Use of laboratory and remote sensing techniques to estimate vegetation patch scale emissions of nitric oxide from an arid Kalahari savanna

Abstract. The biogenic emission of nitric oxide (NO) from the soil has an important impact on a number of environmental issues, such as the production of tropospheric ozone, the cycle of the hydroxyl radical (OH) and the production of NO. In this study we collected soils from four differing vegetation patch types (Pan, Annual Grassland, Perennial Grassland and Bush Encroached) in an arid savanna ecosystem in the Kalahari (Botswana). A laboratory incubation technique was used to determine the net potential NO flux from the soils as a function of the soil moisture and the soil temperature. The net potential NO emissions were up-scaled for the year 2006 and a region (185 km×185 km) of the southern Kalahari. For that we used (a) the net potential NO emissions measured in the laboratory, (b) the vegetation patch distribution obtained from Landsat NDVI measurements, (c) estimated soil moisture contents obtained from ENVISAT ASAR measurements and (d) the soil surface temperature estimated using MODIS MOD11A2 8 day land surface temperature measurements. Differences in the net potential NO fluxes between vegetation patches occur and range from 0.27 ng m⁻² s⁻¹ in the Pan patches to 2.95 ng m⁻² s⁻¹ in the Perennial Grassland patches. Up-scaling the net potential NO fluxes with the satellite derived soil moisture and temperature data gave NO fluxes of up to 323 g ha⁻¹ month⁻¹, where the highest up-scaled NO fluxes occurred in the Perennial Grassland patches, and the lowest in the Pan patches. A marked seasonal pattern was observed where the highest fluxes occurred in the austral summer months (January and February) while the minimum fluxes occurred in the austral winter months (June and July), and were less than 1.8 g ha⁻¹ month⁻¹. Over the course of the year the mean NO emission for the up-scaled region was 0.54 kg ha⁻¹ yr⁻¹, which accounts for a loss of up to 7.4% of the nitrogen (N) input to the region through atmospheric deposition and biological N fixation. The biogenic emission of NO from the soil is therefore an important mechanism of N loss from this arid savanna ecosystem and has the potential to play an important role in the production of tropospheric ozone and the OH cycle.