Combining the N Gas Flux Method and NO Isotopocule Data for the Determination of Soil Microbial NO Sources.

RATIONALE

The analysis of natural abundance isotopes in biogenic NO molecules provides valuable insights into the nature of their precursors and their role in biogeochemical cycles. However, current methodologies (for example, the isotopocule map approach) face limitations, as they only enable the estimation of combined contributions from multiple processes at once rather than discriminating individual sources. This study aimed to overcome this challenge by developing a novel methodology for the partitioning of NO sources in soil, combining natural abundance isotopes and the use of a N tracer (N Gas Flux method) in parallel incubations.

METHODS

Laboratory incubations of an agricultural soil were conducted to optimize denitrification conditions through increased moisture and nitrate amendments, using nitrate that was either N-labeled or unlabeled. A new linear system combined with Monte Carlo simulation was developed to determine NO source contributions, and the subsequent results were compared with FRAME, a Bayesian statistical model for stable isotope analysis.

RESULTS

Our new methodology identified bacterial denitrification as the dominant process (87.6%), followed by fungal denitrification (9.4%), nitrification (1.5%), and nitrifier denitrification (1.6%). Comparisons with FRAME showed good agreement, although FRAME estimated slightly lower bacterial denitrification (80%) and higher nitrifier-denitrification (9%) contributions.

CONCLUSIONS

This approach provides an improved framework for accurately partitioning NO sources, enhancing understanding of nitrogen cycling in agroecosystems, and supporting broader environmental applications.