Ten years of improved-fallow slash-and-mulch agroforestry in Brazilian Amazonia: Do nitrogen-fixing trees affect nitrous oxide and methane efflux?

Slash-and-mulch agroforestry systems can reduce greenhouse gas emissions by mulching the vegetation instead of burning it. This mulch layer then contains greater stocks of organic material than after burning, making it a potential source of N₂O and CH₄ efflux during decomposition. We examined N₂O and CH₄ efflux from slash-and-mulch AFS using a two-way factorial design: with and without P + K fertilization, and with and without a nitrogen-fixing tree (Inga edulis). We hypothesized that inclusion of N-fixing trees would increase N₂O efflux and that CH₄ efflux would increase due to increased soil moisture with mulching. We measured trace gas fluxes prior to the end of Rotation 1, and after mulching to begin Rotation 2. N₂O efflux increased with I. edulis during the year prior to, but not after, mulching. No differences by treatment were detected for CH₄ efflux before or after mulching. Site conversion from secondary forest to Rotation 2 resulted in a 130% increase in N₂O efflux and a 430% decrease in CH₄ efflux. The CO_2e increase of 2,400 kg ha⁻¹ was an order of magnitude less than estimated releases of trace gases from burning (38,400 kg ha⁻¹). For both N₂O and CH₄, land disturbance during mulching led to larger changes in trace gas fluxes than either P + K fertilization or inclusion of the N-fixer. The order-of-magnitude estimates of trace gas release as CO_2e from mulching and the addition of N-fixers appears to be less than that from burning alone.