Optimizing the Carbonic Anhydrase temperature response and stomatal conductance of carbonyl sulfide leaf uptake in the simple biosphere model (SiB4)
Abstract. Carbonyl Sulfide (COS) is a useful tracer to estimate Gross Primary Production (GPP) because it shares part of the uptake pathway with CO2. COS is taken up in plants through hydrolysis, catalyzed by the enzyme carbonic anhydrase (CA), but is not released. The Simple Biosphere model version 4 (SiB4) simulates COS leaf uptake using a conductance approach. SiB4 applies the temperature response of the RuBisCo enzyme (used for photosynthesis) to simulate the COS leaf uptake, but the CA enzyme might respond differently. We introduce a new temperature response function for CA in SiB4, based on enzyme kinetics with an optimum temperature. Moreover, we determine Ball-Berry model parameters for stomatal conductance (gs) using observation-based estimates of COS flux, GPP, and gs along with meteorological measurements in an evergreen needleleaf forest (ENF) and deciduous broadleaf forest (DBF). We find that CA has optimum temperatures of 22 °C (ENF) and 38 °C (DBF) with CA’s activation energy as 40 kJ mol-1, which is lower than that of RuBisCo (45 °C), suggesting that air temperature changes can critically affect CA’s catalyzation activity. Optimized values for the Ball-Berry offset parameter b0 (ENF: 0.013, DBF: 0.007 mol m-2 s-1) are higher (lower) than the original value (0.010 mol m-2 s-1) in the ENF (DBF), and optimized values for the Ball-Berry slope parameter b1 (ENF: 16.36, DBF: 11.43) are higher than the original value (9.0) at both sites. We apply the optimized gCA and gs parameters in SiB4 site simulations, thereby improving the timing and peak of COS assimilation. In addition, we show that SiB4 underestimates the leaf humidity stress under conditions where high VPD should limit gs in the afternoon, thereby overestimating gs. Furthermore, we simulate global COS biosphere fluxes, which show smaller COS uptake in the tropics and larger COS uptake at higher latitudes, corresponding with the updates made to the CA temperature response. This SiB4 update helps resolve gaps in the COS budget identified in earlier studies. Using our optimization and additional observations of COS uptake over various climate and plant types, we expect further improvements in global COS biosphere flux estimates.
Ara Cho et al.
Ara Cho et al.
Ara Cho et al.
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Initial work on the leaf COS uptake was based on the notion that the carbonic anhydrase (CA) conductance (gca) would be relatively large (or the corresponding resistance low) since CA is highly efficient in catalyzing COS. As a consequence, it was assumed that the leaf COS uptake would be mainly limited by stomatal conductance (gs), opening interesting avenues for using the leaf COS uptake as a proxy for transpiration and photosynthesis. By now more and more experimental data are surfacing which suggest that gca may be of similar magnitude as gs or even be the rate-limiting step for leaf COS uptake. There is thus an urgent need to better understand gca, both in terms of inter-specific differences and what these relate to, as well as with regard to the short-term drivers, and this information needs to be included in models which simulate the leaf COS uptake.
The manuscript by Cho et al. makes an important and timely contribution to this field by suggesting a peaked as opposed to the previous purely exponential temperature response of gca in the model SiB4. The updated model is able to reproduce the temperature response of the canopy-scale COS at two different forest study sites and in a global application the COS uptake is increased in higher latitudes and decreased in the tropics. In addition, the authors constrain the parameters of the stomatal conductance model inside SiB4 by means of the COS flux measurements.
Overall, most of my comments are minor, but there are many of these, aimed at improving the clarity of the writing, as summarized below.
The one, possibly, major, comment relates to the fact that the authors optimized parameters affecting the supply side of photosynthesis, i.e. the b1 stomatal parameter, against experimentally derived GPP, but not the demand side, e.g. Vcmax. I presume that all parameters the authors did not optimize, were left at the default values for the corresponding PFTs. This could mean that by optimizing the b1 parameter, the authors might have mapped differences between the (unknown) true and pre-scribed Vcmax into the b1 parameter. Furthermore, since gca is scaled to Vcmax, this might have further consequences for the estimated alpha parameter and possibly even the temperature reponse parameters of gca. I would like the authors to state why they did not choose to optimize some parameter representing the demand side of photosynthesis and discuss what the implications of not doing so might be. Ideally, they would underpin their arguments with some evidence which convincingly shows that any bias in Vcmax does not affect the parameters they retrieve and their interpretation.
Finally, I would like to suggest, following Sun et al. (2022, 10.1111/nph.18178), to replace the term gca with gi as conceptually all conductances/resistances other than ga, gb and gs are mapped into gca, notably the mesophyll conductance.