AGRICULTURAL INTENSIFICATION IN THE AMAZON: TRACKING NITROGEN FERTILIZER FROM SOY-MAIZE DOUBLE CROPPING TO STREAMS
Vanessa Cabrera1, KathiJo Jankowski2, Christopher Neill2, Marcia Macedo3, Linda Deegan2, Paulo Brando3, Sebastio Aviz do Nascimento4, Sandro Rocha4, Ebis Pinheiro do Nascimento4, Darlisson Nunes da Costa4, Michael Coe3.
1University of California, Santa Cruz, Santa Cruz, CA, 2The Ecosystems Center, Woods Hole, MA, 3Woods Hole Research Center, Falmouth, MA, 4Instituto de Pesquisa Ambiental da Amazonia, Canarana, BR.
Globalization and the increasing demand for food create pressure both to expand and intensify agriculture. These changes have potentially large consequences for the solute concentrations and functioning of streams. In the Brazilian Amazon, crop agriculture expanded greatly during the last 20 years. More recently, farmers have intensified production on existing cropland by double cropping of soy and maize during the same year. Maize, a novel crop for the region, requires much higher applications of nitrogen (N) fertilizer than soybeans. To determine whether this novel land use and associated N addition influenced N concentrations in groundwater and stream water, we measured N concentrations in groundwater wells and streams from small headwater watersheds across 3 land uses (soy-maize, soy, and tropical forest) in the Upper Xingu Basin, a region of rapid cropland intensification in the southern Amazon. Each watershed contained 6 groundwater wells arranged in a transect reaching the cropland field edge on either side of the stream. Total inorganic N concentrations were higher in wells adjacent to fields where double cropping occurred, while stream concentrations did not differ overall among land uses. This suggests the riparian zones are critical in the removal of N, but as the intensification of agriculture continues, the ability of the riparian zone to prevent N from traveling to streams is unknown. Their protection is critical to the functioning of tropical watersheds.