Global variation and similarities in nitrogen benefits and impacts

David U. Hooper, Dept. of Biology, Western Washington University, Bellingham, WA, Cargele Masso, International Institute of Tropical Agriculture (IITA), Yaoundé, Cameroon, Luis Lassaletta, Universidad Politécnica de Madrid, Madrid, Spain, Miguel Quemada, School of Agricultural Engineering, Technical University of Madrid, Madrid, Spain, Mekonnen Giweta, International Institute for Tropical Agriculture, Yaoundé, Cameroon and INMS Component 3 Regional Demonstration Projects, UK Centre for Ecology and Hydrology, Edinburgh, United Kingdom

Background/Question/Methods
Reactive nitrogen (Nr) helps provide high yields in modern agriculture, but, in excess, causes a variety of environmental and human health problems. However, both the benefits and negative impacts of Nr are unevenly distributed around the globe. As part of the project “Towards an International Nitrogen Management System”, we collated data on nitrogen use, benefits and impacts, from seven regional demonstration projects, each encompassing international watersheds and covering a range of economic development and nitrogen use patterns: East Asia, South Asia, East Africa (Lake Victoria Basin), Eastern Europe, Western Europe, Latin America (La Plata River Basin), and North America (Nooksack River Basin). Our goal is to identify commonalities and differences among regions to develop regionally-tailored N management strategies. Here we focus on crop yields, N inputs, nitrogen use efficiency (NUE) and N surplus. N surplus equals the difference between N inputs and N in harvested crops, and NUE is share of N inputs retained in crop yields. Though some surplus N may be stored in plants and soils, losses of excess N cause environmental problems. On the other hand, very high NUE (low or negative surplus) can result from “soil mining” of N, leading to reduced long-term soil fertility.
Results/Conclusions
We compared our N indicators for 1) common cereal crops across multiple regions, and 2) variation in cropping systems across regions. N inputs, yield, surplus and NUE varied widely across regions when comparing similar crops. Reflecting known issues, inputs to and yields of maize were much lower in subsistence farms in Kenya (E. Africa demo) than in other regions. Fields with no intentional N input had high NUE, negative surplus N, mining of soil N (NUE >100%) and low yields (<50 kg N/ha/yr). In contrast, maize N yields in high intensity systems in other regions, had high inputs of manure and synthetic fertilizer, high yields and intermediate-to-low NUE. However, variation across locations within the same region (e.g., S. Asia) could equal or exceed variation across global regions. Some crop types, such as fruits and vegetables, had low NUE due to inherently low harvested protein content, but provide other nutritional benefits. Except Africa, all regions had situations exceeding 50 kg N/ha of surplus, which can cause air pollution (PM2.5), excess deposition, groundwater pollution, and surface water eutrophication. In all regions, better understanding of partitioning of N surplus among these fates will help N abatement measures to minimize local and global impact.