Estimation of nitrification rates in flooded soils

Area of Science:

• Soil Science
• Environmental Microbiology
• Biogeochemistry

Background:

• Nitrification is a key process in soil nitrogen cycling.
• Accurate measurement of nitrification rates is essential for understanding soil health and nutrient management.
• Flooded soils present unique challenges for nitrification rate estimation.

Purpose of the Study:

• To evaluate the effectiveness of three techniques for estimating nitrification rates in flooded soils.
• To compare methods based on ammonium oxidation inhibition, nitrite oxidation inhibition, and nitrogen-15 (¹⁵N) isotope dilution.
• To identify potential sources of error and limitations in each technique.

Main Methods:

• Short-term soil incubation experiments were conducted.
• Ammonium oxidation was inhibited using various chemical compounds, including emulsified 2-ethynylpyridine.
• Nitrite oxidation inhibition was attempted with compounds like potassium chlorate.
• Nitrogen-15 (¹⁵N) isotope dilution techniques were employed.

Main Results:

• Emulsified 2-ethynylpyridine effectively inhibited ammonium oxidation without significantly impacting nitrogen mineralization or immobilization.
• Inhibitors of nitrite oxidation proved largely ineffective or non-specific, compromising this approach.
• Isotope dilution methods yielded similar nitrification rate estimates to each other.
• Nitrification rates estimated by isotope dilution were lower than those from ammonium oxidation inhibition in two out of three soils.
• Underestimation by isotope dilution occurred when labeled ammonium originated from dissimilatory nitrate reduction.

Conclusions:

• Emulsified 2-ethynylpyridine is a suitable inhibitor for estimating gross nitrification rates in flooded soils.
• Nitrite oxidation inhibition is not a reliable method for nitrification rate estimation in these conditions.
• Nitrogen-15 (¹⁵N) isotope dilution can underestimate nitrification rates in soils with significant dissimilatory nitrate reduction.
• Careful consideration of potential artifacts is necessary when selecting methods for nitrification rate determination.