Nitrogen pollution accelerates wood decomposition, study finds
The response of wood decomposers to man-made nitrogen inputs in forest ecosystems can help us understand the effects of climate change on forest carbon cycles, according to a new study published by Earthwatch and the University of Oxford, in the international journal Oecologia.
Nitrogen in the atmosphere is converted to a useful form by natural processes like bacteriological nitrogen fixation and lightning strikes, and anthropogenic (human) activities such as agricultural fertilization, fossil fuel combustion and other industry processes. This conversion helps distribute the supply of this nutrient, essential for plant growth and food production.
Dr Dan Bebber
However, air pollution as a result of human activities has led to increased nitrogen input to ecosystems, as it rains down on landscapes. Although nitrogen is a key nutrient, too much anthropogenic nitrogen deposition - the input of converted nitrogen from the atmosphere to terrestrial ecosystems - is detrimental to important ecological processes like decomposition. Biomass production and storage depends on nitrogen availability. Fallen leaves and ‘dead wood’ (fallen branches and trees) release carbon dioxide back into the atmosphere when they decompose. Decomposer species like fungi are key accelerators of the process and crucial in recycling key nutrients like nitrogen, back into the soil.
“We conducted experiments to see how wood decay fungi respond to nitrogen deposition in temperate forests like those in the UK and most parts of Europe” said Dr Dan Bebber, head of climate change research at Earthwatch and author of the study. “Our results show that small increases in nitrogen availability can increase wood decomposition by the decay fungi, and this can affect forest carbon capture and storage.
“Forest design and management can have significant influences on nitrogen availability, and this has implications for a number of ecological processes and organisms that depend on decaying wood as a source of food, nutrients and cover. Sustainable forestry practices already recognize the inclusion of dead wood as a key feature of productive and resilient ecosystems. However, we should be careful to ensure that nitrogen deposition doesn’t reach a level where inputs are in excess of biological demand - both tree growth and wood decay are affected by nitrogen availability, and excess nitrogen pollution could alter the balance between these processes. The biodiversity of forest ecosystems can also be affected as higher nitrogen availability rapidly increases the growth of nitrogen-demanding plants, at the cost of others.”
Mycelial cord system of Phanerochaete velutina emerging from a beech wood block
Decomposition rates are generally greater in plants with low carbon to nitrogen ratios. The two major types of forest litter- plant and wood - have differing levels of carbon and nitrogen, and as such different communities of fungi rely on these litter types for nutrition.
Saprotrophic basidiomycete fungi were the decay organisms used in the study. They play a major role in how ecosystems function, and contribute significantly to the carbon cycle as they are the only species capable of breaking down decaying wood. Basidiomycete fungi reside in the soil, storing and transporting nutrient resources across distances of several metres through their ‘mycelial cords’. These inter-connected networks of resources mean they are very efficient in recycling nitrogen and also adapting to environments with low nitrogen availability.
Dan Bebber added: “Plant litter decomposition using simulated nitrogen levels has been previously studied. However, variations in the results from these experiments could be due to the large amounts of nitrogen added. Our research, using realistic simulations of nitrogen availability in temperate forests, is contributing to the research gap on the role of nitrogen availability on wood decay. The results will be useful for woodland managers, conservationists and policy makers.
“Our experiments were carried out in beech plantations at Earthwatch’s forest research centre near Oxford, one of five around the world. Our centre in Oxford is the base for our forest research in the UK where Earthwatch scientists are carrying out fieldwork with the help of volunteers, to see how Europe’s forests are responding to climate change. In the UK, our research paper will be one of the first to provide a comprehensive picture of the carbon cycle in fragmented forests.
“Forests help stabilise the climate by storing carbon. Understanding the carbon cycle, the movement of carbon through the earth’s system, is a key step to understanding climate change.”