Soil CH4 and N2O Fluxes in an Improved-Fallow Slash-and-Mulch Agroforestry System for One Year in Secondary Forest and One Year of Crop Cultivation in Eastern Amazonia of Brazil.

Soil CH4 and N2O Fluxes in an Improved-fallow Slash-and-mulch Agroforestry System for One Year in Secondary Forest and One Year of Crop Cultivation in Eastern Amazonia of Brazil Aaron Joslin1 Daniel Markewitz1, Lawrence Morris1, Francisco de Assis Oliveira2, Steel Vasconcellos3 and Oswaldo Kato3 Abstract Agricultural practices account for approximately 25% of global GHG emissions. Slash-and-burn preparation methods are common among tropical, smallholding farmers, contributing to CO2, CH4, and NOx gases to the global emissions budget via burning. Slash-and-mulch technology has been developed to replace burning during site preparation, reducing total GHG emissions compared to slash-and-burn practices. Improved-fallow practices incorporate N-fixing plants into the crop-fallow cycle in an effort to increase N stocks and biomass in the agricultural system without the use of mineral fertilizers. Increased system C and N content may also cause increased CH4 and N2O fluxes from the soil, negating GHG sequestration via soil and biomass C storage. We investigated the CH4 and N2O fluxes of an split-plot design improved-fallow, slash and mulch agroforestry system in eastern Amazonia of Brazil for one year prior to, and one year following, conversion of secondary forest to cropped field. Fertilization with P+K fertilizer was the main-plot treatment and the inclusion of an N-fixing tree (I. edulis) in the improved-fallow planting mix was the sub-plot treatment. Samples were taken along a transect, at distances of 0.25, 0.5, 1.0 and 2.0 m from the base of each of 2 trees in each plot. There were no significant increases in CH4 or N2O fluxes during the year pre-conversion in either the main or sub-plot treatments, yet there was a significant increase in the flux of N2O in the N-fixer sub-plot treatment during the year post-conversion. Rates of N2O flux were significantly different, and greatest, only at 1.0 m from the base of the tree. There was a trend of increasing N2O and CH4 fluxes with P+K fertilizer. Keywords: Agroforestry, Greenhouse gas emissions, smallholding agriculture, N-fixing trees, Slash-and-mulch, Improved-fallow, biogeochemistry, Nitrogen cycling, 1University of Georgia, 2 Universidade Federal Rural da Amazônia, 3Embrapa Amazônia Oriental