Tracing nitrogen: a measuring network on the Utrechtse Heuvelrug and Veluwe

The impact of nitrogen on nature is a much-discussed topic. But how does nitrogen actually spread through an area? And how does this relate to measured nitrogen levels in the soil? In her doctoral research at IRAS of the Faculty of Veterinary Medicine, Shirah Buijnink investigates the spatial and temporal variation in nitrogen deposition in the Veluwe and the Utrechtse Heuvelrug.

In the nature areas of the Veluwe and the Utrechtse Heuvelrug, which border urban and agricultural zones, the influence of atmospheric nitrogen deposition is significant, while few deposition measurements are currently being conducted. That’s why Shirah Buijnink, together with colleague Isabella van Schothorst, set up a monitoring network for her doctoral research at 25 locations spread across the Veluwe and the national park Utrechtse Heuvelrug. These natural areas are sensitive to the effects of nitrogen deposition, which leads to soil acidification and leaching of essential nutrients. The monitoring sites are situated at various distances from the Gelderse Vallei—a region with many livestock farms that emit ammonia—in order to better understand how nitrogen spreads.

Nitrogen is often an abstract concept in the debate, but with these measurements, we make it tangible: what actually ends up in nature

Nitrogen reaches the ground in two ways: through precipitation (wet deposition) or as gas and dust particles (dry deposition). The researchers use a combination of five different measuring setups to determine both forms of nitrogen deposition. Within forest plots, rainwater is collected beneath the canopy and along tree trunks to get an impression of the total nitrogen deposition. Bulk collectors and wet-only collectors are placed in open fields near the forest sites to measure how much nitrogen is present in the rainwater. In addition, concentrations of ammonia (NH₃) and nitrogen dioxide (NO₂) in the air are measured in the open field. The deposition measurements are carried out over a period of two years, with rainwater being collected every two weeks.

In this way, the researchers determine how the ratio between dry and wet deposition changes with distance from the Gelderse Vallei. They also examine the influence of vegetation structure, elevation profiles, and weather conditions such as wind, precipitation, and temperature. Additionally, they investigate the contribution of livestock farming to nitrogen deposition by looking at the ratio between ammonium (NH₄⁺) and nitrate (NO₃⁻). Agricultural sources mainly emit NH₄⁺ and NH₃, while industry and traffic are primarily responsible for NOₓ emissions.

Current nitrogen policy is based on a lot of calculations and very few field measurements. The detailed monitoring network allows us to better understand which nitrogen compounds settle where, and what effects this has on the imbalance in the nitrogen cycle of forest ecosystems

To determine whether the measured gradients in deposition are also reflected in the soil, variables such as acidity, nitrogen content, organic matter, and the C:N ratio are examined. This is linked to the research of her PhD supervisor René Verburg on climate-smart forestry, which is part of a collaboration between Utrecht University, Staatsbosbeheer (the Dutch forestry service), and the Utrechtse Heuvelrug National Park Foundation, under the Research & Education Hub Utrechtse Heuvelrug. Among other things, this project investigates the impact of soil nitrogen on tree growth in the area. The shared goal is to link the nitrogen measurements described in this article to the soil and growth data.

So, if you come across Shirah and Isabella’s installations in the forest or field during your walk, don’t worry: this is all part of important work to preserve nature!