Browsing by Subject "Wassergüte"

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    Nutrient flow in improved upland aquaculture systems in Yen Chau, province Son La (Vietnam)
    (2014) Pucher, Johannes Gregor; Focken, Ulfert
    In South-East Asia, pond aquaculture plays an important role in the integrated agriculture aquaculture systems of small-scale farmers and contributes to their food security and income. In mountainous regions, aquaculture differs from aquaculture that is practiced in the lowland due to differences in climate and availability of feeds, fertilizers and water. In Northern Vietnam, the traditional aquaculture is a polyculture of 5-7 fish species. The macro-herbivorous grass carp (Ctenopharyngodon idella) is stocked as the main species. Common carp (Cyprinus carpio), silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis), mrigal (Cirrhinus mrigala), mud carp (Cirrhinus molitorella) and Nile Tilapia (Oreochromis niloticus) are stocked as secondary species and are often insufficiently nourished by farm by-products. Manure is used by farmers as fertilizer for natural food resources. Ponds are managed as a constant water flow-through system. The inflowing water introduces soil particles eroded from the sloping fields of intensively cultured maize and cassava into the ponds, and cause high turbidity that limits both the primary and secondary production. The fish production of this system is low at about 1.5 ± 0.3 t ha-1 a-1 and is mainly limited by the poor quality of pond inputs, low availability of natural food resources, low oxygen production in the ponds and the occurrence of a species-specific disease that causes high mortality in grass carp. To improve the local fish production of small-scale farmers, changes in the traditional pond management were designed and tested in farmers’ ponds in the uplands of Northern Vietnam. These changes included the reduction of water flow through the ponds to reduce the introduction of eroded particles and reduce the turbidity. Chemical fertilizers were added to increase the productivity of natural food resources and encourage higher primary production. The disease-prone grass carp was replaced as the main species by common carp that command a similarly high price on the local markets. To feed the omnivorous common carp, supplemental pellet feeds based mainly on locally available resources were applied to the ponds. In a pond trial, the traditional and modified pond managements were compared for water quality parameters, availability of natural food resources, fish yields, nutrient utilisation efficiencies and monetary net benefit. In a 15N tracer experiment, the nitrogen dynamics in the natural food web in local ponds were compared under the two types of pond management. Acceptability of the modifications by local farmers was evaluated. In a net cage trial, the suitability of earthworm meal as a replacement for fishmeal in supplemental pellet feeds for common carp was tested. In another net cage trial, the effect of pesticide contaminated grass feeds on the feed intake and health condition of grass carp were tested. When compared with traditional pond management, the modified pond management was found to result in reduced water turbidity, deeper phototrophic zones, higher availability of natural food resources, higher primary production and higher fish yield. In addition, the small plankton benefited from the changes and allowed significantly higher growth rates of filter feeding fish. Common carp and grass carp had higher yields due to the changes. Under both types of pond management, nitrogen compounds were assimilated rapidly into the natural food web and there were high rates of sedimentation and re-mobilization of settled nitrogen from the pond bottom. Generally, the modifications to pond management were associated with increased nutrient utilisation efficiencies and resulted in higher net benefits and more stable pond culture conditions. It was shown that plant material from pesticide-treated fields should only be used cautiously as feeds for grass carp because pesticide residues reduce feed intake and adversely affect fish health. Low cost modifications were well accepted by the farmers. Application of supplemental feeds and chemical fertilisers, which required a continual monetary investment, were less well received. The better-educated farmers are more likely to further invest in aquaculture and might act as local adopters. To reduce the costs of feeds for common carp, earthworm has been shown to be suitable as a replacement for fishmeal in feeds. Vermiculture might therefore be a suitable additional farming activity in combination with the implementation of pond management modifications. Formation of fish farmer cooperatives might further increase the acceptability of innovations. The improvements to pond aquaculture that have been developed here may have a beneficial impact on fish production, food security and income of small-scale farmers in the uplands in South-East Asia if the information is suitably transferred through education programmes that train farmers in technologies that have been specially adapted to conditions in the uplands.
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    Sediment, carbon and nitrogen capture in mountainous irrigated rice systems
    (2016) Slaets, Johanna I. F.; Cadisch, Georg
    Anthropogenic influences have caused landscapes to change worldwide in the last decades, and changes have been particularly intense in montane Southeast Asia. Traditional swiddening cropping systems with low environmental impacts have been largely replaced by forms of permanent upland cultivation, often with maize. The associated soil fertility loss at the plot scale is well documented. In valley bottoms of these areas, paddies have been cultivated for centuries, and are considered some of the most sustainable production systems in the world – in part maintained by the influx of fertile sediments through irrigation. Altered cropping patterns on the slopes therefore also have potential repercussions on rice production, and hence on food security, but the consequences of shifted sediment and nutrient redistribution at the landscape scale are not well understood. In order to assess these effects, methodologies were developed in this thesis that enable low-cost, continuous monitoring of sediment and nutrient transport in irrigated watersheds (Chapter 2), as well as quantification of the uncertainty on constituent loads (Chapter 3). These methods are applied in a case study to determine sediment, organic carbon and nitrogen trap efficiency of paddy rice fields in a mountainous catchment in Vietnam (Chapters 4 and 5). The upland area had an average erosion rate of 7.5 Mg ha-1 a-1. Sediment inputs to the paddy area consisted of 64 Mg ha-1 a-1, of which irrigation water provided 75% and the remainder came from erosion during rainfall events. Erosion contributed one third of the sand inputs, while sediments from irrigation water were predominantly silty, demonstrating the protective effect of the reservoir which buffered the coarse, unfertile material. Almost half of the total sediment inputs were trapped in the rice area. As all of the sand inputs remained in the rice fields, the upland-lowland linkages could entail a long-term change in topsoil fertility and eventually a rice yield loss. Quantification of nutrient re-allocation in Chapter 5 showed that irrigation was even more important as a driver of sediment-associated organic carbon and nitrogen inputs into the rice fields, contributing 90% of carbon and virtually all nitrogen. Direct contributions from erosion to the nutrient status of the paddies were negligible, again underscoring the protective function of the surface reservoir in buffering irrigated areas from unfertile sediment inputs. 88% of the sediment-associated organic carbon and 93% of the nitrogen were captured by the rice fields. Irrigation water additionally brought in dissolved nitrogen, resulting in a total nitrogen input of 1.11 Mg ha 1 a-1. Of this amount, 24% was determined to be in the plant-available forms of ammonium and nitrate, a contribution equivalent to 66% of the recommended nitrogen application via chemical fertilizer. The dependence of paddy soil fertility on agricultural practices in the uplands illustrates the vulnerability of irrigated rice to unsustainable land use in the surrounding landscape. Unfortunately, alternatives for upland land use that are not detrimental to soil quality are hard to come by, due to the economic reality of high maize prices on the world market. Conservation measures and agroforestry systems offer potential, but without some form of payment for environmental services, adoption rates remain low. Finding sustainable solutions is especially urgent as climate change is likely to increase the number of extreme rainfall events and hence intensify the redistribution processes already taking place. In this light, the role of trapping elements in landscapes such as paddy fields and surface reservoirs becomes more important as well. As these features are widely spread throughout tropical landscapes, their role in global sediment and nutrient cycles must be taken into account. The methodologies developed in this thesis, for sediment and nutrient transport monitoring and for uncertainty assessment, can aid in closing the data gap that currently hinders a reliable assessment of the consequences of anthropogenic and climate change, both on food security and on environmental impacts, locally, regionally and globally.