Invariant soil water potential at zero microbial respiration explained by hydrological discontinuity in dry soils

Manzoni, Stefano; Katul, Gabriel G.

doi:10.1002/2014gl061467

Cited by 80 publications

(62 citation statements)

References 45 publications

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“…We note that when using the th obtained during calibration (0.063) we also obtained a high fit to the validation data (R 2 = 0.98, data not shown), so the recalibration of th led to a noticeable but small improvement. While the value 0.063 for our soil came close to the water potential of -15 MPa found in previous studies 10 (Manzoni and Katul, 2014), this relationship did not hold for the validation soil, where we assumed a higher clay and silt content from its classification. Thus, a prerequisite for applying our model to other soils is finding a relationship between th and soil type that holds in all cases.…”

Section: Moisture Effects and Diffusion Limitationssupporting

confidence: 73%

“…th is the percolation threshold for solute diffusion (Manzoni and Katul, 2014) was here a calibrated parameter. The diffusive flux of enzyme C between the microbial and the decomposition spaces is then calculated as:…”

Section: Diffusive Fluxesmentioning

confidence: 99%

“…(Allison et al, 2010;Li et al, 2014) VU_ref 0.092 h -1 0.35 Reference rate of carbon uptake (Allison et al, 2010;Li et al, 2014) th 0.063 m 3 m -3 0 Moisture threshold for diffusion (Manzoni and Katul, 2014) Non-calibrated parameters Tref 293 °K -Reference temperature -Ea_m 10 kJ 4.6 Activation energy for rmd_ref (Grisi et al, 1998;Salazar-Villegas et al, 2016) Ea_e 10 kJ 2.2 Activation energy for rmr_ref (Grisi et al, 1998;Salazar-Villegas et al, 2016) …”

Section: Competing Interestsmentioning

confidence: 99%

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Diffusion based modelling of temperature and moisture interactive effects on carbon fluxes of mineral soils

Moyano

Vasilyeva

Menichetti

2018

Preprint

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Abstract. While CO2 production in soils strongly responds to changes in temperature and moisture, the magnitude of such responses at different time scales remains difficult to predict. In particular, little is known of the mechanisms leading to interactions in the effects of these drivers on soil CO2 emissions, even though such observations are common. Here we compare a number of modelling approaches to test which underlying mechanisms best simulate the interactive responses measured in soils incubated under combined levels of temperature and moisture. We found that two model components were critical for reproducing the observed interactive patterns: 1. Michaelis-Menten reaction kinetics, which strongly improved the model fit when applied to decomposition reactions, and 2. diffusion of dissolved C and enzymes. The latter replaces conventional empirical functions as a mechanism relating moisture content with C fluxes. Indeed, empirical functions failed to capture the main observed interactions. After model calibration we were able to explain 84&#8201;% of the variation in the data. Model simulations resulted in a decoupling of decomposition and respiration C fluxes in the short and mid-term, with interaction effects and general sensitivities to temperature and moisture being more pronounced for respiration. Sensitivity to different model parameters was highest for those affecting diffusion limitations, followed by activation energies, the Michaelis-Menten constant, and carbon use efficiency. Model validation resulted in a high fit against independent data (R2&#8201;=&#8201;0.99). The same underlying model parameters resulted here in different apparent temperature sensitivities compared to the calibration step, demonstrating a strong effects of initial soil conditions. With these results we could demonstrate the importance of model structure and the central role of diffusion and reaction kinetics for simulating complex soil C dynamics related to temperature and moisture interactions. Future studies should further validate this mechanistic approach and extend its use to a larger range of soils.

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Section: Moisture Effects and Diffusion Limitationssupporting

confidence: 73%

Section: Diffusive Fluxesmentioning

confidence: 99%

Section: Competing Interestsmentioning

confidence: 99%

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Diffusion based modelling of temperature and moisture interactive effects on carbon fluxes of mineral soils

Moyano

Vasilyeva

Menichetti

2018

Preprint

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“…The breaking of water film continuity that occurs at low soil water content leads to a reduction in microbial activity owing to the spatial separation of the microbes and their respiratory substrates (Manzoni and Katul, 2014). In our case soil water discontinuity should not affect OCS supply as gaseous OCS should be equally available in all soil pores.…”

Section: Consumption and Production Ratesmentioning

confidence: 79%

A new mechanistic framework to predict OCS fluxes from soils

Ogée¹,

et al. 2016

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Abstract.Estimates of photosynthetic and respiratory fluxes at large scales are needed to improve our predictions of the current and future global CO 2 cycle. Carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere and has been proposed as a new tracer of photosynthetic gross primary productivity (GPP), as the uptake of OCS from the atmosphere is dominated by the activity of carbonic anhydrase (CA), an enzyme abundant in leaves that also catalyses CO 2 hydration during photosynthesis. However soils also exchange OCS with the atmosphere, which complicates the retrieval of GPP from atmospheric budgets. Indeed soils can take up large amounts of OCS from the atmosphere as soil microorganisms also contain CA, and OCS emissions from soils have been reported in agricultural fields or anoxic soils. To date no mechanistic framework exists to describe this exchange of OCS between soils and the atmosphere, but empirical results, once upscaled to the global scale, indicate that OCS consumption by soils dominates OCS emission and its contribution to the atmospheric budget is large, at about one third of the OCS uptake by vegetation, also with a large uncertainty. Here, we propose a new mechanistic model of the exchange of OCS between soils and the atmosphere that builds on our knowledge of soil CA activity from CO 2 oxygen isotopes. In this model the OCS soil budget is described by a first-order reaction-diffusion-production equation, assuming that the hydrolysis of OCS by CA is total and irreversible. Using this model we are able to explain the observed presence of an optimum temperature for soil OCS uptake and show how this optimum can shift to cooler temperatures in the presence of soil OCS emission. Our model can also explain the observed optimum with soil moisture content previously described in the literature as a result of diffusional constraints on OCS hydrolysis. These diffusional constraints are also responsible for the response of OCS uptake to soil weight and depth observed previously. In order to simulate the exact OCS uptake rates and patterns observed on several soils collected from a range of biomes, different CA activities had to be invoked in each soil type, coherent with expected physiological levels of CA in soil microbes and with CA activities derived from CO 2 isotope exchange measurements, given the differences in affinity of CA for both trace gases. Our model can be used to help upscale laboratory measurements to the plot or the region. Several suggestions are given for future experiments in order to test the model further and allow a better constraint on the large-scale OCS fluxes from both oxic and anoxic soils.

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“…θ th is the percolation threshold for solute diffusion, for which Manzoni and Katul (2014) reported a value of −15 MPa. This value was not optimal in our case, so θ th was also calibrated.…”

Section: Diffusive Fluxesmentioning

confidence: 99%

Diffusion limitations and Michaelis–Menten kinetics as drivers of combined temperature and moisture effects on carbon fluxes of mineral soils

Moyano¹,

Vasilyeva

Menichetti

2018

Biogeosciences

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Abstract. CO 2 production in soils responds strongly to changes in temperature and moisture, but the magnitude of such responses at different timescales remains difficult to predict. Knowledge of the mechanisms leading to the often observed interactions in the effects of these drivers on soil CO 2 emissions is especially limited. Here we test the ability of different soil carbon models to simulate responses measured in soils incubated at a range of moisture levels and cycled through 5, 20, and 35 • C. We applied parameter optimization methods while modifying two structural components of models: (1) the reaction kinetics of decomposition and uptake and (2) the functions relating fluxes to soil moisture. We found that the observed interactive patterns were best simulated by a model using Michaelis-Menten decomposition kinetics combined with diffusion of dissolved carbon (C) and enzymes. In contrast, conventional empirical functions that scale decomposition rates directly were unable to properly simulate the main observed interactions. Our best model was able to explain 87 % of the variation in the data. Model simulations revealed a central role of MichaelisMenten kinetics as a driver of temperature sensitivity variations as well as a decoupling of decomposition and respiration C fluxes in the short and mid-term, with general sensitivities to temperature and moisture being more pronounced for respiration. Sensitivity to different model parameters was highest for those affecting diffusion limitations, followed by activation energies, the Michaelis-Menten constant, and carbon use efficiency. Testing against independent data strongly validated the model (R 2 = 0.99) and highlighted the importance of initial soil C pool conditions. Our results demonstrate the importance of model structure and the central role of diffusion and reaction kinetics for simulating and understanding complex dynamics in soil C.

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Invariant soil water potential at zero microbial respiration explained by hydrological discontinuity in dry soils

Cited by 80 publications

References 45 publications

Diffusion based modelling of temperature and moisture interactive effects on carbon fluxes of mineral soils

Diffusion based modelling of temperature and moisture interactive effects on carbon fluxes of mineral soils

A new mechanistic framework to predict OCS fluxes from soils

Diffusion limitations and Michaelis–Menten kinetics as drivers of combined temperature and moisture effects on carbon fluxes of mineral soils

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