Level of zinc in maize seeds and maize growing soils of central Mecha, Amhara National Regional State of Ethiopia

Ethiopia is one of the world countries with reported zinc deficiency or high probability of zinc deficiency. Zinc deficiency is an important soil constraint to crop production, food quality and human health. The aim of this study was to evaluate the zinc concentration of different cultivars of maize seeds and soil samples in central Mecha area using a laboratory analysis to establish whether the roles and effects of zinc in crop productivity and quality in the area could be significant due to plant factors (e.g. cultivar) or changes in soil nutrient zinc concentration or both. Thirty representative soil samples from seven kebeles of the central Mecha and six genotypes of maize grains (Bako Hybrid-540, Bako Hybrid-543, Bako Hybrid-660, Pioneer Hybrid-3253, Melkassa Hybrid-2, and Melkassa Hybrid-4) available in the region for farmers archived by Ethiopian Seed Enterprise (ESE) were collected and analyzed in the laboratory. Zinc levels were determined by FAAS using wet digestion and dilute acid extraction methods. The mean pH (KCl) value of the soils indicated the samples studied are acidic (pH = 4.60). The means and ranges of concentrations of total zinc using strong acids wet digestion and available HCl-extractable zinc in soils (mg Kg-1 DW) were 50.49 mg Kg-1 (44.80 to 65.22 mg Kg-1) and 2.95 mg Kg-1 (1.76 to 4.94 mg Kg-1), respectively. ANOVA analysis revealed significant differences (P < 0.05) between maize varieties in zinc levels. There was significant genetic variability in the level of zinc between the maize cultivars from 16.18 mg Kg-1 for Bako Hybrid-540; 20.08 mg Kg-1 for Bako Hybride-660; 21.08 mg Kg-1 for Pioneer Hybrid-3253; 23.26 mg Kg-1 for Bako Hybrid-543; 29.38 mg Kg-1 for Melkassa-2 to 32.52 mg kg-1 for Melkassa-4. Chemical analysis of composite soil samples indicated that inherent zinc level was on the borderline sufficiency to support good crop growth for now, however; the variations in the level of zinc among the different maize cultivars should be significant in limiting high and quality yield for consumption. Adequate zinc in soils and high zinc concentrations in seeds ensure agronomic and nutritional benefits resulting in high yield and nutritional quality crops.


INTRODUCTION
Zinc (Zn) is an essential micronutrient for plants, animals as well as for humans (Hotz and Brown, 2004). It is an essential trace element and a constituent of more than 300 metalloenzymes involved in major metabolic pathways in biological systems that support life. Thus, there is always an optimal concentration of this essential element required by organisms; above or below the optimal concentration, in which a toxic or deficiency state in the ecosystem is developed may compromise health, quality and productivity. A typical doseresponse curve for elements in crops is shown below (Figure 1) (Alloway et al., 2004).
The soil is the primary source of zinc for plants, animals and humans. The zinc mineral that is supplied through the food chain from the soil overlying the surficial lithosphere could be sufficient, deficient or excessive resulting into effects on health, quality and productivity of plants, animals and humans; mostly with hidden symptoms and sometimes with clear subclinical and clinical symptoms.  .
Zinc deficiency in crop production is known in many parts of the world with deficient soils in their underlying agricultural soil. Agricultural productivity in Ethiopia is below the region average because of soil fertility problems due to nutrient depletion or unavailability. An earlier global project  has showed low concentration products for iron and zinc deficiencies (Sillanpaa, 1990). In recent researches to assess the status of micronutrients in central highlands of Ethiopia the available Zn in Nitosols, Luvisols and Cambisols soils in Bale area were low (Teklu Baissa et al., 2005;Teklu Baissa et al., 2007;Asgelil Dibabae et al., 2007;Ashenafi Ali et al., 2010;Mesfin Kebede and Yifru Abera, 2013;Tuma Ayele et al., 2014).
According to a WHO report on the risk factors responsible for development of illnesses and diseases, Zn deficiency ranks 5th among the 10 most important factors in developing countries. Zinc deficiency in animals and human beings due to the lack of Zn in their diet or unavailability for absorption is associated with cessation of growth, psychomotor delay, hypogonadism and suppression of both primary and secondary sexual characteristics.
There are estimates that some two billion people worldwide are afflicted by Zn deficiency, and up to half of the population in developing countries are at risk of Zn deficiency (WHO, 2002).
Ethiopia is one of the world countries with reported zinc deficiency or high probability of zinc deficiency (Welch et al., 1991;Alloway et al., 2004).
Zinc deficiency is an important soil constraint to crop production, food quality and human health. A study carried out in Ethiopia on existence of zinc deficiency showed that high prevalence of parental zinc deficiency exists and that it is one of the major contributing factors for high stunting rates in infants and young children in Ethiopia (Jemal Haider et al., 2005;Afework Kassu et al., 2008;Samson Gebremedhin et al., 2011). Zinc deficiencies are thus known in Ethiopian population particularly in rural and slum areas where people mainly depend on cereals as the main staple food and with little or no access to fruits and vegetables and animal products.
Generally, the distribution of zinc deficiency in the world correlates with the availability of Zinc in the biological ecosystem (Welch et al., 1991;Alloway et al., 2004). The regions with Zn-deficient soils are most of the time the regions where Zn deficiency in human beings is widespread, for example in India, Pakistan, China, Iran and Turkey (Yang et al., 2007). There are suggestions indicating that available Zn in the soils can be a limiting factor for crop production and zinc deficiency in the population who depend on Zn deficient soils.
The main hazards that limit high yield crop production and access to quality food and micronutrients for human and livestock consumption in Ethiopia based their roots from soil fertility challenges including top soil erosion, organic matter, macro and micro nutrients depletion, acidity and an array of pests and diseases including African boll worm, grasshopper, maize stalk borer, aphids and root rot (Yihenew Gebreselassie, 2002;ATA, 2013). However, the empirical relationship between widespread Zn deficiency problem in Ethiopian population and soil concentrations and plant uptake has not been reported or diagnosed.
In this study availability of zinc in the soils of nitosols in central Mecha Kebeles in the south west maize livelihood area in Amhara and zinc accumulation in maize cultivar grains for seeds cropped in the area were diagnosed. The main objective of the study was to identify issues to improve the management of maize production in the study area with respect to yield and nutritional quality increment for human and livestock consumption in the region.

Description of the study area
The South West Maize livelihood zone is one of the historically food secure areas in Amhara Region. It spreads across: West Gojam; Agew; and East Gojam. It is predominantly located in woina dega agro-ecology with some parts kola. The study area, Mecha woreda is located in this maize livelihood zone in the southwest Amhara Regional The region has a high crop production potential.
However, agricultural productivity has not reached its potential so far and people living in the area who depend on staple foods from their agriculture have paradoxically witnessed micronutrient defi-ciencies (a hidden hunger) due to the lack of nutrient dense quality foods (Melaku Umeta, 2003). Oxidable organic carbon (OC) was measured using the method of Walkey and Black dichromate wet oxidation method (Walkey and Black, 1934).  (Mclean and Langille, 1976;Brennan et al., 1993;Srivastava et al., 1996;Shituu, 2013;Tayie et al., 2013). Distilled de-ionized water was used throughout the analysis.

Spectrometry (Buck Scientific Model 210 VGP,
East Norwalk, USA) equipped with deuterium arc background correctors using air-acetylene flame (Lindsay and Norvell, 1978

Methods
To check the efficiency of the procedure in determining the total and phytoavailable zinc in soil and grain samples using Flame atomic absorption spectrometry triplicate measurements of the samples and measurements after appropriate stock solutions of zinc spiked on them were performed. The results of the percentage of recoveries for studied materials were all between 90 to 105%. The detection limit of the instrument was determined to be as low as 0.5 μg g -1 .

Properties of Studied Soils
The soils in central Mecha woreda are largely developed on parent materials of volcanic origin predominantly basalt. In terms of topography, the zone is predominantly plain with some hills between 1500 and 2500 meters altitude (Yihenew Gebreselassie, 2002 average that is ranging from 160 to 560 g kg -1 ).
The soil OC content was generally low (<1%), ranging from 3.57 to 6.2 g kg -1 , which is typical for soils in the region. The pH (CaCl This is a consequence of both the higher zinc concentrations in clay and shale parent materials and also the greater ability of clay-rich soils to adsorb and retain zinc and other elements relative to soils with lower percentages of clay and higher percentages of sand (Alloway et al., 2004).
However, it is important to stress that in many parts of the world, soils tend to be very heterogeneous in their distribution, especially in areas affected by soil formation processes with a wide range of soils developed on drift deposits.
Thus, patches of sandy soil of low zinc and other nutrient element status can often be found amongst more clay-rich soils with adequate levels of available micronutrients. Zinc total value may be very low in highly acidic soils due to the intense soil leaching (Brennan et al., 1993). N, P, K, S   (Brennan et al., 1993). The mean and ranges of concentrations of available HCl-extractable Zn in the studied soils were found to be 2.95 mg Kg -1 (1.76 to 4.94 mg Kg -1 DW) ( Table 3).
The concentrations of 0.1M HCl-extractable zinc used for interpreting soil analyses show levels of plant availability and zinc concentrations (mg Zn Kg -1 dry soil). According to Brennan and Srivastava (Brennan et al., 1993 andSrivastava et al., 1996), the concentrations of 0.1M HCl-extractable Zn of 1.6 -3.0 mg kg -1 are considered medium as it is in between the critical levels for normal crop growth. Available Zn in the studied surface soils varied from 1.76 to 4.94 mg kg -1 with a mean value of 2.95 mg kg -1 . Considering 0.8 mg kg -1 as the lower critical limit of available Zn as suggested by Brennan andSrivastava (Brennan et al., 1993 andSrivastava et al., 1996), the entire representative soils were under sufficient categories. The 0.1M HCl extractable zinc concentrations of studied soil samples were medium it in all instances. HCl-extractable soil metal concentrations were generally medium, as is to be expected for nitosol soils from young basaltic rock origin.
The critical concentrations for the interpretation of soil tests are often highly specific to certain types of soil and crops. It is therefore important for local expert advice to be sought in the interpretation of soil test results and the most appropriate method of treatment, if this is required. Quite often soil pH, clay and organic matter contents will also be taken into consideration. The advantage of soil tests over plant analysis is that they enable possible deficiencies to be predicted in advance of growing the crop so that appropriate fertilisation or other treatments can be made to prevent the yield and/or quality of the future crop being impaired by zinc deficiency.
The phytoavailability of soil micronutrients de-pends on soil properties such as total micronutrient concentrations, pH, calcium carbonate (CaCO 3 ) content, organic matter (OM) content, soil moisture conditions, and available phosphorus (Mahin et al., 2009). Available Zn showed significant correlation coefficient with pH, OC, available N, and available P 2 O 5 . Low solubility of Zn in soils rather than low total amount of Zn is the major reason for

Zinc Concentrations in Seeds of Maize
The concentration of zinc in grain can also be used as a retrospective indication of the zinc status of the previous crop and in identifying areas where future grain crops could suffer from deficiency. A critical value of 15 mg Zn kg -1 has been suggest-ed as a general value for the interpretation of grain analyses (Mahin et al., 2009;Ghulam et al., 2011).
ANOVA revealed significant differences (P < 0.05) between maize varieties in their zinc levels.   It has been indicated that micronutrient levels in maize grains were controlled to a large extent by environmental factors and interactions between the genotypes and the environment (Mahin et al., 2009;Ghulam et al., 2011). Based on a range of reports and this survey study, the average concentration of Zn in whole grain of Maize in various countries was between 16 to 35 mg kg −1 (Rengel et al., 1999;Cakmak et al., 2004). The Zn concentrations in cereals reported so far and found in this study were too low to meet daily human requirement, especially for those consuming a high proportion of cereal-based diets. For a measurable biological impact on human health, the concentration of Zn in whole maize grain needs to be increased at least by approximately 10 mg kg −1 , assuming a 400 g per day intake for adult woman in the countries where whole grain flour is used for making food (Pfeiffer and McClafferty, 2007). Thus, there is increasing interest in the zinc concentration (density) in grains used for human consumption. The aim will be to increase the zinc concentration in grain to 40-60 mg Zn kg -1 which is much higher than the concentration indicating possible yield losses due to zinc deficiency. This can be achieved by either genetic or agronomic biofortification. The use of improved varieties and or application of fertilizers blended with zinc compounds can be used for optimal results. However, it is also important to note that future experimental research on the amount of zinc