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Periodic Report Summary 1 - DIESEL (Developing Improved Estimations of Soil CO2 Effluxes at ecosystem Level)

Summary description of the project objectives

Most studies consider soil CO2 effluxes (Fsoil) as directly corresponding to soil respiration. However, in the short term the Fsoil can deviate from instantaneous soil respiration whenever a change occurs in the amount of CO2 stored in soil pore spaces. Gaps in Fsoil measurements usually are filled using the soil temperature as a predictor. The relationship between soil temperature and Fsoil has been defined using exponential functions, most commonly the Arrhenius functions or Q10 models. However, growing evidence suggests that the Fsoil does not always follow the expected Arrhenius or Q10 temperature response; rather, soil CO2 effluxes show a hysteretic response. This hysteretic response has generated a growing call for deeper understanding of the different factors and processes limiting soil carbon metabolism and the soil Fsoil. The main objective of this proposal is to use a combination of field and controlled-environment experiments to identify and quantify the causes and consequences of temperature hysteresis and soil CO2 storage with regard to the Fsoil, which will lead to improved ecosystem models for regional-to-global carbon cycle quantification.

Description of the work performed since the beginning of the project

To achieve this objective, DIESEL project has been working on some tasks during the outgoing period (first two years) to produce research results of high quality and subsequently, publish them in top international conferences and journals:
• Determine the roles that biotic and abiotic drivers have on soil CO2 flux hysteresis. Measurements of diel Fsoil under grasses, mesquites and in inter-canopy soils: we use two measurement methods, 1) above ground, obtaining Fsoil from a multi-chamber monitoring system, and 2) underground, calculating Fsoil using Fick’s law of diffusion, using CO2 probes installed at different depths in conjunction with soil temperature and moisture probes.
• Develop a dynamic diffusivity algorithm based on soil water content conditions and soil properties. We built on a protocol by conducting a release of known volumes of the trace gas sulfahexafluroride (SF6) throughout different rain events.
• Girdle a subset of mesquites to eliminate photosynthate delivery to soils: we are quantifying diel Fsoil in both soils that receive recent photosynthate (primarily sugars) from shrubs and also in soils from which these products are withheld. Autotrophic respiration will be calculated as Fsoil on the control plots minus Fsoil on girdled plots, providing an independent estimate for heterotrophic versus autotrophic contributions to total FSoil.
• Identify the main drivers controlling CO2 storage and/or emission in the soil. Storage of CO2 in soil pore-space can be affected by abiotic processes like changes in soil water content or pressure pumping. The CO2 storage driven by pressure pumping can be due to three factors: 1) the wind, which penetrates into soils and enhances gas exchange, 2) changes in atmospheric pressure, which compress and expand the soil air allowing its exchange and/or 3) changes in the water table.
• Identify the bias in Fsoil that can result from temperature hysteresis and/or CO2 storage. The combination at the hourly scale of Fsoil, soil temperature and SF6 injections has allowed characterizing errors in Fsoil due to a lag effect.
• Glasshouse experiments independently controlling above and belowground abiotic conditions. We are regulating the environmental conditions to manipulate patterns of vertical temperature gradients in the soils under grasses, mesquites and in bare soils to distinguishe the influence of abiotic and biotic drivers on patterns of hysteresis.

Description of the main results achieved so far

• We found that the application of published diffusion models to obtain Fsoil using the gradient method grossly underestimated cumulative Fsoil. An in situ diffusion model obtained by SF6 injection also did not generate accurate estimations in cumulative Fsoil. Instead, we found great improvements by using the gradient method and chamber measurements to determine an empirical soil CO2 transfer coefficient in situ, which produced accurate cumulative Fsoil. More information can be found in Sánchez-Cañete et al. 2016a.
• In natural ecosystems, we found that the wind can cause increases in the CO2 in the shallow soil layer due to CO2 transport from the deep root zone toward the surface or maybe due to CO2 transport from deeper layers (Sánchez-Cañete et al. 2016b). But also, it can cause decreases in the CO2 molar fraction increasing the errors in Fsoil estimations using the gradient method (Sánchez-Cañete et al 2016a).

Expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far).

All these results can help to improve accurate long-term Fsoil measurements. This is important because Fsoil represents the dominant source of terrestrial CO2 emissions and continuous measurements of this important land-atmosphere exchange are only sparsely available. At the ecosystem scale, the measurement of net ecosystem CO2 exchange (NEE) can be partitioned into ecosystem respiration (Reco) and gross ecosystem production (GEP). Despite advances in NEE partitioning, there are large uncertainties of NEE mainly associated with low-turbulence conditions at night, and these uncertainties are transferred to the partitioning of NEE into ecosystem respiration Reco and GEP. Furthermore, Reco consists of a belowground component, Fsoil, and an aboveground component attributed to plant respiration. Fsoil is commonly measured manually, yielding a low sampling frequency, which translates into annual estimates that are highly uncertain (data lacking for >99% of half-hour periods throughout a year, for biweekly sampling). Therefore, comparative studies between Fsoil at high resolution (spatially and temporally) and ecosystem fluxes are very useful to better understand carbon cycle processes and may influence on how we parameterize and construct models.


Sánchez-Cañete, E.P; Scott, R.L; Van Haren, J; Barron-Gafford, G.A. Improving the accuracy of the gradient method for determining soil carbon dioxide efflux. Journal of Geophysical Research: Biogeosciences. 2016a.

Sánchez-Cañete, E.P; Oyonarte, C; Serrano-Ortiz, P; Curiel Yuste, J; Pérez-Priego, O; Domingo, F; Kowalski, A.S. Winds induce CO2 exchange with the atmosphere and vadose zone transport in a karstic ecosystem. Journal of Geophysical Research: Biogeosciences. 2016b.

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Life Sciences
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