Controls on deep versus near-surface soil CO2 production and soil organic matter turnover (Invited)
Abstract
At the soil profile scale, driving factors of decomposition and C turnover can change independently with depth, making a depth-specific assessment of controlling factors, such as temperature, dry-wet cycles, and vertical soil movement, is essential. In a set of depth explicit experiments and modeling exercises, we have tried to elucidate the effects of these controlling factors within the soil profile. In a first experiment, we combined 13C- and 14C-methodology with fractionation techniques to assess the C turnover of labile versus resistant C across the soil profile. As expected, the turnover of labile C increased drastically with increasing soil depth and had a Q10 of 2.8 in the surface layer. In a second experiment, we measured CO2, O2, temperature, and soil water content in two plots at 15, 30, 50, 70, and 90 cm depths to further elucidate the temperature sensitivity of soil C across the profile. Under non-limiting moisture (θ>20%) the Q10 value for the A horizon was 5.3 decreasing to 1.3 at θ<10%. In the C2 horizon at θ>20%, Q10 was 3.1 indicating that it is less sensitive to temperature variations than topsoil when moisture is non-limiting. In a follow-up third experiment, we conducted a wetting and drying experiment to assess the effect of changing moisture regimes on deep soil C dynamics. We found that soil CO2 production across the soil profile was more sensitive to changes in soil moisture during the drying than wetting phases, especially in the top 30 cm of soil. On the contrary, soil CO2 production across the soil profile was more sensitive to changes in soil temperature during wetting than drying phase. Deep soil CO2 production was significantly more sensitive to changes in temperature during the wetting phase, compared to SOM in topsoil layers. Finally, we developed a depth-explicit SOM model based on carbon isotopes and radionuclides. We found that it was essential to include vertical soil transport and its depth attenuation in order to correctly predict SOM dynamics across soil depths. In conclusion, the dominant underlying mechanisms controlling deep soil C dynamics are different from surface soil C and respond differently to global change factors.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B11J..08S
- Keywords:
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- 0428 BIOGEOSCIENCES Carbon cycling;
- 0466 BIOGEOSCIENCES Modeling;
- 0402 BIOGEOSCIENCES Agricultural systems