Intensive use of fertilizers helps increase agricultural production. These products are known to have an impact on soil and water chemistry. On the other hand, the effects of this artificial enrichment on pollinating insects remain poorly understood. A British team looked into the subject. She examined the relationship between soil fertilization, soil yield and biodiversity. They found that high levels of fertilizer significantly reduce the number of flowers and pollinators.
Pollinating insects, in particular, are essential to agricultural productivity and the maintenance of ecosystems. However, the decline in the abundance and distribution of many pollinator species has caused concern. Intensive agriculture appears to be one of the main causes of this decline. But the effects of artificial soil enrichment on these insects remain poorly understood. British researchers from the University of Sussex looked into the question. Their study is based on Rothamsted's Park Grass Experiment, which is the world's oldest ecological experiment.
A full-scale study initiated in the 19e century
Created in 1856 in the south-east of England, this experimental station aimed to study the effects of organic and inorganic amendments on land yield. It quickly became apparent that these treatments had a spectacular effect on the plant species composition of the soil. Subsequently, Park Grass helped answer a wide range of ecological, environmental and evolutionary questions.
This experience continues to be a valuable resource for gaining new knowledge about agroecological systems. “ The Park Grass data is particularly valuable because fertilizer treatments acted as filters on the original grassland plant community, meaning that the species found on each plot today are naturally assembled together », Explain the researchers in the journal NPJ Biodiversity.
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However, the relationships between changes in plant communities and the wildlife that use them as a resource remain unclear. However, increased soil fertility is thought to affect pollinator communities via three main mechanisms. First, it influences the quantity of food plants. Then, it impacts the nutritional quality of nectar and pollen. Finally, it disrupts the timing of flowering events.
For example, soil enrichment impacts interspecific competition, favoring certain species over others. This can reduce the diversity of flowering plants, leading to a negative impact on pollinator diversity. “ However, to our knowledge, the direct link between soil enrichment and pollinator diversity has not yet been quantified. », Explain the researchers. So they set out to study the impact of fertilization on pollinators and flowering plants in Park Grass.
An abundance of pollinating insects reduced by half
On some 1,400 transects of 40 m², the team recorded 1,285 foraging insects. Bees were the most numerous, constituting 65.8% of individuals (including 32.2% honey bees). This was followed by flies (27.5%), beetles (2.9%), butterflies (2.1%) and wasps (1.6%).
The data showed that pollinator abundance and species richness (i.e. the number of species present) were significantly negatively associated with the DEFRA soil fertility index (based on the amount of phosphorus, potassium and magnesium). , the quantity of nitrogen and the application of agricultural lime. “ As you increase fertilizer, the number of pollinators decreases. It'sis the direct link which, to our knowledge, has never been demonstrated before “, declared to Guardian Dr. Nicholas Balfour, first author of the study.
The team found a near doubling of pollinator abundance and species richness in untreated plots, compared to those receiving high levels of fertilizer. In particular, bees, which are key pollinators, were almost 10 times more numerous! The exception was plots receiving all nutrients except nitrogen. They had relatively high pollinator abundance and species richness.
Relationship between pollinator species richness and (a) soil nitrogen and (b) fertility index. Relationship between flowering plant species richness and (c) nitrogen and (d) fertility index. Credits: Balfour et al., NPJ Biodiversity (2025)
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At the same time, the researchers recorded, over approximately 2500 m², 7016 floral units from 19 species of flowering plants. The most common were Pimpinella saxifraga (21.1%), L. corniculatus (17.8%) and C. nigra (16.0%). As with pollinators, flowering plant abundance and species richness were significantly negatively associated with DEFRA fertility index, nitrogen quantity and lime addition.
Flower abundance and species richness were respectively 5.18 and 8.46 times higher in untreated plots than in those that received the greatest amount of fertilizer, the team said. Not surprisingly, pollinator abundance and diversity were positively related to both flower abundance and diversity per plot.
An “important and significant” compromise between productivity and biodiversity
These results show that fertilizer application reduced the abundance and species richness of flowering plants in the Park Grass experiment. According to the researchers, the use of fertilizer creates conditions that favor a limited number of fast-growing species, such as grasses, to the detriment of flowering plants. Plots receiving the highest levels of nitrogen had higher proportions of Apiaceae.
The results also indicate that increasing soil pH through the application of agricultural lime (calcium carbonate) can mitigate, to some extent, the negative impact of nitrogen fertilizers on flowering plant and pollinator populations. Indeed, plots treated with lime had a greater abundance and diversity of pollinators and flowers.
Additionally, plots fertilized with all nutrients except nitrogen were particularly rich in leguminous species (such as clovers). They were not rich in floral species, but they maintained yields. Additionally, they supported high numbers of pollinators, including bees.
Trade-off between flower and pollinator species richness and grassland yield. Credits: Balfour et al., NPJ Biodiversity (2025)
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Furthermore, the data revealed a large and significant trade-off between productivity and biodiversity. “ PTo maximize flower species richness, and thus also pollinator species richness, a significant decline in fertility is necessary and will reduce yield », Explain the researchers. Therefore, to reduce the use of fertilizers in order to preserve biodiversity, subsidies will be necessary to compensate for economic losses.
The European Union is currently undertaking major ecological reforms of agricultural policies. Soil nutrient management strategies that favor legumes, with added lime, can reduce the trade-off between biodiversity and yield, the team concludes.
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