Hydrological Process In Freshwater Wetland Forest Based On Hydrogen And Oxygen Stable Isotopes

Freshwater wetland forests exert a critical role in regulating climate,controlling flood and drought,and maintaining the balance of ecosystem.With the global precipitation changing,the hydrological processes in freshwater wetland forests also shift,resulting in the degradation of ecological function,consequently threatening ecological security.However,substantial uncertainties exist in the hydrological processes of freshwater wetland forests,especially for the precipitation,soil water,plant xylem water and shallow groundwater in different types of freshwater wetland forests.Here we addressed these issues in two types of wetland forests(pure vs.mixed stand of Populus deltoides)under three rainfall events in the Middle-Lower Reaches of the Yangtze River in China,by employing dual stable isotopes of hydrogen and oxygen coupled with two-end linear mixing model and Bayesian mixing model(Mix SIAR),by combining meteorological data,plant properties and soil variable,and by utilizing a variety of statistical techniques(including structure equation model and variation partitioning analysis).Characteristics of?D and?¹⁸O in the precipitation:Using the stable isotope technique,we analyzed the compositions of?D and?¹⁸O in 154 precipitation samples between 2015 and 2017in Anqing.We also linked the?D(?¹⁸O)with the daily precipitation and temperature to determine whether and how the climatic factors affect the compositions of stable isotopes.Furthermore,the sources of atmospheric water in Anqing was traced by analyzing the D-excess(d)in the precipitation,and verified through HYSPLIT model.Our results revealed that the relationship between?D and?¹⁸O in the precipitation in Anqing was?D=(8.08±0.06)?¹⁸O+(10.84±0.48)(R²=0.99,n=154).Both?D and?¹⁸O were significantly negatively related to temperature(P<0.01),while had no significant relationship with precipitation(P>0.05).D-excess(d)was close to the global average(10‰),suggesting that the precipitation in this area was mainly affected by the ocean monsoon.The air mass back trajectories of precipitation were processed by the HYSPLIT model and showed that there were different sources of atmospheric vapor in Anqing.In spring,the atmospheric vapor was affected by the local evaporation and continental air mass from north and south China,and was likely influenced by the Pacific southeast monsoon when rainfall was heavy.In other seasons,atmospheric vapor sources were mainly impacted by the South China Sea air mass,Pacific southeast monsoon,and southwest monsoon of India Ocean.Contribution of rainfall to the soil water within two types of wetland forests:We employed stable hydrogen isotope to explore the contribution of different magnitudes of rainfall(7.9,18.6 and 34.1 mm)to the soil water in two types of wetland forests(pure vs.mixed stand of P.deltoides).We further used structure equation modelling to determine the relative importance of soil properties and vegetation biomass in affecting the contribution of different magnitudes of rainfall to soil water.Our results revealed that there was no significant difference between these two stand types in the contributions of light and moderate rainfall to soil water,while the contribution of heavy rainfall to soil water(CHRSW)in mixed stand was significantly higher than that in pure stand(74.3%vs.62.9%),suggesting that mixed stand soil has higher water-holding capacity than pure stand soil.The results based on the structural equation model showed that the difference in soil properties between these two stand types was the primary reason for the different capacity of precipitation interception.Water use patterns of dominant trees in two types of wetland forests:By employing dual stable isotopes of hydrogen and oxygen coupled with a Bayesian mixing model(Mix SIAR),we explored the water uptake patterns of P.deltoides in two types of wetland forests(pure vs.mixed stand of P.deltoides)under different magnitudes of rainfall events.We further used both partial correlation and variation partitioning analyses to determine the relative importance of soil variables and plant properties in affecting the proportion of P.deltoides water uptake from different soil layers.Our results revealed that P.deltoides mainly uptake water from deep soil layers(60-100 cm).When it was mixed with Taxodium ascendens and Liquidambar formosana,P.deltoides had a higher proportion of water uptake from shallow soil layer while had a lower proportion from deep soil layer than the pure stand of P.deltoides.Our results also revealed that plant properties such as leaf biomass,fine root biomass,and diameter at breast height were the primary factors influencing plant water uptake.Translation of precipitation,throughfall,soil water,xylem water,shallow groundwater and Yangtze River water in the two types of wetland forests:Based on linear fitting model analysis for both?D and?¹⁸O in each water body,we found that the dual stable isotopes in precipitation were significantly correlated with those in Yangtze River water,throughfall,soil water,xylem water and shallow groundwater,suggesting precipitation was the input of the hydrological process in the wetland forests,and the main source of Yangtze River water,soil water and shallow groundwater.Furthermore,the?D and?¹⁸O values of Yangtze River water were significantly related to those in shallow groundwater,indicating that these two water bodies could be transformed into each other.Despite the precipitation,groundwater was also the main source of soil water.Moreover,soil water was the direct source of P.deltoides,T.ascendens,and L.formosana.Regulation of two types of wetland forests on hydrological processes:We compared the?D and?¹⁸O in precipitation,throughfall,soil water and shallow groundwater in two types of wetland forests after different rainfall events.Our results revealed that?D of canopy throughfall in both pure and mixed stands was slightly poorer than that of precipitation under light rainfall(7.9 mm),while?D of canopy throughfall was similar to that of precipitation under moderate(18.6 mm)and heavy rainfall(34.1 mm).This result illustrated that when the precipitation was light,the wetland forest canopy could exert interception effect on precipitation.With the increase of precipitation,the interception effect of canopy on precipitation removed.Moreover,there was no significant difference in?D of canopy throughfall between pure forest and mixed forest,whether in light,moderate and heavy rainfall.By comparison,wetland forests could influence the rainfall infiltration into soil profile,and the regulatory effect of mixed stand was better than pure stand.In addition,the groundwater level in the study area was generally high,and the groundwater level underlying mixed stand was higher than pure stand,suggesting a more conducive to water conservation.In summary,the mixed stand could exert stronger role in regulating soil water holding capacity,plant water uptake and associated hydrological processes than pure stand in Anqing.Compared with pure stand,mixed stand has better capacity for alleviating floods,which plays an important role in coping with frequent extreme precipitation events in the middle and lower reaches of the Yangtze River.In the future,when we conduct the restoration of degraded wetland forests,more attention should be paid to multi-species allocation.This kind of forest management not only increases the stability of forest ecosystems through raising the species diversity,but also improves the ability of wetland forests to alleviate floods.