11:30 AM - 11:45 AM
[MIS11-10] To close the nitrogen budgets at a rice paddy
Keywords:Rice paddy, Biological nitrogen fixation, Denitrification, Elevated CO2 levels, Climate change, Paddy soil
Input of chemical nitrogen (N) fertilizer to rice paddies to earn high rice yield can cause N pollution such as water pollution and eutrophication. In addition, the increasing atmospheric CO2 levels and climate change can impact on the N cycling in rice paddies. For example, elevated CO2 levels promote photosynthesis and then crop yield (CO2 fertilization effect), whereas the relative deficiency in N and other nutrients reduces the fertilization effect and the protein composition. Warming can accelerate many biogeochemical N processes. To maintain the sustainability of rice paddies with respect to rice production and agro-ecosystems, accumulating knowledge of the N cycling in rice paddies and its response to environmental change is essential. Grasping N budgets (total input – total output = stock change) is fundamental when aiming to maintain soil fertility as a target of the sustainability. Closing the N budgets of rice paddies is to obtain reliable information of the N flows of all processes in rice paddies.
As shown in the figure, a variety of N processes exist in a rice paddy. The values in the figure were obtained from previous studies those correspond to single-cropping rice paddies in the Kanto region of Japan (unit: kg N ha–1 yr–1 as a flux). For rice paddies, there are several processes lacking in quantitative information of their flows, i.e., biological N fixation (BNF) as input to rice paddies and denitrification and ammonia (NH3) volatilization as output from rice paddies. Quantitative information is particularly limited in BNF and denitrification with flows as dinitrogen (N2). Both are microbial processes active in anaerobic conditions, and therefore submerged rice paddies are ideal environment for these processes. However, it is difficult to measure the atmosphere–rice paddy N2 fluxes where 78% of the atmosphere is consisted of N2. Although techniques using N-15 and membrane inlet mass spectrometry enable precise quantification of N2 flows, they are still not suitable for field measurements. The two processes are often ignored in discussions even in the field of soil science. It is unfortunate to exclude them from the discussion only because of the difficultness in measurement. According to the presenter’s research, the net free-living BNF (excluding symbiotic and associative BNF) of surface soil (1 cm thickness) in submerged rice paddy was estimated at ca. 40 kg N ha–1 in three months in the cropping season. Considering the associative BNF with rice plants, the total rate of 100 kg N ha–1 described in the past literature would be reasonable. Owing to the reduced fertilization in recent years with respect to environmental protection, application rates of chemical fertilizer to rice paddies are ca. 50 kg N ha–1 indicating an increase in relative contribution of BNF. Although few examples are available on denitrification in actual rice paddies, paddy soils have a strong dentification potential according to incubation studies. Measuring NH3 volatilization is possible but limited number of studies have been conducted in Japan. Unelucidated processes remain such as the daytime NH3 emissions in summer not ascribed to fertilization (Hayashi et al., 2017). Effects of elevated CO2 levels and climate change on the N cycling in rice paddies are almost unknown. The CO2 fertilization effect can increase rice yield in a short term, but that might accelerate nutrient uptake by rice plants and then reduce soil fertility in a long term. It may not be serious in Japan where fertilization and irrigation are carried out, whereas soil degradation might occur in the future in areas where organic farming and rain-fed cultivation are conducted. Thus, it is expected to elucidate each of the key N processes in rice paddies and its response to environmental changes, to connect the obtained knowledge, and to cooperatively work on synthetic research to enable model evaluation including the effects of agricultural management practices.
As shown in the figure, a variety of N processes exist in a rice paddy. The values in the figure were obtained from previous studies those correspond to single-cropping rice paddies in the Kanto region of Japan (unit: kg N ha–1 yr–1 as a flux). For rice paddies, there are several processes lacking in quantitative information of their flows, i.e., biological N fixation (BNF) as input to rice paddies and denitrification and ammonia (NH3) volatilization as output from rice paddies. Quantitative information is particularly limited in BNF and denitrification with flows as dinitrogen (N2). Both are microbial processes active in anaerobic conditions, and therefore submerged rice paddies are ideal environment for these processes. However, it is difficult to measure the atmosphere–rice paddy N2 fluxes where 78% of the atmosphere is consisted of N2. Although techniques using N-15 and membrane inlet mass spectrometry enable precise quantification of N2 flows, they are still not suitable for field measurements. The two processes are often ignored in discussions even in the field of soil science. It is unfortunate to exclude them from the discussion only because of the difficultness in measurement. According to the presenter’s research, the net free-living BNF (excluding symbiotic and associative BNF) of surface soil (1 cm thickness) in submerged rice paddy was estimated at ca. 40 kg N ha–1 in three months in the cropping season. Considering the associative BNF with rice plants, the total rate of 100 kg N ha–1 described in the past literature would be reasonable. Owing to the reduced fertilization in recent years with respect to environmental protection, application rates of chemical fertilizer to rice paddies are ca. 50 kg N ha–1 indicating an increase in relative contribution of BNF. Although few examples are available on denitrification in actual rice paddies, paddy soils have a strong dentification potential according to incubation studies. Measuring NH3 volatilization is possible but limited number of studies have been conducted in Japan. Unelucidated processes remain such as the daytime NH3 emissions in summer not ascribed to fertilization (Hayashi et al., 2017). Effects of elevated CO2 levels and climate change on the N cycling in rice paddies are almost unknown. The CO2 fertilization effect can increase rice yield in a short term, but that might accelerate nutrient uptake by rice plants and then reduce soil fertility in a long term. It may not be serious in Japan where fertilization and irrigation are carried out, whereas soil degradation might occur in the future in areas where organic farming and rain-fed cultivation are conducted. Thus, it is expected to elucidate each of the key N processes in rice paddies and its response to environmental changes, to connect the obtained knowledge, and to cooperatively work on synthetic research to enable model evaluation including the effects of agricultural management practices.