils with potential for high phosphorus losses, says Frank.

Heavily manured soils have a high risk of leaching phosphorus

Soils that have received excessive amounts of manure over a long period, often decades, can be classified as high-risk soils. Manure is generally rich in nutrients such phosphorus and nitrogen and is often applied in high doses to meet the crop demand for nutrients such as phosphorus.

In the heavily manured soil, phosphorus had accumulated in the upper soil layers, mainly adsorbed to iron and aluminium oxides. These compounds are very important for the phosphorus dynamics in most soils, as they bind or sorb phosphorus of P by reversible means. However, there is a risk that adsorbed phosphorus can be released again after heavy rainfall.

In the manured soil, most aluminium and iron mineral sorption sites are already occupied by phosphorus, which means that the soil’s capacity to retain phosphorus is almost exceeded. Following heavy rainfall, there is a high risk of phosphorus leaching from such soils as well. Here, phosphorus leaching was found to be linked to the pool of phosphorus adsorbed to aluminium and iron minerals.

In heavily manured soils phosphorus may be stabilized as calcium phosphates

Another important phosphorus form identified in manured soil was calcium phosphates, which made up around one-third of the total phosphorus content in the upper soil. At the slightly alkaline pH in the manured soil, calcium phosphates are not readily dissolved, and they did not contribute to phosphorus leaching in the irrigation experiment. Hence, excess phosphorus retained in the form of calcium phosphates in heavily manured soils can be considered rather stable.

­­– However, a future decrease in soil pH could reduce this stability, since the solubility of calcium phosphates is higher under acidic conditions. Ironically, regular manuring may help to stabilise the soil pH in the alkaline range since manure is generally alkaline, says Frank.

Swampy areas pose a risk for leaching

Another high-risk soil type is organic soils, which develop under waterlogged oxygen-free conditions in swampy areas. Organic soils contain only small amounts of iron or aluminium minerals that could function as phosphorus sorption sites. However, there was a much smaller pool of phosphorus adsorbed to iron and aluminium mineral surfaces than in the manured mineral soil. Organic phosphorus was not leached in an irrigation experiment.

– However, if these soils continue to be cultivated, much of the organic phosphorus is likely to be mineralised into inorganic form and can leach out, concludes Frank.

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Contact

frank.schmieder@slu.se, 018-67 23 55