Soil N2O emissions in French Guiana after the conversion of tropical forest to agriculture with the chop-and-mulch method

Abstract In French Guiana, the population growth will result in an increase in demand for agricultural products and thus, will lead to an increase in the amount of tropical forests converted into cropland or pasture. Impacts of different agricultural systems on greenhouse gas (GHG) fluxes have not been studied in French Guiana. In this context, the fire-free chop-and-mulch method was used to convert a tropical forest site to agriculture. This study focused on soil nitrous oxide (N 2 O) emissions and we compared four land uses: (1) the undisturbed tropical forest, (2) recently converted grassland and recently converted croplands (fertilized soybean/maize rotation) with either (3) disk tillage or (4) no tillage. N 2 O measurements were obtained through the chamber technique and conducted over a 1-year period (measurements began 19 months after the forest was cleared). N 2 O fluxes were related to soil parameters measured at each sampling date: nitrate and ammonium contents, gravimetric water content (GWC) and temperature. Through the entire period, the mean (±standard error) and median N 2 O fluxes were 3.8 ± 0.5 and 2.7 g N ha −1  day −1 , respectively for undisturbed tropical forest and 2.4 ± 0.9 and 0.8 g N ha −1  day −1 , respectively for grassland (mowed Brachiaria ruziziensis ). For croplands, no significant difference was found for N 2 O emissions between both agricultural practices. The mean (±standard error) and median N 2 O fluxes were 8.5 ± 1.2 and 4.0 g N ha −1  day −1 , respectively for disk tillage plots and 8.5 ± 1.3 and 3.6 g N ha −1  day −1 , respectively for no tillage plots. Nitrogen inputs (due to the application of fertilizer or due to the mineralization of crop residues) led to higher N 2 O fluxes, resulting in significantly higher mean N 2 O emissions from croplands compared to the forest, when only considering land use effect on N 2 O fluxes in a statistical model. The soil nitrate content, GWC and temperature had a significant positive effect on N 2 O fluxes. Taking into account these soil parameters in another statistical model, N 2 O emissions from croplands were not higher than the natural N 2 O emissions from tropical forest soils. Our results suggest that, if more forest will have to be converted in the course of the expected population growth in French Guiana, it could have low impact on the soil N 2 O fluxes (similar to natural fluxes from forest) with the improving of farming techniques (for example modification of the splitting of N-fertilizer) in the cropping plots.