Phosphorus makes you fat
If you want to protect diversity, you have to know the causes of its decline - otherwise many a rescue operation will go awry. This also includes putting traditional opinions to the test. Because evil sometimes feeds on a completely different root.
These colorful orchid meadows with their impressive diversity of species have become rare. It's hard to believe that they are actually signs of great poverty: the soil near their roots is extremely poor in nutrients such as nitrogen and phosphorus. But just as humans have encouraged it – every mowing or intensive grazing draws nutrients from the system – so it has destroyed it: through fertilizers and air pollution that make the soil richer, it paves the way for more competitive species that crowd out the plant starvation artists. Once dandelions and the like sprout, the fat meadow is perfect.
In order to persuade the lean-eaters to return, conservationists like to prescribe a starvation diet in such areas: They reintroduce the former use as hay meadows or pastures and thus try to reduce the nutrient content. However, the measures are not always successful. Maybe because the effort is focused on the wrong culprit?
Because so far, nitrogen has been considered a deficiency factor in the soil nutrient supply in our latitudes. It would thus also be primarily responsible for the decline in species that thrive on lean food. Numerous studies and field observations have shown a close connection between increasing productivity and nitrogen concentration and decreasing biodiversity.
But Martin Wassen from Utrecht University and his colleagues had their doubts. After all, nitrogen is not always the limiting factor: in water bodies, for example, phosphorus limits rampant growth. So couldn't he also be involved on land?
The botanists analyzed the species composition of various wet locations from the Netherlands and Belgium via Poland to Siberia - from moors and swamps to wet meadows -, counted the Red List representatives as representatives of endangered lean habitats and determined the limiting nutrient using a Elemental analysis of plant biomass. The highlight of the matter: the further east the researchers got, the less nitrogen there was for the plants, since the input through the air is becoming smaller and smaller.
The results initially showed the familiar pattern: endangered species were found, as expected, primarily in areas with a poor nutrient supply – after all, there was no threat of competition here. In the Netherlands and Belgium, by far the most were on soils with low levels of phosphorus. This fits with the assumption that the pronounced nitrogen fertilization through the air enriched poor locations here in the past, increased productivity and thus drove away the hunger artists - unless phosphorus became a limiting factor for the gluttons. And: In Poland and Siberia, where both nutrients are scarce, some species showed no preference for nitrogen or phosphorus limited locations - the main thing was gaunt.
But some things didn't fit into the picture: Among those endangered species that showed a preference for nutrients, those suffering from phosphorus deficiencies predominated. Also, most surprisingly and contrary to expectation, nitrogen was by no means underrepresented in most of the lean corners of the Netherlands and Belgium, but rather nitrogen - where seventy percent of endangered species crowded the phosphorus-limited wetlands. In general, the number of Red List species increased the more phosphorus was revealed to be a limiting factor.
Why is that – are there more phosphorus deficiency specialists than nitrogen deficiency sufferers? This is unlikely, since the number of species in the two types of poor sites would then have to differ significantly - and it does not. The explanation that there are more phosphorus-limited than nitrogen-limited locations also does not fit: In the well-fertilized Benelux region of all places, exactly the opposite is the case.
The researchers conclude that it is most likely that human activities have simply had a greater impact on phosphorus-limited communities. Be it through groundwater abstraction, which reduces the flow of phosphorus-binding iron and calcium ions into wetlands, be it through the input of phosphorus via surface water and a changed dynamic of flooding and dry traps - all processes that increase the plant availability of the nutrient and thus level the ground for the displacement of the lean-eaters. The inevitable consequence is that these will then also fill up the red lists accordingly.
But if phosphorus is the main problem and not nitrogen, then it is also clear why some care measures remain ineffective: Unlike nitrate, which travels quite quickly when dissolved in the soil water, phosphate, closely bound to clay minerals, remains On the spot and therefore available for a long time. Former fields in particular continue to nibble on this internal fertilizer supply for a long time. Therefore, making the starvation diet successful requires a much more comprehensive approach than mowing and grazing.
