Abstract
In order to gain more detailed knowledge of the CO2 concentration gradient in forest soil profiles and to better understand the factors that control CO2 concentration along forest soil profiles, we examined the soil surface CO2 flux, soil properties and soil profile CO2 concentration in upright (CK), inverted and mixed soil columns with a depth of 60 cm in two subtropical forests in China from May 2008 to December 2009. The results showed that: (1) The SOC (soil organic carbon), TN (total N) and microbial biomass were higher in the deeper layers in the inverted soil column, which was consistent with an increase in CO2 concentration in the deeper soil layer. Furthermore, the biogeochemical properties were homogenous among soil layers in the mixed soil column. (2) CO2 concentration in the soil profile increased with depth in CK while soil column inversion significantly intensified this vertical stratification as the most active layer (surface soil) was now at the bottom. The stratification of CO2 concentration along the soil profile in the mixed soil column was similar to that in CK but it was not intensified after soil was mixed. (3) The soil surface CO2 flux did not significantly change after the soil column was inverted. The surface CO2 flux rate of the mixed soil column was higher compared to that of the inverted soil column but was not significantly different from CK. Our results indicated that the profile soil CO2 production was jointly controlled by soil properties related to CO2 production (e.g., SOC content and soil microbial biomass) and those related to gas diffusion (e.g., soil bulk density and gas molecular weight), but the soil surface CO2 flux was mainly determined by soil surface temperature and may be affected by the intensity of soil disturbance.
Highlights
An increase in atmospheric CO2 concentration is considered to be one of the main causes for global warming [1,2]
The soil CO2 concentration was greater in the deeper soil layer regardless of the way that the soil columns were manipulated in the present study
Rainfall could lead to an increase in soil moisture and a decrease in CO2 diffusivity, which would subsequently result in CO2 accumulation in soil and increase the CO2 concentration in deep soil [16]
Summary
An increase in atmospheric CO2 concentration is considered to be one of the main causes for global warming [1,2]. Forests 2019, 10, 192 important in the global carbon cycle [3,4,5,6,7]. All CO2 produced in the soil would be emitted through soil surface efflux on a long-term basis [8]. The soil profile CO2 concentration was reported to drive and accelerate this surface emission process [9] and this would influence the carbon balance of the forest. Some models estimated the soil CO2 effluxes from the soil CO2 concentration profiles [10,11,12]. We need to gain more detailed knowledge of the soil profile CO2 concentration in order to better assess its contribution to soil surface CO2 emission and global warming
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