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Analysing chemical landscapes: a tool to evaluate grasslands

The trapping of volatile organic compounds is a useful tool to evaluate grasslands.

Hameau en automne. Environs de Mandailles (15).. © INRA, VIDAL Louis
Updated on 01/29/2016
Published on 12/22/2015

Imagine the Auvergne, a summer sky over the grasslands where livestock graze peacefully with insects buzz overhead.  An idyllic landscape where the vegetation releases a considerable diversity of volatile organic compounds that form complex environments.  From these scented landscapes the insects can extract signals that are essential to their reproduction or nutrition.

It is these grasslands and their chemical landscapes that INRA scientists and their colleagues have explored, and demonstrated that the trapping of volatile organic compounds can be a useful tool to evaluate grassland environments.

Auvergne, its grasslands and their chemical landscapes

In the heart of the Cantal, the scientists focused on two grassland areas managed differently in terms of fertilisation and grazing.  They compiled a floral and entomological inventory while at the same time analysing the odours present.  Rather than using the headspace analytical technique, they preferred a method based on solid phase microextraction (SPME).

These two grasslands called Montagne and La Prade differ in terms of their botanical composition. Montagne, which had not received any fertilisation for the past 20 years and had been reasonably grazed, was found to harbour some 83 plant species, compared with 65 for La Prade, which was more used to receiving nitrate and manure applications and had been grazed intensively.  Furthermore, Montagne counted an average of 30 species per m2, versus only 19 for La Prade; herbaceous plants dominated in the former area, while legumes such as clover were abundant in the latter.

The scientists were able to detect 67 volatile organic compounds, including hydrocarbons, alcohols and aldehydes (all contributing to "green" odours), acids and their derivatives, and 24 compounds in such small quantities they could not be identified.

All these compounds are characteristic constituents of grasses, flowers and other plants, but although their numbers were identical in both grassland areas and at the same times relative to grazing, their composition varied.

Six compounds, including terpinene (which has a characteristic floral and spicy odour) and 2-phenylethylacetate (whose odour is reminiscent of rose) were more abundant in Montagne than in La Prade.  By contrast, benzoic acid and its "burnt" odour, was abundant in La Prade.

Although the differences between the two grasslands were not as considerable as might have been anticipated from their botanical composition, they were more marked with respect to grazing.  The abundance of 31 volatile organic compounds was affected by the period of grazing: 19 - including terpenes such as delta-bisabolene, alcohols and acid derivatives - were more abundant in the samples collected during cattle grazing, while 12 - including ketones, aldehydes and 1-butanol - were more abundant after grazing.

 

Grasslands, their chemical landscapes and insects

Diversified mountain grazing on the one hand, and more ordinary, semi-natural grassland on the other, Montagne and La Prade were not found to be home to the same insects.  There were larger populations of bumble bees and wild bees in the former than in the latter, unlike other Hymenoptera whose numbers were identical in both areas.  The number of insects correlated with the volatile compounds detected.  Furthermore, the presence of limonene tended to be unfavourable to wild bees, while α-pinene (with a pine odour), limonene (orange odour) or cis 3-hexenyl acetate exerted a negative impact on other Hymenoptera.

Overall, this work has thrown new light on the usefulness of volatile organic compounds to evaluating the functions of grasslands and the stresses experienced by the plants that grow there.  Once again, solid phase microextraction proved to be a powerful tool, and the odorous landscapes revealed using this method offer new perspectives for their study and experimentation in order to report on ecosystem services.

REFERENCE

Agnès Cornu, Anne Farruggia, Ene Leppik, Centina Pinier, Florence Fournier, David Genoud, Brigitte Frérot (2015) Trapping the Pasture Odorscape Using Open-Air Solid-Phase Micro Extraction, a Tool to Assess Grassland Value. PLoS ONE 10(11):e0140600