ABSTRACT
Integrated Soil Fertility Management (ISFM) has been recommended to address challenges of low soil fertility, by incorporating locally available organic resources (ORs) together with inorganic nitrogen (N) fertilizers. Despite ISFM success in field trials, there is limited information on ORs contribution to atmospheric greenhouse gas concentrations through nitrous oxide (N2O) emission. A short-term field study was conducted at two sites with different soil types; silt loam (Aludeka) and silty-clay soil (Sidada), to determine the effects of selected ORs (Calliandra carothyrsus (CL), farmyard manure (FYM) and maize stover (MS)) and their combination with inorganic N fertilizer on soil N2O emissions, available soil nitrogen and maize yields. The study also evaluated the relationship between N2O emissions and soil organic carbon (SOC), total nitrogen (TN), soil temperature, moisture content, soil nitrate (NO3-) and ammonium (NH4+). Static manual chambers were set up in the field to collect gas samples to quantify soil N2O emission. Relative to the control (0.19±0.1 Kg N2O-N ha-1), cumulative N2O emissions were significantly (P= 0.01) higher by 6, 9 and 13 fold under MS +N (1.05±0.8 Kg N2O-N ha-1), FYM +N (1.74±0.8 Kg N2O-N ha-1) and CL +N (2.54±1.2 Kg N2O-N ha-1), respectively at the Aludeka. At Sidada, cumulative N2O emissions were similar across all the treatments (P = 0.149). Approximately 240% and 411% of increase in cumulative N2O emissions across treatments at Sidada and Aludeka, respectively, was related to inorganic N fertilizer application. At Aludeka, cumulative N2O emissions exhibited significant positive relationship with SOC (r = 760, P = 0.029), TN (r = 0.820, P = 0.013), NO -(r = 0.905, P = 0.002) and NH + (r = 0.738, P = 0.036), and negatively correlated with soil C:N ratio (r = -0.710, P = 0.049), soil pH (r = -0.739, P=0.036). At Sidada only NO3- (r = 0.711, P =0.048) exhibited a significant positive correlation with cumulative N2O emissions. In terms of grain yield at Aludeka, there was a significant (P < 0.001) effect of treatments on maize grain yield, with no observed significant effect at Sidada (P>0.05). FYM +N treatment recorded the highest mean maize grain yield at both Aludeka (10.63 t ha−1) and Sidada (9.23 t ha−1). In Aludeka site, treatments with ORs in combination with inorganic N fertilizers increased maize grain yield in comparison with those without. The study suggests that influence of OR on N2O emissions in maize based-cropping system vary depending on the type of soil and increases when OR are applied in combination with inorganic N fertilizers. A more understanding of the prevailing environmental soil conditions especially on soil texture is necessary for finding the best treatment combination in terms of yield and N2O emission reduction under the ISFM approach.
TABLE OF CONTENTS
DECLARATION ii
PLAGIARISM DECLARATION iii
DEDICATION iv
ACKNOWLEDGEMENT v
LIST OF TABLES x
LIST OF ABBREVIATIONS AND ACRONYMS xi
ABSTRACT xii
CHAPTER ONE
1. INTRODUCTION
1.1 Background Information 1
1.2 Problem statement 3
1.3 Justification 4
1.4 Objectives 5
1.4.1 Main Objectives 5
1.4.2 Specific objectives 5
1.5 Hypotheses 5
CHAPTER TWO
2. LITERATURE REVIEW
2.1 Integrated soil fertility Management 6
2.2 The use of Organic Resources to Improve Soil Fertility 7
2.3 The Soil Nitrogen Cycle 8
2.4 Nitrous Oxide and Greenhouse Gases 10
2.5 Pathways of Nitrous Oxide Production 10
2.5.1 Nitrification 11
2.5.2 Denitrification 12
2.6 Nitrous oxide Emissions under cropping systems 13
2.6.1 Nitrous oxide emissions under ISFM practices in maize-based cropping systems 14
2.7 Available Soil nitrogen changes under different Organic Resources 16
2.8 Effects of Organic Resources and inorganic N fertilizer on Maize yields and yield scale emissions 19
2.9 Chamber Techniques 21
CHAPTER THREE
3. MATERIALS AND METHODS
3.1 Description of the study sites 22
3.2 Experimental design and field management 23
3.3 Generation of organic resources 25
3.4 Gas sampling and auxiliary measurements 25
3.5 Soil sampling and Organic Resource analyses 28
3.5.1 Mineral N 28
3.5.2 Total P 28
3.5.3 Total C and N analysis 29
3.5.4 pH analysis 30
3.6 Yield scale Emissions 30
3.7 Statistical analyses 31
CHAPTER FOUR
4. RESULTS
4.1 Chemical characteristics of organic resources 32
4.2 Chemical characteristics of soil 32
4.3 Soil mineral N concentrations during the study period 33
4.4 Soil moisture and temperature during the study period 34
4.5 Soil N2O fluxes under different treatments during the study period 35
4.6 Effect of Organic Resources on mean and cumulative N2O emissions 37
4.7 N2O emissions relationships with soil parameters 39
4.8 Effect of organic resource on maize yield and yield scale emissions 39
CHAPTER FIVE
5. DISCUSSION
5.1 Effect of organic resources and added inorganic N fertilizers on N2O emissions 41
5.2 Effect of soil properties on N2O emissions 45
5.3 Effects of Inorganic and Organic N fertilizers on maize yield 47
5.4 Yield Scaled N2O emissions 50
CHAPTER SIX
6. CONCLUSION AND RECOMMENDATIONS
REFERENCES 53
APPENDICES 71
Appendix 1. Daily precipitation as observed at a) Aludeka and b) Sidada sites 71
Appendix 2. Field activity Schedule 72
Appendix 3. N2O Emission factors 73
Appendix 4. Soil Moisture Anova table 73
Appendix 5. Site map 74
Appendix 6. Soil sampling 74
LIST OF FIGURES
Figure 1. a) Organic resource incorporation at Aludeka b) manual chambers for gas sampling at Sidada 27
Figure 2.Box plots showing NH4+-N (mg/kg) (a, b) and NO3--N (mg/kg) (c, d) for Aludeka and Sidada, respectively, under different organic resource application with and without mineral N fertilizer application 34
Figure 3. Average daily soil moisture and temperature (5 cm depth) at a) Aludeka and b) Sidada sites during the study period 35
Figure 4. Temporal soil N2O flux from 7th March to 3rd May 2018 at Aludeka site under different treatments 36
Figure 5. Temporal soil N2O fluxes from 6th March to 5th May 2018 at Sidada site under different treatments 37
LIST OF TABLES
Table 1. Chemical characteristics of the ORs used in the study 32
Table 2. Soil pH, total phosphorus (P), total carbon (C), total nitrogen (N) and C: N ratio of soils taken at the top 15cm from the two different sites, Aludeka and Sidada before planting 32
Table 3. Cumulative N2O fluxes from March to May 2018, maize yield, yield scale fluxes and total N input for studied treatments at the two sites, Aludeka and Sidada. Total N input refers to input through inorganic fertilizer and organic resources 38
Table 4. Correlation coefficients of linear association between soil properties and cumulative N2O emissions 39
LIST OF ABBREVIATIONS AND ACRONYMS
ANOVA Analysis of Variance
FYM Farmyard Manure
GDP Gross Domestic Product
GHG Greenhouse gas
CL Calliandra calothrysus
ISFM Integrated Soil Fertility Management
LR Long Rains
MS Maize stover
MT Soil Moisture
N Total Nitrogen
N2O Nitrous Oxide gas
NUE Nitrogen Use Efficiency
OR Organic Resource
ORD Organic Resource Database
SOC Soil Organic Carbon
SOM Soil Organic Matter
SPSS Statistical Packages for Social Sciences
SR Short Rains
SSA Sub Saharan Africa
CHAPTER ONE
INTRODUCTION
1.1 Background Information
The African continent relies on agriculture as the major source of livelihood and main contributor of GDPs (Shiferaw et al., 2011). Maize is the main staple crop to most of the Sub Saharan Africa (SSA) population and is grown on about 27 million hectares, representing approximately 30% of the cultivated land under cereals (Cairns et al., 2013). Although such a large area is occupied by maize crop in the region, average yields of 1.9 metric tons per hectare (mt/ha) have been obtained, which is below the world average crop potential of 5.01 mt/ha (Shiferaw et al., 2011). This shows that there is a significant yield gap between the actual and potential maize yield production (Vanlauwe et al., 2011), which shows that there is an unexploited potential for increasing maize production in SSA. Low soil fertility has been cited as the major biophysical cause of declined per-capita food availability in SSA smallholder farmers, with a reduction from 150 to 130 kg per person for the past 35 years of the production (Bationo, 2003). The declined food availability has been attributed to insufficient nutrient supply, poor soil management (Waithaka & Shepherd, 2006), leaching, soil erosion, and gaseous losses (Jaetzold and Schmidt, 2005). Maize research on smallholder farming system in SSA has emphasized the attainment of high yields per hectare to use of increased fertilizer inputs (Kimani et al., 2004). In most of SSA, the use of fertilizer in sufficient amounts is not possible due to the high costs of the fertilizers (Chianu et al., 2012). This has led to farmers relying on locally available organic residues (OR) to address low soil fertility constraints (Vanlauwe et al., 2010).
New technologies such as the Integrated Soil Fertility Management (ISFM), conservation agriculture and agroforestry (Mtangadura et al., 2017), are yet to be fully adopted by the smallholder farmers, who are the significant maize producers (Blackie and Jones, 2015). Recently, SSA agriculture intensification has gained support, partly because there is a growing acceptance that improved farm productivity is an important step in breaking the vicious cycle characterizing rural poverty (Vanlauwe et al., 2010). The use of ISFM in particular, has been of main focus in agronomic research on soil fertility improvement in maize cropping systems in SSA (Vanlauwe et al., 2010). The ISFM mainly involves a combination of inorganic fertilizer and OR to achieve sustainable agricultural intensification and to increase crop productivity and profitability among smallholder farming systems (Muyayabantu et al., 2012; Roobroeck et al., 2016).
However, the role of ISFM in climate change mitigation on N use efficiency has limited investigations through the application of OR amendments on the soil. The ORs have several functions in intermediate microbial reactions resulting in N2O production. N2O is produced due to mineralization of organic N found in OR which releases ammonium resulting in nitrification to nitrate and denitrification of nitrate to molecular dinitrogen (Charles et al., 2017). As a result, environmental benefits with OR amendments of soils can be offset depending on the degree of N2O emissions (Senbayram et al., 2012). An understanding of N fertilizer contribution to N2O emissions in maize cropping systems may assist in minimizing GHG emissions at the same time maintaining an improved crop productivity (Mapanda et al., 2012). An understanding of N release will also help smallholder farmers in managing the diverse OR in a way to maximize the uptake of nutrients and reduce gaseous losses (Kimetu et al., 2006). The study focused on measuring N2O fluxes in the field from maize plots under three OR amendments namely, maize (Zea mays) stover, Calliandra calothyrsus and farmyard manure with and without inorganic N fertilizer. The study aimed at collecting experimental data that is going to contribute to the development of solutions to improve on soil fertility in maize cropping systems and management practices.
1.2 Problem statement
Maize is one of the major staple crops in SSA serving as a source of food and as nutritional security for millions of households (Cairns et al., 2013). In the western region of Kenya, cultivation of this important crop is mainly done by smallholder farmers (Olwande, 2012) who obtain low yields. The low yields are due to limited access to inorganic fertilizer coupled with low soil fertility, soil moisture stress and soil degradation in smallholder farming systems. These challenges will be worsened further due to rising abiotic and biotic stresses as a result of climate change (Butterbach- bahl et al., 2013). This calls for an adoption of soil nutrient management strategies that will result to increased yields at the same time improved soil fertility, mitigation and adaptation of climate change.
However, increase in food production is one of the sources of atmospheric N2O emissions (Mosier, 2001), though the N fertilizer fraction that transitions to N2O (which is a potent greenhouse gas) in maize based cropping systems is not known, because it is dependent on the cropping system in terms of soil properties, climate and management activities. With the uncertainty in N2O emission rates from different ORs, a study for maize systems in SSA is needed to investigate on the relationship between environmental and management factors that control these emissions (Chadwick et al., 2011). ISFM has shown a potential to utilize ORs to improve on crop yield and at the same time reduce GHG emissions in different environments on different types of soil and climatic conditions (Senbayram et al., 2012). However, the role of ISFM in climate change mitigation on N use has limited investigations through application of different OR amendments on soil.
A combination of inorganic N fertilizer with ORs will raise the amount of N available to the crop which may result to soil N loss through N2O emissions (Bru et al., 2008). This shows that an understanding of N2O emissions in the fields is complex because of diverse nature of the factors that regulate N2O production, which include soil type and uncertain physical and chemical characteristics of the ORs (Li et al., 2013). Consequently, the extent and direction to which N2O emissions takes place is dependent on the quality of OR and ratio of organic to inorganic N inputs applied to soil (Baggs et al., 2003; Frimpong & Baggs, 2010). Therefore, the interaction between inorganic N fertilizer and ORs on N emissions in the soils must also be understood. A study is therefore needed to address the problem of agricultural N2O emissions under ISFM approaches with different types of OR amendments.
1.3 Justification
Estimation of N2O emissions under a maize cropping system would generate data required for identification of a good nitrogen nutrient management involving the application of ORs of different qualities in maize cropping systems in SSA. Determination OR with inorganic nitrogen fertilizer that minimizes nitrogen losses due to N2O emissions would increase nitrogen use efficiency and reduce on the costs of fertilizers. Understanding the key drivers responsible for N 2O emissions from different soil and climatic conditions is essential for the development of a good soil structure due to good soil organic matter management. Knowing the best sustainable approach to maize intensification that rely on suitable organic resources for nutrient sources will be particularly important in overcoming hunger and improve on food security of smallholder farmers.
1.4 Objectives
1.4.1 Main Objectives
To determine the relationship between nitrogen losses through nitrous oxide emission and soil nitrogen changes, under different integrated soil fertility management practices in a maize-based cropping system.
1.4.2 Specific objectives
i. To quantify and compare N2O emissions from soils treated with selected ORs and or in combination with inorganic N fertilizer in two different sites.
ii. To determine available soil N in soils amended with selected ORs and inorganic N fertilizer.
iii. To assess the influence of applied OR and inorganic N fertilizer on maize yield and yield scale N2O emissions.
1.5 Hypotheses
i. Application of organic amendments and inorganic N fertilizer has no effect on N2O emissions under maize cropping systems.
ii. Addition of organic amendments and inorganic N fertilizer has no influence on available soil nitrogen.
iii. Maize yield and yield scale N2O emissions are not affected by the organic and inorganic N fertilizer.
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