Estimation and mitigation assessment of N₂O emission and nitrate leaching in a mountainous catchment in South Korea using the LandscapeDNDC model

Abstract The application of excessive N fertilizer is a common farming practice in Southeast Asia to ensure the optimum crop yield. However, surplus N in soil induced from higher fertilization than crop N demand is highly susceptible to loss as N2O emission and nitrate leaching during heavy rainfall events in the monsoon season. Intensive farming conducted in the Haean catchment of South Korea has received much attention due to its geographical importance as an upstream region of the Soyang River Dam, which is used as the major drinking water for urban residents (including Seoul). Taking into account the combination of monsoon climate, intensive N fertilizer use and sand dressing prior to mulching and seeding of upland fields in the Haean catchment are likely to cause significant N loss and soil erosion, which have a high potential for directly impacting on the dam water quality via the Mandae stream. The plastic mulch as well as high N fertilization is a typical agricultural management practice for upland crop cultivation in the Haean catchment. To consider effects of plastic mulch on the dynamics of soil temperature and water content, based on soil measurements, meteorological input data i.e. air temperature and precipitation were adjusted to allow the biogeochemical LandscapeDNDC model differentiating simulations of plastic mulch (row) conditions. Furthermore, the actual weather data was applied for the simulation of interrow conditions. The main parameters such as MaxTDD, Tlimit, OptYield and WUECMAX for the simulation of plant growth of major upland crops (i.e. potato, radish, soybean and cabbage) and dominant tree species (i.e. Quercus Mongolica) of the Haean catchment were newly implemented into the model. Taking into account mulching effects and different agricultural management practices, the LandscapeDNDC was validated against detailed field measurements of N2O emission, nitrate concentration, soil temperature and water content (5, 15 and 30 cm soil depth) and biomass production from potato, radish, soybean and cabbage fields. Furthermore, the LandscapeDNDC was also tested against field data of N2O emission, soil temperature and water content (10 cm soil depth) from temperate deciduous forest sites located at three different altitudes and thus different exposure to atmospheric N deposition (24 - 51 kg N ha-1). Application of the adjusted meteorological data showed better prediction of soil temperature and water content from rows covering with plastic mulch as compared to application of the actual weather data (e.g. adjusted data: r2 = 0.49; actual data: r2 = 0.18). Developmental stages of major upland crops were successfully captured by the LandscapeDNDC and separately simulated above- and belowground biomass were in good agreement with measured biomass (r2 = 0.81 - 0.98). The peak N2O emissions after N fertilization from potato, radish and cabbage fields were generally underestimated, however, with respect to high uncertainties and low frequency of measurements, temporal dynamics and magnitude of N2O emission (r2 up to 0.45; ME up to 0.21) as well as nitrate concentration (r2 up to 0.89; ME up to 0.43) were well captured by the model. Based on the successful site validation, the LandscapeDNDC was connected to a GIS database holding all spatially explicit information on climate, soil, vegetation and management and used for estimating N2O emission, nitrate leaching and crop production from intensively managed upland fields and temperate deciduous forest of the Haean catchment (61.5 km²). The main objectives of this thesis were to estimate N2O emission, nitrate leaching and crop yield from the Haean catchment taking into account different land use and environmental conditions and to evaluate mitigation options to minimize N losses while maintaining the current crop yield. The LandscapeDNDC simulation of the mean annual direct N2O emissions from upland fields and temperate deciduous forest was 2.03 and 0.50 kg N ha-1 through 2009 - 2010, respectively. Simulated mean nitrate leaching rates from upland fields resulted in much higher annual values of 112.2 and 125.4 kg N ha-1 in 2009 and 2010, respectively. In contrast, simulated mean nitrate leaching rates from temperate deciduous forest were negligible (≤ 0.01 kg N ha-1 yr-1) both in 2009 and 2010. Direct N2O emission factors for upland fields of the Haean catchment were 0.80 and 0.94% in 2009 and 2010, respectively, which is slightly lower than the IPCC default value of 1%. However, due to the high nitrate leaching rate estimated indirect N2O emission from nitrate leaching was substantial and was in the similar range of direct N2O emission from upland fields. Simulated upland crop biomass ranged between 5.5 and 17.8 t DW ha-1 with annual mean values of 10.4 and 9.3 t DW ha-1 in 2009 and 2010, respectively. Estimation of area-weighted total N2O emission (sum of direct and indirect N2O emissions) from the Haean catchment was 3.31 and 2.93 t N yr-1 in 2009 and 2010, respectively. About 52% of the total N2O emission was derived from fertilized upland fields, covering only 27% of the catchment area. The model predicted nitrate leaching as the dominant pathway of N loss from the Haean catchment with annual values of 72.0 and 59.5 t N yr-1 in 2009 and 2010, respectively. Fertilized upland fields were the strongest source of nitrate leaching, which accounted for 99% of simulated total nitrate leaching from the Haean catchment through 2009 - 2010. Mainly due to the decrease in total N fertilization rate in response to the reduction of cultivation area, N2O emission and nitrate leaching were about 14% lower in 2010 as compared to 2009. Adopted mitigation options were based on the maximum reduction of nitrate leaching and N2O emission without penalizing the current crop yield. Generally simulations show that N export to the environment could be reduced by overall lowering of fertilization rates by approximately 34% without impacting on current crop yields. Splitting N fertilizer application into 3 times rather than 2 times showed slightly higher potential for minimizing N loss from upland fields of the Haean catchment. By splitting N fertilizer application into 3 times the total nitrate leaching could be significantly reduced by 68% (32.7 t N yr-1) in 2009. Even a higher reduction rate of 78% was achieved for the year 2010. Reduced nitrate leaching would significantly decrease mean nitrate concentrations in the Mandae stream at the Haean 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. Estimated mitigation of N2O emissions from upland fields was 0.93 and 0.78 t N yr-1, which was about 49 and 52% reduction in N2O emissions as compared to farmers` practices in 2009 and 2010, respectively. Taking into account a 47 ton reduction in N fertilization by the adopted mitigation option, fertilizer-induced N loss (sum of nitrate leaching rate and N2O emission) from upland fields of the Haean catchment was projected to decrease significantly by 73% as compared to the N loss from the farmers` practices through 2009 - 2010. To the best of our knowledge this was the first study of upscaling N2O emission and nitrate leaching including assessment of mitigation options in order to reduce N loss from a catchment in South Korea with a process based biogeochemical model. The most remarkable finding of this thesis was to show the significant potential for decreasing N loss without affecting the current crop yield by the application of adopted mitigation option. However, further studies are still required to evaluate additional 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 accurate weather predictions, which can be particularly important under monsoon climate conditions. The finding of this study could be suggested as guidelines for improving farmers` practices while minimizing the N loss from the entire crop field of the Haean catchment.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Kim, Youngsun

Contributors dc:contributor

• Tenhunen, John

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1928/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1928