Human activities changing the nitrogen cycle in Brazil SOLANGE FILOSO 1, *, LUIZ ANTONIO MARTINELLI 2 , ROBERT W. HOWARTH 1 , ELIZABETH W. BOYER 3 and FRANK DENTENER 4 1 Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853 USA; 2 Centro de Energia Nuclear na Agricultura, University of Sa ˜o Paulo, Av. Centena ´rio 303, Piracicaba, SP 13416-000 Brazil; 3 Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720 USA; 4 Joint Research Centre, Institute for Environment and Sustainability, Climate Change Unit, TP280, I-21020 Ispra (Va), Italy; *Author for correspondence (e-mail: sfw6@cornell.edu) Key words: Agricultural expansion, Amazon, Brazil, Cerrado, Deforestation, Nitrogen, Nitrogen budget Abstract. The production of reactive nitrogen worldwide has more than doubled in the last century because of human activities and population growth. Advances in our understanding of the nitrogen cycle and the impacts of anthropogenic activities on regional to global scales is largely hindered by the paucity of information about nitrogen inputs from human activities in fast-developing regions of the world such as the tropics. In this paper, we estimate nitrogen inputs and outputs in Brazil, which is the world’s largest tropical country. We determined that the N cycle is increasingly controlled by human activities rather than natural processes. Nitrogen inputs to Brazil from human activities practically doubled from 1995 to 2002, mostly because of nitrogen production through biological fixation in agricultural systems. This is in contrast to industrialized countries of the temperate zone, where fertilizer application and atmospheric deposition are the main sources of anthropogenic nitrogen. In Brazil, the production of soybean crops over an area of less than 20 million ha, was responsible for about 3.2 Tg N or close to one-third of the N inputs from anthropogenic sources in 2002. Moreover, cattle pastures account for almost 70% of the estimated 280 · 10 6 ha of agricultural land in Brazil and potentially fix significant amounts of N when well managed, further increasing the importance of biological nitrogen fixation in the nitrogen budget. Much of these anthropogenic inputs occur in the Brazilian savannah region (Cerrado), while more urbanized regions such as the state of Sa˜o Paulo also have high rates of nitrogenous fertilizer inputs. In the Amazon, rates of anthropogenic nitrogen inputs are relatively low, but continuing conversion of natural forests into cattle pasture or secondary forests potentially add a significant amount of new nitrogen to Brazil given the vast area of the region. Better measurements of biological fixation rates in Brazil are necessary for improving the nitrogen budgets, especially at a more refined spatial scale. Introduction Over the past century, growing human population and increasing human activities related to the production of food and energy have more than doubled the production rate of reactive nitrogen (Nr) on the land surface of the Earth Biogeochemistry (2006) 79: 61–89 Ó Springer 2006 DOI 10.1007/s10533-006-9003-0