SUPPORTING INFORMATION Accounting for the Biogeochemical Cycle of Nitrogen in Input-Output Life Cycle Assessment Shweta Singh and Bhavik R. Bakshi * William G. Lowrie Department of Chemical and Biomolecular Engineering The Ohio State University, Columbus, OH 43210 June 27, 2013 S.1 Background S.1.1 The Biogeochemical cycle of N Nitrogen is a crucial building block for existence of life and various terrestrial/aquatic ecosystems. It is considered to be a limiting factor for net primary production (NPP) in the biosphere, thus emphasizing the need for availability of nitrogen. Nitrogen exists in various forms in the biosphere but not all its forms are useful [18]. Based on the utility, nitrogen is defined into two basic forms : “inert” and “reactive”. Inert nitrogen is the form that cannot be used by plants or other organisms for production purposes (except for industrial use of gaseous nitrogen). “Reactive” nitrogen on the other hand is the form of nitrogen than can be taken up by plants and living beings and used for production [13]. The largest pool of nitrogen exists in the atmosphere as N 2 gas which is “inert” nitrogen and cannot be directly used for production in ecosystem or anthropogenic systems. The biogeochemical cycle of nitrogen is the major mechanism by which the nitrogen pool from atmosphere becomes available in useful form to living beings. The transformation of inert nitrogen to reactive nitrogen involves many processes like nitrification, ammonification, etc. which change the oxidation state of nitrogen. Due to the existence of nitrogen at valence states ranging from -3(NH 3 ) to +5 (NO - 3 ) numerous transformations of nitrogen are possible. The energy released during these transformations are utilized by different organisms for their metabolic activities [18]. Therefore, these microbial actions play a significant role in the circulation of N in the geobiosphere. The natural processes which are involved in the nitrogen cycle are : N 2 fixation, bacterial degradation, nitrification and denitrification. Nitrogen fixation is mainly the conversion of “inert” nitrogen to “reactive” nitrogen through natural processes of lightning or by free living and symbiotic microbes. The fixed nitrogen is further converted to NH + 4 in a process called ammonification. In presence of nitrifying bacteria like Nitrosomonas, ammonium ions are converted to nitrate ions. This process of the nitrogen cycle is called as nitrification. Further, through the process of denitrification fixed nitrogen is returned to the atmospheric pool in form of N 2 and some loss of reactive N as N 2 O thus closing the nutrient cycle. The natural nitrogen cycle provides important nutrients to plants and animals, thus also supporting human existence. Over the last few decades, due to the increased demand for food and other products that depend on reactive nitrogen its use and flow into the biosphere has increased tremendously [13, 12, 19]. However, the * Corresponding author, e-mail: bakshi.2@osu.edu, tel: 1-614-292-4904, fax: 1-614-292-3769 S.1