Natural sources also contribute to emissions, but people have a strong influence on these in some regions through land-use change, especially cropland expansion (Pacifico et al. 2012; Ciais et al. 2013). Wind-blown dust from natural landscapes and unprotected cropland in arid and semi-arid regions is the largest source of atmospheric PM and the dominant fraction of coarse PM in many regions, such as northern Africa and the Middle East (Ginoux et al. 2012; Albani et al. 2014). Sustainable land and water management practices can decrease sand and dust storms, while contributing to reduced desertification, preserving biodiversity and mitigating climate change. Regional and national action plans, including those developed under the United Nations Convention to Combat Desertification (UNCCD), have the potential to address the underlying causes of sand and dust storms (UNEP, WMO and UNCCD 2016).
Globally, terrestrial vegetation is the dominant source of atmospheric NMVOCs, outweighing anthropogenic sources by a factor of ten (Guenther et al. 2012; Sindelarova et al. 2014). Biogenic NMVOCs tend to be highly reactive and can contribute significantly to O3 and PM formation even in urban areas (Chameides et al. 1988). Soil microbial processes are an important part of the nitrogen cycle and can be a significant source of NOx emissions outside urban areas and the dominant source of nitrous oxide (N2O), a potent GHG, on a global basis (Ciais et al. 2013). Soil NOx emissions are highest in croplands due to increased soil nitrogen content from fertilizer application (Vinken et al. 2014). Deforestation associated with expansion of croplands and pasturelands is estimated to have reduced global annual biogenic NMVOC emissions by 10-35 per cent and increased soil NOx emissions by about 50 per cent since the 1850s, except in parts of the eastern United States and Western Europe where reforestation has taken place (Unger 2014; Heald and Geddes 2016). Bouwman et al. (2013) estimated that agricultural soil N2O emissions increased by a factor of three during the 20th century.
Soil respiration is a major source of CO2 to the atmosphere at a global scale (Hashimoto et al. 2015) that in recent decades has increased its contribution (Bond-Lamberty et al. 2018).
