Consequences of human modification of the global nitrogen cycle

Abstract

The demand for more food is increasing fertilizer and land use, and the demand for more energy is increasing fossil fuel combustion, leading to enhanced losses of reactive nitrogen (N r ) to the environment. Many thresholds for human and ecosystem health have been exceeded owing to N r pollution, including those for drinking water (nitrates), air quality (smog, particulate matter, ground-level ozone), freshwater eutrophication, biodiversity loss, stratospheric ozone depletion, climate change and coastal ecosystems (dead zones). Each of these environmental effects can be magnified by the ‘nitrogen cascade’: a single atom of N r can trigger a cascade of negative environmental impacts in sequence. Here, we provide an overview of the impact of N r on the environment and human health, including an assessment of the magnitude of different environmental problems, and the relative importance of N r as a contributor to each problem. In some cases, N r loss to the environment is the key driver of effects (e.g. terrestrial and coastal eutrophication, nitrous oxide emissions), whereas in some other situations nitrogen represents a key contributor exacerbating a wider problem (e.g. freshwater pollution, biodiversity loss). In this way, the central role of nitrogen can remain hidden, even though it actually underpins many trans-boundary pollution problems.

Changes in the location of biodiversity–ecosystem function hot spots across the seafloor landscape with increasing sediment nutrient loading

“…This is the real-world consequence of shifting biodiversity in habitats that are well recognized for their contribution to nutrient processing [23]. Importantly spatial shifts in functional performance has strong implications for scaling-up BEF relationships, mapping nutrient processing or relating detailed process-based measurements to ecosystem service delivery.…”

“…Coastal sediments are important sites of denitrification, the process that transforms the dissolved forms of nitrogen that fuel plant growth into nitrogen gas. Calculations indicate that at least 80% of terrestrial dissolved inorganic nitrogen can be denitrified in the coastal ocean margin [23]. There is a complicated set of biogeochemical interactions associated with the degradation and transformation of organic matter, and microbial communities are directly involved in these chemical transformations.…”

Enhanced hyporheic exchange flow around woody debris does not increase nitrate reduction in a sandy streambed

“…Although most fertilizer is applied as ammonium, the prevailing redox conditions in surface water allow the complete nitrification of ammonium to nitrate. This mass-delivery of nitrate to aquatic environments has already resulted in devastating ecological effects such as eutrophication (Camargo and Alonso 2006;Erisman et al 2013;McIsaac et al 2001), as well as risks to human health (Powlson et al 2008), and increased costs in the treatment of drinking water (Shrimali and Singh 2001).…”

Nutrients in the nexus

“…At each step in the cascade, reactive-N can cause negative environmental and human health consequences. Reactive-N has been shown to contribute to human disease and global climate change (Townsend et al 2003;Erisman et al 2011Erisman et al , 2013, cause stratospheric ozone depletion (Cowling et al 1998;Ravishankara et al 2009), acidify lakes and streams (Vitousek et al 1997;Camargo and Alonso 2006), and lead to the eutrophication and habitat degradation of coastal systems (Howarth et al 2000;Rabalais et al 2002).…”