The role of a fish pond in optimizing nutrient flows in integrated agriculture-aquaculture farming systems

In the Mekong delta, the Vietnamese government promoted integrated agriculture-aquaculture (IAA) farming systems as an example of sustainable agriculture. An important advantage of IAA-farming is the nutrient linkage between the pond and terrestrial components within a farm, which allows to improve resource use efficiency and income while reducing environmental impacts. This study monitored and analyzed water use in and nutrient flows through ponds that are part of an IAA-farming system. The goal was to improve the nutrient management of ponds which in turn lead to improved water and nutrient use efficiency of the whole IAA-farm. The study included three main parts: (1) understanding the context and characteristics of IAA-systems (chapters 2 and 3), (2) analyzing the performance of IAA-systems, suggesting and testing improvements (chapters 4 and 5), and (3) recommending procedures for the continuous upgrading of existing IAA-farming systems (chapter 6). The research was done on-farm in freshwater areas of the Mekong delta and followed a Participatory Learning in Action approach. Different multivariate statistical methods were applied for data analysis. At community and household level, results showed that the type of IAA-farming systems applied was determined by a mixture of bio-physical, technological and socio-economic factors (chapter 2). Three major IAA-systems were identified: (1) low-input fish farming in fruit-dominated area (system 1), (2) medium-input fish farming (system 2), and (3) high-input fish farming (system 3) in rice-dominated areas. System 1 was commonly practiced in a rural and intensive fruit production area with fertile soils, while systems 2 and 3 were more frequent in peri-urban and in rice production areas with less fertile soils. In the study areas, poor farmers usually did not adopt IAA-farming. With good market accessibility, richer farmers tended to intensify fish farming. The principal factors why farmers did not start aquaculture were the inappropriateness of the technology available, lack of capital, insufficient land holding, poor access to extension services, limited farm management, and a fear of conflicts associated with pesticide use on crops. The main motivations to practice IAA-farming were increased farm resource uses, which resulted in improved income, a better supply of foods for home consumption and a reduction of the environmental impacts from the farming. In low- and medium-input ponds, nutrient inputs, the accumulation of nitrogen (N), organic carbon (OC) and phosphorus (P) and environmental impacts were closely linked (chapter 3). Parameters related to nutrients input levels and water exchange rates in ponds explained most of the variability between farms. Parameters linked to agro-ecological sites, pond physical properties and livestock or human excreta inputs explained most of the remaining variability. A combination of these variables allowed to characterize three indicative integrated systems: (1) the low water-exchange-rate ponds in the fruit-dominated area, (2) the low water-exchange-rate ponds in the rice-dominated area receiving home-made feed, and (3) the high water-exchange-rate ponds in the rice-dominated areas receiving excreta. These systems concurred to a large extend with the systems identified on the basic of the community and household survey. In the rice-dominated area with deep ponds, more livestock or human wastes were supplied, and high water exchange rates were practiced. In these ponds, large excreta-OC loads reduced dissolved oxygen and increased total phosphorus concentrations in the water column and nitrogen, organic carbon and phosphorus accumulation in the sediments. In the rice-dominated area with wide ponds, more home-made feed was applied and low water-exchange rates were practiced, which resulted in a high phytoplankton biomass and primary productivity. On the contrary, in the fruit-dominated area fish were grown in shallow and narrow ditches with a low phytoplankton biomass and only a small fraction of the nutrient input accumulated in the sediments. The water and nutrient budgets of a selected number of ponds, representing either low or high water-exchange systems were determined (chapter 4). The sluice-gate water inflow and outflow largely dominated the total pond water budgets, accounting for 72-97% of the total water budget. On-farm livestock manures were the most important nutrient source for ponds. High water-exchange rate ponds received larger quantities of livestock and/or human excreta and had significantly higher volumes of water passing through ponds than low water-exchange rate ones. Only 5-6% of the total N, OC and P inputs were retained in the harvested fish, but 18-91% accumulated in the pond sediments, the rest was lost through pond water discharges. Fish yields and the quantity of nutrients accumulating in the sediments increased with increasing on-farm nutrient input levels at the cost of higher nutrient discharges. Its was concluded that farmers need to manage water and nutrient flows between the pond and the other IAA-farm components with the goals to maximize productivity and profitability while minimizing nutrient discharges of the farm as a whole. Excreta were the principle type of nutrient input applied to ponds in the study areas. Therefore, the economic and nutrient discharge tradeoffs stemming from the use of livestock and human excreta were analyzed (chapter 5). Data collected during three consecutive production years were combined in the analysis. Results showed that increased excreta input levels resulted in lower dissolved oxygen concentrations, higher water exchange rates practiced, and increased discharge of chemical oxygen demand (COD), N, P and total suspended solids (TSS). Fish yields and the accumulation of N, OC and P in pond sediments, however, increased with increasing excreta input levels. Through regression analysis, it was predicted that with an input of 5 kg N ha-1 day-1, a fish yield of 8379 kg and an economic return of 52 million VND ha-1 yr-1 will be obtained while about 2057 kg COD, 645 kg N, 213 kg P and 39203 kg TSS ha-1 yr-1 will be discharged from the farm. At this input level, about 9% of input-N will be retained in harvested fish, 52% will accumulate in the sediments and 39% will be discharged. Further development of IAA-farming practices should focus on reducing nutrient discharges while maintaining favorable economic returns. In brief, this study demonstrated that the adoption of one type of IAA-system by farmers is determined by a mixture of factors at different scales ranging from the individual pond to community or village level. Within each IAA-system, the pond fulfils multiple roles, in part influenced by the existing resource base, agricultural development pathways and the household's goals and aspirations. An important function of ponds is the trapping and storage of nutrients for subsequent reuse within IAA-systems, which otherwise would be lost. Optimizing nutrient storage in ponds also concurs with best management practices from an environmental and economic point of view. The key challenge to the further development and optimization if IAA-farming is to balance economic, environmental and social interests within a highly dynamic setting of the Mekong delta today.