INFLUENCE OF COMPACTION AND MOISTURE REGIME ON PERFORMANCE OF RHIZOBIUM-INOCULATED SOYBEAN (Glycine max L. Merill) IN AN ALFISOL OF NORTHERN GUINEA SAVANNA OF NIGERIA

ABSTRACT

An experiment was conducted to determine the effect of soil moisture deficit (stress), soil compaction and nitrogen sources on performance of soybean. The trial was conducted in two phases; the first was a screen-house experiment in the Department of Soil Science, Ahmadu Bello University, Zaria while the second was a field trial at the University Research Farm of the Institute for Agricultural Research, Samaru, Zaria. The screen house experimental treatments involved three (3) soil compaction levels [no compaction (C₀), compaction at 1.3 kg F cm^-2 (C₁) and 1.7 kg F cm^-2 (C₂)], three (3) sources of nitrogen [Legumefix (commercial rhizobium inoculants), Mineral nitrogen fertilizer (20 KgN/ha, urea) and negative control (no rhizobium and nitrogen)] and four (4) levels of available soil moisture deficit [0%, 25%, 50% and 75%] arranged in a factorial combination using randomized complete block design (RCBD) and replicated three times. The field experiment was laid in RCBD in split-split plot arrangement with four (4) available soil moisture deficit level [ASMD (0, 25%, 50%, and 75%) ] as main treatments, two compaction levels [no compaction (C₀) and compaction at 1.5 kgF cm² (C₃) (representing conventional and minimum tillage)] as sub-treatments and three nitrogen sources (sub-sub treatment) involving Rhizobium, Mineral nitrogen (20 kg N/ha) and a negative control (without nitrogen). In both experiments, parameters observed were plant height, root length, shoot and root fresh weight, shoot and root dry weight, leaf area, nodule number, nodule fresh and dry weight, bulk density and penetration resistance, nitrogen concentration of the plant. The amount of nitrogen fixed, chlorophyll index, total dry matter, grain yield and hundred seed weight were only observed in the field trial. In the screen-house, soil compaction at C₁ significantly (P≤0.05) increased root length by 7.77% and decrease by 5.09% at C₃ relative to the control (C₀)_. The result showed that compaction at C₀ and C₁ were statistically similar in leaf area and both were higher than C₃. There was a decrease in nodule number and nodule fresh weight with an increase in soil compaction level. Plant height, root length, shoot and root fresh weight, shoot and root dry weight, leaf area, nodule number, nodule fresh weight, nodule dry weight were found to decrease with each increase in the soil moisture deficit. Nitrogen sources significantly (P≤0.05) influence root length, nodule number and nodule fresh weight. Rhizobium gave the highest mean value for both nodule number and nodule fresh weight compared to mineral N. However, in the field trial, soil compaction had significant (P≤0.05) influence on plant height, root length, root fresh weight, chlorophyll content, soil bulk density (BD) and penetration resistance (PR). There was increase in soil BD with increase in soil compaction level at 0-5 cm, 5-10 cm and 10-15 cm depths except at 15-20 cm where soil BD decreases with increase in soil compaction level. Up to 15.11% increase in PR was observed in the compacted soil over the un-compacted soil. Available soil moisture deficit significantly (P≤0.05) affected plant height, root length, root fresh weight, shoot dry weight, nodule number, total dry matter, and leaf area. The results also showed no significant influence of N sources on these parameters except total dry matter. Accumulation in soybean mineral nitrogen significantly (P≤0.05) increased total dry matter by 27.82% over the unfertilized controlled. The amount of nitrogen fixed by the soybean ranged between 77.02 kg/ha to 152.22 kg/ha. It can be concluded that stressing soybean plant to 25% ASMD would result in similar or even higher yield characters than with full irrigation (0% ASMD). In some cases soybean performance was found to be better in moderately compacted soil (compaction value of 1.3 kg F cm^-2). The three nitrogen sources only significantly (P≤0.05) affected total dry matter.

CHAPTER ONE

1.0 INTRODUCTION

Soybean (Glycine max L.) is the most important and most widely grown of the grain legumes worldwide (Giller and Wilson, 1991) of great nutritional value and enormous uses. It is a source of protein in human food, animal feed and industrial products. Proximate composition of soybean is 40% protein, 20-21 % fat (oil), 32-35% carbohydrate, 5% ash (mineral) and 3% fibre (Anonymous, 2011). Soybean is native to East Asia and first introduced to Africa in late 1800s (Shurtleff and Aoyagi, 2007), and to Nigeria in the year 1904 (Ezedimma, 1964). Nigeria is the largest soybean producing country in Sub-Saharan Africa. A total of about 661,000 ha of soybean were harvested in West Africa out of 99,501,101 ha cultivated worldwide. Nigeria accounts for 95% and the remaining 5% in the rest of the West African countries (FAO, 2009).

Soybean is one of the major legume crops cultivated in northern Guinea Savannah (NGS) of Nigeria. Although its production among legumes requires assimilation of large quantity of nitrogen for maximum yield, soils of this region are poor in nutrient status, especially total N (Machido et al., 2011; Laditi et al., 2012). The situation is further worsened by nutrient depletion by crops and other related processes, such as leaching, denitrification, volatilization and removal of crop residues for alternative uses (Yakubu et al., 2010).

Among the means available to supply and improve soil nitrogen status, fertilizer plays an important role. However, the production and use of chemical nitrogen fertilizers is historically influenced by changing; and often interrelated factors such as increasing populations and economic growth, agricultural production, prices, and government policies (FAO, 2011). Their production requires a great consumption of fossil fuels (1-2 % global fossil fuel) and is subjected to constant variations in prices (Vieira et al., 2010). The comparison, in terms of economic and ecological costs, between chemical and biological nitrogen fertilizers shows that biological nitrogen fertilizers represents an economic, sustainable and environmental friendly resource to guarantee the nitrogen requirement of an agro-ecosystem. It has been reported that significant portion of soybean N (up to 80%) (Salvagiotti, 2008) is derived from biological nitrogen fixation (BNF) when grown in association with effective and compatible soil bacteria known as Bradyrhizobium (Chianu et al., 2009). Although, yields of legumes can be improved by addition of appropriate rhizobium inoculants, this can only be sustained and assured under suitable soil environment. Suitability of the soil environment depends soil management practices. An important soil management practice that influences soil quality is tillage (Mahdi and Hanna, 2004). The traditional tillage practice in this zone involves manual hoe ridging and weeding. These are done with no special attention to conservation measures against soil nutrient depletion (mining), soil erosion and runoff (Kirchhof and Odunze, 2003) and many changes in soil physical qualities.

However, the introduction of agricultural machinery into the country has led to increased level of mechanized farming with the aim to ease and hasten the processes of cultivation. Heavy machines are extensively used in land cultivation, from sowing to harvesting. This result in varying degrees of soil compaction that causes profound changes in soil structure. Soil structure is important and must not be damaged because it determines the ability of soil to hold and conduct water, nutrients, and air necessary for plant root activity. Compaction affects not only the physical, chemical and hydraulic properties of the soil, but also seed germination, root growth, water utilization, nutrient uptake by crop (Sataranayana and Ghildyal, 1970) and activities of soil microorganisms. It had been reported that the use of machineries and fertilizers may not preserve productivity if significant soil deterioration occurs (Lal, 1979).

Therefore, there is the need to develop better soil management practices that prevent or reduce the effect of soil compaction on soils and crops. This mainly involves management measures aimed at controlling traffic on soils during and after cultivation. This may also include adoption of agronomic practices that would improve soil physical condition such as conservation farming approaches based on no-tillage or minimum tillage. No-tillage practice refers to zero tillage (zero disturbances on the soil) with direct application of seeds into the soil that aims at 100% ground cover with no plow or disk used. Even with the best soil physical quality, crop productivity depends to a great extent on availability of soil moisture.

Moisture deficiency is one of the most important environmental factors affecting agricultural productivity around the world and may result in considerable yield reductions if unchecked. The need to produce more food with less water poses vast challenges to reassign existing water supplies, encourage more efficient use and promote natural resource protection (Hussain et al., 2007). One of the water conserving irrigation scheduling techniques is deficit irrigation which provides a means of reducing water consumption while minimizing adverse effects on yield and the environment (Ghinassi and Trucchi, 2001; Kirda, 2002; Panda et al., 2003). The main objective of deficit irrigation is to increase the water use efficiency (WUE) of a crop by eliminating irrigations that have little impact on yield. The resulting yield reduction may be small compared with the benefits gained through diverting the saved water to irrigate other crops for which water would normally be insufficient under traditional irrigation practices. This objective will be achieved through improvements in agronomic practice, cultivation of superior legume varieties, and increased efficiency of the nitrogen-fixing process itself by better management of the symbiotic relationship between the legumes and bacteria.

1.1 Problem Statement:

Food production capacity is faced with an ever-growing number of challenges, including a world population expected to grow to nearly 9 billion by 2050 and a falling ratio of arable land to population (PDESAUNS, 2007). Crop production in the northern Guinea Savanna of Nigerian is increasing in scope and intensity and crops are commonly grown under rainfed conditions. The major crops include maize, sorghum, rice, cowpea, groundnut, cotton, and soybean. However, the soils are increasingly being degraded by poor management practices. The soils consequently do not contain sufficient plant nutrients to support vigorous crop growth and high yield (Kowal 1972; Jones and Wild, 1975). There is need to ensure adequate food production using sustainable technologies. This may include application of chemical nitrogen fertilizers. However, chemical nitrogen fertilization is associated with environmental problems such as watershed contamination by nitrogen leaching, volatilization and de-nitrification and all these can be source of environmental pollution (Herridge et al., 2008). Rhizobium inoculants are widely used in agriculture for production as to improve soil fertility because of their ability to fix atmospheric nitrogen in association with legume crops. The products are environmentally friendly and cheaper source of nitrogen.

Successful inoculation and establishment of effective legume-rhizobium symbiosis can only be achieved in the presence of favourable soil physical conditions such as soil porosity, moderate bulk density, moisture content and soil temperature which are all influenced by soil tillage systems. Effects of these soil physical properties and processes can be expressed as changes of soil microbiological activity, soil respiration and consequently changes in plant growth and development

1.2 Justification:

The production of soybean in Northern Guinea Savanna of Nigeria is mainly under rainfed conditions. However climate uncertainty is growing, resulting in inconsistency of rainfall amount and distribution pattern (Nicou et al 1999, Nicholson et al, 2000). Hence there is need to plan for solutions to mitigate these effects of dry spell that may occur within the production period. In this regard, supplementary or full irrigation may be required in order to maintain high yield for this important crop. However, it may not be feasible to practice irrigation to meet the full crop water requirement due to limitations and competition for fresh water between various sectors. This will likely continue to increase pressure on all disciplines to use water resources more efficiently. Deficit irrigation, a practice to apply water which exposes crops to certain (predetermined) levels of water stress during either a particular growth or development stage or throughout the irrigation period during the growing season with an insignificant reduction in yield need to be adopted. In addition to deficit irrigation agronomic measures such as reduced (minimum) or no-till practices can reduce irrigation requirements. This is because it has been reported that no-till improve soil water holding capacity (Heidarpour, 2004). Therefore, field under no-till will have greater potential for maintaining adequate soil moisture under deficit irrigation. Moraru and Rusu (2012) reported that soils subjected to no-tillage system are compacted due to reduce or complete elimination of soil perturbation. This results in high penetration resistance that reduces root growth (Cornish and Lymbery (1987); Moraru and Rusu, 2010), consequently affecting water and nutrient uptake by crops. This high resistance to penenetration can adversely affect emergence and seedling growth under no-tillage (Munawar et al., 1990).

Therefore, the effect of tillage on the aforementioned soil physical properties (soil moisture and penetration resistance) that in turn affect plant performances may also significantly affect biological nitrogen fixation due to modification of the rhizobial environment (soil). Hence the study of soil compaction and soil moisture level as influenced by tillage on the performance of rhizobium inoculated soybean.

Thus, this research was based on the following null- hypothesis:

1. Soybean responds equally to application of rhizobium inoculants and mineral nitrogen.
2. Production of soybean is not influenced by soil compaction
3. Production of soybean is not influenced by moisture deficit.

1.3 Objectives:

The general objective of this study is to assess the effect of soil compaction and varying moisture conditions on performance of soybean treated with different nitrogen sources and selected soil physical properties while the specific objectives are:

1. To assess the response of soybean to rhizobium inoculation and mineral nitrogen.
2. To determine the effect of soil compaction on the productivity of soybean.
3. To determine the effect of soil moisture deficit on the productivity of soybean.
4. To determine the effect of soil compaction and soil moisture regime and nitrogen sources on some selected soil physical properties.

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