Deposition of Nutrients from Harmattan Dust in Ghana, West Africa

Pedosphere 25(4): 613–621, 2015 ISSN 1002-0160/CN 32-1315/P c 2015 Soil Science Society of China Published by Elsevier B.V. and Science Press

Deposition of Nutrients From Harmattan Dust in Ghana, West Africa

Henrik BREUNING-MADSEN1,∗, Theodore Wola AWADZI2 and Gry LYNGSIE1 1Institute of Geosciences and Natural Resource Management, University of Copenhagen, Oster Voldgade 10, 1350 Copenhagen K (Denmark) 2Department of Geography and Resource Development, University of Ghana, PO Box LG 59, Legon, Accra (Ghana) (Received September 10, 2014; revised April 2, 2015)

ABSTRACT In order to measure dust’s nutrient input on farmland in different agro-ecological zones, Harmattan dust was sampled by mats with plastic straw in Ghana between 2002–2006. The inputs of total nutrients by Harmattan dust in Ghana per Harmattan period were about 1–2 kg Ca ha−1, 0.5–2 kg K ha−1, 0.5–1.5 kg Mg ha−1 and less than 0.5 kg P ha−1. Compared with the annual input of nutrients by precipitation, the dust accounted for 10% or less of Ca, Mg and K but approximately 20%–40% of P. The input of nutrients by dust was only valid for areas with vegetation, because in areas with none or sparse vegetation, loss of soil due to wind erosion and hereby loss of nutrients might be significant. In farmland areas with bare and vegetated fields there seemed to be an internal redistribution of the nutrients and not a net gain of nutrients from outside the area (long-range transported dust). The input of P by dust might be of some importance in the traditional shifting cultivation systems, while the inputs of other three nutrients of Ca, Mg and K were so low that they must be considered insignificant. In the intensive agriculture systems with huge inputs of manures and fertilizers the nutrient input by dust is insignificant and could be neglected. Key Words: deposition rate, dust deposition, input of nutrient, long-range transported dust, nutrient concentration, traditional agricultural system

Citation: Breuning-Madsen H, Awadzi T W, Lyngsie G. 2015. Deposition of nutrients from Harmattan dust in Ghana, West Africa. Pedosphere. 25(4): 613–621.

INTRODUCTION parts of the year. In Ghana the Harmattan wind is ex- perienced in December to March, replacing the south- Ghana is located in West Africa and stretches for west monsoon wind. Throughout that period, storm about 1 000 km from the Gulf of Guinea towards the activities in the Bodélé Depression, located in the north. The traditional agricultural system throughout Chad Basin of Africa, raise large amounts of dusts into the country is shifting cultivation, a system characteri- the atmosphere, which are transported towards south- zed by a few years of cultivation followed by many years west by the predominant trade winds (Kalu, 1979; Mc- of fallow. It is a system of subsistence agriculture and Tainsh, 1980; Afeti and Resch, 2000; Washington et harvest crops are mainly for domestic use with little al., 2006; Warren et al., 2007). During a Harmattan surplus for sale. In this system, nutrient lost to the dust plume’s movement towards southwest, the coarse surroundings is limited and import of fertilizers and grain dust settles quickly and only the most fine grain manure is therefore not necessary. The loss of nutrients fraction stays in the atmosphere and can be transpor- due to leaching and soil erosion are replaced by atmos- ted thousands of kilometers as reported by Koren et pheric input from precipitation and dust deposition, al. 5 (2006) and Ben-Ami et al. (2010), who pointed and by weathering of primary subsoil silicates available out that the Bodélé Depression is the main source of (Szott et al., 1991). The atmospheric input of nutrient dust, and hereby nutrients, to the Amazon forests. is measured normally by the chemical compositions of The input of nutrients by dust can be determined rainwater, but wind-blow dust is an important agent by measuring the concentrations of nutrients in dust that can contribute with a significant portion of nu- deposited. The sampling strategy is important when trients to soil in some areas, i.e., countries along the determining the amount of deposited dust and the sam- Gulf of Guinea. In these areas, a dry dust-laden wind, plers should mirror the surface of the ground as good as the Harmattan wind, blows from the Sahara Desert in possible. In researches on Harmattan dust, dust sam-

∗Corresponding author. E-mail: [email protected]. 614 H. BREUNING-MADSEN et al. ples were collected mostly in different types of containers or funnels, some dry, others filled with different liquids (McTainsh and Walker, 1982; Møberg et al., 1991; Tiessen et al., 1991; McTainsh et al., 1997; Ade- tunji et al., 2001; Breuning-Madsen and Awadzi, 2005; Goossens and Rajot, 2008). For determination of dust deposition in forests the drip canopy method was used (Stoorvogel et al., 1997). Several studies on Harmattan dust have been car- ried out in Ghana. Tiessen et al. (1991) investigated soils in northern Ghana and compared their chemical properties with that of Harmattan dust captured on plastic sheets or funnels. Pelig-Ba et al. (2001) showed, indirectly, the influence of airborne dust on the amount of nutrients (Ca, Mg and K) and other elements (Fe, Al, Mn, Zn, V, Cr, Cu and Pb) deposited around Tamale, Ghana. Breuning-Madsen and Awadz- i (2005) measured, at a regional scale, the amount of dust trapped in bowls with water, mats with plastic straw and wooden plates and showed the regional variation in texture and organic matter content of dust. He et al. (2007) determined the clay mineralogy of Har- mattan dust in northern and southern Ghana. Lyngsie Fig. 1 Map showing the agro-ecological zones in Ghana and the et al. (2011) used water-filled bowls to capture dust for location of sampling sites. mineralogical investigation of dust in the Harmattan and southwest monsoon seasons at Tamale. Breuning- a long dry season. Madsen et al. (2012) used bowls and mats to collect In the coastal area of southeastern Ghana, the an- dust for determining the amount of dust deposited in nual precipitation is lower than 1 000 mm and a nar- Lake Volta. Few of studies investigated the annual and row coastal savannah zone with grasses and few trees is regional variation of nutrients due to Harmattan dust found. In the southwestern corner of Ghana, the vege- deposition. This variation may be significant for sub- tation is tropical rainforest (Fig. 1). This is surroun- sistence agriculture and natural systems, as mentioned ded by a moist semi-deciduous tropical forest with pro- above. Thus, the objectives of this work were to in- nounced litter fall in the dry season. Towards the north vestigate the annual amount of nutrients deposited via the moist semi-deciduous tropical forest turns into a Harmattan dust on surfaces with short vegetation like Guinea savannah with grasses, scrubs and a few trees. grasses in different agro-ecological zones of Ghana in In the dry north, the Guinea savannah zone turns into West Africa, and to evaluate if Harmattan dust de- a Sudan savannah zone. position had any significant influence on keeping soil In December–February the northeast Harmattan nutrients at an adequate level for plant production. wind dominates in northern Ghana, while the southwest monsoon wind dominates in southern Ghana. MATERIALS AND METHODS This means that most areas of Ghana are affected by Study areas the Intercontinental Convergence Zone (ITCZ). The location of the border between the two wind systems, Ghana is located in West Africa on the Gulf of Inter Tropical Discontinuity (ITD), is movable during Guinea and lies between 6◦–11◦ Nand1◦ E–3◦ W the Harmattan period (Engelstaedter et al., 2006; Sun- (Fig. 1). The mean monthly temperature is between nu et al., 2008). The ITD reaches its most southern lo- 25 and 30 ◦C. In the south of Ghana, two pronounced cation of Ghana, approximately 5◦ N, in the beginning rainy seasons exist, one in April to June and the other of January (Sunnu et al., 2008), but can move further in September to November. The minor dry season is toward south and the northeast Harmattan wind rea- in July to August, while the major dry season is in ches the Gulf of Guinea. Thus, the northeast Harmat- December to February. In the north of Ghana, there is tan wind dominates in northern Ghana (Bawku and only one rainy season from July to October, followed by Tamale) on a monthly basis in December–February, NUTRIENTS DEPOSITED FROM DUST 615 while it reaches the Central Ghana (Kintampo and occur during the Harmattan period, especially in the Kete Krachi) every year although it does not domi- south. The tube fed the water to a 36-L plastic contai- nate in all the three months (Awadzi and Breuning- ner. Second, at the other 4 sites located in Central and Madsen, 2007). The southwest monsoon dominates at southern Ghana another type of mat sampler was used the coast (Accra in southern Ghana) on a monthly ba- because erratic showers during the Harmattan period sis in December–February, but in more inland areas will fill the 36-L plastic containers, and thereby reduce at Kpong and Kade northerly wind blows occasional- the possibility of accurately calculating the amount of ly on a monthly basis (Awadzi and Breuning-Madsen, dust retained. A plate of plywood with a diameter of 2007). A study of Lyngsie et al. (2013) indicated that about 55 cm was placed in a bowl 2 cm from the top. ITD most likely works as a barrier for Harmattan dust Holes for drainage were drilled in the plate and on the plumes, which means that areas below or south of the plate a mat fitting the size of the plywood was placed. ITD would not be influenced by long-range transpor- When it rained the water passed through the holes in ted Harmattan dust. the plywood and collects in the bowl. These samplers can easily absorb all the precipitation in erratic show- Dust collection ers. In order to determine the nutrient input due to dust The two types of mat samplers were installed at the deposition during the Harmattan periods in Ghana, end of November in each year and collected on March 1 sampling sites have been established in different agro- of the following year. At the end of the sampling period ecological zones in Ghana during a 10-year period from the mats were placed in plastic bags to avoid contami- 2000 to 2009. In some years the sampling was conduc- nation during their transportation to the laboratory ted in specific agro-ecological zones like the savannah in Accra. This sampling approach is believed to give zone, but from 2001 to 2007 the nationwide dust sam- an estimate of dust which is captured by low vegeta- pling was conducted all over Ghana. Results from six tion during the Harmattan period (Breuning-Madsen sites are presented in this study, and the main features and Awadzi, 2005). It must be emphasized that this of these sites can be found in Table I. approach cannot be used as a measurement of the All sampling sites are covered with vegetation and amount of dust deposited on bare ground, forests or they are all located more than 200 m away from pub- water bodies. For these surfaces, wooden plates, the lic roads and major towns in order to minimize the drip canopy method and bowls with water, respective- input of local dust (Tiessen et al., 1991; Erell and ly, should be used (Stoorvogel, 1997; Breuning-Madsen Tsoar, 1999). The dust was trapped on plastic mats and Awadzi, 2005). 1 m above the ground and the following two setup- Soil sampling s were used. First, at Bawku and Tamale in northern Ghana, plastic mats covering 0.63 m2, 70 cm broad and Topsoil was collected from the four sites of Bawku, 90 cm long were used. The mats had plastic straws like Tamale, Kpong and Kade as one bulk sample mixed a grass lawn. There were 8 plastic straws cm−2. Each by three jacks from within a radius of 10 m from the straw was 1.5 cm long and had a surface area of about dust samplers. 1cm2. Thus, the surface area of the mat was about Physical and chemical analyses 8 times that of bare ground. The mats were placed in a 2-cm high wooden frame. A drain was fitted by con- In the laboratory, the mats were carefully washed necting a tube to a drilled hole in the frame in order with distilled water. In some cases, the samples were to drain the water from erratic showers which might contaminated with remnants of straw and starch from

TABLE I A summary description for the study sites in Ghana

Location Soil type Soil colour Vegtation zone Type of site Bawku, northern Ghana Acrisola) Brown to reddish brown Sudan savannah Agricultural experimental station Tamale, northern Ghana Acrisola) Brown to reddish brown Guinea savannah Agricultural experimental station Kintampo, Central Ghana Lixisolb) – Guinea savannah Local farmer’s field Kete Krachi, Central Ghana Lixisolb) – Moist semi-deciduous forest Local farmer’s field Kpong, southern Ghana Vertisola) Black Coastal savannah Agricultural experimental station Kade, southern Ghana Lixisola) Reddish brown Moist semi-deciduous forest Agricultural experimental station a)Classified according to World Reference Base for Soil Resources (IUSS Working Group WRB, 2006); b)Cited from Jones et al. (2013). 616 H. BREUNING-MADSEN et al. farming activity in the vicinity of sampling sites. Be- ter using distilled water. The concentration of total P fore analyses, most of these remnants were removed in the extracts was determined spectrophotometrically by a floating procedure, in which samples were put by the molybdenum-blue method (Murphy and Riley, into distilled water-filled beakers (250 mL) and the 1962). The concentrations of total Ca, K and Mg in remnants floating in the surface water could be re- the extracts were determined by AAS. The concen- moved. The water was subsequently evaporated to get trations of slowly plant-available K, Ca and Mg were a dried sample prior to analyses. The dried sample determined as the difference between the total and the was weighed and the amount of dust was calculated readily plant-available K, Ca and Mg. Slowly plant- in g m−2. The samples were tested for carbonates available nutrients are mainly organic bound nutrients by adding 3 mol L−1 HCl. All samples were negative that will only be available for plants after mineraliza- on the presence of carbonates. As only small amounts tion of the organic matter (Breuning-Madsen et al., of dust were collected (5–15 g), analytical procedures 2012). involving small amounts of soil material were chosen. Statistical analysis Alternatively, standard analytical procedures were modified; i.e., the sample amounts and extraction liquids Descriptive statistics were conducted in Excel soft- were reduced while keeping the ratio the same as in ware. Confidence interval was given at a 95% confi- the standard procedure. dence level. The dust particle-size was determined by the laser diffraction method using a Malvern Mastersizer 2000 RESULTS AND DISCUSSION (Malvern Instruments, UK), as this method requires Regional variations in texture and amount of carbon in only small amounts of dust samples. One g of dust dust samples were treated with hydrogen peroxides for re- moval of organic matter, dispersed with 0.1 mol L−1 Table II shows the amounts of dust trapped on the Na4P2O7, and treated with ultrasound for 3 min before mats in Harmattan periods of 2002–2006. In general, analysis. A standard procedure was used for operating the deposition rate of Harmattan dust was the high- the Malvern Mastersizer 2000 and the diffraction pat- est in the north and decreased towards the south. At tern was converted to a grain size distribution using the Bawku and Tamale in the north, the average deposition Mie diffraction theory (Agrawal et al., 1991; McCave amount per Harmattan period was about 160 kg ha−1, and Syvitski, 1991). while it was about 100 kg ha−1 at Kete Krachi and Total organic carbon (TOC) of soil and dust was Kintampo in Central Ghana. In the south, the average analyzed using the dry combustion method at 1 250 ◦C dust deposition amount at Kpong and Kade was only in oxygen on an Eltra SC-500 analyzer (ELTRA Gm- 60 kg ha−1. The north-south variations might be due bH, Germany), with an accuracy of ±0.2% (ELTRA, to a more sparse vegetation cover in the north, resul- 1995). As no carbonates were present in the samples ting in more wind erosion. The higher deposition rate the total carbon content is equivalent to the TOC con- in the north was consistent with the findings of Klose tent. et al. (2010), who found that the zone of maximum The pH of soil and dust was determined potentio- dust event activity was located north of ITD based metrically in a suspension of dust or soil and distilled on synoptic station data from 1983 to 2008. Further- water giving a solid-liquid ratio of 1:5 (Thomas, 1996). more, Klose et al. (2010) found that this zone was close- The exchangeable bases, also referred to as readily linked to the spatiotemporal evolution of ITD and ly plant-available Ca, Mg and K, were extracted with that this was especially evident in the months between −1 1 mol L ammonium acetate (NH4C2H3O2)atpH7 November and March, when Ghana was influenced by (Chapman, 1965) and the concentrations were deter- ITD. The variation in dust deposition density was not mined by atom absorption spectrophotometry (AAS) synchronic in the country; e.g., the atmosphere du- using a Perkin Elmer AAnalyst 400 (Perkin Elmer Inc., ring the Harmattan periods was the most dust-laden in USA). 2004–2005 at Bawku and Tamale, in 2005–2006 at Kete The concentrations of total P and non-silicate Krashi and Kpong, and in 2003–2004 at Kade. This bound Ca, Mg and K, referred to as total Ca, Mg meant that there was no connection for maximum dust and K were determined as follows: 0.5 g dust or soil retention between the Harmattan periods in the north samplewasheatedinanovenat550◦Cinorder and south. The variations in dust deposition among the to reduce the organic matter to ash, followed by a sampling locations may likely be due to the ITD po- −1 ◦ treatment with 10 mL 6 mol L H2SO4 at 80 C sition over Ghana during the Harmattan period, sepa- for 1 h, and then washed several times through a fil- rating the northern part where northeasterly winds NUTRIENTS DEPOSITED FROM DUST 617

TABLE II Total amounts of dust deposited at 6 sampling sites in the Harmattan periods of 2002–2006

Location 2002–2003 2003–2004 2004–2005 2005–2006 Average CIa) kg ha−1 Bawku, northern Ghana 187 156 196 191 183 154–212 Tamale, northern Ghana 141 117 152 117 132 103–161 Kintampo, Central Ghana 57 123 136 138 114 54–174 Kete Krachi, Central Ghana 112 53 78 108 88 43–133 Kpong, southern Ghana 51 100 93 102 87 49–125 Kade, southern Ghana 32 77 55 31 49 14–84 a)Confidence interval at a 95% confidence level (n =4). dominate from the south where southwest monsoon color notation the colour of dust at Bawku was often winds dominate (Engelstaedter et al., 2006; Sunnu et pale brown to brown; at Tamale it was brown; in the al., 2008; Lyngsie et al., 2013). south it was dark greyish brown. The grain size distribution of dust trapped on mats Regional variation in nutrients deposition due to dust in the Harmattan periods of 2002–2006 is shown in Table III. In the north the mean grain size of dust is The concentrations of nutrients in dust deposited approximately 14 μm. The grain size distribution is at different sites are presented in Table IV, together rather close to a log normal distribution but with a with the total amounts of nutrients in kg ha−1.For little tail of coarser materials (Breuning-Madsen and comparison the readily plant-available nutrients (ex- Awadzi, 2005). The mean grain size of dust was ap- tracted by ammonium acetate), Ca, Mg and K in soil proximately 11 μm in Central Ghana (Kintampo and were also determined for the topsoil at four of the Kete Krashi) and was slightly less than 8 μminthe six sites, along with the total P concentration. The south (Kade and Kpong). The variance in grain size concentration of readily plant-available nutrients in could be due to a lower wind speed in the south com- dust was in general the highest in Central or south- pared to the north in the Harmattan periods, as de- ern Ghana, which was in line with the higher TOC scribed by Lyngsie et al. (2013) for the Harmattan pe- and fine textures of dust deposited in these areas. In riods in 2006–2007. the north (Bawku and Tamale) the concentration of The TOC content in dust was the lowest in the readily plant-available Ca in dust was somewhat high- north and increase toward the south. The relative in- er than readily plant-available K and about 10 times crease in TOC may be due to the differences in the higher than readily plant-available Mg. In Central and deposition rate of mineral and organic composition of southern Ghana, the concentration of readily plant- dust and/or an enrichment of the dust with organic available K in dust was higher than readily plant- matter passing through Ghana in the Harmattan pe- available Ca. This was especially pronounced in the riods, when the stubble was burned in the fields. The south, and like in the north the concentration of readily increasing content of TOC towards the south was also plant-available Mg was the lowest. The concentration reflected in the colour of dust. Using the Munsell soil of total Ca (extracted by H2SO4) in dust exceeded the

TABLE III Some physical and chemical properties of dust and soil collected at 6 sampling sites in the Harmattan periods of 2002–2006

Location Dust Soil

Grain size TOC pH TOC pH

Mean CIa) Mean CI Mean CI μm gkg−1 gkg−1 Bawku, northern Ghana 13.2 11.3–15.1 42 26–48 6.8 5.8–7.8 8.8 5.2 Tamale, northern Ghana 13.7 10.4–17.0 57 51–63 7.5 6.1–8.9 8.1 5.7 Kintampo, Central Ghana 10.4 10.1–10.7 125 95–155 7.8 7.2–8.4 - - Kete Krachi, Central Ghana 11.9 7.9–15.9 170 102–238 8.3 7.5–9.1 - - Kpong, southern Ghana 7.7 5.2–10.2 158 101–215 7.9 7.4–8.4 11.7 6.4 Kade, southern Ghana 7.2 3.9–10.5 156 123–189 7.2 5.6–8.8 25.7 6.3 a)Confidence interval at a 95% confidence level (n =4). 618 H. BREUNING-MADSEN et al. total K concentration at all sampling sites except Kade deposited K was rather close to that of Ca with total (6.72mgCag−1 and 9.86 mg K g−1). Again, the total K between 0.4–1.8 kg ha−1 and readily available K of Mg concentration (1.15–5.81 mg g−1) was lower but 0.3–1.3 kg ha−1. The amount of readily plant-available exceeded the concentration of total P (0.88–3.80 mg Mg was very low with 0.05–0.1 kg ha−1 and a depo- g−1) at each site. A comparison of the total and readi- sition of total Mg between 0.5 and 1.5 kg ha−1.The ly plant-available nutrient concentrations given in Ta- amount of total P was almost negligible, less than 0.5 ble IV showed that, contrary to Mg, most of K was kg ha−1. generally readily plant-available. Similar studies on the concentration of the nu- In general, the concentrations of nutrients in the trients in dust in West Africa had been reported by deposited dust were much higher than those in the Møberg et al. (1991) in Nigeria, Drees et al. (1993) corresponding local soils (Table IV). Again, this was in Niger, and Stoorvogel et al. (1997) in Ivory Coast, partly due to the higher TOC and fine texture of dust, but different sampling strategies and analytical me- both of which favoured higher amounts of nutrients. thods have been used. Stoorvogel et al. (1997) used The amount of readily plant-available and total the canopy drip method to estimate the amount of nu- nutrients in dust in kg ha−1 per Harmattan period trients that reached the forest floor and found 3.5 kg showed that the soils got an average of 0.7 kg ha−1 Ca ha−1, 0.4 kg Mg ha−1, 2.5 kg K ha−1, and 0.11 readily plant-available Ca and 1–2 kg ha−1 total Ca in kg P ha−1. The discrepancy in results of Stoorvogel et north and Central Ghana. In the south the soils got al. (1997) and this study can be explained by different only 0.3 kg ha−1 total Ca at Kade. The amount of sampling methods, changes in dust chemistry when it

TABLE IV

Mean concentrations and amounts of nutrients extracted by NH4Ac (readily plant-available nutrients) and H2SO4 (total nutrients) in dust and soil collected at 6 sampling sites in the Harmattan periods of 2002–2006

Location Readily plant-available nutrients in dust Total nutrients in dust Nutrients in soil

Concentration Amount Concentration Amount Readily Total available Mean CIa) Mean CI Mean CI Mean CI mg g−1 kg ha−1 mg g−1 kg ha−1 mg g−1 Ca Bawku, northern Ghana 3.71 0.56–6.86 0.68 0.09–1.27 6.25 1.22–11.28 1.14 0.15–2.13 0.06 – Tamale, northern Ghana 5.08 −0.31–10.47 0.67 −0.16–1.50 9.35 1.30–17.40 1.23 0.07–2.39 0.05 – Kintampo, Central Ghana 6.04 2.53–9.55 0.69 0.32–3.06 18.35 3.41–33.29 2.08 −0.47–4.63 – – Kete Krachi, central Ghana 8.31 4.39–12.23 0.73 0.09–3.37 20.92 7.14–34.70 1.84 −0.04–3.72 – – Kpong, southern Ghana 3.83 1.59–6.07 0.33 0.03–2.63 15.34 3.22–27.46 1.33 −0.14–2.79 0.51 – Kade, southern Ghana 2.83 0.27–5.39 0.14 −0.13–1.41 6.72 1.33–12.11 0.33 0.00–0.66 0.18 – Mg Bawku, northern Ghana 0.38 0.19–0.57 0.07 0.07–0.07 1.50 0.28–2.72 0.27 0.03–0.51 0.01 – Tamale, northern Ghana 0.37 0.04–0.70 0.05 0.05–0.05 1.85 0.31–3.39 0.24 0.03–0.45 0.02 – Kintampo, Central Ghana 0.89 0.40–1.38 0.10 −0.06–0.26 4.31 −0.34–8.96 0.49 −0.24–1.22 – – Kete Krachi, Central Ghana 0.69 −0.71–2.09 0.06 −0.10–1.22 5.81 −0.62–12.24 0.51 −0.25–1.27 – – Kpong, southern Ghana 1.15 0.27–2.03 0.10 −0.06–0.26 4.49 2.71–6.27 0.39 0.10–0.68 0.12 – Kade, southern Ghana 1.50 0.37–2.63 0.07 0.07–0.07 3.12 1.93–4.13 0.15 0.01–0.29 0.03 – K Bawku, northern Ghana 1.85 0.04–3.66 0.34 0.02–0.66 2.99 0.29–5.69 0.55 0.01–1.09 0.01 – Tamale, northern Ghana 4.16 0.37–7.95 0.55 0.04–1.06 4.96 1.81–8.11 0.65 0.24–1.06 0.02 – Kintampo, Central Ghana 11.30 −3.78–26.38 1.28 −0.55–3.11 15.62 0.00–34.07 1.77 −0.87–4.41 – – Kete Krachi, Central Ghana 9.22 4.73–13.71 0.81 0.13–1.49 11.70 2.71–20.69 1.03 −0.15–2.21 – – Kpong, southern Ghana 11.63 1.80–21.46 1.00 0.33–1.67 14.60 6.60–22.60 1.26 1.01–1.51 0.03 – Kade, southern Ghana 8.29 2.31–14.27 0.40 −0.03-0.83 9.86 8.14–11.58 0.48 0.16–0.80 0.01 – P Bawku, northern Ghana – – –– 0.88 0.26–1.50 0.16 0.05–0.27 – 0.08 Tamale, northern Ghana – – –– 1.35 0.52–2.19 0.18 0.07–0.29 – 0.12 Kintampo, Central Ghana – – –– 3.80 0.51–7.09 0.43 −0.11–0.97 – – Kete Krachi, Central Ghana – – –– 3.47 −0.91–7.85 0.31 −0.20–0.82 – – Kpong, southern Ghana – – –– 3.12 2.07–4.17 0.27 0.08–0.46 – 0.13 Kade, southern Ghana – – –– 1.99 −0.16–4.14 0.10 −0.04–0.24 – 0.34 a)Confidence interval at a 95% confidence level (n =4). NUTRIENTS DEPOSITED FROM DUST 619 passes the canopy (Nye, 1960), different land-uses (total Ca, Mg and K) were between 11–18 kg ha−1 around the sampling sites, and the different wind di- year−1, while the input of total P was very low, less rections through the Harmattan periods. than 0.5 kg ha−1 year−1. The nutrient inputs of Ca, Mg and K by Harmattan dust were 10% or less compared Nutrient input by dust compared to precipitation to the inputs by the precipitation on a yearly basis. Data of nutrient concentrations in rainwater is li- The total P input from Harmattan dust was signifi- mited in Ghana and data only from Kade and Tamale cant compared to the input by precipitation, which are available at the 6 sampling sites. Nye (1960) de- accounted for 20%–40% of the P input. termined the amount of nutrients deposited via preci- The input of nutrients by dust in this study was pitation at Kade Research Station on a monthly basis limited to the Harmattan period. In the 9 months out- in 1959. At Tamale, Pelig-Ba et al. (2001) collected side the Harmattan period dust was also deposited, rainwater samples at about 30 sites from Tamale town but only a few data were available. However, Breuning- in 1997 and 1998 to investigate elemental contamina- Madsen et al. (2012) showed that a similar amount of tion of rainwater by airborne dust. The results showed dust was deposited in the 9 months outside the Har- that rainwater had low amounts of total dissolved mattan period. Thus, it is necessary to measure the solids and low concentrations of major elements but annual amount of nutrients deposited by dust, espe- contained significantly higher quantities of trace ele- cially for P, the input of which by dust might be close ments. The rainwater’s nutrient concentrations were to the level for the precipitation. almost similar at Kade and Tamale as shown in Table V, except Ca. Compared to the results in Burkina Faso Do nutrients deposited by dust have any significant in- and Ivory Coast, the K concentration in the Ghanaian fluence in a cultivation system? rainwater was higher, whereas the P concentration was In traditional shifting cultivation systems, the in- lower (Pieri, 1985; Stoorvogel et al., 1997). put of nutrients by dust and precipitation is believed TABLE V to be of great importance as it is an external input of nutrients to the soil. The precipitation input can Concentrations of nutrients deposited via precipitation at Kade and Tamale in Ghana be considered as a net input at a village scale as the nutrients in the rainwater have been transported over Location K Ca Mg P long distances. However, the measured input of nutri- μgg−1 ents by dust is not straightforward like precipitation. Kade, yearlya) 1.0 0.7 0.6 0.02 Kade, April–Septembera) 0.9 0.6 0.5 0.02 A study of Burkina Faso Visser and Sterk (2007) at a Tamale, April–Septemberb) 0.8 1.7 0.5 - village scale showed that significant soil and nutrient losses from bare fields were due to wind erosion and fur- a)Source from Nye (1960); b)Source from Pelig-Ba et al. (2001). ther that this local dust was deposited on neighbouring To investigate the relationship between dust and fields with vegetation cover. This pattern could also be rainwater nutrient input to soil we look at the two sam- expected in Ghana as the shifting cultivation systems pling sites, Kade and Kete Krashi, in Central Ghana as include periods of exposed ground and wind erosion shown in Table VI. As both sampling sites are located (Quansah, 1990). Thus, the mat-sampled dust in this in the moist semi-deciduous forest zone and receive the study was a mix of long-range transported dust and same amount of precipitation, it was assumed that the local dust from the nearby fields. Therefore, the nutri- chemical composition in rainwater at both sites was ent input measured is not a net gain of nutrients from similar. Rainwater inputs of Ca, Mg and K nutrients outside the area because a part of the nutrients is of

TABLE VI Total inputs of nutrients by Harmattan dust and precipitation at Kade and Kete Krashi, Ghana

Location Nutrient input Ca Mg K P kg ha−1 year−1 Precipitation, yearlya) 12.7 11.2 17.5 0.4 Kade Harmattan dust 0.3 (2%) 0.2 (2%) 0.5 (3%) 0.1 (20%) Total input of nutrient 13.0 11.4 18.0 0.5 Kete Krashi Harmattan dust 1.8 (12%) 0.5 (4%) 1 (5%) 0.3 (42%) Total input of nutrient 14.5 11.7 18.5 0.7 a)Source from Nye (1960) and Pelig-Ba et al. (2001); b)Values in the parenthesis represent the percentages of input by dust. 620 H. BREUNING-MADSEN et al. local origin. The share of local dust versus long-range shifting cultivation systems, but in modern intensive a- transported dust will vary with vegetation cover, land griculture systems using manures and fertilizers it was use, distance to roads or villages, wind speed, and found to be of no significance. wind directions. Thus, the measured amount of nutrients in dust is only valid if the input of local dust is ACKNOWLEDGEMENT insignificant. In southern Ghana dominated by south- This study was supported by the Danida-Enreca west winds, local dust must dominate as the distance project “Ecological Laboratory” of the University of to the sea is short and the northeast Harmattan wind Ghana, Legon, Accra, Ghana. is only blowing erratically in December to February. In northern Ghana, where the northeast Harmattan wind dominates in December to February, we could expect REFERENCES a huge input of long distance dust from the Bodélé Adetunji M T, Martins O, Arowolo T A. 2001. Seasonal variation Depression. However, based on a chemical and mine- in atmospheric deposition of nitrate, sulphate, lead, zinc, and ralogical investigation of dust at Tamale in northern copper in southwestern Nigeria. Commun Soil Sci Plan. 32: Ghana Harmattan dust deposited was primarily of lo- 65–73. Afeti G M, Resch F J. 2000. Physical characteristics of Saharan cal origin (Lyngsie et al., 2011). This is in line with dust near the Gulf of Guinea. 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