Mind your manure: Impacts of biochar and dry-wet cycling on microbial processing of dual-labelled 13C-15N poultry manure compost

Nutrient enrichment, driven by agricultural intensification and urbanisation, is a major contributor to water quality deterioration globally. Excessive nitrogen (N) and phosphorus (P) inputs lead to eutrophication, hypoxic zones, and harmful algal blooms, while also representing economic losses for farmers and environmental challenges due to the reliance on inorganic fertilisers. External Organic Amendments (EOAs), such as compost and manure, offer a sustainable alternative by recycling nutrients, improving soil health, and reducing greenhouse gas emissions. However, the variability in nutrient release from EOAs, particularly N mineralisation, poses challenges for their effective use. Composted and biochar amendments have shown potential to stabilise nutrient release and mitigate environmental losses, but their biogeochemical dynamics remain poorly understood, especially under future climate scenarios involving intensified drying- rewetting (DRW) cycles. This study aims to address critical knowledge gaps by employing dual- labelled (13C-15N) composted and fresh poultry manure to quantify gross N and C mineralisation rates, assess microbial and biochemical mechanisms, and evaluate nutrient leaching potentials under constant moisture and DRW cycles. A combination of laboratory incubations and greenhouse soil column experiments will be conducted to compare the efficiency of composting and biochar amendments in improving fertiliser-use efficiency and reducing nutrient losses. Advanced isotopic tracing techniques will be used to disentangle amendment-derived nutrient dynamics from native soil pools, providing novel insights into the fate of dissolved organic matter and nutrient transport in agricultural soils. The findings will inform sustainable nutrient management practices and contribute to mitigating nutrient pollution under changing climatic conditions.