Phosphorus-rich soils support larger invertebrates

In a recent study, researchers have defined the relationship between soil conditions and nutrients with the health of soil ecosystems. The results suggest that organic grassland, rich in phosphorus, is supportive of large populations of bigger invertebrates.

All living things are made up of chemical elements in certain proportions and the availability of these elements in the environment can affect growth rate. For example, it is thought that organisms with increased growth rates need higher proportions of phosphorus at the genetic level to support the greater protein production needed for growth.

The researchers investigated whether there was a link between soil acidity, the proportions of carbon, nitrogen and phosphorus in soil and the abundance of different soil invertebrates and the microbes that they eat (such as bacteria and fungi). For instance, in sites lacking in phosphorus bacteria are expected to grow relatively slowly and have a lower phosphorus content. This may limit food quality and development for other creatures higher up the food web, such as nematodes (roundworms) or microarthropods (e.g. soil mites) that feed on the bacteria.

The sandy soils of 12 organic grasslands under low intensive management and 10 ex-organic farms that had been abandoned for over ten years in the Netherlands were sampled for carbon, nitrogen and phosphorus content. In addition, the number of all invertebrates, bacteria and fungi in the soil was measured. The complete data is online and available to download1.

Across the soil samples there were significant differences in the concentrations of the three elements. On average, carbon and nitrogen concentrations were highest in the abandoned grasslands that had not been fertilised for over a decade and lowest in the managed grasslands. In contrast, total soil phosphorus was lowest in the abandoned grasslands and highest in the managed grasslands that had received organic fertilisers. In abandoned grasslands, soil was almost seven times more acidic than that of the managed grasslands receiving lime.

Overall, the abundance of different sizes of invertebrates was closely related to soil acidity and the relative proportions of carbon, nitrogen and phosphorus that were available. The study found that the higher the level of available phosphorus (from organic fertilisers in less acidic soils), the greater the population density or abundance and size of the larger invertebrates. Conversely, when phosphorus levels were limited (in more acidic soils), there were comparatively fewer larger-sized invertebrates but more smaller-sized invertebrates.

Soils rich in organic phosphorus had higher quality organic matter for invertebrates allowing the maintenance of food webs with increased numbers of larger invertebrates. This suggests that smaller bodied invertebrates, such as nematodes, cope better with nutrient-poor conditions in which phosphorus is limited, than larger bodied invertebrates, such as microarthropods.

The researchers suggest that the results of the study reflect the habitat response relationships at individual and population levels found in the food webs of soils of managed and unmanaged grasslands.

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