Microbial biomass stoichiometry and proportion of Fe organic complexes separately shape the heterogeneity of mixotrophic denitrification and net N2O sinks in iron-carbon amended ecological ditch

Bi-Ni Jiang; Ying-Ying Zhang; Yan Wang; Hai-Qin Liu; Zhi-Yong Zhang; Yi-Jing Yang; Hai-Liang Song

doi:10.1016/j.watres.2024.122945

Microbial biomass stoichiometry and proportion of Fe organic complexes separately shape the heterogeneity of mixotrophic denitrification and net N₂O sinks in iron-carbon amended ecological ditch

Water Res. 2024 Dec 9:272:122945. doi: 10.1016/j.watres.2024.122945. Online ahead of print.

Authors

Bi-Ni Jiang¹, Ying-Ying Zhang², Yan Wang², Hai-Qin Liu², Zhi-Yong Zhang³, Yi-Jing Yang¹, Hai-Liang Song⁴

Affiliations

¹ Institute of Agricultural Resources and Environment, Jiangsu Academy of Agricultural Sciences, Key Laboratory of Agro-Environment in Downstream of Yangtze Plain, Ministry of Agriculture and Rural Affairs, Nanjing, 210014, , PR China; School of Environment, Nanjing Normal University, Jiangsu Province Engineering Research Center of Environmental Risk Prevention and Emergency Response Technology, Jiangsu Engineering Lab of Water and Soil Eco-remediation, Wenyuan Road 1, Nanjing 210023, PR China.
² Institute of Agricultural Resources and Environment, Jiangsu Academy of Agricultural Sciences, Key Laboratory of Agro-Environment in Downstream of Yangtze Plain, Ministry of Agriculture and Rural Affairs, Nanjing, 210014, , PR China.
³ Institute of Agricultural Resources and Environment, Jiangsu Academy of Agricultural Sciences, Key Laboratory of Agro-Environment in Downstream of Yangtze Plain, Ministry of Agriculture and Rural Affairs, Nanjing, 210014, , PR China. Electronic address: 20070015@jaas.ac.cn.
⁴ School of Environment, Nanjing Normal University, Jiangsu Province Engineering Research Center of Environmental Risk Prevention and Emergency Response Technology, Jiangsu Engineering Lab of Water and Soil Eco-remediation, Wenyuan Road 1, Nanjing 210023, PR China. Electronic address: hlsong@njnu.edu.cn.

PMID: 39674145
DOI: 10.1016/j.watres.2024.122945

Abstract

Coupling of iron-carbon can form a mixotrophic denitrification and is regarded as a promising solution for purifying nitrate-rich agricultural runoff. However, its prevalence and efficacy of the synergistic augmentation of nitrogen elimination and net N₂O sinks remain crucial knowledge gaps in ecological ditches (eco-ditches). Here, we investigated the underlying variability mechanisms by implementing sponge iron (sFe)-coupled Iris hexagonus (IH)- or Myriophyllum aquaticum (MA)-derived biochar produced via microwave-assisted (MW) pyrolysis and conventional pyrolysis. Surprisingly, unamened eco-ditch became net N₂O sink while exhibiting a significant increase in total nitrogen (TN) removal rate of 319 % (P < 0.001) compared to soil ditch. The integration of MW pyrolyzed IH-derived biochar with sFe to amend eco-ditch achieved synchronous enhancement in net N₂O sinks (P < 0.01) and TN removal rate (P < 0.001), whereas the remaining amended eco-ditches that significantly intensified TN removal performance, were N₂O emitters. Such heterogeneity primarily depends on Fe organic complexes (Fep) / the total reactive Fe oxides (Fed) ratio, rather than the prevailing nosZ gene, underscoring that low density metastable reactive iron plays a more important role than biological reactions during the mixotrophic denitrification process. As such, iron oxides are not necessarily a bottleneck for denitrification and contribute to N₂O sinks. Conversely, microbial biomass C:(C + N), together with nirK and nosZ genes, mainly explain the TN removal heterogeneity of sFe-biochar eco-ditch. This study revisits the discrepant resilience of iron-carbon coupling to N abatement and N₂O sink-induced cooling and has significant practical implications for better understanding the cascading effects of mixotrophic denitrification driven by iron-carbon interactions.

Keywords: DOM fraction; Fe oxides; Iron-carbon micro-electrolysis; Mixotrophic denitrification; Net N(2)O sink; Vegetated drainage ditch.