According to a study, immense amounts of nitrogen compounds have found their way into the oceans over the past few decades, mainly as a result of excessive use of fertilizers. The annual input in the form of ammonia in 2018 was about 89 percent higher than in 1970, a research team reports in the journal Proceedings of the US National Academy of Sciences ("PNAS").
Ammonia (NH3) is a gaseous nitrogen compound whose worldwide emissions are almost exclusively due to the agricultural sector. Nitrogen has been used as a fertilizer for decades to maximize food production. In addition to that of ammonia, the release of climate-damaging nitrous oxide (N2O) and nitrates can also be traced back to it.
In coastal regions in particular, ammonia is now ahead of nitrogen oxide emissions (NOx) from the combustion of fossil fuels, reports the predominantly Chinese scientists led by Lei Liu from Lanzhou University. The over-fertilization of the seas has developed into a global problem. It triggers numerous biogeochemical feedback loops, including ocean acidification, degradation of shallow-water habitats, and the development of harmful algal blooms and oxygen-poor zones.
Study: Large amounts of fertilizer could simply be left out
According to the study, China had the highest NH3 and NOx emissions of all countries in 2018, followed by India, the European Union, the USA and Brazil. For 2010, the calculation found that about 38 percent of agricultural nitrogen fertilizer use was excessive - meaning that this amount could be omitted without reducing yield. Excessive use is mainly found in China, India and the USA, for example in the cultivation of maize and rice for direct consumption and for animal fattening.
According to experts, the problem is still large in Germany. "We know that agriculture accounts for the largest share of nitrogen inputs in the North and Baltic Seas," says Julian Mönnich from the Federal Environment Agency (UBA). According to this, about 80 percent of the German nitrogen inputs in the North Sea and Baltic Sea come from agriculture.
Oxygen minimum zones in the oceans
Too high a proportion of nitrogen in bodies of water accelerates algae growth, explains Mönnich. "If the algae grow too much, not enough light gets underneath them. Then not much life is possible for creatures that depend on light." The algae eventually die, sink and are broken down by bacteria, which use oxygen to do so. "If too much of it is broken down, then, as is often the case in the Baltic Sea, there are oxygen minimum zones," says Mönnich.
The long-term trend observed in the study continues, says Wera Leujak from the UBA. "The problem is ultimately the growing world population with its increasing protein requirements and the associated need for more food, more agriculture and, of course, more and more meat consumption."
A study by Lei Liu's team, published about a year ago, showed that between 1980 and 2010, ammonia emissions increased by almost 80 percent worldwide. "Agriculture is responsible for about two-thirds of global reactive nitrogen pollution," the group wrote in PNAS at the time. While global food production has doubled over the past four decades, the use of synthetic nitrogen fertilizers has tripled.