Neglected Nitrogen Escalates Environmental Crisis

According to experts and scientists, there are nine planetary boundaries that people should not cross to protect human existence. The Earth's environment is now in danger because nitrogen (N) and phosphorus (P) waste have far exceeded the planetary threshold. The Colombo Declaration was adopted in October 2019 to compel South Asian nations to do a thorough analysis of N-related policies, management, and scientific issues to progress towards sustainable N management. This highlights the skyrocketing N emissions, related issues, and urgency for sustainable N management, in particular in South Asian nations. Nitrogen is one of the most common elements that make up 78.1per cent of the Earth's atmosphere and is an essential and limiting agent for many ecosystems. However, a small increase in N content, especially reactive N species such as gaseous ammonia and oxides of N in the atmosphere, leads to negative consequences for both terrestrial and aquatic biodiversity. In contemporary times, where everyone is focusing on carbon emissions, surging N pollution is often ignored, and thus its impacts are less understood. The 6th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) highlighted that the global warming potential (GWP) of nitrous oxide is 273 times that of carbon dioxide for a 100-year time scale. Moreover, the UN Environment Programme highlighted N pollution as one of the critical issues facing humanity and provided the four critical reasons why the world needs to limit N pollution; among many, N pollution is contributing to climate change, disrupting life on land and water, threatening human health, and weakening the global economy.

Sources of nitrogen

Both autochthonous (N fixation, plant decay, and dry and wet deposition) and allochthonous (fossil fuel burning and fertiliser inputs) sources of N are common. Although the synergies are poorly understood, it is quite likely that reactive nitrogen deposition interacts synergistically with other stressors, including climate change, acid deposition, and ground-level ozone. Moreover, the amount of N in both terrestrial and aquatic habitats has significantly increased as a result of the incorrect use and abuse of fertilisers, poor handling of animal waste, excessive protein intake, wasteful food waste, and haphazard industrial activities. The leaching of nitrogen into the soil is another environmental issue. Moreover, it may also be released into the atmosphere, buttress GHG emissions, and interact with other pollutants to impair air quality. The element is particularly exceptional since it may be lost through a variety of biological processes from the moment it enters an inorganic form until it is mineralized from decaying organic materials. All aforementioned sources create cascading effects of N on the Earth, exacerbating issues in both terrestrial and aquatic habitats.

Nitrogen availability 

In developing countries like Nepal, the shortage of nitrogen-based fertilisers is one of the most common problems among farmers. However, among the many, agriculture has been a key factor in the development of excessive nitrogen in industrialised parts of the world. At present, N concentrations in the atmosphere, especially gaseous ammonia, are concentrated around the Indo-Gangetic plain, which is now considered one of the hotspots, and some recent studies have highlighted the exceedance of the N and ammonia critical loads in the atmosphere. On the other hand, there is currently a scarcity of N in other parts of the world, such as sub-Saharan Africa. This is mostly caused by the soil's poor fertility, which is exacerbated by the lack of fertiliser and the negligent application of the limited amount that is available. This insufficiency leads to low crop yields and food problems for a population that is rapidly growing. Nonetheless, N may still have adverse impacts on the environment even in times of scarcity if it is permitted to release GHG into the atmosphere and cause other types of pollution. The global N use efficiency is only around 20 per cent, and the remaining 80 per cent is lost to the environment. Nitrous oxide (N2O), a strong greenhouse gas and the main source of stratospheric ozone depletion emissions, returns around 4 per cent of the anthropogenic N in agricultural systems to the atmosphere globally. Some of the most optimistic projections for the need for nitrogen fertiliser have been surpassed by the present trajectory.

Impacts on biodiversity 

From genes and genomes to ecosystems, the fluxes, reactions, and availability of N significantly affect ecological processes and ecosystem functioning. Although N cycling and N stocks in terrestrial ecosystems significantly differ between different ecosystem types, the addition of N to any terrestrial ecosystem supported by anthropogenic activities gives rise to a multitude of effects, including nutritional imbalances, modifications to the health of the forest, declines in crop yield and natural vegetation, and losses of biodiversity. However, some evidence suggests biological communities react to the soil's accumulated supply of nitrogen that is accessible to plants. Moreover, it is not yet known if various wet-deposited types of N (for example, nitrate and ammonium) affect biodiversity in different ways. However, through direct foliar damage, gaseous ammonia can be extremely detrimental to vegetation, especially lower plants. Although there are some blatant instances of faunal variety declines that may be connected to N deposition, overall, our understanding of faunal effects is still sparse. 

Eutrophication, algal bloom, deoxygenation, and loss of aquatic biodiversity have been brought on by the leaching of excessive N and P fertilisers used on agricultural land. On the other hand, the leaching of these chemical fertilisers and rampant waste disposal into water bodies is a driver for severe ecological issues in aquatic habitats. Although P is thought to limit nutrients for primary productivity in freshwater systems, the leaching of N and its compounds such as nitrate and ammonia that contribute to the acidification of freshwaters (both surface and groundwater) can contribute to augmenting pollution. Further, the impacts of excessive N in the freshwater system can be seen through the nutrition enrichment, limitation of plankton, and decline in fish growth. Moreover, the depletion of oxygen by excessive N availability hinders the microbial processes that are the principal regulators of ecological processes. Besides, excessive nitrogen can damage coral reefs in the marine ecosystem. Additionally, the impacts of N, especially oxides of N, ammonia, and N-induced particulate matter, and ozone-enhanced GHG emissions prop up climate change, which causes numerous complications in both the environment and human health.

Ways forward

Scientists are trying to persuade the globe to minimise waste with significant changes to farming practices to combat the impacts of N pollution. To help lower N emissions, it is proposed that consumers on the street limit their use of foods high in nitrogen, seek to wean themselves off fossil fuels, and optimise the operation of power plants and other industrial facilities. Additionally, enhancing the usage of N-based fertilisers, altering lifestyles, and reducing industrial emissions might all increase the efficiency of N use globally. Even though it might be challenging to decide how to restore habitat, educating stakeholders about biodiversity may be able to aid with sustainable N management, but this requires ongoing work and improvement. Future research should therefore focus on gaining a holistic understanding of the scope of the problem of atmospheric N and the synergistic connections between N deposition and other factors by using proxies including vegetation, soil and aquatic chemistry, and atmospheric chemistry. To accomplish sustainable N management, N-related policies should also be emphasised and regulated in every country.

(Pradhan is a Research Associate and Project Coordinator at UKRI-GCRF-South Asian Nitrogen Hub, Kathmandu University, Dhulikhel.)