Seasonal variation of carbon fluxes in a sparse savanna in semi arid Sudan

BackgroundLarge spatial, seasonal and annual variability of major drivers of the carbon cycle (precipitation, temperature, fire regime and nutrient availability) are common in the Sahel region. This causes large variability in net ecosystem exchange and in vegetation productivity, the subsistence basis for a major part of the rural population in Sahel. This study compares the 2005 dry and wet season fluxes of CO2 for a grass land/sparse savanna site in semi arid Sudan and relates these fluxes to water availability and incoming photosynthetic photon flux density (PPFD). Data from this site could complement the current sparse observation network in Africa, a continent where climatic change could significantly impact the future and which constitute a weak link in our understanding of the global carbon cycle.ResultsThe dry season (represented by Julian day 35–46, February 2005) was characterized by low soil moisture availability, low evapotranspiration and a high vapor pressure deficit. The mean daily NEE (net ecosystem exchange, Eq. 1) was -14.7 mmol d-1 for the 12 day period (negative numbers denote sinks, i.e. flux from the atmosphere to the biosphere). The water use efficiency (WUE) was 1.6 mmol CO2 mol H2O-1 and the light use efficiency (LUE) was 0.95 mmol CO2 mol PPFD-1. Photosynthesis is a weak, but linear function of PPFD. The wet season (represented by Julian day 266–273, September 2005) was, compared to the dry season, characterized by slightly higher soil moisture availability, higher evapotranspiration and a slightly lower vapor pressure deficit. The mean daily NEE was -152 mmol d-1 for the 8 day period. The WUE was lower, 0.97 mmol CO2 mol H2O-1 and the LUE was higher, 7.2 μ mol CO2 mmol PPFD-1 during the wet season compared to the dry season. During the wet season photosynthesis increases with PPFD to about 1600 μ mol m-2s-1 and then levels off.ConclusionBased on data collected during two short periods, the studied ecosystem was a sink of carbon both during the dry and wet season 2005. The small sink during the dry season is surprising and similar dry season sinks have not to our knowledge been reported from other similar savanna ecosystems and could have potential management implications for agroforestry. A strong response of NEE versus small changes in plant available soil water content was found. Collection and analysis of flux data for several consecutive years including variations in precipitation, available soil moisture and labile soil carbon are needed for understanding the year to year variation of the carbon budget of this grass land/sparse savanna site in semi arid Sudan.

[1]  Jonas Ardö,et al.  Soil Carbon Sequestration in Traditional Farming in Sudanese Dry Lands , 2004 .

[2]  W. Oechel,et al.  Energy balance closure at FLUXNET sites , 2002 .

[3]  Benjamin Smith,et al.  Precipitation controls Sahel greening trend , 2005 .

[4]  R. Lal,et al.  POTENTIAL OF DESERTIFICATION CONTROL TO SEQUESTER CARBON AND MITIGATE THE GREENHOUSE EFFECT , 2022 .

[5]  M. G. Morgan,et al.  Managing Soil Carbon , 2004, Science.

[6]  W. Oechel,et al.  Diurnal, seasonal and annual variation in the net ecosystem CO2 exchange of a desert shrub community (Sarcocaulescent) in Baja California, Mexico , 2005 .

[7]  J. Lloyd,et al.  On the temperature dependence of soil respiration , 1994 .

[8]  D. O. Hall,et al.  The global carbon sink: a grassland perspective , 1998 .

[9]  D. Dokken,et al.  Climate change 2001 , 2001 .

[10]  T. Vesala,et al.  Deriving a light use efficiency model from eddy covariance flux data for predicting daily gross primary production across biomes , 2007 .

[11]  Dan Yakir,et al.  Carbon sequestration in arid‐land forest , 2003 .

[12]  K. Cook,et al.  Climate science: The mysteries of Sahel droughts , 2008 .

[13]  C. Tucker,et al.  Desertification in the Sahel: a reinterpretation of a reinterpretation , 2007 .

[14]  Mike Hulme,et al.  Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1083 COMMENT THE RECENT SAHEL DROUGHT IS REAL , 2004 .

[15]  J. Gebauer,et al.  Non-timber Forest Products: Opportunities and Constraints for Poverty Reduction in the Nuba Mountains, South Kordofan, Sudan , 2004 .

[16]  J. Ardö,et al.  A recent greening of the Sahel—trends, patterns and potential causes , 2005 .

[17]  M. Saier,et al.  Climate Change, 2007 , 2007 .

[18]  R. Lal,et al.  Soil structure and management: a review , 2005 .

[19]  W. Oechel,et al.  Seasonality of ecosystem respiration and gross primary production as derived from FLUXNET measurements , 2001 .

[20]  Andrew E. Suyker,et al.  Year‐round observations of the net ecosystem exchange of carbon dioxide in a native tallgrass prairie , 2001 .

[21]  L. Hein,et al.  Desertification in the Sahel: a reinterpretation , 2006 .

[22]  Lars Eklundh,et al.  Vegetation index trends for the African Sahel 1982–1999 , 2003 .

[23]  Olaf Kolle,et al.  Seasonal variation in energy fluxes and carbon dioxide exchange for a broad‐leaved semi‐arid savanna (Mopane woodland) in Southern Africa , 2004 .

[24]  Benjamin W. Heumann,et al.  AVHRR Derived Phenological Change in the Sahel and Soudan, Africa, 1982 - 2005 , 2007 .

[25]  Yakov A. Pachepsky,et al.  Software to estimate -33 and -1500kPa soil water retention using the non-parametric k-Nearest Neighbor technique , 2008, Environ. Model. Softw..

[26]  Tobias Hinderson.  Analysing environmental change in semi-arid areas in Kordofan, Sudan , 2004 .

[27]  R. Lal,et al.  Soil carbon sequestration in sub‐Saharan Africa: a review , 2005 .

[28]  Hank A. Margolis,et al.  Ecosystem-level CO2 fluxes from a boreal cutover in eastern Canada before and after scarification , 2006 .

[29]  Niall P. Hanan,et al.  Photosynthesis and carbon balance of a Sahelian fallow savanna , 1998 .

[30]  Hans Papen,et al.  Diurnal, seasonal, and interannual variation in carbon dioxide and energy exchange in shrub savanna in Burkina Faso (West Africa) , 2008 .

[31]  Jay Apt,et al.  Ecology. Managing soil carbon. , 2004, Science.

[32]  John A. Taylor,et al.  Sources and Sinks of Atmospheric CO2 , 1992 .

[33]  Mark A. Sutton,et al.  Partitioning European grassland net ecosystem CO2 exchange into gross primary productivity and ecosystem respiration using light response function analysis , 2007 .

[34]  R. Leuning,et al.  Carbon and water fluxes over a temperate Eucalyptus forest and a tropical wet/dry savanna in Australia: measurements and comparison with MODIS remote sensing estimates , 2005 .

[35]  Jean L. Steiner,et al.  Can carbon sequestration markets benefit low-income producers in semi-arid Africa? Potentials and challenges , 2007 .

[36]  Ü. Rannik,et al.  Estimates of the annual net carbon and water exchange of forests: the EUROFLUX methodology , 2000 .

[37]  C. Tucker,et al.  Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981–2003 , 2005 .

[38]  M. Sjöström,et al.  Investigating vegetation changes in the African Sahel 1982-2002: a comparative analysis using Landsat, MODIS and AVHRR remote sensing data , 2004 .

[39]  E. Boegh,et al.  Carbon dioxide flux, transpiration and light response of millet in the Sahel , 1997 .

[40]  S. Running,et al.  Global products of vegetation leaf area and fraction absorbed PAR from year one of MODIS data , 2002 .

[41]  P. Gerakis,et al.  The influence of Acacia senegal on the fertility of a sand sheet (‘goz’) soil in the central Sudan , 1970, Plant and Soil.

[42]  Jonas Ardö,et al.  Soil Carbon Sequestration in Degraded Semiarid Agro-ecosystems—Perils and Potentials , 2002, Ambio.

[43]  V. H. Boughton.  Phyllode Structure, Taxonomy and Distribution in Some Australian Acacias , 1986 .

[44]  A. Mohamed.  IMPROVEMENT OF TRADITIONAL ACACIA SENEGAL AGROFORESTRY: Ecophysiological characteristics as indicators for tree-crop interaction on sandy soil in western Sudan , 2005 .

[45]  John Gaunt,et al.  Bio-char Sequestration in Terrestrial Ecosystems – A Review , 2006 .

[46]  J. Ardö,et al.  Broad‐scale increase in NPP quantified for the African Sahel, 1982–1999 , 2006 .

[47]  James A. Brass,et al.  Fluxes of carbon, water and energy over Brazilian cerrado: an analysis using eddy covariance and stable isotopes , 1997 .

[48]  A. Verhoef,et al.  Spatial and temporal variations in net carbon flux during HAPEX-Sahel. , 1997 .

[49]  Christopher A Williams,et al.  Africa and the global carbon cycle , 2007, Carbon balance and management.

[50]  José J. San José,et al.  Productivity and carbon fluxes of tropical savannas , 2006 .

[51]  W. Cohen,et al.  Evaluation of MODIS NPP and GPP products across multiple biomes. , 2006 .

[52]  Jonas Ardö,et al.  Assessment of soil organic carbon in semi-arid Sudan using GIS and the CENTURY model , 2003 .

[53]  M. Hadley,et al.  The Ecology of Tropical Savannas , 1970 .

[54]  A. Verhoef,et al.  Fluxes of carbon dioxide and water vapour from a Sahelian savanna. , 1996 .

[55]  J. Tenhunen,et al.  Responses of Acacia tortilis and Acacia xanthophloea to seasonal changes in soil water availability in the savanna region of Kenya , 2005 .

[56]  O. Luukkanen,et al.  The influence of different Acacia senegal agroforestry systems on soil water and crop yields in clay soils of the Blue Nile region, Sudan , 2007 .

[57]  Allison L. Dunn,et al.  A satellite‐based biosphere parameterization for net ecosystem CO2 exchange: Vegetation Photosynthesis and Respiration Model (VPRM) , 2008 .

[58]  P. Tschakert.  Environmental services and poverty reduction: Options for smallholders in the Sahel , 2007 .

[59]  T. A. Black,et al.  The use of remote sensing in light use efficiency based models of gross primary production: a review of current status and future requirements. , 2008, The Science of the total environment.

[60]  Brian C. Murray,et al.  Economic consequences of consideration of permanence, leakage and additionality for soil carbon sequestration projects , 2007 .

[61]  Jonas Ardö,et al.  Disentangling the effects of climate and people on Sahel vegetation dynamics , 2008 .

[62]  W. Rawls,et al.  Estimating generalized soil-water characteristics from texture , 1986 .

[63]  M. Hulme.  Rainfall changes in Africa: 1931–1960 to 1961–1990 , 1992 .

[64]  S. Goetz,et al.  Satellite remote sensing of primary production : an improved production efficiency modeling approach , 1999 .

[65]  J. Kessler,et al.  The potential benefits of agroforestry in the Sahel and other semi-arid regions , 1997 .

[66]  Robert J. Scholes,et al.  The Carbon Cycle and Atmospheric Carbon Dioxide , 2001 .

[67]  Jonas Ardö,et al.  The potential for soil carbon sequestration in three tropical dryland farming systems of Africa and Latin America: A modelling approach , 2007 .