C natural abundance: rooting medium matters

• Tree root respiration utilizes a major portion of the primary production in forests and is an important process in the global carbon cycle. Because of the lack of ecologically relevant methods, tree root respiration in situ is much less studied compared with above-ground processes such as photosynthesis and leaf respiration. • This study introduces a new 13 C natural tracer method for measuring tree root respiration in situ . The method partitions tree root respiration from soil respiration in buried root chambers. • Rooting media substantially influenced root respiration rates. Measured in three media, the fine root respiration rates of longleaf pine were 0.78, 0.27 and 0.18 mg CO 2 carbon mg − 1 root nitrogen d − 1 at 25 ° C in the native soil, tallgrass prairie soil, and sand–vermiculite mixture, respectively. Compared with the root excision method, the root respiration rate of longleaf pine measured by the field chamber method was 18% higher when using the native soil as rooting medium, was similar in the prairie soil, but was 42% lower if in the sand–vermiculite medium. • This natural tracer method allows the use of an appropriate rooting medium and is capable of measuring root respiration nondestructively in natural forest conditions.