The effects of intensive farming practices on N2O emission, nitrate leaching and biomass production of the Haean catchment in South Korea were tested and analyzed by the LandscapeDNDC model in this thesis. The first study (Chapter 3) focused on adaptation of the LandscapeDNDC model for simulation of spatial difference in N2O emission and nitrate leaching between row with plastic mulch and interrow without mulch with regard to four different N treatments of a radish field. In the second study (Chapter 4), the site application of the LandscapeDNDC was extended to other upland crops (i.e. cabbage, potato, radish and soybean) and temperate deciduous forest sites constituting the main land use types in the Haean catchment. After successful site validation the LandscapeDNDC model was used to upscale crop yields, N2O emissions and nitrate leaching for the main land uses of the Haean catchment. Finally, mitigation options for N2O emission and nitrate leaching without penalizing crop yields were evaluated to potentially guide improvement of farmers` practices in order to reduce N loads to the environment of the Haean catchment.
Study 1 Conclusions: Site scale simulation of radish cultivation under plastic mulch
The first study showed the successful application of the LandscapeDNDC model to the distinctive environmental conditions (e.g. monsoon season) and farming practices (e.g. plastic mulch, excessive N fertilizer use, sand dressing, etc) of the Haean catchment in South Korea. Taking account of different soil environmental conditions between plastic mulch and no-mulch by adjusting maximum air temperature and annual precipitation, the LandscapeDNDC simulation of N2O emission, nitrate leaching, soil temperature and water content and radish biomass in response to 50, 150, 250 and 350 kg N ha-1 treatments agreed well with the measured values. Simulated nitrate leaching rate was revealed as the significant source of N loss from the radish field with the average value of 352 kg N ha-1 yr-1. Due to this high nitrate leaching rate, estimated annual indirect N2O emission was higher than the annual direct N2O emission with the values of 2.6 and 2.4 kg N ha-1 yr-1, respectively. Estimated total N2O emission (sum of direct and indirect N2O emissions) was 5 kg N ha-1 yr-1. Both measured and simulated radish biomass demonstrated the necessity of improving farmers` practices of the Haean catchment due to no significant difference of radish biomass among four N treatments. Radish biomass slightly increased from 0.5 to 1.1 t
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DW ha-1, whereas increasing N fertilization rates by 3 to 7 times. This finding could be implemented to the further study, which is supplemented with more detailed scenarios for mitigating nitrate leaching rate and N2O emission while optimizing the crop yield. It was a challenge to apply the plastic mulch into the LandscapeDNDC model due to its complicated effect on soil biogeochemical processes and the horizontal movement of nutrient and water between plastic mulched-row and no-mulched-interrow was hard to be considered in this study. The further research would be required to be connected with hydrological and thermal models for better prediction of the effects of plastic mulch on soil nutrient cycling and associated N2O emission and nitrate leaching.
Study 2 Conclusions: Regional scale simulation of major upland crop fields and temperate deciduous forest and assessment of mitigation options
Taking into account different environmental conditions and agricultural managements, the LandscapeDNDC was successfully applied to the simulation of N2O emission, nitrate leaching and crop yield from major upland crop fields and deciduous forest of the Haean catchment for two consecutive years 2009 and 2010. With respect to no significant difference of weather condition (air temperature 8.2 and 7.9°C; precipitation: 1527 and 1522 mm) between 2009 and 2010, about 23% reduction in cultivation area led to less N2O emission, nitrate leaching rate crop production in 2010 as compared to 2009.
Estimated total N2O emissions (sum of agricultural and forest N2O emissions) from the Haean catchment were 3.3 and 2.9 t N yr-1 in 2009 and 2010 with 52% of the total N2O emissions originating from upland fields covering only 27% of the total catchment area. The LandscapeDNDC simulation of nitrate leaching was revealed as the most significant pathway of N loss from the Haean catchment. The total nitrate leaching rates from the Haean catchment were 72.0 and 59.5 t N yr-1 in 2009 and 2010, respectively.
Thereby, 99% of the leaching was induced from the fertilized upland fields. These numbers agreed very well with NO3-N loads calculated from discharge and concentration measurements at the catchment outlet with 89.9 and 81.5 t N yr-1 in 2009 and 2010. Overall, estimated total N losses from the Haean catchment were 101.7 and 87.7 t N yr-1 in 2009 and 2010, respectively. This study demonstrates that the use of the IPCC default N2O EFd of 1% would significantly overestimate direct N2O emissions from the Haean catchment characterized by sandy soils and a monsoon climate. However, with nitrate leaching as the main fate of N fertilizer application indirect N2O emissions from the Haean catchment are in the same range of direct N2O emissions and thus, cannot be neglected.
Decrease in N fertilization rates with splitting fertilizer into 2 and 3 applications were tested as mitigation options for reducing N loss from the upland crop cultivation in the Haean catchment. Adopted mitigation
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option showed the significant potential for minimizing nitrate leaching rate and N2O emission with maintaining the current crop yield mainly by reducing fertilization rates by about 35%. Splitting fertilizer application into 3 times (Split3) rather than 2 times (Split2) revealed even better results of decreasing nitrate leaching rate, N2O emission through 2009 and 2010. Application of Split3 could reduce about 47 t N fertilizer and associated nitrate leaching rate and N2O emission by 62% as compared to the application of conventional farming practices. This finding demonstrated the necessity to improve the farming practices of the Haean catchment. Reduced nitrate leaching would significantly decrease mean nitrate concentrations in the Mandae stream at the catchment outflow from 3.5 to about 2 mg l-1, which is much closer to the quality standard of inland water of 1.5 mg l-1. To the best of our knowledge this was the first study of upscaling N2O emission and nitrate leaching including testing of mitigation options in order to reduce N losses from a catchment in South Korea by use of a process-based biogeochemical model.
However, further studies are required to evaluate more detailed mitigation options such as cover crops (e.g. rapeseed and winter wheat which have been already started by a cultivation experiment in Gangwon Province) and reduced tillage both potentially contributing also to increase of soil carbon stocks and soil fertility. Furthermore, adaptation of fertilizer management with fertilization only into the plant holes of rows and adjustment of timing of fertilization depending on actual weather predictions, which can be particularly important under monsoon climate conditions.
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