Differences and influencing factors for underground water carbon 1 uptake by karsts in Houzhai Basin , southwest China

9 Karst geological carbon sink is an important part of the global carbon sink, so how to get the 10 accurate carbon sink of karst ecosystem has become the core issue of the research. We used flow 11 and carbon ion concentration data from three stations with different environmental background 12 conditions in the Houzhai basin to analyze the differences in carbon uptake between stations and 13 their impact factors. Results show that carbon sink discharge was mainly controlled by the flow of 14 each site. The rapid increase in flow only has a partial dilution effect on ion concentrations, 15 preliminary analysis considered due to the high speed and stability of chemical carbonate 16 weathering. LUCC type has important effects on the bicarbonate ions concentrations, if runoff is 17 stable, the influence of flow variation on ion concentration will be less than the effects of chemical 18 carbonate weathering in different environmental conditions (comparison of Laoheitan and Liugu 19 station results is 150%) on bicarbonate ion concentrations. However, if runoff increases 20 significantly, the impact of runoff variation on bicarbonate ions will be greater than the effects of 21 chemical carbonate weathering by different environmental conditions (comparison results of 22 Laoheitan and Maoshuikeng station). This work provides a reference for the calculation of karst 23 geological carbon sink. 24


Introduction
Global warming from emissions of greenhouse gases has become the core issue of global environmental change.One of the most pressing concerns in the science of global climate change is the effective accounting of the global budget for atmospheric CO 2 (Schindler,1999;Melnikov and Neill,2006;Liu et al.,2010;Kao et al.,2014), since in order to control global warming, it is necessary to control emissions of carbon dioxide through carbon capture and storage (CCS) technology.In addition to developing CCS technology, an understanding of a number of natural ecological and geological processes such as rock weathering, plant growth, and other physical, chemical, and biological processes can also improve CCS (Hoffmann et al.,2013).Carbonate weathering in rock weathering processes is considered to be both an important source and sink of CO 2 (Zeng et al.,2015;Lian et al.,2011;Liu and Zhao,2000;Serrano-Ortiz et al.,2010;James et Solid Earth Discuss., doi:10.5194/se-2016-37, 2016 Manuscript under review for journal Solid Earth Published: 10 March 2016 c Author(s) 2016.CC-BY 3.0 License.modifications (Calmels et al.,2014).Therefore, in this study we used flow and carbon ion concentration data from three observation stations with different environmental background conditions in the same karst groundwater basin in order to analyze the differences in carbon uptake between stations and their impact factors.This work also provides a reference for improving the calculation accuracy of karst geological carbon sink.

Study Area
Houzhai basin is located in Puding county in the middle of Guizhou province (26 ° 13 '-26 ° 15' N, 105 ° 41 '105 ° 43' E).The total area of the basin is 80.65 km2, and the length of the main river is about 12 km (including the ground and underground river) (Figure 1).The southeastern portion of the basin is lower than the northwestern portion.The relative elevation of the basin is about 150 m, and its average altitude is 1250 m.A typical hoodoo depression physiognomy is distributed in the east of the basin where the main land-use type is forest vegetation, while karst is distributed in the west of the basin where the main land-use type is farmland.It has a subtropical humid climate; the average rainfall is 1316.8mm and the average temperature is 15.5 °C .The rainy season occurs from May to October and the dry season from November to April.
Precipitation during the rainy season accounts for more than 80% of annual rainfall.Bedrock in the basin is composed of mainly carbonate rock formed during the Triassic.As a result of lithology and geological structure, karstification is strong and karst formation is widely developed in the basin.Hydrological runoff processes are significantly influenced by karst underground space (gap and pipe) and its distribution characteristics.There is no obvious surface river valley upstream, and although there is a river valley midstream and downstream, seasonal runoff only appears temporarily, and leakage pits are arranged along the riverbed.
Figure 1.The distribution of drainage systems and weather hydrological stations.

Date Sources
The main data are derived from three groundwater hydrology and water quality monitoring In a karst environment, carbon dioxide dissolves in water and undergoes a reversible chemical process (2) with calcium carbonate： Under a steady state, the quantity of carbon dioxide dissolved in karst water is equal to the discharge of CO 2 from the atmosphere.That discharge in g C m −2 time step −1 is calculated according to the following formula (3) (Yan et al.,2011;Amiotte-Suchet and Probst,1993).where c is the concentration of bicarbonate ions(g/m 3 ); q is the production flow (m 3 /time step); M C and M HCO3 are the molecular weights of C and HCO 3 − , respectively, and 1/2 means that 1 mol of bicarbonate needs only half a mole of CO 2 from the soil or atmosphere.Additionally, karst water is generally alkaline.The content of CO 3 2-C in dissolved inorganic C is very small, so we did not need to consider it in the DIC calculation (Gelbrecht et al.,1998;Yan et al.,2011).In this study, we used the formula F 1 below to calculate net carbon uptake by karst, using the estimates of year mean [HCO3 − ], ion concentration during the dry-wet season, and the mean daily where c is either the annual average bicarbonate density or the ion concentration in the dry-wet season (mg/L), and q is the average daily excretion( m 3 /s, n=365 day).

Dry -Wet Seasonal and Inter-annual Variations of Ion Concentration and Discharge
For each site during the study period, the ion concentration in the wet season was slightly smaller than in the dry season.LHT station, which had the longest study period, exhibited the highest and lowest values for bicarbonate ion concentration, which were 240.5mg/L (1994) and 201.7 mg/L (1999) in the rainy season, 259.6 mg/L (2002) and 234.7mg/L (1991) in the dry season, and 248.3 mg/L (1994) and 218.8 mg/L (1999) for the whole year, respectively.Moreover, there was a negative correlation between ion concentration and discharge (Figures 2, 3, and 4).
From 1992-1996, the annual average concentration of bicarbonate ions in the rainy season, dry season, and whole year were 228.8 mg/L, 249.3 mg/L, 239.1 mg/L, respectively for LHT station and 222.0 mg/L, 253.5 mg/L, 237.8 mg/L, respectively at LG station.Although there is little difference in ion concentration between the two stations, when considering the stability of ion concentration changes (Table 1), LG station was more stable than LHT station.During the same period, from 1996-2001, the annual average ion concentrations in the rainy season, dry season, and whole year were 217.8 mg/L, 247.4 mg/L, and 232.6 mg/L, respectively for LHT station, and 209.9 mg/L, 226.4 mg/L, 218.2 mg/L, respectively for MSK station.Table 1 shows that MSK station was more stable than LHT station with respect to the standard deviation of ion concentration variation.Although, the difference in ion concentrations between LHT and LG was smaller than that between LHT and MSK, differences in the site as a whole were small.The discharge from MSK station, which is located at the outlet of the underground river basin, was larger than the discharge from LG andLHT. From 1996-2001, the   From Table 1, we can determine the stability of flow as follows: MSK>LG>LHT.    .Comparing the results for the same period, we found that the annual carbon sink discharge in the rainy season, dry season, and whole year for LHT station were greater than those for MSK and LG stations.However, with respect to the stability of carbon discharge (Table 1), MSK was the most stable in the rainy season while LG was the most stable in the dry season.

Flow and Ion Concentration Change and its Effects on Carbon Sink
According to the flow trend of each station, we can see that the flow in the rainy season is consistent with the flow trend for the whole year, suggesting that the runoff from precipitation in the rainy season accounts for the majority of the annual runoff.This is mainly a result of the monsoon climate where summer (May-September) precipitation levels are significantly higher than in the winter (December-February); however, the flow trend in the dry season was smooth (figure 5) because of less rainfall in the dry season when the runoff was mainly supplied by soil We then contrasted the results of each site.From 1992-1996, the annual average carbon ion concentration was 237.8 mg/L for LG station and 239.1 mg/L for LHT station.The annual average flow of LHT station was 1.37 times that of LG station, but the ion concentration did not decline significantly due to the increase in flow.The basin area controlled by LHT station is characterized by peaks and valleys, which have good vegetation cover that recovers rapidly.Previous studies have shown that the concentration of HCO 3 -is vulnerable to LUCC (land cover and land use change) and other environmental changes (Zhao et al.,2010;Lan et al.,2015).In particular, the fast recovery of vegetation can significantly promote the dissolution of carbonate and thus increase bicarbonate ion concentration in karst groundwater (Liu et al.,2010;Berner,1997).This suggests that when there is little change in flow, the effect of flow increase on ion concentration dilution is smaller than the environmental effects of chemical carbonate weathering.environmental conditions are stable.In addition, although studies have shown that under the conditions of a stable LUCC, the strength of the carbon sink from rock weathering will depend on the climate (e.g.temperature T, precipitation P) (Hagedorn and Cartwright,2009;Gislason et al.,2009;Tipper et al.,2006), the annual average carbon sink trend for LHT station, which had the longest study period (1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002), differed significantly from the annual average temperature trend.
However, this may be a result of time resolution limitations of the monitoring data.

Variation in Carbon Sink Discharge for Each Site
The carbon sink discharge for each site in the rainy season was greater than for that of the dry season and the annual average, while the carbon sink discharge in the dry season was less than the annual average (figure 6).This shows that the karst carbon sink (karstification's absorption of atmospheric CO 2 and soil CO 2 ) changes significantly with the seasons and exhibits striking seasonal patterns.The reason for this is that the considerable summer rainfall runoff significantly increases the amount of carbon sink discharge in the rainy season.The annual average carbon sink discharge of all the stations during study period shows that LHT>MSK>LG; however, comparisons cannot be made due to the different study periods.
During the same period, the carbon sink values for LHT station in the rainy season, dry season, and whole year were greater than both LG and MSK stations.From 1992-1996, the flow for LG in the rainy season and dry season were both significantly less than for LHT (figure 6).The annual average concentration for LG in the rainy season and dry season were 239.1 mg/L and 237.8 mg/L, respectively, and are slightly less than LHT (figure 6).The difference in carbon sink discharge between the two stations results from differences in flow.Furthermore, the LHT control basin station is surrounded mainly by forest vegetation while the LG control basin is surrounded mainly by dry farmland, and the different LUCC types may further increase differences in carbon sink discharge between the two stations.MSK than for LHT.On the one hand, the fact that the carbon sink discharge for MSK was less than LHT might be linked to the water conveying distance and LUCC type of the control area.The carbon sink for MSK, which is the groundwater outlet for the whole basin, was influenced by the landform and LUCC type of the entire river basin (Figure 1).Previous research has shown that karst erosion rates under soil vary significantly for different LUCC types in karst watersheds, and the averages for cultivated land, thickets, secondary forests, grassland, and forest were found to be 4.02, 7.0, 40.0, 20.0 and 63.5 t km 2 a -1 , respectively (Zhang,2011), with the erosion rate of carbonate karst under the cover of cultivated land being the lowest (Yan et al.,2014).Previous research has also shown that vegetation can increase the speed of weathering by 3-10 times (Berner,1997).According to monitoring data from Guilin province in China vegetation restoration can significantly increase the average annual concentration of soil CO 2 (increased by 266% in 10 years).The increase in CO 2 promotes the dissolution of carbonate rock and greatly increases HCO 3 concentrations in groundwater (Liu,2012;Waterson and Canuel,2008).Research in the Houzhai valley has shown that forest recovery causes more carbon dioxide (CO 2 ) to be dissolved in karst water, which in turn allows for carbon uptake by forests (Yan et al.,2014).This research also suggested that karst hydro-geochemistry and the karst-related carbon cycle could be regulated effectively by different LUCC types (Zhao et al.,2010).On the other hand, in the process of runoff converging at the outlet, much of the water flows into the surface river and flows across the thick soil of paddy fields, but our calculation method only considers carbonate weathering carbon sinks (water -rock -gas interaction) and not the organic processes, which may affect calculation results.
Research has shown that aquatic photosynthesis uses dissolved inorganic carbon to synthesize organic carbon (Waterson and Canuel,2008;Tao et al.,2009), and this is also one of the factors affecting the results.In addition, differences in basin surface water and groundwater proportions controlled by geological landform could also affect the calculation results.
To sum up, the calculation results for carbon sink discharge from karstification using watershed monitoring data in areas limited to a dominant single LUCC type may differ in a small watershed where geomorphology, hydrology, and land use cover are different.This is one of the reasons why there is such a large deviation in China's total carbon sink discharge estimated by using carbon sink data from a single watershed in a karst region.Therefore, considering the diversity of landform types and surface covers in the southwestern karst area, it is important to develop a monitoring network in different topographical and surface cover regions, using a variety of monitoring technologies to improve the accuracy of karst carbon sink estimates.

Conclusion
It is important basic significance to determine the main factors that affect the karst geological carbon sink and understand the mechanism of their effects on the karst geological carbon sink.Through the contrast analysis of flow, bicarbonate ion concentrations and carbon sink In addition, this study without considering the proportional distribution problem of the surface and underground runoff in catchment area of each monitoring sites, which may have influence on the results.Therefore, it is necessary to monitor runoff and bicarbonate ions of the surface and underground simultaneously, but unfortunately the monitoring data we used did not achieve it.
annual average flow values of MSK in the rainy season, dry season, and whole year were 282.5 m 3 , 121.3 m 3 , 403.9m 3 , respectively, and the flow in the rainy season was significantly greater than in the dry season.The flow of LG and LHT in the rainy and dry seasons exhibited the same trend (Figure 2, 3, and 4).
Table1.Standard deviation of production flow, ion concentration, and carbon sink for each station in the dry and wet seasons and over the whole year

Solid
Earth Discuss., doi:10.5194/se-2016-37,2016 Manuscript under review for journal Solid Earth Published: 10 March 2016 c Author(s) 2016.CC-BY 3.0 License.water and fissure/pore water.It also suggests that the composition of underground karst aquifer medium structures have important effects on the dry season flow.However, due to the difference between the control area and the surface to underground diversion ratio, the flows between sites cannot be compared.The trends in annual runoff among sites are consistent with carbon sink discharge but differ from the trends for bicarbonate ions (figure2, 3, 4).This suggests that the effect of flow change on carbon sink is greater than on ion concentration.According to changes in ion concentrations in the rainy season, dry season, and whole year (figure2, 3, and 4), flow correlated negatively with carbon ion concentration, but if there was a significant difference between flow in the rainy season and in the dry season, bicarbonate ion concentrations would not decrease when the flow increased rapidly.Although we found differences between bicarbonate ion concentrations in the dry and wet seasons (ion concentrations in the dry season are greater than in the rainy season), they were small.

From 1996 -
2001, annual average carbon ion concentrations at LHT and MSK stations were 232.6 mg/L and 218.2 mg/L, respectively, but the average annual flow for LHT was only 115.6 m 3 , while MSK exhibited an annual flow of 3.49 times that value.Similarly, ion concentration did not decline significantly as a result of the increase in flow.MSK station is located at the edge of a paddy field, which has thicker soil coverage, and the underground rivers have more biological carbon sources that could produce more HCO 3 -in the ground water compared to LHT.Therefore, the flow only has a partial dilution effect on ion concentration.Meanwhile, the effect of flow increase on ion concentration dilution exceeded the environmental effects of carbonate weathering.This shows that the dilution effects of the flow change on ion concentrations were not multiplicative.That is to say, the flow was just a part of the dilution effect on ion concentrations, and thus carbonate weathering was significantly affected by factors other than flow.Bicarbonate ion concentrations of karst underground water may have a relatively stable extremum when Solid Earth Discuss., doi:10.5194/se-2016-37,2016 Manuscript under review for journal Solid Earth Published: 10 March 2016 c Author(s) 2016.CC-BY 3.0 License.

Figure 5 .Figure 6 .Figure 7 .
Figure 5. Variation in flow among sites in the rainy season, dry season, and whole year during the study period (a.LHT; b.LG; c. MSK).
From 1996-2001, the carbon sink discharge for LHT station in the dry and wet seasons and whole year were greater than for MSK station.The annual average concentration for LHT (230.4 mg/L) was greater than for MSK (218.2 mg/L), while the runoff was significantly greater for Solid Earth Discuss., doi:10.5194/se-2016-37,2016 Manuscript under review for journal Solid Earth Published: 10 March 2016 c Author(s) 2016.CC-BY 3.0 License.

Solid
Earth Discuss., doi:10.5194/se-2016-37,2016 Manuscript under review for journal Solid Earth Published: 10 March 2016 c Author(s) 2016.CC-BY 3.0 License.discharge between the different sites in three stations located upstream, midstream and downstream of Houzhai basin, respectively, we analyzed the reasons for the difference of flow, bicarbonate ion concentrations and carbon sink discharge.The preliminary conclusions are as follows: (1) The carbon sink discharge was mainly controlled by the flow of each site, and LUCC type has important effects on the bicarbonate ions concentrations in each site.(2) The large difference in flow among sites did not lead to significant differences in bicarbonate ion concentrations in the sites, showing that the rapid increase in flow only has a partial dilution effect on ion concentrations.Due to the high speed and stability of chemical carbonate weathering, bicarbonate ion concentrations did not change significantly, and thus did not affect carbon sink discharge.(3) For different LUCC conditions, if runoff is stable, the influence of flow variation on ion concentration will be less than the effects of chemical carbonate weathering by different environmental conditions (comparison of LHT and LG results is 150%) on bicarbonate ion concentrations.However, if runoff increases significantly, the impact of runoff variation on bicarbonate ions will be greater than the effects of chemical carbonate weathering by different environmental conditions (comparison results of LHT and MSK).