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.

Similar articles

High-quality atmospheric CO2 measurements are sparse across the Amazon rainforest. Yet they are important to better understand the variability of sources and sinks of CO2. And indeed, one of the reasons ATTO was built was to obtain long-term measurements in such a critical region. Santiago Botía and his colleagues now published the first 6 years of continuous, high-precision measurements of atmospheric CO2 at ATTO.

The Amazon rain forest plays a major role in global hydrological cycling. Biogenic aerosols, such as pollen, fungi, and spores likely influence the formation of clouds and precipitation. However, there are many different types of bioaerosols. The particles vary considerably in size, morphology, mixing state, as well as behavior like hygroscopicity (how much particles attract water) and metabolic activity. Therefore, it is likely that not only the amount of bioaerosols affects the hydrological cycle, but also the types of aerosols present.

GRAEGOR detector unit for measuring trace gases like nitrogen in the laboratory container. © Robbie Ramsay / University of Edinburgh

Ramsay et al. measured inorganic trace gases such as ammonia and nitric acid and aerosols in the dry season at ATTO. They are to serve as baseline values for their concentration and fluxes in the atmosphere and are a first step in deciphering exchange processes of inorganic trace gases between the Amazon rainforest and the atmosphere.

Smoke from biomass burning rises into the atmosphere over the central Amazon basin. © NASA

Soot and other aerosols from biomass burning can influence regional and global weather and climate. Lixia Liu and her colleagues studied how this affects the Amazon Basin during the dry season. While there are many different interactions between biomass burning aerosols and climate, they found that they overall lead to fewer and weaker rain events in the Amazon rainforest.

Graphical Abstract

In a new study, Dr. Haijie Tong and co-authors studied a subset of PM2.5, the so-called highly oxygenated molecules (HOMs) and its relationship with radical yield and aerosol oxidative potential. They analyzed fine particulate matter in the air in multiple locations. This including the highly polluted megacity Bejing and in the pristine rainforest at ATTO. They wanted to get insights into the chemical characteristic and evolutions of these HOM particles. In particular, they wanted to find out more about the potential of HOMs to form free radicals. These are highly reactive species with unpaired electrons.

Wu et al. collected and analyzed aerosols in two locations: the city of Manaus, a large urban area in Brazil, and the ATTO site in the heart of the forest. The aerosol compositions varied largly. At ATTO most aerosols were emitted by the forest itself, while in Manaus, anthropogenic aerosols were very common. The results were published in ACP.

Mira Pöhlker and her team continuously measured aerosols and their properties in the atmosphere at the 80 m tower at ATTO, thereby created the first such long-term record in the Amazon. They analyzed the data in two subsequent paper. The second was now published in ACP.

Moran-Zuloaga et al. analyzed the coarse fraction of aerosols every 5 minutes for over 3 years at ATTO. They found that the composition remains fairly constant throughout the year, except for a short period in the wet season when Saharan dust occurs regularly. They published their results in ACP.