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.

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. 

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’s soil samples in the 3^rd 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 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.