Abstract
Green house and field experiments were undertaken in this study. The green house study was done in International Institute of Tropical Agriculture (IITA), Ibadan, green house, while the field experiments were conducted in the Department of Soil Science, University of Nigeria, Teaching and Research Farm and Dominican Centre for Human Resources Development, Moniya-Ibadan Farm in 2008 and 2009 crop years. The green house study was a 2 x 3 x 2 factorial in completely randomized design (CRD), comprising of two crop rotations, three nitrogen levels and two residue management options as factors, replicated thrice to give 36 pots. In the field experiment, the design was a 4 x 2 x 2 factorial in randomized complete block design (RCBD) with four crop rotations, two nitrogen levels and two residue management options as factors and replicated three times making 48 plots. Each year involved two growing periods comprising initial growth of velvet-bean, cowpea, soybean and maize and subsequent residual growth of maize in all the plots. Soil samples at a depth of 0 to 15cm were collected at onset of the experiment and at the end of each rotation cropping for 2 years for determination of soil properties. Green house experiment had 37 samples while field experiment had 97 samples in each location. In addition, maize dry matter and grain yields were collected at the end of each rotation cropping. The difference between the grain yields of legume/cereal and maize/maize rotations divided by the grain yield of maize/maize rotation was used to calculate rotation benefit. The soil properties and maize yields were subjected to analysis of variance (ANOVA) and stepwise regression, significantly different means were separated using Fischer’s least significant difference (f-LSD 0.05). At the end of the green house experiment, velvet-bean/maize rotation significantly (p < 0.05) increased maize yield over continuous maize by 13.59 %, nitrogen (N) benefit accounted for 4.56 % while non-N benefit accounted for 9.03 %. In 2008, in the UNN soil, the values of N (0.107
%), Ca (0.83 cmol/kg), Mg (0.59 cmol/kg), ECEC (2.86 cmol/kg) and SMBP (0.0024 %) were significantly (p < 0.05) higher in the legume rotation systems while pH (4.7) was significantly (p <0.05) higher in continuous maize. In 2009, the values for N (0.07 %), P (15.64 mg/kg), Mg (0.26 cmol/kg), SMBP (0.0007 %), Ksat (27.5 cm/hr) and AS (21.97 %) were significantly (p < 0.05) higher in legume rotations than in continuous maize. Moreso, in 2008, Moniya soils had ECEC (3.22 cmol/kg) and SMBP (0.0016 %) significantly (p < 0.05) higher in legume rotations while in 2009, SMBC (0.026 %), Pt (44.48 %) and Ksat (25.76 cm/hr) were significantly higher (p < 0.05) in legume rotations. Regression analyses showed that in Moniya, N and TVS contributed 44 % changes in dry matter while N, P and TVS contributed 74 % of the changes in grain yield in 2008. In 2009, SMBC and N contributed 62 % of the changes in dry matter and SMBC, EA and Ksat contributed 61 % of the changes in
grain yield. In 2008, N and Mg UNN soil contributed 51 % of the changes in dry matter and 44 % of the changes in grain yield while in 2009, EA, MBC and Ksat contributed 57 % of the changes in dry matter and SMBC, SMBN and Ksat contributed 69 % in grain yield. Comparatively, velvet-bean/maize rotation had highest percentage rotation benefits (255 %) relative to cowpea/maize (25 %) and soybean/maize (43 %) rotations. Non-N benefits had increase in exchangeable K, Mg, available P, MBC and Ksat.
1.0 INTRODUCTION
CHAPTER ONE
Widespread disappearance of soil fertility restoring practices such as fallowing, inadequate and inappropriate nutrient adding and nutrient saving practices were the major cause of soil fertility decline (Gichuru et al., 2003) . The fact is that management tools and external inputs are insufficiently available to meet intensification and continuous cultivation of soils. Currently, farmers are experiencing decline in yield in most countries of sub-Saharan Africa.
Notably, integrated plant nutrient management strategies with legume rotation systems have been successful in increasing yield and subsequently reducing chemical nitrogen (N) fertilizer requirement of crops (Smaling et al., 1997). Biological N-fixation by legumes in cereal based cropping systems of West African savanna enables them to fix substantial amount of nitrogen into the cropping system from atmospheric nitrogen gas. Decomposition of legume residue then releases this N to the following crop. Sustainable crop production researches in the tropics have focused on the role of planted fallows and their spatial arrangement (e.g., as in alley cropping) for many decades. More researches need to be done in area of grain and dual- purpose legumes such as soybean and cowpea since they equally improve soil fertility in cereal/legume rotation systems (Sanginga et al., 2001). Moreover, they have the advantage of giving more rapid return on investment and are increasingly more popular in West Africa (Sanginga et al., 2003; Buerkert et al., 2001; Franke et al., 2004).
Yield increases in cereals, following legumes in rotation have been reported for West Africa but little progress has been made to explain the mechanism behind them (Sanginga et al.,
2003; Buerkert et al., 2001; Franke et al., 2004). Researchers had attributed this to improved N nutrition of the cereal crop by the legume (Power, 1990; Karlen et al., 1994), subsequently; research shows that the N left behind by the N –fixing legumes does not entirely explain the growth enhancement of the rotating cereal crop (Sanginga et al., 2001). Yield increase in maize following soybean occurs even in situation where the N contribution of the legume is negative (Sanginga et al., 2001). This suggests that there must be other benefits derivable from cereal/ legume rotations that bring about increase in cereal yield.
Generally, most recognized benefit of a legume crop to a succeeding cereal is improvement in yield, known as rotation benefit. This benefit results from either improvement of N and non-N soil components. N benefit is the yield advantage associated with extra soil N
availability to a cereal crop succeeding a legume. For example, wheat following pea accumulated approximately 50 kg/ha more N compared to wheat in a wheat/wheat rotation (Evans et al., 1991). Non-N benefit of a legume in a legume-cereal rotation is that portion of the yield increase not explained by extra N accumulated by a succeeding cereal crop or that portion of the yield advantage relative to a cereal/cereal rotation that cannot be accounted for by the addition of fertilizer N (Bullock, 1992).
There is an ongoing argument as to what these ‘non-N benefits or other rotation benefits’ are. Proper understanding of these other rotation benefits will lead to effective management and utilization of legume/cereal rotation for sustainable crop improvement. These ‘other rotation benefits’ have been attributed to chemical and biological factors such as enhanced P nutrition (Alvey et al., 2001), weed control (Tarawali et al., 1999) and reduction of soil- borne diseases and parasitic nematodes (Vargas-Ayala et al., 2000; Bagayoko et al., 2000). Influence of legume-cereal rotations on soil properties, which enables the cereal crop to exploit the soil, better, might be the crucial result of legume-cereal rotation in improving cereal yield. Yanni et al. (2001) noted that rhizobia endophytically colonize the root of cereal and change their root structure, which enables them to absorb nutrients and water better. As at present, fundamental and complete understanding of the beneficial rotation benefits are lacking, and remains scientific challenge and is necessary for proper utilization of legume/cereal rotation system. This gap in knowledge stimulated this research. These other benefits not well understood are termed ‘other rotation effects’ or ‘non-nitrogen benefits’ in this research.
Therefore, the general objective of this research was to evaluate the effects of legume-cereal rotation systems on soil bio-physico-chemical properties and their contribution to cereal yield. The specific objectives were to:
(i) determine the contributions of N and non-N rotation benefits to increased maize yield, (ii) evaluate the effect of rotation on soil chemical properties (pH, organic carbon (OC), N, exchangeable calcium (Ca), magnesium (Mg), potassium (K), available phosphorus (P), effective cation exchange capacity (ECEC) and exchangeable
acidity (EA)), soil physical parameters (saturated hydraulic conductivity (Ksat), aggregate stability (AS) and total porosity (Pt)) and soil biological properties (mycorrhiza population, soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN), soil microbial biomass phosphorus (SMBP)) and their dynamic contribution to rotation benefits and
(iii) compare the beneficial effect of cowpea and soybean in relation to velvet-bean in contributing to rotation benefits in an integrated nutrient management.
(iv) determine the effect of mycorrhiza inoculation, different P combinations and cereal- legume rotation system on subsequent maize N and P
This material content is developed to serve as a GUIDE for students to conduct academic research
ASSESSMENT OF SOIL BIOLOGICAL AND PHYSICO-CHEMICAL BENEFITS OF LEGUME-CEREAL ROTATION SYSTEMS IN DERIVED SAVANNA ZONE OF NIGERIA
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