An international team of researchers has now gained a comprehensive picture of global biological nitrogen fixation. Carla Reis Ely from Oregon State University (USA) led the study, which involved a total of 24 scientists: “Our new estimate of natural nitrogen fixation, based on improved scientific understanding, suggests that less new nitrogen enters natural ecosystems.” As a result, these ecosystems may store less carbon dioxide and mitigate climate change to a lesser extent.
“The amount of data has quadrupled over the past 25 years. In addition, significantly more habitats were taken into account and realistic average values were calculated, enabling us to describe global processes much more accurately,” explains Bettina Weber, co-author of the paper. “These data provide an essential basis for global calculations and modelling of Earth system processes. They are needed to understand how global biogeochemical cycles function and to take effective countermeasures in the event of undesirable developments.”
Importance of soil crusts
Worldwide, more than half (56 percent) of the nitrogen fixation by microorganisms takes place in natural landscapes, such as forests and soils without human use. However, this figure is up to two-thirds lower than was assumed 25 years ago. In this context, soils contribute 26 per cent, trees 19 per cent, herbs 17 per cent and shrubs nine per cent. Biological soil crusts have a previously unrecognised importance. They account for 18 per cent of the nitrogen fixation in natural environments. Biologist Bettina Weber: “This community of bacteria, fungi, mosses, lichens and algae, which occurs everywhere in arid areas, acts as a kind of local fertiliser, ensuring that other plants can settle and thus preventing desertification.”
Balance of the Earth's atmosphere
In addition to natural areas, nitrogen fixation also plays a role in fields and pastures, where special plants, known as legumes, have fixing bacteria in nodules on their roots. Scientists attribute the 75 percent increase over the past two decades to intensification and the near doubling of the area under soybean cultivation. Soybeans are now responsible for 60 percent of nitrogen fixation in the agricultural sector, while 33 percent is accounted for by meadows and pastures, mainly clover, and seven percent by other agricultural products.
“If too much nitrogen is added to the soil, conversion processes can release more gaseous compounds into the atmosphere,” warns Bettina Weber, referring to the emission of degradation products such as nitrogen oxides and nitrous oxide. These may interfere with the balance of the Earth's atmosphere and alter or disrupt climate processes.