The increase in N fertilizer application is the key to solving the problem of food security and global warming in rice paddy soils with high initial N concentrations.
Authors: Lee Hyun Ho , Choi Eun Jung , Ju Ok Jung , Hong Chang Oh , Kang Hojeong 
Published: February 2023

It is essential to reduce methane (CH4) emissions to minimize the contribution of global warming from rice paddies because CH4 contributes approximately 87% to the global warming potential (GWP). An increase in N fertilizer application could reduce CH4 emissions for several reasons: (1) affect the biomass enhancement of crops to store carbon, (2) decrease methanogenesis activity due to increased nitrate respiration, (3) occur nitrate- and nitrite-dependent anaerobic CH4 oxidation. According to the global scale meta-analysis, increasing N application reduces CH4 emissions. However, the reducing effect of nitrogen (N) fertilization on CH4 emissions from rice paddies needs to be confirmed. The changes in CH4 emission induced by N have a reasonably wide margin of error because the research results were not controlled for the direct factors related to CH4 emissions. Hence, although the increased N fertilizer can increase crop productivity and reduce GWP, it is not introduced as an appropriate agricultural practice.&#160;To elucidate the effects of N fertilization on CH4 emissions from rice paddies, field-based manipulation experiments were conducted in three regions where N fertilizer was applied under the control of the other factors. In addition, we aimed to identify why CH4 emissions differ depending on the amount of N fertilizer through the result of the microbial-mediated CH4 cycle.Urea was selected as N fertilizer and application rates were at 0, 90, 135, and 180 kg ha-1 (N0, N1, N2, and N3, respectively). Irrigation, rice species (Oryza sativa cv. Samkwang), cultivation period, and input of P, K, Ca, and Si fertilizer were identically managed in 3 sites in Miryang, Wanju, and Hwaseong for three years. DNA/RNA co-extraction was performed using three region&#8217;s soil samples in the 3rd year. Real-time qPCR was performed to quantify the mcrA and pmoA gene copy numbers to identify the abundances of methanogens and methanotrophs, respectively.Cumulative CH4 emissions increased with increasing N fertilizer in Miryang. In contrast, cumulative CH4 emissions significantly decreased with increasing N fertilizer application rates in Hwaseong and Wanju. Especially, N2 and N3 showed CH4 emissions lower than N0 in Hwaseong. In Wanju, the decrease in the abundance of mcrA and subsequently methanogenesis after N1 may cause a reduction in CH4 emissions, while in Hwaseong, the increase in quantification of pmoA and the subsequent higher CH4 oxidation may induce a decline in CH4 emissions. In Miryang, higher mcrA may promote methanogenesis and higher CH4 emissions with an increasing N application. The different responses in methanogens and methanotrophs with different N fertilizer application rates could be due to the initial N concentration. The correlation analysis between initial N including ammonium and nitrate&#160;and CH4 emissions from the literature showed a negative relationship at a global scale, suggesting the mechanism based on our experimental results is robust in global rice paddies.In conclusion, additional N fertilizer application considering the initial N concentration can not only increase crop yield in rice paddies but also reduce global warming by reducing CH4 emissions.&#160;