The steep moisture gradient induced by biochar amendment in the capillary zone significantly reduced N₂O emissions in constructed wetlands

Nan Wang ¹, Xin Wang ², Zhiqiang Shen ³

⁰College of Environmental Science and Engineering, Guilin University of Technology, Guilin, 541004, China.

Environmental Research +

|

November 22, 2025

View abstract on PubMed

Summary

Adding biochar to constructed wetlands (CWs) improves total nitrogen (TN) removal and reduces nitrous oxide (N2O) emissions. Biochar in the capillary zone is especially effective, creating conditions that promote nitrogen removal and minimize N2O release.

Area Of Science

Environmental Engineering
Water Treatment Technologies
Biogeochemistry

Background

Constructed wetlands (CWs) struggle to simultaneously achieve high total nitrogen (TN) removal and low nitrous oxide (N2O) emissions.
Biochar, a high water holding capacity material, is explored for its potential to mitigate these challenges in CWs.

Purpose Of The Study

To investigate the impact of biochar incorporation into different zones of CWs on nitrogen transformation and N2O emissions.
To determine the optimal biochar configuration for synchronous TN removal and N2O emission reduction.

Main Methods

Biochar was added to the capillary zone and surface layer of CWs.
Nitrogen removal efficiency and N2O emissions were monitored and compared to control CWs.
Changes in moisture gradients, redox reactions, microbial communities (specifically N2O-reducing bacteria), and cytochrome c content were analyzed.

Main Results

Biochar in the capillary zone significantly enhanced TN removal by 28.1% and reduced N2O emissions by 92.6% relative to TN removed.
Biochar created a moisture gradient, balancing redox reactions and suppressing N2O generation.
Biochar boosted electron transfer for N2O-reducing bacteria, increasing cytochrome c by 102.0% and enriching specific bacterial genera (Rhodanobacter, Comamonas, Flavobacterium, Flavihumibacter, Simplicispira) carrying nosZ, forming an N2O emission barrier.
Biochar in the surface layer improved nitrification but disrupted redox balance.

Conclusions

Biochar application in the capillary zone of CWs is a promising strategy for achieving synchronous high TN removal and low N2O emissions.
The mechanism involves promoting a favorable moisture gradient, balancing redox conditions, and enhancing N2O reduction by specific microbial communities.
This approach offers a theoretical basis for optimizing biochar use in CWs for improved wastewater treatment and reduced greenhouse gas emissions.

Keywords:

Biochar Capillary zone Constructed wetland Moisture N(2)O

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