Temperature sensitivity of nitrate removal in woodchip bioreactors increases with woodchip age and following drying–rewetting cycles

Abstract

Woodchip bioreactors are a beneficial management practice with increasing use for the sustainable reduction of nitrate in waters discharged from agriculture and urban landscapes. Previous research has shown an interaction between temperature and carbon quality with respect to microbial respiration, which may affect performance of woodchip bioreactors. This study used two previously published data sets of woodchip bioreactors in Spain and the United States that were exposed to weekly drying–rewetting cycles, to better understand the processes driving changes in temperature sensitivity of nitrate removal. The factor by which nitrate removal increased given a 10 °C increase in temperature (Q₁₀) was used as a metric for temperature sensitivity. Values of Q₁₀ for nitrate removal in both experiments ranged from 1.8–3.1 and generally increased over time as woodchips aged. In field bioreactors, mean nitrate removal rate at temperatures 10–15 °C and 22–27 °C decreased by 36% and 7%, respectively, from the first to second year. Values of Q₁₀ increased with amount of time since resaturation of the woodchips following a drying–rewetting cycle. Dynamic calculations of Q₁₀ showed changes in Q₁₀ were not unidirectional. Subsetting the datasets showed that Q₁₀ was temperature-dependent and varied according to minimum temperature value and total range in temperature. Results suggest temperature sensitivity of nitrate removal was related to short and long-term changes in carbon quality or availability, consistent with the carbon-quality-temperature hypothesis. When sizing woodchip bioreactors, water quality managers should consider that long-term declines in efficiency will be greatest at lower temperatures.