Floodplain reforestation reduces nitrate loss through soil microbial pathways
Canning, Adam, and Tink, Michelle (2026) Floodplain reforestation reduces nitrate loss through soil microbial pathways. Ecological Solutions and Evidence, 7. ee70266.
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Abstract
1. Reducing nutrient losses from agricultural catchments is critical for improving water quality issues in rivers, estuaries and oceans. Floodplain reforestation has been proposed as a potential nature-based solution to reducing nutrient loss. However, the underlying microbial and nutrient mechanisms of floodplains remain poorly understood.
2. We conducted a field experiment in North Queensland comparing nitrate leaching and soil microbial assemblages (bacteria, fungi, nematodes) between intensively farmed sugarcane and adjacent mature Melaleuca forest restored on floodplain land. Five nitrate application rates (0–100 kg N/ha) were applied to replicate 4 m × 4 m plots, with nitrate leaching assessed using ion-exchange resins and microbial communities characterised by amplicon-based metabarcoding.
3. Nitrate leaching increased linearly with fertiliser load in sugarcane plots but remained low and unresponsive to load in the Melaleuca forest. Differences were attributed to higher soil organic carbon in Melaleuca plots, supporting decomposer-dominated microbial communities that immobilise nitrogen during organic matter breakdown, in contrast to sugarcane soils dominated by ammonia-oxidising bacteria that rapidly convert ammonium to leachable nitrate.
4. Microbial community composition differed significantly by vegetation type across all three taxonomic groups. Melaleuca plots were enriched in bacterial decomposer traits including chitinolysis and cellulolysis, while sugarcane plots were dominated by nitrification-associated taxa, with these functional differences correlated with measured nitrate leaching and organic matter decomposition rates. Fungal communities in sugarcane plots were dominated by disturbance-tolerant Ascomycota, while Melaleuca plots supported more stable Basidiomycota-rich assemblages. Nematode diversity increased with nitrate loading but was lower in Melaleuca soils.
5. Soil microbial communities showed strong concordance in compositional shifts across bacteria, fungi and nematodes, suggesting coordinated food web responses to vegetation type and nitrogen load. Functional trait analysis revealed that microbial traits, rather than diversity alone, may better explain nitrate retention and loss dynamics.
6. Practical implication: Reforesting floodplains with native vegetation such as Melaleuca can significantly reduce nitrate leaching by promoting microbial processes that retain nitrogen. These findings support the use of floodplain restoration as a nature-based solution for improving water quality outcomes. Microbial traits potentially offer practical indicators for monitoring the effectiveness of restoration and indication of ecosystem services.
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