Soil greenhouse gas fluxes following conversion of tropical forests to fertilizer-based sugarcane systems in northwestern Uganda

Abstract

Tropical deforestation for fertilizer-based agriculture has greatly increased in the last decades resulting in significant greenhouse gas (GHG; carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) emissions. Unfortunately, empirical studies on soil GHG fluxes from African deforestation hotspots are still limited, creating uncertainties in global GHG budgets. Therefore, we assessed how soil GHG fluxes along with their auxiliary controls (water filled pore space (WFPS), temperature, and mineral nitrogen (N)) differed between the forest and sugarcane plantations. This assessment was based on monthly (forest) and intensive (sugarcane) GHG and auxiliary measurements between May 2019 and June 2020. Measurements were conducted in four reference forest plots and 12 sugarcane plots randomly assigned to three fertilization treatment groups (low, standard, and high), representing the fertilization gradient used by sugarcane farmers in Uganda. Despite the use of different fertilization rates as treatments for the sugarcane experiment, neither auxiliary controls nor soil GHG fluxes significantly differed among the treatments. Soil CO2 effluxes were higher under sugarcane (17.6 ± 0.0 Mg C ha-1 yr-1) compared to forest (14.5 ± 0.1 Mg C ha-1 yr-1; p < 0.001) because of the higher autotrophic respiration from the sugarcane’s fine root biomass and the microbial decomposition of the sugarcane’s larger soil organic carbon (SOC) stocks. Conversely, soil CH4 uptake under sugarcane (−1.1 ± 0.0 kg C ha-1 yr-1) was three times lower than under forest (−3.1 ± 0.0 kg C ha-1 yr-1; p < 0.001), owing to the likely alteration of methanotroph abundance upon conversion. Likewise, soil N2O emissions were smaller under sugarcane (1.3 ± 0.0 kg N ha-1 yr-1) compared to forest (1.8 ± 0.0 kg N ha-1 yr-1; p < 0.001) because excess N from fertilizer addition in the sugarcane was either lost through leaching or taken up by the sugarcane crop. Only seasonal variability in WFPS, among the auxiliary controls, affected CH4 uptake at both sites (p < 0.001) and soil CO2 effluxes under sugarcane (p = 0.018). Noteworthy, soil N2O fluxes from both sites were unaltered by the seasonality-mediated changes in auxiliary controls. We conclude that even with the increased SOC sequestration and the lower N2O emissions under sugarcane compared to forest, the forest-sugarcane conversion resulted in a yearly net C loss of 2.8 Mg CO2-eq ha-1 from soils to atmosphere, largely arising from the higher soil CO2 effluxes and to a smaller extent from a reduced CH4 uptake under sugarcane.