Impact of soil collar insertion depth on microbial respiration measurements from tropical peat under an oil palm plantation

Carbon dioxide (CO2) emissions from peat are commonly measured using closed chambers, using a collar insertion depth of 5–20 cm. However, measured emission values at these depths are likely to be a mixture of root (autotrophic) and microbial (heterotrophic) respiration. Using a deeper collar insertion depth of 60 cm may minimise the influence of root respiration. We compared CO2 fluxes measured from six shallow (20 cm insertion depth) and six deep (60 cm insertion depth) collars. The collars were installed permanently in a mature (25-year old) oil palm plantation; 450 cm from the base of individual palm trees. Carbon dioxide fluxes were measured in the field bi-weekly from May to October 2017 using an Infrared CO2 Gas Analyser (IRGA). For each measurement date, CO2 fluxes from the shallow collars were consistently higher than from the deep collars. On a daily basis, the mean CO2 emission rate from the deep collars was 448 ± 25 mg m-2, 29 % lower than mean emissions recorded from the shallow collars (634 ± 30 mg m-2). The significant difference (p = <0.001) in emissions between the different insertion depths implies that even at a 450 cm distance from the tree (approximately mid-way between fully mature palms) root respiration influences CO2 fluxes. This suggests that in order to minimise overestimation of soil CO2 produced via microbial respiration, root trenching or the use of deep insertion collars is important. Emissions from the deep collars may better represent soil microbial respiration than shallow collars.