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Globally, crop and aquaculture systems are highly vulnerable to changing environments, market dynamics, land use, and socio-economic conditions. The increasing vulnerability is a formidable challenge for ensuring food and nutrition security, livelihood improvement, and achieving sustainable development goals, particularly in developing countries like Bangladesh. Therefore, identifying sustainable farming systems for a specific location – and the associated drivers, risks, and constraints – is necessary to maximise land and water resource productivity and alleviate farm business vulnerability. Deltaic environments are often highly productive and densely populated but are especially vulnerable to environmental change. In response to climatic changes, salinisation, scarce fresh-water, extreme weather events, and market dynamics, farmers in southwest coastal Bangladesh have been modifying their farming practices and food systems. However, farming systems adaptation in the region has often relied on opportunistic responses, driven by market demands, and lacking clear policy oversight. This has created agronomic, hydrological, and socio-ecological concerns, threatening smallholder livelihoods and the environment.

The overall objective of this study was to provide a deeper understanding of the dynamics of farming systems intensification and diversification in southwest coastal Bangladesh. To develop appropriate support mechanisms, this research evaluated cropping and aquaculture adaptation in southwest coastal Bangladesh, exploring drivers, risks, constraints, and socioecological impacts of rice, non-rice, and shrimp/prawn aquaculture farming systems. The specific aims were to:

1. Review the status, challenges, and adaptation of the resilient rice production system under changing climate, salinisation, and socio-economic changes in Bangladesh"

2. Evaluate the trajectories of cropping system intensification and diversification in southwest coastal Bangladesh under changing biophysical and socio-economic conditions"

3. Evaluate the economic viability of rice and non-rice cropping systems incorporating risks, adaptation, and variability in southwest coastal Bangladesh" and

4. Evaluate the economic viability of shrimp and prawn-based farming systems under changing environments and socio-ecological dynamics.

To achieve the research objectives, a mixed-methods approach was used, including a literature review, the compilation and analysis of secondary data, and a series of primary data collection activities. Secondary data on temperature and rainfall were collected from the Bangladesh Meteorological Department and Bangladesh Agricultural Research Council. Soil salinity data were collected from the Soil Resources Development Institute. Cropping and aquaculture data were obtained from the Bangladesh Bureau of Statistics, Food and Agricultural Organisation of the United Nations, Fisheries Resources Survey System, Department of Agricultural Extension, and Department of Fisheries. Three villages (Atlia, Jharjhoria, and Dhanibunia) of Dumuria Upazila under the Khulna district were selected for primary data collection. Primary data were collected from November to December 2018 and November to December 2019. Multiple data collection activities, such as a household questionnaire survey (n = 180" 60 cropbased farmers, 60 shrimp-based farmers, and 60 prawn-based farmers), a household case study (n = 27), focus group discussions (n = 54), and key informant interview (n = 21) were applied to collect primary data on the perceptions of climatic changes, salinisation, impacts on farmer's views of risks and adaptation strategies, and the cost-benefits of major cropping and aquaculture systems. Descriptive statistical analyses were carried out to summarise the socio-economic features of farm households, their experience of climate change, risk perceptions, and adaptation responses. Enterprise budgets were developed using in-depth case study data on inputs and labour use patterns, wages, prices of farm inputs and outputs, and productivity to assess the relative profitability of farm enterprises. Using Monte Carlo simulation, risk simulation was conducted based on farmers' perceived seasonal yield and price variability across different seasonal conditions to assess the risk-return trade-off of the farm enterprises.

A review of the status, availability, challenges (climatic, socio-ecological, and market), and adaptations of the rice-based farming system in Bangladesh is presented in Chapter 2. Farmers adopted planned and autonomous adaptation strategies to reduce the risk. However, agricultural production growth was found to slow down, and the resilience of rice production systems is threatened by rising temperatures, anomalous rainfall patterns, extreme weather events, increasing salinisation, unstable prices, new marketing options, and agricultural land-use change. Farmers' current adaptation strategies were inadequate to sustain productivity growth in rice production systems, particularly in the resource-scarce and environmentally fragile southwest coastal ecosystem. Policies related to research and development (e.g. soil and water management, varietal improvement, insect and pest control), infrastructure maintenance and expansion, and market access and engagement are required to achieve sustainable development goals and maintain smallholder livelihoods. Farmers' adoption of autonomous adaptation strategies was noted, highlighting the importance of locally designed interventions. These findings can support policy design and decision-making for a sustainable rice production system in changing environments and markets.

Chapter 3 explores the intensification trajectories of the rice (Oryza sativa), non-rice crops (pulses, oilseeds, vegetables, jute), shrimp (Penaeus monodon) and prawns (Macrobrachium rosenberghii) systems under changing temperature, rainfall, and salinisation. Farmer perceptions of increasing temperature, decreasing rainfall with anomalous patterns, increasing soil salinisation, and increased extreme weather events were consistent with observed data. These environmental changes potentially impact farming operations, irrigation water supplies, and natural enemies, consequently reducing the yield of crops and aquaculture. Over the last thirty years (1990- 2018), the dry season (DS) rice and brackish water shrimp area increased by 17% and 11%, respectively, with the overall cropping intensity increasing from 135% in 1990 to 175% in 2018. Non-rice crop area (e.g. pulses, oilseeds, watermelon) was found to increase gradually. The expansion of fresh-water prawns was slower than shrimp. The area under the rain-fed wet season rice and partially irrigated early wet season rice decreased by 18% and 2%, respectively. The main drivers behind this intensification (DS rice and non-rice crops) were environmental changes, socio-economic factors (e.g. improved market access, increased population), varietal improvements, use of modern agricultural technologies, extension support (e.g. training, farmer field schools), infrastructure development (e.g. embankments, irrigation and drainage facilities) and policy support. This research highlighted the need for targeted research (e.g. developing stress-tolerant varieties, insect and disease control), smart farm management technologies, enhanced extension services, improved value-chain, and continuing irrigation and other infrastructure development for sustainable intensification in climatevulnerable and salinity-affected southwest coastal Bangladesh.

The profitability and riskiness of major rice and non-rice cropping systems (incorporating risks and prices and yield variations) were evaluated for a range of common and emerging cropping options in southwest coastal Bangladesh (Chapter 4). The core production and market risks identified were rainfall anomalies, extreme weather events, seasonal price fluctuations, and limited market access. Risk management strategies included adjusting sowing/transplanting dates, changing crop cultivars, increasing chemical applications, storing irrigation water, and seeking market updates. The dominant systems (rice-rice-fallow, rice-fallow-fallow) had the lowest profitability with the greatest chance of incurring a loss, while systems with vegetables and non-rice crops (jute, watermelon, mustard) had the higher profitability and low chance of income risks. However, expanding the cropping systems with higher profit potential is constrained by: higher input costs, lack of capital, labour-intensive practices, market access, and persistent environmental risks. This research suggested agricultural policies and intervention programs for addressing production risks and facilitating access to market, credit, and extension services.

Chapter 5 evaluated shrimp- and prawn-based farming systems (gher farming system), incorporating production and market risks, adaptation strategies, profitability, the chance of incurring losses, and socio-ecological dynamics. Gher is a shallow (1.0 to 1.5 metre depth) pond-like structure for farming shrimp and prawns. Major risks identified in gher farming were disease outbreaks, lack of tidal water, salinisation, heat extremes, irregular and reduced rainfall, and price fluctuations. The main risk management strategies adopted by farmers were modifying overall production systems, adjusting the timing of operations, maintaining the gher, dike cropping, and water storage. Prawn-based systems were found to be economically more viable than shrimp-based systems, with three times higher net income. Depending on the resource availability, the expansion and intensification of the shrimp-based system in areas with high salinity and prawn-based systems in areas with low to moderate levels of salinity may offer sustainable farming systems in southwest coastal Bangladesh. However, key constraints to be addressed included limited availability of disease-free post-larvae, the low yield potential of black tiger shrimp, and financial barriers, and market volatility.

In summary, cropping and aquaculture adaptation by smallholder farmers in southwest coastal Bangladesh is vulnerable to multiple and inter-related risks. Bio-physical or environmental threats included increased temperature, altered rainfall, hydrological patterns, extreme weather events, and coast-specific issues such as storm surges, sea-level rise, saline inundation, and water erosion. Socio-economic threats in the region consisted of fluctuating prices for produce, lack of financial resources to adapt, low market access, and inadequate or poorly targeted support mechanisms. Farmers were found to intensify their cropping and aquaculture systems autonomously and associated with planned support activities. The expansion of brackish water shrimp was mainly driven by economic returns" however, significant socio-ecological impacts were also reported. In contrast, diversification with non-rice crops, although often profitable, was at a slower rate because of uncertain production and marketing, sensitivity to salinity and labour shortages. Prawn-based systems were found to be profitable with positive socioecological impacts, but expansion was slow due to financial barriers for smallholder farmers. Access to natural capital (e.g. land, water), physical capital (e.g. irrigation infrastructure), financial capital (e.g. credit, subsidy), social capital, and human capital is required to strengthen the adaptive capacity for the sustainable farming system.

The findings and insights from this study have important implications for designing policies and actions for resilient cropping systems in vulnerable delta regions, which could contribute to achieving sustainable development goals and food security. These findings are relevant to farmer groups, government agencies, agricultural and aquaculture research organisations, commercial actors, and development partners. Policy preparedness for resilient farming systems facing environmental and socio-economic changes is essential for sustaining coastal agriculture and livelihoods. As fresh-water availability is a key factor for cropping intensification and diversification in the area, this study has highlighted the importance of supporting infrastructure in surface water management and rainwater harvesting. The environmental impacts of irrigated dry season rice and brackish water shrimp farming point to a necessity to regulate their rapid and unplanned expansion in water scarce and salinity-affected southwest coastal Bangladesh. Cropping systems with vegetables and non-rice crops with higher income and lower price risk offer alternative adaptation pathways" however, expanding these systems is constrained by capital, labour, and market access. This research highlighted on climate-smart agriculture, easy access to information about technical innovations and marketing, and capacity building to develop resilient cropping and aquaculture production systems.

Rhizoctonia solani AG-3PT causes stem canker and black scurf disease of potato which results in economic yield loss of up to 35-50% around the world. Until now, progress in the management of these diseases has been slow. This study looked at the effects of biological control strategies for stem canker and black scurf of potato caused by R. solani AG-3PT using soil organisms alone or in combination with resistant potato varieties or fertilizers or intercropping systems with Brassica plants.

Trichoderma species were studied because they have a known role in minimising the impacts of pathogens and improving plant growth. Eight isolates of Trichoderma obtained from healthy tubers and soil reduced the severity of disease and promoted growth of potato plants in laboratory and glasshouse experiments. T. harzianum strain T5 and T. hamatum strain T8 had the best performance in prevention of disease and enhancing the plant growth and potato yield.

The information on comparative sensitivity of varieties of potato will assist farmers to make informed decisions in terms of black scurf disease management. Six potato varieties utilized in this study showed a range of sensitivity reactions to R. solani AG-3PT, but none of these varieties were completely resistant to the disease. The Sapphire and Royal Blue varieties showed the lowest level of infection of tubers by sclerotia of R. solani AG-3PT at harvest, whereas Sebago and Desiree varieties showed the highest level of sclerotia on tubers.

Laboratory experiments showed that Sebago sprouts contained materials that reduced the pathogen growth and prevented the growth of biocontrol agents compared with other potato varieties. The interaction between two Trichoderma isolates (T5 and T8) and the pathogen in glasshouse experiments with three potato varieties (Sebago, Desiree and Sapphire) showed the same disease reduction and plant growth promotion for all varieties. There was therefore no interaction between plant resistance and biocontrol.

In culture experiments, there were small but significant effects of seven nutrients (N, P, K, Ca, Mg, Fe and Mn) on the interactions between Trichoderma species and the pathogen. One series of pot trials on the effects of N, K and Mn on biocontrol did not detect an interaction between the nutrients and disease control or growth promotion. However, a field trial did show that the effect of T. hamatum T8 on growth promotion of potato in the presence of the pathogen was greater at low levels of NPK fertilizer. As a result, farmers may reduce fertilizer level when the biocontrol agent is applied.

Biofumigation has been used as an alternative method for controlling soilborne plant pathogens. Biocontrol agents T5 and T8 were not pathogenic to broccoli or cabbage and promoted their growth. R. solani was sensitive to compounds from the root tissues of broccoli and cabbage, whereas Trichoderma isolates, especially T8, were tolerant to compounds from root tissues. In a glasshouse experiment either intercropping with cabbage or broccoli, or inoculation with T8, prevented disease symptoms on potatoes. In a field trial, intercropping with cabbage or treatment with T8 controlled the stem canker and black scurf disease caused by R. solani AG-3PT. Isolate T8 also greatly increased the growth rate of the cabbages. The growth of potato plants was reduced by competition with the cabbage, indicating the need for further work on refining the intercropping system.

Overall this study provides insight on the efficacy of Trichoderma isolates for biological control, and the effect of Trichoderma isolates on potato varieties resistance, fertilizers and cropping system to control stem canker and black scurf of potato caused by R. solani AG-3PT.

Agricultural research is vital for sustainable food production. However, due to changing driving factors, researchers and funding bodies have to prioritize research. We conducted horizon scanning to determine how rice systems might change by 2050 and to prioritize research gaps. The study involved 101 rice experts from 31 countries. Experts identified research gaps and prioritized these based on novelty and relevance. The top 25 research gaps were about sustainability, agricultural development, rice crop science (genetics, breeding and crop physiology), and policies. By addressing these research gaps, researchers will tackle the most pressing issues and contribute towards the sustainability of rice systems.