This dataset captures one year of seasonal biogeochemical monitoring in the root zones of English oak (Quercus robur) trees affected by Acute Oak Decline (AOD), Chronic Oak Decline (COD), and healthy control trees in Writtle Woodlands, Essex, UK. Measurements include: (1) total soil nutrients at the start and end of the sampling period (2) Monthly bioavailable soil nutrients, moisture, pH, conductivity, temperature in surface soil (0-10 cm). (3) Seasonal bioavailable soil nutrients at multiple depths (0-40 cm), (4) seasonal litterfall mass and composition (leaves, seeds, wood, flowers), (5) litter chemistry (i.e., C, N, pH, and metals) (6) greenhouse gas (GHG) fluxes (CO2, CH4, N2O) from static chambers, and (7) microbial community structure derived from phospholipid fatty acid (PLFA) profiling. Data were collected between October 2022 and August 2023 at three replicated trees per health status category (AOD, COD, healthy). Measurements also include: (8) High frequency nitrate concentrations and soil moisture content obtained using in situ sensors (one sensor per tree per health status) and daily rainfall totals. The data enable assessment of temporal changes in soil chemistry, litter dynamics, microbial communities, and GHG fluxes in relation to tree health. Full details about this dataset can be found at https://doi.org/10.5285/26bbd3ed-adbd-464c-b31a-bbfb7052e9bf