Temperature‐Dependent Effects of Laboratory‐Heated Dissolved Organic Matter on Aquatic Microbiome Composition and Nitrogen Cycling Dynamics

Wildfires generate partially combusted biomass that mobilizes to aquatic ecosystems as dissolved pyrogenic organic matter (dPyOM) in runoff, where it can impact surface water quality. However, the influence of dPyOM on aquatic microbiome composition, functional gene distribution, and biogeochemical processes remains poorly understood, with unknown implications for post‐wildfire water quality perturbations. To address this issue, soil and litter collected from a conifer‐dominated experimental forest were heated at low (250°C) and moderate (450°C) temperatures to simulate wildfire combustion conditions and prepare dissolved organic matter (DOM) leachates. A 42‐day batch reactor incubation was conducted using a native river microbiome inoculum, with unheated DOM, low‐ or moderate temperature dPyOM as primary carbon sources. Microbiome composition was assessed via Illumina MiSeq 16S rRNA amplicon sequencing. Putative biodegraders were identified using Sanger sequencing, and nitrogen cycling gene abundance (nitrification: amoA , amoB , nxrA ; denitrification: narG , nirS , nirK ; nitrogen fixation: nifH ) was quantified by qPCR. By Day 42, moderate temperature substrates supported microbiomes showing the strongest evidence of compositional divergence, which may reflect the distinct chemical composition and lower molecular weight carbon species characteristic of moderate temperature combustion products than low temperature and unheated treatments. Functional gene analysis showed that nitrification gene abundance decreased significantly with increasing heating temperature ( amoA : r s = −0.82; amoB : r s = −0.91), while denitrification was positively correlated ( narG : r s = +0.94), suggesting a temperature‐driven shift toward nitrate‐reducing conditions. Nitrogen fixation gene abundance also decreased with temperature. Findings demonstrate that dPyOM may exert temperature‐dependent influences on aquatic microbiome composition and nitrogen cycling, with implications for post‐wildfire ecosystem management.