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Johannes Gutenberg-Universität Mainz (2009)

Soil biogenic emissions of nitric oxide from arid and semi-arid ecosystems

Feig, Gregor Timothy

Titre : Soil biogenic emissions of nitric oxide from arid and semi-arid ecosystems

Auteur : Feig, Gregor Timothy

Université de soutenance : Johannes Gutenberg-Universität Mainz

Grade : Doktor der Naturwissenschaften Im Promotionsfach Geographie 2009

Nitric oxide (NO) has an important influence on a number of environmental processes including the production of ozone in the troposphere, the cycling of the hydroxyl radical and the production of nitric acid. The biogenic production of NO in the soil accounts for between 10% and 40% of the global total. A large degree of the uncertainty in the estimation of the biogenic emissions stems from a shortage of measurements in arid regions, which comprise a substantial proportion of the earth’s land surface area, but have been neglected from many of the major atmospheric chemical models. This study examined the emission of NO from three ecosystems in southern Africa which cover an aridity gradient from semi-arid savannas in South Africa through arid Kalahari savannas in Botswana and the hyper-arid Namib Desert in Namibia. A laboratory method was used to determine the release of NO as a function of the soil moisture and the soil temperature. Various methods were used to upscale the net potential NO emissions determined in the laboratory to the vegetation patch, landscape or regional level. The importance of landscape, vegetation and climatic characteristics is emphasized. In addition NO measurements were made in other regions such as in Israel, in the Amazon and in European temperate forests and pastures ; these measurements did not form a major part of this PhD but are included in the discussion. The first study occurred in a semi-arid savanna region in South Africa, where soils were sampled from 4 different landscape positions along a catenal sequence in the Kruger National Park. The maximum NO emission occurred at soil moisture contents of between 10% and 20% water filled pore space (WFPS). The highest net potential NO emissions came from the low lying Footslope landscape positions, which have the largest nitrogen (N) stocks and the largest input of N into the soil, the net potential NO emissions from the Midslope and Crest landscape positions were considerably lower. Net potential NO fluxes obtained in the laboratory were converted in field fluxes for the period 2003-2005, for the four landscape positions, using soil moisture and temperature data obtained in situ at the Kruger National Park Flux Tower Site. The NO emissions ranged from 1.5-8.5 kg ha-1 a-1 with the highest emissions occurring in the Footslope positions. The field fluxes were up-scaled to a regional basis using geographic information system (GIS) based techniques, this indicated that the highest NO emissions occurred from the Midslope positions due to their large geographical extent in the considered research area. Total emissions ranged from 20 x 103 kg in 2004 to 34x103 kg in 2003 for the 56000 ha Skukuza land type. The second study occurred in an arid savanna ecosystem in the Kalahari, near the town of Tsabong, Botswana. In this study we collected soils from four differing vegetation patch types including : Pan, Annual Grassland, Perennial Grassland and Bush Encroached patches. The maximum net potential NO fluxes determined in the laboratory ranged 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. The net potential NO emissions were up-scaled for the year December 2005-November 2006 and a region 185km x 185km in the southern Kalahari. This was done using 1) the net potential NO emissions determined in the laboratory, 2) the vegetation patch distribution obtained from LANDSAT NDVI measurements 3) estimated soil moisture contents obtained from ENVISAT ASAR measurements and 4) soil surface temperature measurements using MODIS 8 day land surface temperature measurements. This up-scaling procedure gave NO fluxes which ranged from 1.8 g ha-1 month-1 in the austral winter months (June and July) to 323 g ha-1 month-1 in the austral summer months (January-March). Differences occurred between the vegetation patches where the highest NO fluxes occurred in the Perennial Grassland patches and the lowest in the Pan patches. Over the course of the year the mean up-scaled NO emission for the studied region was 0.54 kg ha-1 a-1 this is within the range of pyrogenic NO emissions in African savannas and accounts for a loss of approximately 7.4% of the estimated N input to the region through atmospheric deposition and biological N fixation. The biogenic loss of NO is therefore a potentially important loss mechanism for N within this region. The third study occurred in the hyper-arid Namib Desert in Namibia. Soils were sampled from three ecosystems ; Dunes, Gravel Plains and the Riparian zone of the Kuiseb River. The net potential NO flux measured in the laboratory was used to estimate the NO flux for the whole of the Namib Desert for the year 2006 using modelled soil moisture and temperature data from the European Centre for Medium Range Weather Forecasts (ECMWF) operational model on a 36km x 35km spatial resolution. The maximum net potential NO production occurred at low soil moisture contents (<10%WFPS) and the optimal temperature was 25°C in the Dune and Riparian ecosystems and 35°C in the Gravel Plain Ecosystems. The maximum net potential NO fluxes ranged from 3.0 ng m-2 s-1 in the Riparian ecosystem to 6.2 ng m-2 s-1 in the Gravel Plains ecosystem. Up-scaling the net potential NO flux to the whole of the Namib desert gave NO fluxes of up to 0.062 kg ha-1 a-1 in the Dune ecosystem and 0.544 kg h-1 a-1 in the Gravel Plain ecosystem for 2006. From these studies it is shown that NO is emitted ubiquitously from terrestrial ecosystems from hyper-arid regions such as the Namib Desert to topical forests, as such the NO emission potential from deserts and scrublands should be taken into account in the global NO models. The emission of NO is influenced by various factors such as landscape, vegetation and climate. This study looks at the potential emissions from certain arid and semi-arid environments in southern Africa and other parts of the world and discusses some of the important factors controlling the emission of NO from the soil.


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