Fluxes of CO2, CH4 and N2O from soil of burned grassland savannah of central Africa

Abstract

Grassland savannah ecosystems subject to frequent fires are considered to have an almost neutral carbon balance, as the C released during burning mostly balance the C fixed by the photosynthetic process. However, burning might modify the net soil-atmosphere exchange of GHGs in the post burning phase so that the radiative balance of the site might shift from neutrality. In the present study the impact of fire on soil fluxes of CO2, CH4 and N2O was investigated in a grassland savannah (Congo Brazzaville) where high frequency burning is the typical management form of the region. An area was preserved for one season from annual burning and was used as "unburned" treatment. Two field campaigns were carried on at different time length from the fire event, 1 month, in the middle of the dry season, and 8 months after, at the end of the growing season. CO2, CH4 and N2O fluxes, as well as several soil parameters, were measured in each campaign from burned and unburned plots. Rain events were simulated at each campaign to evaluate magnitude and length of the generated GHG flux pulses. In laboratory experiments, on soil samples from the two treatments, microbial biomass, net N mineralization, net nitrification, N2O, NO and CO2 emissions were analyzed in function of soil water and/or temperature variations. Results showed that fire had a significant effect on GHG fluxes but the effect was transient, as after 8 months differences between treatments were no longer significant. One month after burning CO2 soil emissions were significantly lower in the burned plots, CH4 fluxes were dominated by net emissions rather than net consumption in the unburned area and fire shifted the CH4 flux distribution towards more negative values. No significant effect of fire was observed in the field on N2O fluxes. It was assumed that the low water content was the main limiting factor as in fact laboratory data showed that only above 75% of water saturation, N2O emissions increased sharply and more strongly in the soil from burned plots. This soil water content was hardly reached in the field even in the watered plots. Burned also stimulated NO production in the laboratory, which was more evident at low water content. Differently from N2O, 25% of water saturation was sufficient to significantly stimulate CO2 production in the laboratory and rain simulation in the field stimulated soil respiration. However in the laboratory the highest fluxes were measured in burned soil whereas in the field the opposite was observed. Increasing the incubation temperature from 25 °C to 37 °C affected negatively microbial growth and activities (mineralization and nitrification) but stimulated gas production (N2O and CO2). Overall, data indicate that fire would have a reductive or null impact on soil GHG emissions in savannah sites presenting similar soil characteristics (acidic, well drained, nutrient poor) and land management (high fire frequency). [ABSTRACT FROM AUTHOR] Copyright of Biogeosciences Discussions is the property of Copernicus Gesellschaft mbH and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)