The Difference Of Carbon Shift And Its Influencing Factors Under Different Land Use Types-A Case Study Of Karst Areas In Nanchuan County,Chongqing

Due to global warming and frequent extreme events, carbon cycle studies have attracted more and more attention. As the worldwide largest carbon reservoir, carbonate plays a very important role in global carbon cycle. Carbonate consumes the CO₂ derived from atmosphere and soil by karstification, which can be considered as a carbon sink for atmospheric CO₂. Affected by vegetation type, soil properties, tilling, HNO3 and H2SO4, the production and migration of soil CO₂ are distinct under different land use types. Therefore, finding out the carbon shift characteristics and its influencing factors under different land use types can both help to accurately estimate the amount of karst carbon sink and provide theoretical basis of improving the land use types to enhance karst carbon sink.In this paper, Baishuwan spring, Lanhuagou spring and Hougou spring were selected as the objects to monitor the hydro-chemistry so as to understanding the karst carbon sink distinction under different land use types. Moreover, by multi factors（like soil physicochemical properties, vegetation type and CO₂ concentration effects）comparative study on spring area and stable carbon isotope tracing carbon sources from different sphere layers, the carbon migration characteristics and its influencing factors under different use land types were analyzed.Hydrochemisty in springs was distinct under different land use types. For these three karst springs, Ca2 + were the dominant ion, while the composition of anions varied greatly. In Baishuwan spring catchment where the hydro-chemistry was rarely influenced by human activities, HCO3- was the dominant anions and the hydrochemisty type is HCO3--Ca. However. In Lanhuagou spring catchment and Hougou spring catchment, where hydro-chemistry parameters were affected by the using of nitrogen fertilizer and the input of industrial solid waste（slag pile）, the hydrochemisty type were HCO3·SO4-Ca. The amount of carbon sink in these three areas can be calculated through hydrochemistry properties. The result showed that the soil CO₂ net consumption was distinct under different land use types. Baishuwan spring had the largest soil CO₂ net consumption, followed by Lanhuagou spring and Hougou spring. In addition, the soil CO₂ net consumption by karstification may change in different seasons in these three springs. The CO₂ net consumption in Baishuwan spring increased from 2.17 mmol·L-1 in dry season to 2.31 mmol·L-1 in rainy season. But CO₂ net consumption in Lanhuagou spring and Hougou spring decreased from 0.58 mmol·L-1 and-0.15 mmol·L-1 in dry season to 0.27 mmol·L-1 and-0.27 mmol·L-1 in rainy season, respectively.Soil CO₂ concentrations of three spring catchments had obvious seasonal variation. Soil CO₂ concentrations during rainy season was significantly higher than it during dry season. The positive correlation between soil CO₂ concentration and soil temperature was found in Lanhuagou catchment and Hougou spring catchment（0.05<P<0.1）. During rainy season, the high soil temperature stimulated the production of soil CO₂, while the CO₂ net consumption did not increase in these catchments due to the high soil CO₂ efflux. According to the comparison, soil CO₂ concentration in Baishuwan spring catchment was higher than it in Lanhuagou catchment and Hougou spring catchment. At 20 cm and 50 cm depth, soil CO₂ concentration at Lanhuagou spring catchment was less than Hougou spring catchment during the dry season, while it was greater than Hougou spring catchment during the rainy season. Bi-directional gradient was found at Baishuwan spring catchment during the whole year, and the soil CO₂ concentration increased with the increasing soil depth at Lanhuagou spring catchment. At Hougou spring catchment, soil CO₂ concentration showed a bi-directional gradient during the dry season, while it increased with the increasing soil depth during the rainy season. The overlying vegetation and soil properties were the main reasons that resulting in the variation characteristic of soil CO₂ concentration in different land use types.In Baishuwan spring catchment, the karstification consumed more soil CO₂ than other spring catchments, and the soil CO₂ net consumption in rainy season was larger than it in dry season which consistent with the change of soil CO₂ concentration. However, in Lanhuagou and Hougou spring catchments, the soil CO₂ net consumption was opposite to the change of soil CO₂ concentration. In addition, the soil CO₂ net consumption in Lanhuagou spring catchment was larger than it in Hougou spring catchment, which was not consistent with the soil CO₂ concentration. It indicated that the soil CO₂ concentration was not the main factor affecting the karstifacation in these spring catchments. And under different land use types, due to the different soil properties and human activities, the direction of carbon shift and the affecting factors were distinct.To analyze the affecting factors of karstification and carbon shift, a simulation experiment was conducted and 6 soil columns were designed. Results showed that the increasing concentration of H2SO4 enhanced the karstification, but the net consumption of CO₂ by karstification didn’t increase. And the net CO₂ consumption by karstification decreased with the increasing H2SO4 concentration when the CO₂ derived from carbonate rock dissolution could not take part in the karstification. Precipitation brought about dilution effect on the hydro-chemistry properties, namely increasing precipitation reduced the ion concentration of the infiltrated water. In addition, the thickness of soil could influence karstification and carbon shift by soil CO₂ concentration and H2SO4 concentration.Monitoring the δ¹³C of soil CO₂ and the δ¹³CDIC of springs, we found that plant residues were transported into the bottom of the soil in the form of DOC during rainy season. It caused the change of δ¹³C of CO₂ at the bottom soil. Thus, whether CO₂ produced by HNO3 or H2SO4 dissolving carbonate rock returned to soil was not sure if the carbon was traced only by the δ¹³C of CO₂. However, the hydro-chemistry properties and δ¹³CDIC values of Lanhuagou spring and Hougou spring indicated that part of CO₂ produced by HNO3 or H2SO4 dissolving carbonate rock was not sequestrated in the spring water in the form of HCO3-. Due to the longer water-rock contact time, the contribution of soil CO₂ on the dissolution of carbonate rock was increased with the precipitation reduction. At the same time, more CO₂ produced by HNO3 or H2SO4 dissolving carbonate rock on carbonate dissolution was input into water in the form of HCO3-. In addition, because of the fact that surface soil was acidic, dehydration and degassing effect induced that the lighter carbon escaped from water and the δ¹³CDIC of spring water became heavier. Therefore, the karst carbon sink effect estimated by δ¹³CDIC of spring water might be inaccurate because it was also affected by other factors.From what have been discussed above we conclude that soil properties, overlying vegetation and human activities can affect carbon shift and the karst carbon cycle is distinct under different land use types. The net soil CO₂ consumption by karstification is mainly controlled by soil CO₂ concentration in forest, while it mainly controlled by precipitation in agricultural land. Furthermore, the CO₂ escaping from water leads to the inaccurate estimation of karst carbon sink effect. Hence, the CO₂ shift direction should be considered in the further studies.