Abstract
In karst landscapes, soil CO2 is a key factor in weathering processes and carbon cycling, where its distribution and migration characteristics directly affect fluxes in carbon source–sink dynamics. We measured the CO2 emission and dissolution rates of carbonate tablets in calcareous soil developed from limestone and red soil developed from clastic rock, in karst and non-karst subtropical forests, in Guilin, southwest China between 2015 and 2018, to analyze their CO2 transfer characteristics and source–sink effects. The results showed similar average soil respiration rates between calcareous soil and red soil, with an average CO2 emission flux of 1305 and 1167 t C km−2 a−1, respectively. Carbonate tablet dissolution rates were bidirectional with increasing depth and were greater in red soil than calcareous soil, averaging 13.88 ± 5.42 and 7.20 ± 2.11 mg cm−2 a−1, respectively. CO2 concentration was bidirectional with increasing soil depth, reaching a maximum at the base of the soil–atmosphere interface (50–60 cm), and the bidirectional gradient was more distinctive in red soil. Change in the carbon isotope value of soil CO2 was also bidirectional in calcareous soils, for which the overall average was 0.87‰ heavier in calcareous than red soil. The carbon sink in calcareous soil in karst regions was estimated to be 11.97 times that of red soil in non-karst regions, whereas its role as a carbon source is just 1.12 times that of red soil, thus indicating the key role of karst soil in the reduction of atmospheric CO2.
Highlights
As the largest carbon pool of the terrestrial ecosystem, the amount of total carbon stored in soil (2300 Pg) is about two times and three times higher than the amount stored in the atmosphere (750 Pg) and in living biomass (650 Pg), respectively [1]
There was a single peak in the soil respiration rate in one year of the karst and non-karst soil profiles, and soil respiration rates varied among the three calcareous soil profiles
No difference in the mean soil respiration rate was found between the two soil types, the two soil types, but the variation was greater in calcareous soil
Summary
As the largest carbon pool of the terrestrial ecosystem, the amount of total carbon stored in soil (2300 Pg) is about two times and three times higher than the amount stored in the atmosphere (750 Pg) and in living biomass (650 Pg), respectively [1]. Global soil CO2 emission rates are up to approximately 98 ± 12 Pg a−1 , which is an order of magnitude greater than total annual CO2 emissions from human activities (6.8 Pg a−1 ); small fluctuations in soil CO2 emissions can greatly affect concentrations of atmospheric CO2 [2,3]. CO2 from the weathering of carbonate rocks (karstifiction) can reach 36 million t C a−1 [6], accounting for 48% of China’s forest carbon sink (75 million t C a−1 ) [7] between 1981 and 2000. The karst area of eight provinces in southwest China totals 1.12 million km , of which the bare karst area, depicted as carbonate rock exposed to the atmosphere directly, is 460,000 km2 [5]. The CO2 concentration in the limestone soil profile has a bidirectional gradient, with
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