Quantifying climate-dependent ammonia emissions from global agriculture: from development of the AMmonia–CLIMate (AMCLIM) model to its application and implications

Abstract

Ammonia (NH3) is one of the primary forms of reactive nitrogen and can negatively affect the environment and human health. It has adverse impacts on air, water, soil quality and ecosystems. Emissions of NH3 mainly originate from agricultural activities and are found to be strongly dependent on environmental conditions. Current emission inventories often consider the effects of environmental factors in a limited way. To address this deficiency in existing estimates, a dynamic, process based emissions model, AMmonia–CLIMate (AMCLIM) has been developed to quantify agricultural NH3 emissions. AMCLIM simulates important physical, chemical and biological processes that are sensitive to climatic conditions in agricultural systems, and focuses on major livestock farming and synthetic fertilizer use. The model has been applied to different scales, with modelled results evaluated by comparison with site measurements, showing close agreement. When applied at the global scale, AMCLIM operates at high spatial and temporal resolution, and AMCLIM is thought to be the first model that simulates NH3 emissions from all individual sectors using a consistent process based modelling approach, with high levels of detail. For the year 2010, global agricultural NH3 emissions estimated by AMCLIM are 44.9±4.4 Tg N yr-1, equivalent to 22±2 % of agricultural nitrogen input (synthetic fertilizer and livestock excreta) being lost through NH3 volatilization. The global estimates of AMCLIM are consistent with other models and studies. Around 1/3 of the NH3 emissions result from synthetic fertilizer use, with 2/3 associated with livestock farming (including housing, manure management, land application of manure and grazing). China, India, US, Brazil and Pakistan result in the largest estimated emissions, together accounting for nearly 60 % of global NH3 emissions. Cattle are the largest emitter group among livestock, followed by pigs, chicken, sheep and goats. Emissions of NH3 not only show large spatial variations but also exhibit significant seasonal variation. The highest estimated NH3 emissions are found in July, mainly driven by the planting season and high temperatures in the northern hemisphere. The impacts of temperature, wind speed and water availability on NH3 volatilization have been investigated. Increasing temperature and wind speed facilitates volatilization to cause more NH3 emissions, with temperature normally being the most critical factor especially under cold conditions. A global sensitivity test to temperature indicates that annual NH3 emissions may increase by around 7 % due to a (uniform) 2 °C warming, compared to the base value of year 2010. Ammonia emissions tend to be larger under drier conditions, but wetter soils can either result in higher or lower emissions, depending on complex interactions with other variables. Soil pH is also a critical factor as alkaline soils typically lead to more intense volatilization. Mitigation measures have been simulated and have been found to be effective in reducing NH3 emissions. These include decreasing nitrogen application rates and improving livestock feeding materials, covering stored manure and better land application techniques (e.g., incorporation and deep placement). It is estimated that a potential 40 % abatement of global NH3 emissions can be achieved when applying a suite of the tested measures. Using AMCLIM, it is estimated that NH3 emissions have increased from 39.8 Tg N yr-1 in 2000 to 45.2 Tg N yr-1 in 2018, and future emissions by 2100 are projected to go up to 51 to 55 Tg N yr-1 due to warming alone (with 2018 activity) and can reach 59 to 102 Tg N yr-1 when also combined with growing livestock numbers and synthetic fertilizer usage. It is suggested that climate change and food production pose risks for future agricultural NH3 emissions at different spatial scales. Regional and local environment and agricultural systems may suffer the consequences of the warming effect, while the globe may face challenges due to increased food production.