Biogeochemical Characteristics Of Carbon And Its Response To Rainstorms In A Karst Critical Zone

The Earth's Critical Zone(CZ)is a heterogeneous environment where living organisms,air,water,soil and rock interact,covering the energy and mineral resources required for the sustainable development of society.This is important for humans and environments.Carbon is an essential element of living organisms,its biogeochemical fate in different media in CZ affects the environment and climate change.Karst critical zone(K-CZ)is a fragile ecosystem,widely distributed soluble rocks and strong water-rock interactions make carbon cycle and water environment variation different from non-karst CZ and sensitive to climate change.Therefore,studying the biogeochemical characteristics of carbon and its dynamic changes in a K-CZ is conducive to identifying the role of momentous processes in the carbon fate,and provide a scientific basis for accurately modeling regional/global carbon cycle,helping understanding how climate change affects water environment variations and protecting environmental ecosystem.There are few systematic studies on carbon fate in a small catchment based on the concept of K-CZ.This study was conducted in a typical karst catchment which owns a critical zone observatory(CZO).This catchment is in Southwestern China and has been fundamentally investigated for?40 years.Both soil and water were the research objects in this study.On the one hand,soil profiles of four land uses were dug to analyze the dynamic processes of soil organic carbon(SOC)and soil inorganic carbon(SIC),quantify the distribution of their storage,discuss major controls and research the potential transportation of carbon between soil and water.On the other hand,high-resolution data were obtained through in-situ sensors and high-frequency sampling for analyzing the dynamic processes of hydrochemistry,dissolved organic carbon(DOC)and dissolved inorganic carbon(DIC)within the surface-underground system,and exploring their chemostatic response and hysteresis behavior during discharge variations.Additionally,continuous monthly sampling for two years was conducted on a larger spatial scale for depicting spatiotemporal variation characteristics of hydrochemistry and dissolved carbon and revealing the relevant influence factors.Main conclusions in this study are briefly descripted as follows:(1)The concentration and storage of SOC are much higher than that of SIC and decrease with increasing depth,while that of SIC changes with an opposite trend.There are no significant seasonal differences in SOC,SIC and corresponding isotope.The turnover rate of SOC is not completely consistent with the fractionation of its isotope(?¹³C_SOC),this is associated with the thin soil cover and significant heterogeneity of this karst system,and mainly controlled by overlying vegetation type,biomass and microbial communities in soil.The labile SOC pool is mainly located in topsoil(“new”carbon accounts for 34±10%).The sources and dynamic processes of SIC are simpler relative to SOC and largely depends on SIC concentrations and the relative ratio of pedogenic SIC to lithogenic SIC,which is affected by long-term dynamic equilibrium between soil respiration,precipitation of secondary carbonate in soil and its redissolution.Pedogenic SIC is the dominant component of SIC and more sensitive to external variation,which is related to the availability of biogenic CO₂ and Ca²⁺in karst soil.The variation of SOC also exert potential influence on SIC.In this catchment,the SOC storage(20.29±1.66 kg m^-2)is relatively higher than most areas in China or around the world.The topsoil of cropland losses SOC to some extent,but in the early stage(10 years)of natural restoration after ceasing tillage,the increasing rate of“new”SOC(0.18 kg m^-2yr^-1)is relatively fast and the decomposition rate constant of SOC(0.01)is relatively low.This is associated with the high permeability of karst soil.While this catchment has a lower SIC storage(1.22±0.34 kg m^-2)than most other areas,this is associated with the climate and the degree of karst soil weathering.(2)Solute concentrations in water have no significant difference in the interannual scale but normally lower in wet season than in dry season(except NO₃^-).All solutes show chemostatic responses with different intensities to discharge variations.The chemostatic behaviors of Na⁺,K⁺and SO₄^2-are weaker and largely controlled by rainwater dilution.NO₃^-and Cl^-have similar agricultural sources or transport processes,the flushing of increasing discharge enables them to migrate from surface water to groundwater,showing a heterogeneous response.Hysteresis behaviors of NO₃^-and Cl^-are also different from other ions.Concentrations of Ca²⁺,Mg²⁺and HCO₃^-are normally higher in groundwater than in surface water and mainly derived from soil CO₂dissolution and carbonate weathering,showing stronger chemostatic response.(3)DIC has a strong chemostatic response and is higher in groundwater than in surface water.Soil CO₂ and Ca CO₃ are dominant contributors to DIC in karst water,the former accounts for a higher proportion,especially in the wet season.Pedogenic SIC(secondary carbonates)also contributes to DIC in infiltration water.Atmospheric CO₂is another potential source of DIC in surface water.Biogenic carbon plays a leading role in DIC variation with discharge.Hysteresis behaviors of DIC and weathering products differ among different rain events,this is related to antecedent hydrological conditions and controlled by a process-limited regime.Hydrogeological conditions(e.g.,porosity,hydraulic conductivity and water head)control the transit pathway(matrix or conduit)of fluid and its velocity,affecting source areas and time available for transportation and deeper fluid-rock interaction.Additionally,dedolomitization might also modulate DIC in a deep confined spring draining gypsum stratum.(4)There is mutual transformation between DOC and DIC,but DOC concentrations are much lower than DIC.DOC concentrations are higher in surface water than in groundwater and higher in wet season than in dry season,both are the opposite of DIC.During the movement of rainwater to groundwater,there are continuous imports of“new”DOC,the relative ratios of“new”and“old”DOC after mixing can change with discharge variations.During permeation processes,DOC can also be partially withheld in the aquifer or adsorbed/utilized on the way,potentially as a continental source of underground water.The labile SOC pool in topsoil is one of DOC sources.The fluctuating trend in fluorescence intensities of various FDOM(fluorescent dissolved organic matter)components demonstrates the importance of terrestrial sources inputting through surface runoff,while the autochthonous contribution is relatively small.DOC in spring water has an accumulation effect with increasing discharge,the change of its hysteresis behavior in different rainstorm events is regulated by the availability,storage and transport during storms,which is mainly influenced by transport-limited regime and related to storm intensity/magnitude.(5)These findings suggest that the K-CZ characterizes soil erosion and limited labilesoil carbon,long-term tillage can also cause soil carbon loss which is unlikely to be recovered naturally within short term,degrading soil quality.Additionally,the thin soil cover and mature underground drainage systems enable NO₃^-,Ca²⁺and Mg²⁺to accumulate in karst groundwater and then cause higher pollution and hardness of water,deteriorating regional water quality during a certain period.Therefore,cost-effective land management policies and optimal measures of water utilization are both imperative for improving soil fertility and water quality in K-CZ.Overall,the carbon biogeochemical fate in K-CZ is active,carbon and environmental hydrochemistry have strong dynamic responses to hydrological variations,affecting global climate and changing environment quality of both soil and water.