The data consists of, standing aboveground biomass, and belowground biomass measurements, from sites in the Conwy catchment. Standing aboveground biomass was measured at 7 sites and belowground biomass measurements were made at 8 sites. Data were collected in 2013 and 2014. The sites were chosen to represent habitat types and the terrestrial productivity gradient in Britain from intensive agriculturally managed lowland grasslands through to montane heath. Standing aboveground biomass (grams of dry mass per metre square) in habitats dominated by herbaceous biomass was measured on 1x1 metre quadrats in four plots per site. Standing aboveground biomass in woodlands was measured in two 200 square metre areas by means of stem cores and litter collections. Belowground total root biomass (grams of dry mass per square metre) was assessed for the topsoil 0-15 centimetres in three plots per site. Measurements were undertaken by trained members of staff from Bangor University and the Centre for Ecology & Hydrology. This data was collected for the NERC project 'The Multi-Scale Response of Water quality, Biodiversity and Carbon Sequestration to Coupled Macronutrient Cycling from Source to Sea' (NE/J011991/1). The project is also referred to as Turf2Surf. Full details about this dataset can be found at https://doi.org/10.5285/46bb0117-ed5d-4167-a375-d84d1237cf21