Production and consumption of N₂O during denitrification in subtropical soils of China

Abstract

Purpose

Nitrous oxide (N₂O) production and reduction rates are dependent on the interactions with each other and it is therefore important to evaluate them within the context of simultaneously operating N₂O emission and reduction. The objective of this study was to quantify the simultaneously occurring N₂O emission and reduction across a range of subtropical soils in China, to gain a mechanistic understanding of potential N₂O dynamics under the denitrification condition and their important drivers, and to evaluate the potential role of the subtropical soils as either sources or sinks of N₂O through denitrification.

Materials and methods

Soils (45, from a range of different land uses and soil parent materials) were collected from the subtropical region of Jiangxi Province, China, and tested for their potential capacity for N₂O emission and N₂O reduction to N₂ during denitrification. N₂O emission and reduction were determined in a closed system under N₂ headspace after the soils were treated with 200 mg kg⁻¹ NO ⁻₃ -N and incubation at 30 °C for 28 days. The soil physical and chemical properties, the temporal variations in headspace N₂O concentration, and NO ⁻₃ -N and NH ⁺₄ -N concentrations in the soil slurry were measured.

Results and discussion

Variations in N₂O concentration (N) over incubation time (t) were consistent with an equation in which average R ² = 0.84 ± 0.11 (p < 0.05): $N = A \times \left( {1 - \exp \left( { - {k_1} \times t} \right)} \right) - B \times \exp \left( {{k_2} \times t} \right)$, where A is the total N₂O emission during the incubation, B is a constant, and k ₁ and k ₂ are the N₂O emission constant and reduction constants, respectively. The results of the simulation showed that k ₁ was greater than k ₂. The reduced amount of NO ⁻₃ -N in the first 7 days of incubation and the N₂O emission rate (the percentage of A value relative to the amount of NO ⁻₃ -N reduced during the 28-day incubation, R _n) were able to explain 82.9 % (p < 0.01) of the variation in total N₂O emission (A) during the incubation for the soil samples studied, indicating that the total amount of N₂O emitted was determined predominately by denitrification capacity. Soil organic carbon content and soil nitrogen mineralization are the key factors that determine differences in the amounts of reduced NO ⁻₃ -N among the soil samples. The R _n value decreased with increasing k ₂ (p < 0.01), indicating that soils with higher N₂O reduction capacity under these incubation conditions would emit less N₂O per unit of denitrified NO ⁻₃ -N than the other soils. Results are valuable in the evaluation of net N₂O emissions in the subtropical soils and the global N budget.

Conclusions

In a closed, anaerobic system, variations in N₂O concentration in the headspace over the incubation time were found to be compatible with a nonlinear equation. Soil organic carbon and the amount of NH ⁺₄ -N mineralized from the organic N during the first 7 days of incubation are the key factors that determine differences in the N₂O emission constant (k ₁), the N₂O reduction constant (k ₂), the total N₂O emission during the incubation (A) and the N₂O emission rate (R _n).