Non-destructive estimates of soil carbonic anhydrase activity and associated soil water oxygen isotope composition

Abstract

Abstract. The contribution of photosynthesis and soil respiration to net land-atmosphere carbon dioxide (CO 2 ) exchange can be estimated based on the differential influence of leaves and soils on budgets of the oxygen isotope composition (δ 18 O) of atmospheric CO 2 . To do so, the activity of carbonic anhydrases (CAs), a group of enzymes that catalyse the hydration of CO 2 in soils and plants, needs to be understood. Measurements of soil CA activity typically involve the inversion of models describing the δ 18 O of CO 2 fluxes to solve for the apparent, potentially catalysed, rate of CO 2 hydration. This requires information about the δ 18 O of CO 2 in isotopic equilibrium with soil water, typically obtained from destructive, depth-resolved sampling and extraction of soil water. In doing so, an assumption is made about the soil water pool that CO 2 interacts with, which may bias estimates of CA activity if incorrect. Furthermore, this can represent a significant challenge in data collection given the potential for spatial and temporal variability in the δ 18 O of soil water and limited a priori information with respect to the appropriate sampling resolution and depth. We investigated whether we could circumvent this requirement by inferring the rate of CO 2 hydration and the δ 18 O of soil water from the relationship between the δ 18 O of CO 2 fluxes and the δ 18 O of CO 2 at the soil surface measured at different ambient CO 2 conditions. This approach was tested through laboratory incubations of air-dried soils that were re-wetted with three waters of different δ 18 O. Gas exchange measurements were made on these soils to estimate the rate of hydration and the δ 18 O of soil water, followed by soil water extraction to allow for comparison. Estimated rates of CO 2 hydration were 6.8-14.6 times greater than the theoretical uncatalysed rate of hydration, indicating that CA were active in these soils. Importantly, these estimates were not significantly different among water treatments, suggesting that this represents a robust approach to assay the activity of CA in soil. As expected, estimates of the δ 18 O of the soil water that equilibrates with CO 2 varied in response to alteration to the δ 18 O of soil water. However, these estimates were consistently more negative than the composition of the soil water extracted by cryogenic vacuum distillation at the end of the gas measurements with differences of up to −3.94 ‰ VSMOW-SLAP. These offsets suggest that, at least at lower water contents, CO 2 -H 2 O isotope equilibration primarily occurs with water pools that are bound to particle surfaces and are depleted in 18 O compared to bulk soil water.

Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest

“…It is thus plausible that in roots, containing a higher fraction of parenchyma cells than stems (Morris et al, 2016), a relatively higher volume of water moving through the symplast could cause a strong depletion of bulk wood water, which is the water sampled during cryogenic extraction. Interestingly, ray and axial parenchyma can account for around 31 % of total xylem tissue volume in both F. sylvatica and Q. robur (Morris et al, 2016) whilst storage water in the stem can account for up to 16 % of daily transpiration in F. sylvatica (Köcher et al, 2013) and contribute even more in some subtropical tree species (Oliva Carrasco et al, 2015). Thus future studies are now required to explore the role of symplastic water transport and storage as a potential mechanism leading to the depletion of bulk wood water δ 2 H compared to the actual source water signal.…”

Transport and Use of Bicarbonate in Plants: Current Knowledge and Challenges Ahead

“…Biological activity contributes to HCO 3 − build-up in soil solutions by hydrating CO 2 from the atmosphere and from the respiratory activity of plant roots, microorganisms, and soil fauna. The CO 2 hydration process catalyzed by soil carbonic anhydrase activity provided mostly by soil cyanobacteria and microalgae [ 6 ] can be considerably higher than the un-catalyzed process [ 7 ].…”

The role of soil pH on soil carbonic anhydrase activity

“…However, in-between the different soils, it seemed that those with the highest CA activity (Planguenoual, Folleville) also had the smallest offset (Table S2). Also for LeBray soil, Jones et al (2017) showed that the offset between δ sw and δ sw-eq decreased when the soil was approaching saturation. The exact reason for this offset between δ sw and δ sw,eq is still unknown.…”