Little Effect of Land Use on N2O and NO Emission Pulses Following Rewetting of Dry Soils Across Seasonally Dry sub-Saharan Africa

In seasonally dry ecosystems, which are common in sub-Saharan Africa, precipitation after dry periods can cause large pulses of nitrous oxide (N2O), a greenhouse gas, and of nitric oxide (NO), a precursor to tropospheric ozone pollution. Agricultural practices can change soil characteristics, affecting trace N gas emissions. To evaluate the effects of land use on trace gas pulses at the start of the rainy season, we conducted laboratory measurements of N2O and NO fluxes from soils collected from four pairs of agricultural and natural savannah sites across the Sudano-Sahelian zone. We also conducted in situ wetting experiments, measuring NO fluxes from fallow sandy soils in Tanzania and NO and N2O fluxes from clayey soils in Kenya with different histories of fertilizer use. In incubation studies, NO increased by a factor of 7 to 25 following wetting, and N2O fluxes shifted from negative to positive; cumulative NO fluxes were an order of magnitude larger than cumulative N2O fluxes. In Kenya and Tanzania, NO increased by 1 to 2 orders of magnitude after wetting, and N2O increased by a factor of roughly 5 to 10. Cumulative NO fluxes ranged from 87 to 115 g NO-N ha(-1) across both countries-a substantial proportion of annual emissions-compared to roughly 1 g N2O-N in Kenya. There were no effects of land use or fertilization history on the magnitude of NO or N2O pulses, though land use may have been confounded with differences in soil texture potentially limiting the ability to detect land use effects. Plain Language Summary In ecosystems with distinct rainy and dry seasons, the first rainfall events stimulate biological activity in soils, leading to pulsed emissions of two nitrogen-based gases from soils: nitric oxide (NO), which leads to ozone pollution, and nitrous oxide (N2O), a powerful greenhouse gas. In farms where little fertilizer is used, such as in sub-Saharan Africa, soil may become depleted of nitrogen, which could result in smaller pulses of these gases. We measured emissions of NO and N2O in the laboratory after experimentally wetting soils from natural savanna and farm sites across the Sudano-Sahelian zone. We also measured emissions of both gases at farms in Kenya and Tanzania. In both studies, emissions of NO increased by a roughly 7 to 100 times after wetting. In the field studies, total emissions during the pulses-which lasted less than a day-were equivalent to about 1/4 to 1/3 of the NO emitted during the first 2 months of the cropping season, when most of the annual NO emissions are expected to occur. Emissions of N2O also increased after wetting, but the increases were smaller. No differences in the size of the laboratory emission pulses were found between the farm and savanna soils.

Automated summary

In seasonally dry ecosystems, rainfall events can stimulate significant emissions of nitric oxide (NO) and nitrous oxide (N2O), with nitrogen gas emissions not significantly affected by land use or fertilization history, possibly due to confounding differences in soil texture.