Denich, M.

Fosu, M.

Kugbe, J

Tamene, Lulseged

Vlek, Paul L.G.

Tamene, Lulseged D.

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 1 Tropentag 2012, Göttingen, Germany September 19-21, 2012 Conference on International Research on Food Security, Natural Resource Management and Rural Development organised by:  Georg-August Universität Göttingen and University of Kassel-Witzenhausen    Nutrient deposition during the harmattan dry season across the northern region of Ghana  Kugbe*a, Joseph, Lulseged Desta Tameneb, Manfred Denicha, Mathias Fosuc, Paul LG Vleka  a Center for Development Research (ZEF), Division of Ecology and Natural Resources Management, University of Bonn, Walter Flex Str. 3, D53113,Bonn, Germany. *Email joekugbe@yahoo.com, jkugbe@uni-bonn.de  b International Center for Tropical Agriculture (CIAT), Chitedze Agricultural Research Station, Lilongwe, Malawi  c Savanna Agriculture Research Institute, Nyankpala, P. O. Box 52, Tamale Ghana  Introduction  Dry atmospheric nutrient deposition is an essential source of soil nutrient replenishment in the northern region of Ghana during the harmattan dry season (Kugbe et al. 2012). The source of the nutrient is attributed to external dust, aerosols, and the redistribution of nutrients from local soils (Baker et al. 2006). The quantities of nutrient deposits are mostly estimated at single sites or in a single direction (e.g., north to south). Though such estimates are representatives of the sampled sites, there are no detailed studies on the two-dimensional-geospatial and temporal deposition of these nutrients. Given that dry nutrient deposition is a fundamental component of the underlying nutrient balance of the West African region, filling this knowledge gap is essential in understanding the holistic cycle of each nutrient element.  The aims in this study were to quantify the dry nutrient deposition (concentration, quantity), and how the deposition varies with month of deposition and geographical space, and relate these to the potential sources of the deposited nutrients.  Materials and Methods  A 24 cm surface-diameter dry-deposition sampler was used to collect the monthly deposits using the water-filled basin method. A complete description of the sampler, sampling sites and sampling process is provided in Kugbe et al. (2012). In summary, a modified stratified contiguous unit-based spatial sampling technique was used in this study, where the study area was stratified into 15 strata (three longitudinal zones and five latitudinal zones, Figure 1). Data collected from any point within a stratum were deemed representative of that stratum and different from data collected from any other strata. The technique is based on Tobler’s First Law of Geography, which argues that for a given parameter, nearby units are more related to each other and hence share similar information compared to units that are far apart, especially so if the information is spatially correlated. There were three replications per sampling site. Nutrient extraction and chemical analyses follows the procedure described in Kugbe et al. (2012).  Statistical comparisons were made by the ANOVA and pair-wise t-testing procedure at alpha level of 0.05. Post hoc tests were run with the Duncan’s multiple range test. Temporal and geospatial linear relations in nutrient concentrations and depositions were analyzed using Spearman’s correlation.  2 Results and Discussion  Temporal nutrient concentration and deposition The estimated average dust fall rate during the harmattan season was 0.4-1 g m-2 day-1. This value is higher than the 0.25 g m-2 day-1 reported by Lyngsie et al. (2011) at a single location (Nyamkpala), and is attributed to the variations in geospatial dust fall rate across the study area.  0o , 30’-0o, 30’1o, 30’2o, 30’8o9o10o NW ENorthern region of GhanaGhanaFebruaryJanuaryDecemberNovemberBandaNkwantaWa AdiboPongTamalePionSakpa0246810Novemb cember ry F aryNutrient concentration mg g-1Month1NovDecJanFeb200100020010002001000MgCaKPNitrate-N2001000Month1NovDecJanFebNutrient deposition (kg km-2)SoilDustMaggnesiumCalciumPotassiumPhosphorusNitrate-Nnesium0Dust Soil1234Nutrient concentration (mg g-1) Figure 1: Study region showing the sites (*) for collection of deposited samples (upper left). The figure also shows the monthly concentration (upper right) and deposition (lower left) of nutrients in the dry deposits, and the comparison of nutrient concentration of the dry deposits with that of soils sampled at same site (lower right).  The concentration of individual nutrients varied from one month to another (Figure 1). Besides nitrate-N which showed significant (P < 0.01) differences in inter-monthly mean concentrations in the order November > December > January > February, there were no significant monthly differences (P > 0.05) in the mean nutrient concentrations for all other nutrients.  Contrary to the results of Chineke and Chimeka (2009), who observed high Ca concentrations in harmattan-dust across Nigeria of magnitude ~21 times the concentration of K, this study shows that K concentration in the dry deposits was consistently higher than that of Ca. Elemental concentrations then followed the order Ca>Mg>nitrate-N>P as observed by Breuning-Madsen et al. 2012 (excluding nitrate-N). The mean nutrient concentrations of the dry deposits were  3 multiples the concentration of soils sampled at same geographic site and extracted similarly. Unlike the soil-available nutrients, which did not show any wide variation or peculiar outliers in nutrient concentration, the dry deposits showed wide variations in elemental concentration, and suggest differences in the sources of the deposited nutrients. The high elemental concentrations compared to those of the soils (Figure 1), and to soils of harmattan dust sources (Goudie and Middleton 2001), suggest other sources for the elements than harmattan-dust and the redistribution from the local soils: or that harmattan picks up the finer particles of soil that are richer in nutrients than the bulk soil. An estimated area of 45-60% of total land of the northern region is annually burned during the harmattan dry season (Kugbe et al. 2012). The concentrations of all extracted nutrients of the dry deposits were consistently lower than the corresponding concentrations in ash of burned vegetation across the study area in the same period (Bagamsah 2005). This observation suggests the return of a mixture of fire-induced transferred-elements and low-nutrient source materials (probably external harmattan-dust) into the system. Though these high nutrient-concentrated materials are added to the soil, the net supply to the soil’s available pool is minimal, given the low amounts of total dry deposits (53 -122 Mg km-2).   Spatio-temporal distribution of dry deposits across northern Ghana The month of deposition had higher influence on the concentration of dry deposits than the geospatial location of sample collection. Latitude was a greater geospatial predictor of deposited dust/nutrients, with more dust falling closer to the Saharan dust source in the north due to the influence of gravity, and less dust as one approaches the equator (Table 1).   Table 1: Pearson's correlation matrix for the spatial distribution of dry atmospheric nutrient concentrations in the northern region of Ghana during the 2010-2011 harmattan dry season. N = 60, 3n = 44. Values are coefficients, significant level (two-tail) in brackets.   Ca Mg K Na NO3- PO43- Longitude Latitude Dust (Mg km-2) -0.26 -0.21 -0.045 **0.40 -0.22 0.13 0.22 **0.66  (0.05) (0.11) (0.72) (0.00) (0.13) (0.32) (0.09) (0.00) Ca 1 **0.77 -0.05 *-0.31 0.01 0.03 -0.24 -0.15   (0.00) (0.73) (0.05) (0.97) (0.83) (0.06) (0.27) Mg  1 -0.08 -0.18 -0.03 0.02 -0.22 -0.10    (0.55) (0.27) (0.82) (0.99) (0.09) (0.44) K   1 *0.31 0.08 0.10 0.07 0.12     (0.05) (0.58) (0.47) (0.61) (0.35) 3Na    1 -0.17 -0.05 *0.30 0.14      (0.34) (0.70) (0.02) (0.39) NO3-     1 -0.03 0.08 -0.12       (0.86) (0.58) (0.40) PO43-      1 -0.20 *0.23               (0.15) (0.04)  The total dry deposits and the concentration of phosphate (PO43-) increased with increasing latitude, while potassium (K) magnesium (Mg), calcium (Ca) and nitrate (NO3-) concentrations were not significantly affected by geographical space (Table 1). The positive inter-nutrient concentration correlation between Ca and Mg suggests a potentially common source for these nutrient elements. So, also was the relation between Na and K. The negative relations between Ca and Mg concentrations on the one hand with that of Na and K on the other for all sites suggest that substantial amounts of Na and K may not have come from same sources as Ca and Mg. The deposit of individual nutrients should follow similar temporal trends as those of total dry deposition (January> February> December> November) if the nutrients come from the same source. Nutrient concentration should be about the same if the source is same while they may vary for different nutrient sources (e.g., burned debris, aeolian transport from external sources,  4 local dust redistribution). If the bulk of nutrients deposited in a given month is from different sources, then the concentration of the various nutrients in the different sources will be the predominant determining factor of the quantity of each nutrient deposited as well as the total dust deposit collected in that month. In this case, the relative fraction of total dry deposits from the different nutrient sources will be a predictive factor for the quantity of each nutrient deposited in the given month. The period of dominance for the different sources of nutrients during the harmattan dry season subsequently defines the major nutrients that are deposited across the region in a given month. For instance, fire activity is highest during the last week of December (Kugbe et al. 2012). One would expect that redistribution of fire-transferred particulate elements (N, P, K, Ca, Mg and Na) and the debris from burned vegetation will contribute to high nutrient deposition between December and January. On the other hand, local dust redistribution and external aerosol deposits will be the major sources of nutrient deposits in early November and late February when there are relatively fewer fires.   Conclusions and Outlook  The differences in order of monthly dry-deposits for different nutrients compared to the trend in total dry deposition show that all nutrients were not from the same source, and that the different sources of nutrients dominate deposition at different periods during the harmattan dry season. Redistribution of nutrients transferred into the atmosphere during vegetation burning might be the major source of the high nutrient concentrations of dry atmospheric deposits.  References  BAGAMSAH, T.T. (2005). The impact of bushfire on carbon and nutrient stocks as well as albedo in the savanna of northern Ghana. Ecology and Development series (25). Cuvillier Verlag, Göttingen. BAKER, A.R., JICKELLS, T.D., BISWAS, K.F., WESTON, K. AND FRENCH, M. (2006). Nutrients in atmospheric aerosol particles along the Atlantic Meridional Transect. Deep Sea Research PT II: Topical Studies in Oceanography 53 (14-16): 1706-1719.  BREUNING-MADSEN, H., LYNGSIE, G. AND AWADZI, T.W. (2012). Sediment and nutrient deposition in lake Volta in Ghana due to Harmattan dust. Catena 92: 99-105. CHINEKE, T.C. AND CHIMEKA, I.U. (2009). Harmattan particulate concentration and health impact in sub-saharan Africa. African Physical Review 3 (0018): 125-135. GOUDIE, A.S. AND MIDDLETON, N.J. (2001). Saharan dust storms: nature and consequences. Earth-Science Reviews 56: 179-204. KUGBE, J.X., FOSU, M., TAMENE, L.D., DENICH, M. AND VLEK, P.L.G. (2012). Annual vegetation burns across the northern savanna region of Ghana: period of occurrence, area burns, nutrient losses and emissions. Nutrient Cycling in Agroecosystems 93 (3): 265-284. LYNGSIE, G., AWADZI, T., AND BREUNING-MADSEN, H. (2011). Origin of Harmattan-dust settled in Northern Ghana-Long transported or local dust? Geoderma 167-168: 351-359. 

Nutrient deposition during the harmattan dry season across the northern region of Ghana

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Nutrient deposition during the harmattan dry season across the northern region of Ghana

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