Atmosphere-biosphere exchange of nitrogen in the tropical rainforest

Scientists developed a new model to access nitrogen exchange between atmosphere and biosphere based on observations at ATTO. This model includes parameters controlling both nitrogen deposition and emissions in tropical forests.

Nitrogen in the environment

Agriculture is one of the main sources of nitrogen to ecosystems around the world because ammonia (NH3) is often used in fertilizers. Forests were once considered to be an overall sink for ammonia, but recent studies found bidirectional transport. (Semi-)natural sources such as wildfires and the excreta of wild animals, but also plants and decomposing leaf litter emit small amounts of ammonia. How much nitrogen forests emit through this process is currently unclear. Plants are affected by the total nitrogen concentration in the atmosphere is in several ways. Up to a certain point, increased nitrogen deposition will increase vegetation growth rates and thus, carbon uptake. But beyond a certain threshold, it may lead to a decrease in plant productivity. In addition, nitrogen deposition may lead to changes in species composition with effects on biodiversity and ecosystem functions.  Therefore, it is important to get a clear picture of nitrogen exchange between the atmosphere and the biosphere.
GRAEGOR detector unit for measuring trace gases like nitrogen in the laboratory container. © Robbie Ramsay / University of Edinburgh
GRAEGOR detector unit for measuring trace gases like nitrogen in the laboratory container. © Robbie Ramsay / University of Edinburgh

Development of a bidirectional model

Robbie Ramsay and his colleagues now measured nitrogen fluxes (mainly NH3 and NH4+) at ATTO. This remote site offers the opportunity to collect data free of agricultural pollution and at a tropical rainforest site with unique meteorological conditions. They collected data for an entire month in the dry season with hourly resolution, as well as several meteorological parameters. And indeed, the team found bidirectional transport: On the one hand, deposition of nitrogen occurs on plant surfaces, thereby removing it from the atmosphere. On the other hand, they also found emissions. Their data indicate that they might be emitted from plant stomata when they are open for photosynthesis.

The scientists then used their data to develop a model that would simulate their observations. Such local and regional models are required to quantify how much nitrogen is deposited from the atmosphere in the Amazon rainforest, and how much the forest emits back. Previous models had used relative humidity or vapor pressure deficit to simulate surface exchange. The model by Robbie Ramsay and his co-authors instead used a novel leaf wetness parameter that captures variation with greater accuracy and defines a set of parameters necessary to capture the variation in the data obtained at ATTO.

The study “Measurement and modelling of the dynamics of NH3 surface–atmosphere exchange over the Amazonian rainforest” by Ramsay et al. is available Open Access in Biogeosciences.

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