CO₂ and CH₄ emission from paddy field play a crucial role in the global greenhouse gas emission and terrestrial ecosystem C cycle.  

The emitted C mainly derives from rhizosphere respiration and microbial decomposition of soil organic matter (SOM). Living roots can regulate SOM decomposition via rhizosphere processes, which is defined as rhizosphere priming effect (RPE).  

However, much less attention has been paid to RPE and CH₄ emission in flooded paddy soil. Therefore, quantifying the RPE on CH₄ emission with synergistic effects of rice growth and N fertilizers in rice paddies requires further investigation. 

Researchers in the Institute of Subtropical Agriculture (ISA) of the Chinese Academy of Sciences invented a CO₂ and CH₄ trapping system and employed ¹³C continuous labeling techniques to investigate how N fertilization influence RPE on CO₂ and CH₄ release in a paddy field ecosystem.  

The team found that rice root growth and rhizodeposits affected the direction of the RPE on CO₂, which first decreased below zero (before 40 days), but rose above zero (after 52 days). The RPE on CH₄ was always above zero, and gradually increased with rice growth in relation to the amount of rhizodeposits and the decrease of soil redox potential.  

Furthermore, N fertilization reduced N limitation, decreased the potential of microorganisms to decompose SOM to meet their N needs, and lowered the magnitude of RPE for both CO₂ and CH₄. Overall, N fertilization and rice growth affected the RPE for CO₂and CH₄ by altering microbial activity in paddy soil. 

This study firstly showed that root-microbial interactions stimulated SOM mineralization in rice paddies through rhizosphere priming effects not only on CO₂ but also on CH₄. The RPE however decreased with N fertilization.  

The important implication is that "Optimized N fertilization is necessary to mitigate greenhouse gas emissions from rice field ecosystems by maintaining high C input by roots and so, high C sequestration” said ZHU Zhenke, an assistant researcher in ISA. 

The study titled "Rice rhizodeposits affect organic matter priming in paddy soil: The role of N fertilization and plant growth for enzyme activities, CO₂and CH₄emissions" has been published in Soil Biology and Biochemistry. 

Conceptual diagram of rhizosphere priming effect for CO₂ and CH₄ depending on rice growth an N fertilization. (Image by ISA)