Variation Characteristics Of Soil CO₂ And CH₄ And Carbon Sink Effect In Karst Rocky Desertification Control Area

The emission of CO₂ and CH₄ is closely related to the global climate change and the carbon cycle of karst ecosystem.The production and emission of CO₂ and CH₄ in the soil profile of karst rocky desertification control area is an important part of the carbon balance of karst ecosystem.The study of their change characteristics is of great significance to confirm the missing carbon sink and its size on land.The typical area of Karst Rocky Desertification in Southwest China-experimental area of rocky desertification control in Nanchuan,Chongqing is selected as research area in this study,which is divided into experimental area of rocky desertification control?poplar YSL,Zanthoxylum HJ,honeysuckle JYH,slope to terrace PGT?and control area without treatment?wasteland HD,abandoned farmland QGD,non slope to terrace fpgt?.The CO₂ and CH₄ in soil profile were monitored by L-type PVC pipe self-made device,and the CO₂ and CH₄ emitted from surface soil were monitored by static box.The variation characteristics and influencing factors of CO₂ and CH₄ concentration in soil profile and CO₂ and CH₄ emission flux in seasonal and daily scales were compared and analyzed.At the same time,the karst carbon sink in different areas of test area and control area were measured by standard limestone dissolution test piece method to explore the influence of different ways of controlling rocky desertification on the effect of karst carbon sink.The main research conclusions are as follows:?1?The seasonal difference of CO₂ concentration in different soil depths was obvious,winter<spring<autumn<summer.The annual average concentration of CO₂ in the control area is larger than that in the test area(1785.59 mg·m^-3).The change from large to small in all kinds of land is:abandoned farmland>honeysuckle>wasteland>poplar forest>prickly ash forest>non slope change ladder>slope change ladder.With the increase of soil depth,the annual average value of soil CO₂ concentration began to increase and then decreased?50cm<10cm<20cm?.The main factors influencing the seasonal variation of soil CO₂ concentration are soil moisture content,soil temperature and air temperature.The CH₄ concentration of soil in different depth had no obvious seasonal change,and the concentration of CH₄ in different depth was between 0.17 and 2.51 mg·m^-3.The annual mean CH₄ concentration of different soils in the experimental area and the control area was little different,from big to small:Pepper forest>honeysuckle>wasteland>non slope change ladder>abandoned land>slope change ladder>poplar forest.The average annual CH₄concentration of soil decreased gradually with the increase of depth and then increased significantly?20cm<10cm<50cm?.The main factors influencing the seasonal variation of soil CH₄ concentration were soil temperature,atmospheric temperature and soil water content.The daily variation of soil CO₂ concentration in different depths of slope to terrace in the rocky desertification control area is irregular in summer,the highest and lowest values appear at 18:00 and3:00.The daily mean of soil CO₂ concentration changes with the depth of 10 cm>20 cm>50 cm,and decreases with the increase of soil depth.The important factor influencing the daily change of soil CO₂ concentration is temperature.The dynamic change of CH₄ concentration with time in different depth of soil in a day also has no uniform rule.The minimum value is 50 cm(1.36 mg·m^-3)at 15:00,and the highest value is 10 cm(1.64 mg·m^-3)at 18:00.The daily mean CH₄ concentration of soil changes with depth,which is 20cm>10cm>50cm,and the surface soil is larger than the deep soil.Soil temperature or CH₄ concentration is an important factor for daily change of soil CH₄concentration.During the observation period of one year,in 10 cm,20 cm and 50 cm soil samples the concentrations of CO₂ were negatively correlated with the concentrations of CH₄.As the concentration of CO₂ in soil increased,the concentration of CH₄ decreased,and vice versa.The correlation of the control area is more significant and stronger than that of the test area,especially at20cm of the control area.During the day and night observation period,the positive correlation between CO₂ concentration and CH₄ concentration was significant at 10cm,a weak correlation between CO₂ concentration and CH₄ concentration at 20cm and 50cm,a negative correlation between them at 50cm and a positive correlation between them at 20cm.?2?The total CO₂ flux of the experiment area and the control area showed obvious seasonal difference,winter<spring<autumn<summer,summer was 1.5 to 8 times the rest of the season.Both the experimental area and the control area are CO₂ emission sources.The annual average value of soil surface CO₂ emission flux in the experimental area is smaller than that in the control area.The annual mean of CO₂ emission flux in all kinds of land is from high to low:wasteland>slope to terrace>honeysuckle>poplar forest>abandoned land>non slope to terrace>prickly ash forest.The main factors influencing the seasonal variation of surface CO₂ flux are air temperature,soil temperature,water content and CO₂ concentration in soil.There was little difference between the seasons of CH₄ flux,and there were two absorption peaks?from December to February and from July to September?.In the whole year,the surface soil in the experimental area and the control area showed weak absorption sink of atmospheric CH₄,the absorption sink was wasteland>honeysuckle>abandoned farmland>poplar forest,and the emission source was slope change ladder>prickly ash forest>non slope change ladder.The main factors that affect the seasonal variation of CH₄ flux are the content of soil organic carbon and soil moisture.On the sampling day,the soil on the slope change terrace was the source of CO₂ emission,and the daily variation of CO₂ emission flux under the condition of grass and no grass coverage had obvious regularity,showing a"V"type trend,the highest and lowest values appear at 15:00 and 6:00.There was a significant positive correlation between the surface CO₂ emission flux and the soil CO₂concentration,and the main factor affecting the surface CO₂ emission flux is soil CO₂concentration.The daily variation of CH₄ fluxes in the surface soil with and without grass cover also has obvious regularity,showing a double peak trend.The total daily CH₄ fluxes are positive fluxes,and the soil mainly releases CH₄?emission source?.The correlation between CH₄ flux and CH₄concentration in soil was low,but not related.During the observation period of one year,except for honeysuckle,slope terrace and wasteland,the surface CO₂ fluxes with and without grass cover tended to be negatively correlated with the surface CH₄ fluxes,which was not significant.There was a positive correlation between the daily variation trend of CO₂ flux and CH₄ flux of the surface soil in the slope to terrace area of the experimental area.The change of sloping land to terraced land may increase the daily emission of CO₂ and CH₄ in the surface soil in summer,and lead to decrease of CH₄ absorption in soil.?3?The dissolution rate of karst did not show a uniform change rule with the increase of soil depth.The highest value is 20cm deep(48.75 mg·cm^-2·a^-1)under poplar forest soil,and the minimum value appeared in the depth of 10cm(-5.10 mg·cm^-2·a^-1)under non slope terrace soil.According to the order of karst carbon sink from high to low,the order of each land is:abandoned farmland>poplar forest>pepper forest>honeysuckle>slope to terrace>wasteland>non slope to terrace.The CO₂ consumption per unit area in the control area of rocky desertification is 2.14mg·cm^-2·a^-11 higher than that in the contrast area through the process of rocky desertification control,and the karst carbon sink in the control area of rocky desertification is 4.97 mg·cm^-2·a^-1 higher.There was a significant positive correlation between the content of CO₂ and the dissolution rate in the control area of rocky desertification,but no significant correlation between the content of CO₂and the dissolution rate in the contrast area.One of the important driving forces of the dissolution rate in the control area is the soil CO₂ concentration.When the concentration of CO₂ in soil of the control area increases,more CO₂ will be involved in the karst process,and the dissolution rate under the soil is also increased.The prevention and control of rocky desertification in karst area has played a role in promoting karst carbon sink,which has a strong carbon sink effect.Poplar forest and pepper forest have the greatest impact,followed by honeysuckle,and slope change ladder has a small impact.