Soil extractable organic nitrogen (EON) is widespread in agricultural soils, and its dynamics are closely related to N mineralization, transformation, and leaching.  

Quantifying the origin of EON is the key to evaluating its ecological and environmental function in soil, but the relative proportions of soil EON derived from exogenous N, as well as the related formation mechanisms, remain poorly understood due to the complexity of N transformation in soil, particularly in over-fertilized and intensively cultivated agricultural soil. Fertilizer N might have largely transformed to EON and then migrated to deep soils or surrounding water body. 

Assistant Professor QUAN Zhi from research group of Stable Isotope Ecology, Institute of Applied Ecology (IAE), under the instruction of Prof. FANG Yunting, conducted a 120-day incubation experiment with an agricultural soil to track the dynamics of ¹⁵N tracers in six different soil N pools from ¹⁵N-labeled ammonium and/or ¹⁵N-labeled ryegrass.  

The initial hypothesis was that exogenous N would be largely transformed to soil EON via microbial N uptake and release, thus microbial biomass N is an important source of soil EON pool. 

In contrast to the hypothesis, researchers quantitatively demonstrated that soil EON is a stable pool in agricultural soils and is mainly composed of inert soil-derived ingredients.  

Exogenous inorganic N, organic N or their combination contributed little to the EON pool in the studied soil during a 120-day incubation. Microbial N uptake and release also contributed little to soil EON production.  

The research results were published in a top journal of soil science Soil Biology and Biochemistryas a Short Communication Paper entitled "Formation of extractable organic nitrogen in an agricultural soil: a ¹⁵N labeling study".  

The work was supported by Strategic Priority Research Program, National Key Research and Technology Development Project, National Natural Science Foundation of China and the Liaoning Science and Technology Project Program.

 Fig. 1 Relative contributions of added ammonium-N, ryegrass-N and soil native N to the extractable organic nitrogen (EON) and microbial biomass N (MBN) in soil with ammonium (80 mg N kg-1) and/or ryegrass (160 mg N kg-1) addition. Numbers on bars are contributions of related sources, which are not shown when < 5% (Image by QUAN Zhi).

Fig. 2 Relationships among the transformations of extractable NH4+ and NO3-, extractable organic nitrogen (EON), microbial biomass N (MBN), mineral fixed NH4+ (MFN) and non-microbial organic N (NMON) after 15N-ryegrass and/or 15NH4+ addition. The solid blue lines indicate the known pathways. The dotted red lines indicate the unclear pathways(Image by QUAN Zhi).