Study on the changes of carbon flux in key zones of typical northern karst under the background of ecological restoration

Ecological restoration facilitates the increase in carbon flux in karst regions, yet the underlying carbon sequestration mechanisms remain elusive, and relevant research is particularly scarce in the northern karst areas of China. Jiguan Cave in Henan Province(33°46′N, 111°34′E) is a typical representative of the karst critical zone in northern China, where continuous cave monitoring has been carried out since 2010. In this study, we integrated 15 years of cave monitoring data(including stable carbon isotopes, dissolved inorganic carbon, pH and other hydrochemical parameters)with remote sensing satellite image data(Net Primary Productivity(NPP)and Normalized Difference Vegetation Index(NDVI))to analyze the variations in carbon flux and its influencing factors in the karst critical zone against the backdrop of ecological restoration. The main findings are as follows: (1)From 2010 to 2024, the Normalized Vegetation Index(NDVI)of the Jiuguan Cave roof exhibited an average annual increase of 0.0056. Ecological restoration is the key factor in the growth of the vegetation index in Jiguan Cave, and it is also affected by the climate, with temperature being the main factor influencing the vegetation index at the top of Jiguan Cave. Among climate factors, temperature was the dominant factor affecting the NDVI of the cave roof. (2)The ecosystem carbon flux, as reflected by the Net Primary Production(NPP)of vegetation, increased from 643.01 gC/m2 in 2010 to 723.01 gC/m2 in 2024, with an average annual increment of 5.33 gC/ (m2·a). This increase was mainly attributed to the vegetation restoration in Jiuguan Cave. (3)Long-term monitoring has revealed that with the recovery of vegetation and the increase in vegetation NPP, the dissolved inorganic carbon(DIC)in Jiaguan Cave has gradually increased. At the drip water site LYXS, the DIC rose from 5.1 mmol/L in April 2010 to 8.4 mmol/L in December 2024. Meanwhile, the stable carbon isotope showed a negative trend, from -12.60‰ in April 2010 to -17.21‰ in December 2024. (4)Nevertheless, under the influence of extreme climate events, such as the extremely arid year of 2013, the NDVI and NPP of vegetation decreased significantly, the DIC content decreased, and the carbon isotope shifted towards more positive values.In summary, under the overarching context of ecological restoration, the increase in the ecosystem carbon flux within the karst critical zone of Jiuguan Cave influences the variations in DIC and carbon isotopes in the cave water. The carbon fixed by plants enters the cave through the soil and rock layers, thereby exerting an impact on the carbon cycle of the karst cave system and contributing to an increase in the karst carbon sink.