Abstract
 
A long-term field experiment was established in 2003 at the Institute for Agricultural Research, (I.A.R) Samaru, northern Guinea savanna of Nigeria to evaluate the effect of N fertilizer on maize grown in rotation with maize, cowpea and soybean. The trial was modified to accommodate tillage as an additional experimental factor in 2009. Ten years later (in 2013), soil and crop data were collected to determine the influence of the experimental factors namely; crop rotation, tillage and N fertilization on soil quality, maize grain and stover yield. To achieve these objectives, data were taken from the maize phase of the rotation where maize was grown in rotation with maize, maize in rotation with soybean and maize in rotation with cowpea, under conventional and reduced tillage, with N fertilization of 0 kg N ha-1 and 90 kg N ha-1. The trial was arranged in a split plot design, replicated three times, with the rotation and tillage in the main plot while N fertilization was in the sub-plot. Soil quality was determined using the procedure described by Parr et al. (1992). Soil quality indicators used were soil physical properties (bulk density, saturated hydraulic conductivity and moisture content), soil biological properties (urease, soil microbial biomass C and soil microbial biomass N) and soil chemical properties (total N. soil organic C). The study showed that the experimental soil was low in quality (SQ = 4) under continuous maize cultivation; however, crop rotation of soybean and cowpea improved the soil quality to moderate status (SQ = 3). Tillage had no effect on soil quality but N fertilization improved soil quality only in the rotation involving legumes (SQ = 3). To improve the soil quality in the northern Guinea savanna of Nigeria over a long period of time, farmers are recommended to practice crop rotation of maize in rotation with cowpea and maize in rotation with soybean at 90 kg N ha-1 under conventional tillage practices. Farmers are also recommended to practice crop rotation of maize in rotation with cowpea (2,791 kg ha-1) and maize in rotation with soybean (3,262kg ha-1) under conventional tillage (3,401 kg ha-1) and the application of N fertilizer at 60 kg N ha-1 (3,564 kg ha-1) for higher grain yield of maize in the northern Guinea savanna of Nigeria.

CHAPTER ONE

 

Introduction

Long-term field experiments are expected to provide important information regarding soil properties as affected by cropping system and soil management practices. Legumes such as cowpea and soybean add both organic matter and nitrogen (N) to the soil (Omay et al., 1997; Sainju et al., 2003) and increase soil fertility. Maize is the second most essential cereal crop after sorghum in sub-Saharan Africa; it is grown much more intensively than it was estimated since 1985 (FAO, 1999). In Nigeria, maize yield averages about 1.4 tons per hectare and this is only about 20% of the average in Canada and other parts of the world where intensive cereal production is carried out (Afolami and Fawole, 1991; FAO, 1999). Studies have shown that legume-cereal rotation produce relatively higher grain yields than either crop grown alone (Rao and Mathuva, 2000; Olufemi et al., 2001; Mpairwe et al., 2002; Dapaah et al., 2003). Rotating maize with grain legumes is often targeted towards utilizing biologically fixed-nitrogen by legumes for the benefit of the maize (Yusuf et al., 2009). Nitrogen fertilizers are most effectively used as part of a balanced fertilization plan that aims to maximize economic return of a cereal–legume rotation system; Nitrogen fertilizer, apart from increasing the content of nitrate in soil that leads to its leaching (Porter et al., 1996), results in changes in soil pH and many other soil properties (Brady and Weil, 2002). Long-term field experiments with N fertilization can give valuable information about how those changes occur and indicate the trends of the changes (Dragan et al., 2010).
Nitrogen (N) is most often the yield limiting nutrient with respect to crop production (Factsheet, 2014). Nitrogen contributes firstly to grain yield and forage biomass production, and at the same time to protein (Eche, 2011). Nitrogen is essential for seed formation and maturity. A steady supply is needed during the early growth stage and this steady supply can be provided by the action of soil microorganisms on the soil organic matter (SOM) or by application of inorganic fertilizers (Eche, 2011). Nitrogen stress
 
was observed to cause reduction in the number of grains per ear (Lemcoff and Loomis, 1994). However, nitrogen fertilizers can be permanently lost through ammonia volatilization, denitrification, leaching and run-off if not well managed. It could also result in changes in soil pH which affects the availability of other plant nutrients. Nitrogen can be fixed in the soil through symbiotic association of some microorganism (e.g rhizobia) and leguminous plants (Brady and Weil, 2002). Therefore proper management of soil in terms of method of tillage practice and cropping system is very important in determining the amount of N in the soil (Veenstra et al., 2006).
Tillage is any physical, chemical or biological soil manipulation to optimize conditions for germination, seedling establishment and crop growth. Tillage practice is very important in relation to cereal – legume rotation; it could be divided into conventional, reduced and zero tillage. Conventional tillage refers to tillage operations considered standard for a specific location and crop; it tends to bury the crop residue, usually considered as a base for determining the cost effectiveness of erosion control practices (Jasper, 2005). Reduced tillage involves no ridging and less weeding compared to the conventional tillage; it also refers to any method of soil cultivation that leaves the previous year’s crop residue on fields before and after planting the next crop to reduce soil erosion and runoff (MDA, 2014). Conventional tillage produces a reduction of organic matter content due to enhanced mineralization of crop residues, disruption of soil aggregates and increasing aeration (Sainju et al., 2006). Conservation (reduced or zero) tillage increases soil organic carbon in the surface layer (Six et al., 1998; Sainju et al., 2006; Melero et al., 2009; Lo´pez-Bellido et al., 2010), improves soil aggregation (Coulombe et al., 1996), and preserves soil resources better than conventional tillage practices (Six et al., 1998). Conventional tillage helps in loosening and aerating the top layer of the soil, in destroying weed mechanically and in mixing crop residue, organic matter (humus), and nutrients evenly into the soil (Ray, 2013). One goal of soil quality research is to learn how to manage soil in a way that improves soil function; therefore
 
soil tillage is among the important practices affecting soil quality and crop yield. It contributes up to 20% of all crop production factors (Khurshid et al., 2006).
 
Soil quality is defined as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans (NRCS, 2014). Soil contains living organisms that provide the basic necessities of life. Healthy soil gives clean air and water, bountiful crops and forests, productive grazing lands, diverse wildlife, and beautiful landscapes. Soil does all these by performing five essential functions: regulating water, sustaining plant and animal life, filtering and buffering potential pollutants and cycling nutrients. Soils respond differently to management depending on the inherent properties of the soil and the surrounding landscape. Understanding soil health means assessing and managing soil so that it functions optimally and is not degraded for future use (NRCS, 2011). Soil quality indicators are used to evaluate how well soil functions since soil function often cannot be directly measured. Measuring soil quality is an exercise in identifying soil properties that are responsive to management, affect or correlate with environmental outcomes, and are capable of being precisely measured within certain technical and economic constraints (Doran and Parkin, 1996). Dynamic soil quality is how soil changes depending on how it is managed. Management choices affect the amount of soil organic matter, soil structure, soil depth, and water and nutrient holding capacity (NRCS, 2011).
 

Statement of the Problem

 
In Africa, three quarter of farmland is severely degraded due to poor soil quality (Eswaran, 1997). Agricultural production in Nigeria is mainly constrained by low levels of soil organic matter, low nutrient status and water holding capacity. Due to the fragile nature of the soils, they degrade rapidly under  continuous  and  intensive  cultivation  (Abu  and  Abubakar  2013).  It  is,  however,  a  known
phenomenon that savanna soils in Nigeria have low inherent fertility status, low organic matter, low
 
total nitrogen and phosphorus, low water infiltration capacity, poor internal drainage due to poor structure (Lombin et al., 1991). As a result of the low fertility level in the soil, farmers apply fertilizer to make up for the deficient nutrient. Recent studies have shown that biologically fixed-N from cowpea or soybean is not enough to meet all the N demand of subsequent maize crop. Supplementary fertilizer N is necessary for optimum maize yield under crop rotation, though the quantity required is lower than that under continuous maize cultivation (Yusuf et al., 2009). However, these fertilizers are not applied at recommended rates. Hence, crop productivity declines gradually due to poor soil quality.
 

Justification

 
Crop rotation with grain legumes helps to improve soil fertility, through biological nitrogen fixation while tillage helps in soil aeration and soil aggregation thereby improving the soil physical properties (soil texture and soil structure). Tillage practices also affect soil organic matter in surface soils (Paustian et al., 1997). Studies have also shown the effects of different tillage management systems on soil quality under different climatic conditions and fertilization (Kandeler et al., 1999; Rolda´n et al., 2005; Melero et al., 2009). Soil quality helps to indicate the productivity potential of soils. Perhaps, long-term continuous mono-cropping may contribute to rapid decline in soil productivity as these biological, physical and chemical properties continue to degrade (Tarawali et al., 2001). However, information on changes produced by the interaction of tillage system, crop rotation, rate of N fertilisation and soil quality over a long time is very scarce (Melero et al., 2009; Melero et al., 2011). Therefore in an effort to reduce land degradation and soil nutrient depletion problems in the northern Guinea savanna of Nigeria, the contributions of crop rotation under conventional and reduced tillage practices in improving soil quality and yield of maize using different N fertilizer rates were investigated.
 

Objectives

The main objective of this research was to determine the effect of long term rotation, nitrogen fertilizer and tillage on soil quality and maize yield in the northern Guinea savanna of Nigeria.
The specific objectives of this research were:
 

1. To determine the influence of long-term crop rotation, tillage and N fertilization on soil quality in the northern Guinea savanna of
2. To determine the influence of long-term crop rotation, tillage and N fertilization on maize grain and stover yield.

To determine the relationship between soil quality indicators and maize grain yield as affected by crop rotation, tillage and N fertilizer.