A decade of non-sorted solid urban wastes inputs safely increases sorghum yield in periurban areas of Burkina Faso

Abstract

Non-sorted solid urban wastes (SUW) are used by periurban cereal farmers in Africa. There is however limited information on how these SUW affect soil quality and cereal production and quality. In order to answer this question we identified around Ouagadougou (Burkina Faso) sites cultivated with sorghum (Sorghum bicolor (L.) Moench) that had received SUW for less than 5 years, for more than 5 years and less than 10 years and for more than 10 years. We sampled soils at 0–15 and 15–30 cm depth and we analysed pH, total carbon (C), nitrogen (N) and inorganic phosphate (P) content and P and heavy metals availability. We also measured at some of these sites sorghum production and nutrient and metal contents in sorghum grain and straw. Our results show that the 0–15 cm horizon of the soils that had received SUW for less than 5 years had lower pH, available P and heavy metals contents and produced lower yields than those that had been amended with SUW for more than 10 years. Maximum grain yield was observed in the sites that had been amended for more than 5 years but less than 10 years. There were no clear effects of SUW application time on the heavy metal contents of sorghum grain and straw. The increases in nutrient and heavy metals content observed in the 15–30 cm horizon of soils that had been amended for more than 10 years point out to the risks of element transfer to deeper horizons. Our results suggest that a complete sorting of organic matter from SUW and its further composting as presently recommended, is not necessary. Simply removing dangerous items from the SUW such as plastics, glass and batteries, would be sufficient. Adding this sorted substrate for 5 to 10 years to cereal fields would be sufficient to reach optimal yields, thereafter this substrate should be added to other surfaces.

Keywords:

• Burkina Faso
• heavy metals
• non-sorted solid urban wastes
• periurban agriculture
• phosphate
• soil and crop pollution
• sorghum

Introduction

The urban population is increasing very rapidly in developing countries (UNDP, 1996). This increase is leading inevitably to an increase in solid and liquid wastes production. Research conducted in the frame of the APUGEDU project (Potential for Development of Urban and PeriUrban Agriculture in Relation to Urban Waste Management in West Africa) showed that the household solid waste production in Ouagadougou (Burkina Faso) ranged between 0.54 and 0.85 kg per person per day (Eaton, 2003) corresponding to about 900 tonnes of non-sorted solid wastes (SUW) produced per day for the entire population of Ouagadougou.

Eaton and Hilhorst (2003) report that in Burkina Faso and Mali periurban cereal farmers use SUW as fertilizers for cereals because of their organic matter and nutrient contents. However these farmers apply often large amounts of SUW (Eaton, 2003) which in the long-term could lead to the losses of nutrients to the environment (Huang et al., 2006) and to the accumulation of heavy metals in the food chain (Madrid et al., 2008). In south-eastern Nigeria, Anikwe and Nwobodo (2002) showed that the long-term application of solid municipal wastes increased not only soil organic matter, total N and cation exchange capacity, but also available heavy metals (Pb, Cu, Fe and Zn) by 214 to 2040% in dump site soils compared with non-dump site soils. Adjia et al. (2008) in Cameroon also showed that the long-term application of SUW led to strong increases in soil total and available heavy metals (Cu, Zn, Cd and Pb) contents.

Since solid urban wastes contain pollutants, most of the studies have focused on how to sort and compost SUW and on the effects of these composts on soil fertility and crop production (see e.g. Annabi et al., 2007; Kaboré et al., 2010). The compost produced from SUW however is too costly for urban farmers (Eaton & Hilhorst, 2003) making the use of non-sorted solid urban wastes a reality in most Sahelian cities. Another approach would be to evaluate, considering farmers’ practices, the number of years of SUW inputs needed to reach an optimum crop yield and how these inputs would affect soil quality and heavy metals content in the plant. Unfortunately, to our knowledge, there is no published information in the Sahel relating the time of SUW inputs to soil quality, crop yield and nutrient and heavy metal uptake by crops in on-farm studies.

Our objectives were (i) to assess changes in soil quality (pH, total C, N and inorganic P, and available P and heavy metals) in the 0–15 and 15–30 cm depth of soils sampled in sites where non-sorted solid urban wastes (SUW) had been applied for less than 5 years, between 5 and 10 years and more than 10 years, (ii) to measure sorghum (Sorghum bicolor (L.) Moench) yield and nutrient uptake at these sites and finally (iii) to assess the impact of SUW application on heavy metals contents in sorghum grain and straw.

Materials and methods

Description of the study area

The periurban agriculture area of Ouagadougou where cereals are grown in the presence of solid urban wastes (SUW) covers a surface of about 8500 ha located a few kilometres north of the city (Eaton, 2003). The soils in this area are classified as Lixisols in the FAO classification and as Alfisols in the USDA soil taxonomy (FAO-UNESCO, 1994).

According to Eaton and Hilhorst (2003), periurban farmers in Ouagadougou purchase on average 18 T of SUW per hectare and per year. These SUW are quite variable. For instance their sand content can vary between 76 and 29% for low- and high-income families respectively (Eaton, 2003). Despite this variability Eaton and Hilhorst (2003) report that these SUW have on average an organic matter content of 110 mg kg⁻¹, a total N content of 2.9 mg kg⁻¹ and a total P content of 1.6 mg kg⁻¹ and that their heavy metals and human pathogens contents are close to the limits given for composts in Europe and by the World Bank. These SUW are brought during the dry season to farmers’ fields by municipal trucks or carts pulled by donkeys. When the rainy season is approaching the SUW are spread on the fields without sorting and ploughed in with hoes at about 15 cm of depth. Sorghum is sown with the first rains and the cowpea (Vigna unguiculata) or peanut (Arachis hypogaea) are generally sowed 2–3 weeks after in remaining free spaces between the sorghum plants.

Selection of fields, soil sampling and yield assessment

In May 2007, we selected with farmers around the research station of INERA Kamboinsé (12° 28 N, 1° 32 W, and 296 m altitude) three fields that had received SUW for more than 10 years (noted SUW >10 years), five that had received SUW for more than 5 years and less than 10 years (noted SUW 5–10 years), and six fields that had received SUW for less than 5 years (noted SUW<5 years). The fields did not show strong spatial heterogeneity and had an average size of about 0.5 ha. Since the fields were almost rectangular we considered in each field the two diagonals and we sampled soils in the centre and 2 metres away from each corner of the rectangle along the diagonals. Soil samples were taken with an auger from the 0–15 and 15–30 cm depth. The subsamples were mixed, yielding one sample per field for the 0–15 cm depth and one for the 15–30 cm depth. Stones, glass, plastic and debris were removed by hand, the samples were air-dried at room temperature, sieved at 2 mm and then analysed for their texture, nutrients and heavy metals content.

Sorghum yields were measured in October 2008. For this purpose, two microplots of 10 m² each were installed in the three fields that had received SUW for more than 10 years, in 4 of the fields that had received SUW for more than 5 years and less than 10 years, and in four of the fields that had received SUW for less than 5 years. The microplots were systematically installed around two of the soil sampling points used in 2007. The sorghum straw and grains were harvested separately in each microplot and air-dried for 2 weeks. The yield of each field was calculated as the average of the yields obtained in the two microplots. Sorghum grain and straw were sampled after yield determination, and packaged in small plastic bags for the later measurement of nutrients and heavy metal contents.

Soil analysis

Soil particle size distribution (clay, silt, sand) was determined with the Robinson pipette method after destruction of soil organic matter with hydrogen peroxide and dispersion in sodium hexametaphosphate (Mathieu & Pieltain, 1998). Soil pH was measured with a pH meter ORION model 720A in soil water suspensions with 1 g of soil in 10 ml of distilled water. The total carbon and nitrogen contents were determined by CN analyser (Flash EA, 1112 series) on ball-milled samples. Total inorganic soil P was estimated by a NaOH-EDTA extraction (3 g dry soil in 30 ml of 0.25M NaOH and 0.05M EDTA solution) during 16 hours (Bowman & Moir, 1993). The extracts were then filtrated with vacuum-filtration through cellulose nitrate filter (0.8 µm) and the P concentration was measured in colorimetry with the malachite green method (Van Veldhoven & Mannaerts, 1987).

Soil available heavy metals (Cd, Cu, Zn, Pb, Ni, Cr), were extracted during 1 hour in soil/solution ratio of 1 g to 10 ml in an acetate ammonium (0.5M) – EDTA (0.02M) solution at pH 4.65 (Afnor 1994). Solutions were then filtered with 8 µm filters and metal concentrations measured with inductively coupled plasma mass spectrometer (ICP-MS; Agilent 7500 Ce). Soil P availability was measured with two approaches: the Bray 1 extraction and the isotope exchange kinetics method (Compaoré et al., 2003). Bray 1 P was extracted on 3 g soil with 21 mL of solution containing 0.025M HCl and 0.03M NH₄F during 5 min according to Bray and Kurtz (1945). The extracts were then filtrated through Whatman no. 42 filters and the P in the extracts was measured in colorimetry with the malachite green method.

The principles of the isotope exchange kinetics method have been described by Fardeau (1996) and Frossard et al. (2011). We give a short summary of this experiment and of the parameters it delivers. Ten grams of air-dried soil were weighed in 250 ml plastic bottles and equilibrated with 99 ml of de-ionized H₂O for 16 hours at 22 °C on a rotating shaker. At time t=0, 1 ml containing 0.1–1 MBq mL⁻¹ of carrier-free ³³P (as H₃ ³³PO₄, specific activity at delivery >100 TBq ³³P mg^{−1 31}P, Hartmann Analytic, GmbH) was injected in the soil suspensions and about 5 ml samples were taken with a polyethylene syringe after 1, 10, 30 and 120 minutes of exchange. The suspensions were rapidly filtered through a 0.2 µm cellulose-acetate membrane filter (Sartorius) and ³³P was measured by scintillation counting in the filtrates. Solution P concentration (Cp) was measured after the 120 minutes sampling in colorimetry with the malachite green method when Cp was higher than 10 µg L⁻¹. For concentrations lower than 10 µg L⁻¹ a method used to measure very low P concentrations in water (Afnor, 1997) was applied. In this method a blue phosphomolybdate complex is developed in a high volume of soil extract and subsequently concentrated in hexanol which is then analysed in colorimetry (Frossard et al., 2011).

The decrease of ³³PO₄ ³⁻ in solution with time is assigned to isotopic exchange with ³¹PO₄ ³⁻. In soils such as those considered in this paper, the decrease of radioactivity with time from the soil solution can be described with the following equation (Compaoré et al., 2003):

1

Where R is the total amount of radioactivity introduced in the soil solution suspension (MBq), r_t is the amount of radioactivity remaining in the solution after t minutes, n and m are soil specific constants, Cp is the concentration of P in the soil water extract (mg P L⁻¹) and total inorganic P (mg P kg⁻¹ soil) is approximated by the amount of inorganic P that is extracted by the NaOH-EDTA mixture. The factor 10 arises from the soil solution ratio of 1 g of soil in 10 mL of water. The amount of isotopically exchangeable P (E_t, mg P kg⁻¹ soil) is calculated as follows:

2

Following Fardeau (1996) we used this equation to calculate the amount of P exchangeable over different time scales. We calculated first the amount of P exchangeable within 1 min (E_{1 minute}) as it is totally and immediately plant available, then we calculated the amount of P exchangeable between 1 minute and 1 day (E_{1 minute–1 day}) i.e. during a period equivalent to the time of active P uptake by a single root hair or a single mycorrhizal hyphae and the amount of P exchangeable between 1 day and 3 months (E_{1 day–3 months}) i.e. during the period of P uptake by the entire root system of an annual crop. The amount of P that cannot be exchanged within 3 months (E_{>3 months}), i.e. which has a limited availability to annual crops, was calculated as the difference between total inorganic P and E_{3 months}.

Plant analysis

Samples of sorghum grain and straw (200 mg) were ashed at 550 °C during 3 hours in an electric furnace. The ashes were extracted with 2 mL concentrated HNO₃ (65%) completed to 100 mL with distilled water. The extracts were then filtrated through a 0.8 µm nitrocellulose membrane and analysed with ICP-MS for Cu, Zn and Cd. The filtered solutions were also analysed in colorimetry for P using the malachite green method. Finally, the N content of sorghum grain and straw was measured with a CN analyser (Flash EA, 1112 series). A reference plant material (hay powder, IAEA-V-10) was extracted and analysed at the same time as our samples to check for the correctness of the measurements of metal contents. We present the Cu, Cd and Zn contents in plant samples as we obtained a correct recovery for these elements.

Calculation and statistical analyses

The statistical analysis was performed with the software SAS 9.2. Two-way ANOVA was carried out with the soil data on the duration of SUW application and the sampling horizon. One-way ANOVA on the duration of SUW application was carried out with plant data. When the interaction between the duration of SUW application and the sampling horizon was significant for a given parameter, ANOVA was carried out for each sampling horizon separately considering only the duration of SUW application as the independent factor. All the means values were compared by the Newman–Keuls test at 95% of confidence level when the factor effect was significant.The parameters m and n of the isotopic exchange kinetics were calculated from non-linear regression using Equation 1 as model with the software STATGRAPHICS plus for Windows 3.1. The validity of regressions was evaluated by comparing the predicted and experimental values and by examining the residuals.

Results

Table I, Table II, Table III, Table IV and Figure 1 show the results of soil and plant analyses. The results of ANOVA are reported in Table V.

Figure 1.  Cd, Cu and Zn contents in the sorghum grains and straw from sites that have received non-sorted solid urban wastes (SUW) for less than 5 years, for more than 5 years and less than 10 years and for more than 10 years, in periurban areas of Ouagadougou.

Table I. Clay, total C and N, NaOH extractable inorganic P and pH of the 0–15 and 15–30 cm layer of soils from sites that have received non-sorted solid urban wastes (SUW) for less than 5 years, for more than 5 years and less than 10 years and for more than 10 years, in periurban areas of Ouagadougou.

	Field codes	Sampling horizon	Clay	Total C	Total N	NaOHPi	pH
Duration of SUW application		cm		g kg^−1
	F9		263	11.3	0.72	0.23	7.7
	F10	0–15	100	9.50	0.64	0.18	8.5
	F11		85	14.5	0.96	0.42	8.5
> 10 years	F9		100	6.13	0.39	0.19	8.3
	F10	15–30	110	5.91	0.42	0.07	8.5
	F11		133	7.76	0.49	0.18	8.6
	F2		103	5.05	0.25	0.03	7.9
	F4		208	6.88	0.47	0.07	7.9
	F6	0–15	258	8.52	0.57	0.06	8.4
	F14		345	10.1	0.83	0.17	8.4
	F13		178	8.11	0.50	0.06	8.2
5-10 years	F2		175	3.07	0.19	0.01	7.2
	F4		140	5.38	0.38	0.02	7.1
	F6	15–30	385	3.86	0.25	0.02	7.5
	F14		103	6.78	0.38	0.02	7.1
	F13		158	6.96	0.47	0.07	8.3
	F1		Nm	7.13	0.44	0.01	6.3
	F3		123	6.64	0.43	0.03	6.8
	F5		110	5.01	0.32	0.01	6.9
	F7	0–15	163	6.44	0.37	0.03	7.6
	F8		113	5.92	0.41	0.04	8.1
	F12		180	15.8	1.09	0.07	7.7
< 5 years	F1		113	4.44	0.25	0.01	6.1
	F3		265	5.90	0.37	0.02	6.5
	F5		108	4.91	0.31	0.01	7.0
	F7	15–30	155	5.10	0.29	0.02	7.0
	F8		83	5.15	0.31	0.02	7.8
	F12		223	9.00	0.52	0.02	6.6
Nm: not measured.

Clay, total C, N and inorganic P and soil pH

Soils from the studied fields contained on average 150 g kg⁻¹ clay (Table I). No significant differences were observed between the soil samples that had received SUW for different periods of time and between the two horizons regarding soil clay content.

Table II. Bray 1 P, water extractable P (Cp) and isotopically exchangeable P contents of the 0–15 and 15–30 cm layer of the soils from sites that have received non-sorted solid urban wastes (SUW) for less than 5 years, for more than 5 years and less than 10 years and for more than 10 years, in periurban areas of Ouagadougou.

		Sampling horizon	Bray 1 P	Cp	n	m	E1min	E1min to 1day	E1day to 3 months	E>3 months
Duration of SUW application	Field codes	cm	mg kg^−1	µg l^−1				mg kg^−1
	F9		26.3	124	0.10	0.95	1.37	1.29	1.45	224
	F10	0–15	26.8	166	0.08	0.80	2.06	1.60	1.56	172
	F11		52.2	518	0.11	1.04	5.42	5.19	6.29	405
> 10 years	F9		10.5	34.5	0.12	0.50	0.69	0.97	1.19	189
	F10	15–30	9.91	15.0	0.26	0.12	1.28	6.21	11.9	55.1
	F11		27.8	105	0.08	0.79	1.33	1.05	1.03	175
	F2		8.01	38.8	0.10	0.90	0.44	0.44	0.48	30.5
	F4		8.72	16.7	0.09	0.84	0.20	0.19	0.20	66.0
	F6	0–15	14.3	79.1	0.09	0.93	0.87	0.76	0.81	57.7
	F14		15.3	89.2	0.15	1.09	0.95	1.52	2.40	163
	F13		10.1	23.3	0.09	0.75	0.31	0.29	0.30	63.4
5–10 years	F2		2.63	14.2	0.09	0.68	0.21	0.19	0.19	12.8
	F4		1.10	2.29	0.26	0.15	0.15	0.78	1.72	15.0
	F6	15–30	2.88	12.2	0.09	0.76	0.16	0.14	0.14	19.0
	F14		1.32	7.80	0.27	0.16	0.48	2.45	4.32	12.3
	F13		7.74	24.6	0.24	0.34	0.71	3.21	6.55	60.3
	F1		1.07	1.29	0.30	0.30	0.04	0.33	0.97	9.92
	F3		2.78	15.3	0.18	0.32	0.46	1.20	1.84	23.7
	F5	0–15	1.04	6.85	0.27	0.51	0.14	0.72	1.58	10.0
	F7		3.58	38.7	0.11	0.94	0.43	0.46	0.55	32.9
	F8		5.17	12.8	0.12	0.85	0.15	0.21	0.27	40.3
	F12		7.25	19.4	0.19	0.27	0.72	2.08	3.45	59.3
<5 years	F1		0.43	7.02	0.31	0.09	0.70	2.90	2.19	1.49
	F3		1.01	1.25	0.49	0.08	0.16	3.95	8.56	4.50
	F5		0.37	19.8	0.39	0.14	1.21	5.45	1.96	0.56
	F7	15–30	1.62	11.4	0.10	0.89	0.13	0.13	0.15	15.5
	F8		2.02	8.47	0.30	0.18	0.45	3.01	5.48	10.6
	F12		1.07	8.00	0.42	0.07	1.15	10.4	7.43	2.51

Total N and C contents of soils increased with the duration of SUW application but the differences were not statistically significant. Soil total N and C contents were significantly higher in the 0–15 cm than in the 15–30 cm. Total inorganic P (NaOHPi) was significantly higher in the SUW>10 years field than in SUW 5–10 and SUW<5 years fields. The horizon 0–15 cm showed significant higher inorganic P than the horizon 15–30 cm. Soil pH was significantly higher in the SUW>10 years fields than in the SUW 5–10 years and SUW<5 years fields.

Table III. EDTA extractable heavy metals (Cd, Cu, Zn, Pb, Ni and Cr) contents of the 0–15 and 15–30 cm layer of the soils from sites that have received non-sorted solid urban wastes (SUW) for less than 5 years, for more than 5 years and less than 10 years and for more than 10 years, in periurban areas of Ouagadougou.

		Sampling horizon	Cd	Cu	Zn	Pb	Ni	Cr
Duration of SUW application	Field codes	cm	mg kg^−1
	F9		0.04	2.93	25.9	7.57	0.63	0.09
	F10	0–15	0.31	5.86	40.7	56.3	1.02	0.11
	F11		0.08	3.71	43.5	92.6	1.22	0.19
> 10 years	F9		0.02	1.71	20.4	3.89	0.49	0.07
	F10	15–30	0.06	3.09	15.3	23.2	0.92	0.02
	F11		0.03	12.95	16.3	28.7	0.76	0.13
	F2		0.01	0.91	5.20	1.51	0.48	0.03
	F4		0.04	5.43	13.4	4.97	0.31	0.01
	F6		0.01	0.91	5.20	1.51	0.48	0.03
	F14	0–15	0.11	2.12	23.7	9.79	0.49	0.13
	F13		0.03	7.91	27.4	12.5	0.44	0.04
5–10 years	F2		0.01	0.79	1.79	1.21	0.32	0.05
	F4		0.01	1.46	2.24	2.21	0.12	Nd
	F6	15–30	0.01	0.76	16.2	2.13	0.16	Nd
	F14		0.01	1.61	1.74	2.17	0.14	0.03
	F13		0.04	1.08	10.3	4.00	0.32	0.02
	F1		0.03	0.69	0.49	2.26	0.17	0.02
	F3		0.01	1.41	1.69	1.88	0.65	0.01
	F5		0.01	0.94	1.12	1.81	0.53	0.01
	F7	0–15	0.01	0.99	4.00	2.61	0.57	Nd
	F8		0.02	0.97	9.74	2.68	0.23	0.01
	F12		0.03	2.45	18.7	6.70	0.54	0.06
<5 years	F1		0.004	0.72	0.67	3.04	0.08	0.04
	F3		0.004	1.59	0.72	1.86	0.51	0.04
	F5		0.004	1.04	0.72	1.64	0.37	Nd
	F7	15–30	0.01	0.72	0.94	1.69	0.64	0.002
	F8		0.02	1.01	9.55	3.26	0.18	0.03
	F12		0.004	1.37	1.61	2.53	0.18	0.06
Nd: not detected.

Soil phosphate availability

Bray 1 P, water soluble P, E_{1 minute}, and E_{>3 months} were significantly higher in the SUW>10 years fields compared to the SUW 5–10 years fields and to the SUW<5 years fields (Table II). No differences were observed between the SUW 5–10 years fields and the SUW<5 years fields regarding P availability. The duration of SUW application did not affect E_{1 minute to 1 day} or E_{1 day to 3 months}. The water soluble P, Bray 1 P and the E_{> 3 months} from the horizon 0-15 cm were significantly higher than those from the 15–30 cm horizon. The interactions between the duration of SUW application and the sampling horizon were significant for soil water soluble P and Bray 1 P. For both sampling horizons, Bray 1 P and water soluble P in the SUW>10 years fields were significantly higher than in the SUW 5–10 years and<5 years fields.

Table IV. Sorghum grain and straw yields and N, P contents in the grain and straw from sites that have received non-sorted solid urban wastes (SUW) for less than 5 years, for more than 5 years and less than 10 years and for more than 10 years, in periurban areas of Ouagadougou.

		Grain yields	Straw yields	N in grain	N in straw	P in grain	P in straw
Duration of SUW application	Field codes	t ha^−1		g kg^−1 dry matter
	F9	2.8	7.1	20.4	8.8	1.0	0.7
> 10 years	F10	2.0	3.7	18.9	4.1	1.5	1.1
	F11	1.9	3.7	17.8	4.4	1.0	0.8
	F2	2.5	6.1	17.7	5.4	0.7	0.5
	F4	2.5	6.8	17.2	4.4	0.7	0.5
5–10 years	F6	2.2	5.8	17.7	8.5	1.0	0.8
	F14	2.4	6.2	17.5	4.4	0.8	0.5
	F1	1.0	3.3	16.2	4.8	1.0	0.5
	F3	1.0	4.4	15.3	5.6	0.9	0.9
<5 years	F5	0.9	4.2	17.0	5.4	1.0	0.6
	F7	0.9	4.0	16.2	4.3	1.0	0.6

Soil heavy metals availability

Heavy metals (Cd, Zn, Pb, Ni, Cr) contents were significantly higher in the SUW>10 years fields compared with the SUW 5–10 years and the SUW<5 years fields (Table III). No differences were observed between the SUW 5–10 years and the SUW<5 years fields for Cd, Zn, Pb, Ni, Cr, and between the three types of fields for Cu. The mean values of available heavy metals of the SUW>10 years fields were at least twice those of the SUW 5–10 years and the SUW<5 years fields. Although the mean available heavy metals contents were higher in the 0–15 cm than in the 15–30 cm horizons, there was no statistically significant difference between the two horizons for Cd, Cu, Pb, Ni or Cr. Only Zn content was significantly higher in the 0–15 cm horizon than in the 15–30 cm horizon. No interactions were found between the duration of SUW application and the sampling horizon regarding soil heavy metals contents.

Table V. Results of ANOVA showing the source of variation of the measured parameters on soil and plant samples from sites that have received non-sorted solid urban wastes (SUW) for less than 5 years, for more than 5 years and less than 10 years and for more than 10 years, in periurban areas of Ouagadougou.

	Variables measured on soil samples
Source of variation	Clay	Total C	Total N	NaOHPi	pH
Horizon	ns	**	**	**	ns
Duration of SUW application	ns	ns	ns	***	***
Duration of SUW application×Horizon	ns	ns	ns	ns	ns
	Bray1 P	CP	n	M	E1 min	E1 min to 1 day	E1 day to 3 months	E>3 months
Horizon	***	**	ns	***	ns	ns	ns	**
Duration of SUW application	***	***	**	ns	***	ns	ns	***
Duration of SUW application×Horizon	*	*	ns	ns	ns	ns	ns	ns
	Cd	Cu	Zn	Pb	Ni	Cr
Horizon	ns	ns	**	ns	ns	ns
Duration of SUW application	*	ns	***	***	***	***
Duration of SUW application×Horizon	ns	ns	ns	ns	ns	ns
	Variables measured on plant samples
	Grain yields	Straw yields	N Grain	N straw	P Grain	P Straw
Duration of SUW application	***	ns	**	ns	ns	ns
	Cd Grain	Cd Straw	Cu Grain	Cu Straw	Zn Grain	Zn straw
Duration of SUW application	ns	ns	ns	ns	ns	ns
*P=0.01–0.05, **P=0.01–0.001, ***P<0.001, ns: not significant.

Sorghum production and nutrients and metals content in the grains and straw

The mean sorghum grain yields reached 2.3, 2.4 and 1.0 t ha⁻¹ for the fields SUW>10 years, SUW 5–10 years and SUW<5 years, respectively (Table IV). The average grain yield from the fields SUW<5 years was significantly lower than those from the fields SUW>10 years and SUW 5–10 years. No significant differences were observed for the straw yield.

The average N and P contents were 17.5 and 1 g kg⁻¹ dry matter in the grain, and 5.5 and 0.7 g kg⁻¹ dry matter in the straw (Table IV). The average N content in the grain from the SUW>10 years fields was significantly higher than those from the fields SUW 5–10 years and SUW<5 years. No statistical differences were observed for N content in straw and for P contents in grain and straw. The results showed in Figure 1 indicate that Cd content tended to be higher in the sorghum growing in the SUW>10 years fields compared with the sorghum growing in the SUW 5–10 years and SUW<5 years fields. The Cd, Cu and Zn contents of the grain and straw however were not statistically affected by the duration of SUW application.

Discussion

Effects of non-sorted solid urban wastes on soil nutrient content and availability

Our results show increased pH, total C, N, NaOH extractable inorganic P and available P in the 0–15 cm horizon of soils that had received SUW for more than 10 years. Furthermore, correlations analyses conducted on all soil samples taken in the 0–15 cm horizon show that total N, total inorganic P, water extractable P, E_{1 minute}, available Zn, Pb and Cr were statistically significantly correlated, either linearly or logarithmically depending on the studied parameter, to total soil C. As the main source of C in these soils was SUW, we conclude that SUW applications increased soil nutrient and heavy metal contents. Indeed, if we consider the data provided by Eaton and Hilhorst (2003) as correct, then each year the 18 T of non-sorted solid urban wastes would bring about 52 kg N ha⁻¹ and 29 kg P ha⁻¹.

The water extractable P (Cp) and E_{1 minute} values observed in our work for soils that had received SUW for less than 5 years were similar to those observed by Compaoré et al. (2003) in non-fertilized soils. The highest Cp and E_{1 minute} values observed in our study after more than 10 years of SUW inputs were comparable to those observed by Compaoré et al. (2003) in the long-term experiment of Saria in the treatment where 15 kg P ha⁻¹ yr⁻¹ and 5 T of manure ha⁻¹ every second year had been applied between 1960 and 1994. It is interesting to note that the Bray 1 P values reported by Compaoré et al. (2003) for their fertilized soils were lower than the values we are reporting for the soils that have received SUW for more than 5 years and less than 10 years although the E_{1 minute} reported by both works were comparable. This can be attributed to the presence of HCl in the Bray 1 extraction leading to the solubilization of soil P in these alkaline soils amended with SUW whereas the isotopic exchange kinetics conducted in water do not solubilize P (Demaria et al., 2005).

The pH increase observed in our study can be explained by the presence of large quantities of ashes in these SUW coming from the kitchen of the households as observed by Sérémé and Mey (2008) in the SUW composts from Bobo Dioulasso.

Finally, besides increasing soil nutrient content, we should mention that long-term SUW inputs also improved soil structure as shown by the formation of numerous aggregates in the SUW>10 years fields. At the sites where SUW have been applied since less than 5 years soil structure remained however very weakly expressed, as often observed in these sandy lixisols. The improved soil structure can be related to the increased pH and soil organic matter content in the sites amended since more than 10 years.

Effects of SUW on sorghum production

The average grain yield of sorghum in Burkina Faso is estimated to 0.8 t ha⁻¹ (Dakouo et al., 2005). Eaton (2003) reports that crop production was no longer possible 10–20 years ago in the periurban area of Ouagadougou and that the use of SUW as fertilizers made it again possible to grow crops. Indeed, we observe that the use of SUW for more than 5 years leads to sorghum yields that are higher than those commonly recorded in lixisols without fertilization (Ouédraogo et al., 2001; Zougmoré et al., 2003). Regression analyses conducted on our data showed that grain yields were logarithmically positively correlated to Bray 1 P, water extractable P and EDTA extractable Zn. This suggests that one or both of these elements were limiting sorghum growth in the absence of SUW.

The differences in sorghum yields were not significant between the fields that had been amended for more than 10 years and those that had been amended for more than 5 years but less than 10 years. It seems therefore that after 5 to 10 years of SUW application the quantities of nutrients accumulated in the soils exceed the needs of these local sorghum cultivars. As the SUW are an important resource for periurban farmers it would make sense to optimize their use and therefore to decrease or stop their application after 10 years. This would have an impact on farmer's income since these SUW are expensive (Danso et al., 2006). Besides, a better use of SUW will probably reduce the risks of environmental pollution and food chain contamination by toxic elements.

Risks of environmental pollution due to the use of SUW

Higher contents of available P and available metals were found in the 15–30 cm horizon of the soils that had received SUW for more than 10 years than in the 15–30 cm of the soils that had received SUW for less than 5 years, although farmers do not access this horizon when preparing the soil with their hoe. This points out to a transfer of P and metals to this lower horizon through leaching or biological activity. Given the low P sorbing capacity of these soils (Frossard et al., 1993) there is a risk that P added in excess of plant needs will be leached from the soil profile and be transferred into surface or ground waters negatively affecting their quality.

The increase in heavy metals availability in soils with the duration of SUW application corroborates the findings of Anikwe and Nwobodo (2002) and Adjia et al. (2008) respectively in Nigeria and Cameroun and suggests a potential risk of environmental pollution. Increased heavy metals content in the upper soil horizon might indeed lead to increased direct contamination of city dwellers through soil particles inhalation during windy periods. In Burkina Faso, as in other West African countries, the dry and dusty wind called ‘Harmattan’ blowing during the dry season when soils are not covered by any vegetation, can transport soil particles rich in heavy metals. Pruvot et al. (2006) mentioned that the ingestion or inhalation of soil particles containing high heavy metals content could represent a significant transfer of heavy metals to the population, particularly to children, living in contaminated areas.

Other risks linked to the use of SUW in the periurban cereal farms which should be mentioned here are the numerous plastics contained in these SUW. The ingestion of these plastic by cattle is a very important problem around Ouagadougou. In addition, these SUW contain glass, medical wastes and metallic pieces which can hurt the farmers who handle SUW without protection.

Risks of food chain pollution due to the use of SUW

The contents of Cd, Cu and Zn in sorghum grains and straw were lower than the contamination limits of foodstuff reported for many countries, such as Malaysia, Brazil, Australia, Czech Republic and the United States (Ustyak & Petrikova, 1996; Yap et al., 2004). Whereas the actual risk linked to the consumption of sorghum grain can be considered as low, the trends of Cd content increase in the aerial parts of sorghum grown in the fields which have been fertilized for a long time with SUW should be considered as an early indicator. Further SUW applications will lead to increased levels in heavy metals in plant products which will finally be detrimental to human health. Furthermore, farmers often cultivate other crops in these fields as eggplants, okra, groundnut and cowpea which may be more strongly contaminated. Indeed it was shown by Bingham (1979) that Cd accumulation by crop decreases in the following order: leaf vegetables>root vegetables>grains crops. Therefore it would be safe to avoid the cultivation of vegetables and roots crops in periurban fields which have been fertilized with SUW for more than 10 years.

In conclusion, the results obtained in this research show that SUW applications have a positive effect on nutrient availability and crop production, and when applied for less than 10 years they lead to limited accumulation of heavy metals in soils and plants and of P in soils. Therefore, following Eaton and Hilhorst (2003), we suggest that it is not necessary to carry out an intensive sorting of organic matter and composting from these SUW. It would be sufficient to simply remove the most dangerous items from the SUW such as plastics, glass and batteries, before recycling the rest in agriculture. Adding this substrate for 5 to 10 years to cereal fields would be sufficient to reach optimal yields, thereafter this substrate should be added to other surfaces.

Acknowledgements

The authors are grateful for the funding from the Swiss Government (Federal Commission for Scholarships for foreign students, FCS) and French embassy funding (Fond de Solidarité Prioritaire, projet FSP/déchets). We thank Astrid Oberson and Else K. Bünemann for scientific advice. We are also grateful to the urban farmers for their good collaboration and to the INERA technicians in particular Moyenga Momini and Ramdé Martin for their help during the soil sampling and Soubeiga Omer for the soil texture analysis. Many thanks to Thomas Flura, technician from the group of plant nutrition ETHZ, who helped with soil and plant analyses.