Internal respiration of Amazon tree stems greatly exceeds external CO<sub>2</sub> efflux

Abstract

Respiration in tree stems is an important component of forest carbon balance. The rate of CO₂ efflux from the stem has often been assumed to be a measure of stem respiration. However, recent work in temperate forests has demonstrated that stem CO₂ efflux can either overestimate or underestimate respiration rate, because of emission or removal of CO₂ by transport in xylem water. Here we used the ratio between CO₂ efflux and O₂ influx in stems of tropical forest trees to better understand respiration in an ecosystem that plays a key role in the global carbon cycle. This ratio, which we defined here as apparent respiratory quotient (ARQ), is expected to equal 1.0 if carbohydrates are the substrate for respiration, and the net transport of CO₂ in the xylem water is negligible. However, using a stem chamber approach to quantifying ARQ we found values of 0.66 ± 0.18. These low ARQ values indicate that a large portion of respired CO₂ (~35%) is not emitted locally, and is probably transported upward in the stem. ARQ values of 0.21 ± 0.10 were found for the steady-state gas concentration within the tree, sampled by in-stem equilibration probes. These lower values may result from the proximity to the xylem water stream. In contrast, we found ARQ values of 1.00 ± 0.13 for soil respiration. Our results indicate, for the first time, the existence of a~considerable internal flux of CO₂in the stem of tropical trees. If the transported CO₂ is used in the canopy as a substrate for photosynthesis, it could account for several percent of the C fixed by the tree, and perhaps serve as a mechanism that buffers the response of the tree to changing CO₂ levels. Our results also indicate, in agreement with previous work, that the widely used CO₂ efflux approach for determining stem respiration is unreliable. We demonstrate here a field applicable approach for measuring the O₂ uptake rate, which we suggest to be a more appropriate method to estimate stem respiration rates

Nitrogen Budget and Topographic Controls on Nitrous Oxide in a Shale‐Based Watershed

“…Carbon dioxide, on the other hand, is produced in soil by cellular respiration and diffuses to the atmosphere. Thus, when only respiration and diffusion drive the concentration gradients of soil O 2 and CO 2 , a linear relationship is expected in a plot of pCO 2 versus pO 2 from different soil locations (e.g., depths or landscape positions) or sampling times (Angert et al, , ). Some processes cause CO 2 to vary, without affecting O 2 (e.g., CO 2 dissolution in water or metabolism with non‐O 2 electron acceptors, like NO 3 − ), which causes the relationship between O 2 and CO 2 to deviate from the expected linear relationship.…”

Contribution of soil respiration to the global carbon equation

“…Nevertheless, some studies have shown that the ratio of soil surface CO 2 efflux to O 2 influx (a better indicator for biological activities) is around 1.0 (0.9-1.06) in various ecosystems (Seibt et al, 2004;Angert et al, 2012;Ishidoya et al, 2013). Since most soil respiration measurements have been conducted at the soil surface as CO 2 efflux, we do not differentiate soil respiration from soil surface CO 2 efflux and use the two terms loosely and interchangeably in this review.…”

Transporte y reciclaje de CO2 en el interior del árbol: factores que complican la estimación de la respiración leñosa a través de la emisión radial de CO2

“…Angert et al (2012b) estimaron que la liberación de CO 2 equivalía a 2/3 del consumo de oxígeno, debido al reciclaje y transporte axial del CO 2 respirado. Aunque más precisa, esta técnica es también más exigente técnicamente.…”

“…Solo así se podrá seguir proporcionando información que resulte útil en la investigación de la respiración de los órganos leñosos; un tema del que se conoce muy poco, y en el que es necesario profundizar mucho más para llegar a conocer su regulación metabólica, modulación ambiental e implicaciones ecológicas. La cuantificación de la composición isotópica del carbono respirado (Ubierna et al 2009, Kuptz et al 2011, Maunoury-Danger et al 2013, Mildner et al 2014) o del oxígeno consumido (Angert et al 2012b) por el tronco o por sus componentes con tejidos vivos -cambium, floema y albura -, constituye otro paso más hacia la comprensión del proceso de la respiración leñosa.…”