Explicit consideration of microbes in quantification of gross N rates based on N-tracing model

Abstract

When studying the nitrogen cycle in soils, the principle of 15N isotope dilution and enrichment is commonly used to measure gross N transformation rates. The measuring data of both organic and inorganic nitrogen substrates are therefore be analysed by Ntrace model. Although all transformations between organic and mineral N pools are microbial, the measurement of microbial 15N has rarely been done due to technical difficulties and uncertainties. To gain a deeper understanding of the effects of microbes in the mineralization-immobilization turnover, a new version of the Ntrace model was developed. By considering microbial 15N in the tracing model, this research investigates whether explicit consideration of microbial biomass is essential during the quantification of gross N rates and the practicability of our new model. In this study we used previously published data to test the new developed model Nmicr compared to the frame conceptual 15N trace model Nbas. By adding two additional parameters: release of ammonium from the microbes (RNH4) and the release of organic N from microorganisms to the organic nitrogen pool (INmic), a total of eight N transformation rates were included in the Nmicr. To reassure the basic rule of organic nitrogen assimilates before conversion to ammonium in Nbas, we also made a third model Nmit with a direct route from organic N to NH4 +. In conclusion, The Nmicr simulation is generally convincing with well-defined probability density functions (PDFs). Our result showed an inseparable interaction between the organic N pool and the microbial biomass in Nmicr simulation: (1) Strong correlation coefficient (≒0.96) of gross rate within the efflux (MNorg) and influx (INmic) of organic nitrogen pool in the permanent grassland soil samples (data GM). (2) INmic and MNorg were both presenting the largest or the second largest gross rate in both datasets. (3) The calculated gross rate of INmic was bigger than RNH4 in both datasets. This study also suggested the importance of including explicit microbial biomass in the simulation: The result of Nmit simulation indicated a necessary efflux coming from the organic pool to the microbial pool (MNorg) which could lead to a downward trend in microbial 15N. Further research could be carried out with two or more organic N pools. It would also be interesting to include the oxidation of organic N pool to nitrate (ONorg) and the dissimilatory reduction of nitrate to ammonium (DNO3).