Unravelling drivers of stream microbial-biogeochemical cycling along a land-use gradient: Effects of organic matter quality and chemodiversity on greenhouse gas fluxes

Stream microbial communities play a vital role in ecosystem functioning, contributing to nutrient cycling, organic matter decomposition, and overall ecological health. Despite this, biogeochemical cycling is typically investigated independently to microbial communities, reducing our understanding of the drivers of microbially-mediated biogeochemical reactions, including those which produce and/or consume greenhouse gases. Given the connectivity of stream ecosystems, microbial communities and chemical substrates (e.g. DOM, nutrients) are also susceptible to influences from land-use changes that occur in the wider watershed. While many studies have examined stream microbial community structure and function along a land-use gradient, few have considered their connectivity with nearby riparian zones, nor conducted microbial diversity surveys in conjunction with biogeochemical measurements. Additionally, recent advancements in high-resolution organic matter characterisation have enabled investigation of the importance of organic matter quality and key metabolites in driving ecosystem function. Here, we examined microbial communities, DOM chemodiversity, and nutrient and DOC concentrations in the water column, streambed sediments, and adjacent riparian zone sediments in 16 headwater streams across a land-use gradient (categorised by percent agriculture, residential, industrial, and human development). We performed incubations with paired streambed and riparian sediments to quantify potential greenhouse gas production (carbon dioxide, methane, and nitrous oxide) and assess the relationship between microbial community structure, potential functional capacity, and greenhouse gas fluxes. We subsequently used high-resolution organic matter characterisation techniques (FTICR-MS and LC-MS) to investigate organic matter quality and key metabolites and how these changed with land-use to also affect microbial communities and greenhouse gas emissions. This work underscores the importance of combining microbial and biogeochemical measurements and how organic matter quality drives ecosystem function, especially in highly connected and complex systems that experience human-driven impacts across scales.