N2O Emissions in Southeastern Amazonia: The Effect of Agricultural Intensification

Tuesday, 16 December 2014
Christine O'Connell1, Paulo M Brando2, Carlos E Cerri3, Michael Thomas Coe4, Eric A Davidson5, Gillian L Galford6, Marcia Macedo4, Christopher Neill7 and Rodney T Venterea8, (1)University of Minnesota Twin Cities, Minneapolis, MN, United States, (2)Carnegie Institution for Science, Washington, DC, United States, (3)USP University of Sao Paulo, São Paulo, Brazil, (4)Woods Hole Research Center, Falmouth, MA, United States, (5)Woods Hole Research Center, East Falmouth, MA, United States, (6)University of Vermont, Burlington, VT, United States, (7)Marine Biological Laboratory, Ecosystems Center, Woods Hole, MA, United States, (8)USDA/ARS-Soil & Water Mgmt, Saint Paul, MN, United States
The Amazon is not only an exceptionally biodiverse and carbon-rich tract of tropical forest, it is also a case study in land use change. Over the last 30 years, Amazonia has been home to extraordinary growth in agricultural production, in part from agricultural expansion, but also due to more intense management on Amazonia’s existing croplands. We use a year-long campaign and approximately 500 field chamber measurements to estimate how cropland intensification in Mato Grosso, Brazil affects the emission of nitrous oxide (N2O) and soil N dynamics. In this system, soybean cropland intensification occurs when double cropping is introduced, in which maize is planted directly after soybean harvest and fertilized twice with inorganic N. We find that dry season N2O emissions in single-cropped (soybean only) fields, double-cropped (soybean/maize) fields and reference tropical forest are uniformly near zero, or ~0-0.5 ngN/cm^2/hr. Surprisingly, wet season emissions rates remain low as well, between 1-4 ngN/cm^2/hr, for both cropland types and reference forest. By contrast, isolated post-fertilization spikes in N2O emissions are large, with a maximum increase of ~800% and a mean increase of ~400%, though these flux increases resolve rapidly and rates return to their low baseline within days. Finally, we explore the role that soil moisture, soil N availability, and soil C availability play in regulating N2O fluxes in reference forest, soybean fields and intensified soybean-maize fields. Open questions surround how the Amazon’s land resources can be leveraged to increase agricultural production at the least harm to the environment. Here, we quantify the consequences of land use change on N2O, a powerful greenhouse gas, in a critical ecosystem undergoing novel agricultural intensification. These results may inform both greenhouse gas accounting and our understanding of the effects of Amazonia’s changing agricultural landscape on the nitrogen cycle.