An optode sensor array for long-term in situ oxygen measurements in soil and sediment

Long-term measurements of molecular oxygen (O) dynamics in wetlands are highly relevant for understanding the effects of water level changes on net greenhouse gas budgets in these ecosystems. However, such measurements have been limited due to a lack of suitable measuring equipment. We constructed an O optode sensor array for long-term in situ measurements in soil and sediment. The new device consists of a 1.3-m-long, cylindrical, spear-shaped rod equipped with 10 sensor spots along the shaft. Each spot contains a thermocouple fixed with a robust fiberoptic O optode made by immobilizing a layer of Pt(II) meso-tetra(pentafluorophenyl)porphine in polystyrene at the end of a 2-mm polymethyl methacrylate plastic fiber. Temperature and O optode readings are collected continuously by a data logger and a multichannel fiberoptic O meter. The construction and measuring characteristics of the sensor array system are presented along with a novel approach for temperature compensation of O optodes. During in situ application over several months in a peat bog, we used the new device to document pronounced variations in O distribution after marked shifts in water level. The measurements showed anoxic conditions below the water level but also diel variations in O concentrations in the upper layer presumably due to rhizospheric oxidation by the main vegetation The new field instrument thus enables new and more detailed insights to the in situ O dynamics in wetlands.