Export And Evolution Of Karstic Soil Organic Matter

Soil organic carbon(SOC)pool is a key part of the terrestrial carbon pool.The export of SOC not only plays an essential role in the carbon cycle,also has a significance in assessing the soil fertility and the quality of water as a potable water resource.Therefore,understanding the dynamics and export processes of SOC is essential to improve the assessment of SOC cycle and ecological studies.However,the spanning of many long-term monitoring programs about natural organic matter(NOM)(typically representing change over the last two decades)in water samples collected from soil leachate or aquatic systems were relative short,making it difficult to detect the drivers of long-term trends of NOM.In karstic cave systems,stalagmites fed by the dripwaters supplied by the overlying aquifer,can record the dynamic change of the karstic ecosystem according to the accurate chronological order.Thus,based on the modern cave monitoring of NOM in dripwaters and corresponding calcite to constrain interpretations of NOM within stalagmites,extracting the information of NOM within stalagmites is expected to recover the past SOC variation and assess the effect of climate change on the SOC dynamics.In this study,we seek to elucidate the NOM record measured by thermoluminescence method within Heshang(HS)stalagmites(HS4 and HS6),along with systematic cave monitoring of NOM(including dissolved organic matter(DOM)and Colloidal organic matter(COM))in dripwaters and corresponding calcite precipitation measured by fluorescence spectrum method.We got some new knowledge for the mobilization mechanism,transport processes of DOM(<0.22?m)and COM(>0.22?m)in dripwaters,preservation of NOM from dripwaters to corresponding calcite precipitation,and correct interpretation of stalagmite NOM records.1.The spatio-temporal characterization of DOM in dripwaters.Here we present a nine-year DOM fluorescence data from two drip sites(HS4 and HS6)between 2005.02 and 2014.02,and a two-year DOM fluorescence data from four drip sites and one flowwater site(F1,D9,HS4,HS6,D5 from bottom to entrance of the cave)to investigate the spatio-temporal variation of DOM in dripwaters.On the spatial scale,electrical conductivity(EC)of water from five sites showed a prominent increased trend from bottom to entrance of HS cave;whereas,the DOC concentration of dripwaters presented a distinct decreased trend.The geochemical variation of four dripwaters and one flowwater,suggested that from bottom to entrance of the cave,the residence time of underground water was longer,consequently,the mineralization degree of underground water was also higher,and then causing the concentration of DOM was lower.The variation trends of the EC and DOM can mirror the transport direction of local underground water-from bottom to entrance of the cave,which is in accordance with the actual flow direction of local underground water.On the temporal scale,cross-correlograms showed that the fluorescence intensity of both dripwaters lagged behind rainfall by~1 year(~11 months lag for HS4,and~13months for HS6),whereas drip rates responded quite quickly to rainfall(0 months lag for HS4,and~3 months for HS6),based on optimal correlation coefficients.The rapid response of drip rates to rainfall is related to the change of reservoir head pressure,and drip rates gradually slow to a constant value associated with base flow from the overlying reservoir-this effect being most evident on inter-annual timescales(R~2=0.80for HS4 and R~2=0.86 for HS6,n=9,p<0.01).We ascribed the~1 year lag of fluorescence intensity to the effect of the soil moisture deficit and the karst process on delaying water and solute transport.After eliminating the one year lag,the congruent seasonal pacing and amplitude between fluorescence intensity and rainfall observed suggests that the seasonality of fluorescence intensity was mainly controlled by the monsoonal rains which can govern the output of DOM from the soil,as well as the residence time of water in the unsaturated zone.On inter-annual scales,a robust linear relationship between fluorescence intensity and annual(effective)precipitation amount(R~2=0.86 for HS4 and R~2=0.77 for HS6,n=9,p<0.01)was identified,implying that annual(effective)precipitation is the main determinant of DOM concentration in the aquifer.Conversely,the insensitivity of fluorescence intensity to variations in the pH of local rainfall suggested that acid rain over the study period(~pH 5.6 to~4.5)had no discernable effect on the quantity and quality of DOM soil and soil solution,likely being strongly buffered by soil carbonates.2.The temporal characterization of COM in dripwaters.Here a consecutive monthly monitoring study of HS4 drip site was conducted from February 2005 to February 2014 to evaluate mobilization of colloidal organic matter(COM)for the duration.COM concentrations in dripwaters were measured by synchronous and emission fluorescence spectra with the difference of raw and filtered(0.22?m)samples.The robust linear relationship between two different fluorescence methods suggested that two methods both can be as a proxy of the concentration of COM in Heshang dripwaters.The monitoring results showed that all COM peaks occurred prior to the rising limb of the dripwater hydrograph.Moreover,there were no evident COM peaks during three relatively extreme hydrologic years(2006 and 2013dry years,and 2009 wet year).Combined with the lack of geochemical response at HS4drip site to the transition from drought to wet condition,the characteristic of COM concentration in response to hydrograph suggested that the source of COM derives from the resuspended materials within overlying karst aquifer rather than soil leachate.The mobilization of COM reflected the movement of the air-water interface(AWI)related to the infiltration fronts which can sorb and mobilize colloids from rock surface due to capillary forces,and hydrodynamic shear on the release of colloids.In addition,the peak COM concentration increased with increased antecedent drying duration(shorter than critical drying duration),possibly because of breaking of more micropores walls under capillary stress as the drying duration increased.The AWI and antecedent drying duration were both connected with a fluctuating water level in a restricted karst aquifer controlled by outside weather conditions.Thus,according to the monitoring result of COM in HS4 dripwater,it is cautious to applying the COM and COM-facilitated metals to reconstruct paleo-rainfall events.3.Drivers of Preservation of NOM from dripwater to calcite precipitationHere we present a six-year monthly monitoring study of D5 drip site including collecting dripwater and corresponding calcite precipitation on glass plate.The invariable NOM(<0.22?m)concentration time series in D5 dripwater suggested that the overlying reservoir had been homogenized.According to the results of NOM concentration in dripwaters and NOM concentration in acidified calcite solution,we calculated the partition coefficient of NOM(),and found that temperature near D5 drip site,drip rate,the growth rate were all related to the:temperature and growth rate correlated negatively with,whereas the drip rate correlated positively with.Through the multi-factor analysis,the result showed thatcan be related to temperature and drip rate two factors toether preferably(R~2=0.36,p<0.01).Previous studies about minerals within soil adsorbing soil organic matter(SOM),suggested the special surface area(SSA)was an essential factor to control the mass of SOM.For calcite on the glass plate,temperature makes for the growth of calcite crystal particle,and then decreases SSA;whereas,fast drip rate was in favor of inhibiting the growth of calcite crystal particle,consequently will increase SSA.Therefore,it seemed probable that the two factors controlled the SSA together,and then affect the.For annual laminae stalagmite characterized with seasonal growth rate distinctly,the variable seasonal growth rate actually controls seasonal SSA variation,and then control the seasonal distribution of NOM in annual laminae stalagmites.4.Export and evolution of karst SOC in the last hundred years.Here we present two annual laminae stalagmites(HS4 and HS6)to study the NOM variation in the last hundred years.NOM concentration in stalagmites was measured by thermoluminescence method.On the basis of the results of modern cave monitoring of NOM,we reconstructed the time series of NOM in HS4 dripwater between 1905 and1990.The modelled time series of NOM showed high values from 1920 to 1940,which could mirror the high temperature during 1920-1940.After 10-year running average,the modelled concentration of NOM had a robust relationship with local temperature(R~2=0.76,p<0.01),which suggested that temperature was the major factor controlling the output of karstic NOM on decadal time scale.Therefore,it has a great prospect in applying stalagmite NOM to investigate the dynamics and drivers of SOC change.