N2O source partitioning in soils using (15)N site preference values corrected for the N2O reduction effect

Area of Science:

• Environmental Science
• Geochemistry
• Soil Science

Background:

• Nitrous oxide (N2O) is a potent greenhouse gas.
• Soil denitrification significantly influences N2O isotope ratios.
• Accurate N2O source partitioning is crucial for climate change mitigation.

Purpose of the Study:

• To assess the impact of isotope fractionation during N2O reduction on N2O site preference (SP) values.
• To quantify the bias introduced in SP-based N2O source partitioning.
• To compare different models for correcting N2O reduction effects on SP values.

Main Methods:

• Analyzed 431 N2O SP values from six soil incubation studies using isotope ratio mass spectrometry (IRMS).
• Measured N2 and N2O concentrations via gas chromatography.
• Applied closed-system, open-system, and dynamic apparent net isotope effect (NIE) models to compensate for isotope fractionation during N2O reduction.

Main Results:

• Recalculated average SP0 values varied by model: -2.6‰ (closed-system), 2.9‰ (open-system), and 1.7‰ (dynamic apparent NIE).
• N2O source contributions from nitrification/fungal denitrification differed: 18.7% (closed-system) vs. 31.0% (open-system) and 28.3% (dynamic apparent NIE).

Conclusions:

• A closed-system model may overestimate N2O reduction effects on SP values, particularly at high reduction rates.
• Soil heterogeneity necessitates accounting for variable reduction rates in soil micropores.
• A dynamic apparent NIE function provides a more accurate compensation for N2O reduction effects, improving source partitioning.