How does microclimatic variability link to fine-scale spatial variation in soil respiration? A field campaign in urban green space

Many cities are currently interested in becoming climate-neutral (European Commission 2022), which has increased the need to understand the biogenic carbon cycle in urban areas; a topic still lacking a comprehensive, measurement-based understanding. Soil respiration, i.e. the biogenic carbon flux through soil surface as a result of belowground plant and microbial activity, is a key part of the carbon cycle (Ryan and Law 2005). Its magnitude is principally regulated by soil temperature, soil moisture, soil microbial community, and decomposable substrate availability, as well as the dominant vegetation and its metabolism. Microclimatic variability is likely to induce fine-scale spatial variation in soil respiration through various pathways, as all of these drivers are either part of the naturally varying microclimate (soil moisture, soil temperature), or subjected to it (substrate availability, plant and microbial communities and their metabolism) (Bramer et al. 2018). To study the connection of microclimate and soil respiration especially in urban areas, we performed a field measurement campaign over two consecutive growing seasons (2023-2024) in Kumpula botanic garden in Helsinki, Finland. A network of 46 microclimate stations was established to continuously measure soil moisture, soil temperature, and near-surface air temperature in high temporal resolution and dense spatial coverage across the garden. Manual chamber measurements of soil respiration were conducted bi-monthly in June-August at 33 measurement points divided between six measurement sites within the garden, together with manual measurements of soil moisture and temperature. The soil respiration measurement sites were situated under tree canopies: three sites under conifers and three sites under broadleaved trees. Stand characteristics were inventoried at all sites, and soil organic carbon and nitrogen contents were determined from soil samples. We aimed to quantify: how much of the observed variation in soil respiration was attributed to variation in soil temperature and soil moisture and how large the role of the site-specific soil and stand characteristics was. how much of the observed variation in soil respiration was attributed to variation in soil temperature and soil moisture and how large the role of the site-specific soil and stand characteristics was. Based on this, we estimate whether the microclimatic data collected by the logger network could be utilised for reliable spatial upscaling of the manual soil respiration measurements to cover the entire garden area. Overall, our results shed light on urban soil respiration characteristics and help in establishing a more comprehensive understanding of the biogenic carbon cycle in urban green space.