Ten Years of Improved-Fallow Slash-and-Mulch Agroforestry in Brazilian Amazonia: Do Nitrogen-fixing Trees affect Nitrous Oxide and Methane Efflux?

Abstract Agriculture, and associated land-clearing, account for approximately 25% of global greenhouse gas (GHG) emissions, including trace gases nitrous oxide (N2O) and methane (CH4). Agroforestry systems (AFS) have been proposed to reduce GHG emissions of agricultural practices. Smallholding farmers of the global tropics often utilize slash-and-burn AFS to prepare land for planting, releasing CO2, CH4, and N2O during burning, and from soil during agricultural and secondary forest regrowth phases. Slash-and-mulch AFS eliminate losses of GHGs via burning by mulching the vegetation instead. This mulch layer contains greater stocks of organic material, rich in C and N, than after burning, making it a potential source of N2O and CH4 efflux from litter and soil during decomposition. Slash-and-mulch systems that incorporate N-fixers for improved fallows may further augment N2O fluxes. To examine rates of N2O and CH4 efflux from slash-and-mulch AFS, four treatments were applied in a two-way factorial design: with and without P+K fertilization, and with and without a nitrogen-fixing tree (I. edulis). The inclusion of N-fixing trees increased N content of fallow forests, and thus was hypothesized to increase N2O efflux. We hypothesized that CH4 efflux would increase due to increased soil moisture with mulching. Trace gas flux measurements were taken over two years; prior to mulching to end Rotation 1 in Year 9 and after mulching to begin Rotation 2. Increased rates of N2O efflux were detected in the presence of I. edulis during the year prior to, but not after, mulching. No differences were detected for CH4 efflux before or after mulching. Site conversion from secondary forest to Rotation 2 was accompanied by 130% increase in N2O efflux and 430% decrease in CH4 efflux. Despite the large increase in N2O efflux, the CO2e of this increase was an order of magnitude less than estimated releases of trace gases from burning.