Dynamics And Predication Of Soil Organic Carbon In Coastal Reclaimed Lands,Eastern China

Soils is the largest pool of organic carbon in terrestrial ecosystems.The changes in soil organic carbon(SOC)are crucial for global climate change and soil fertility.Determining the importance of potential drivers of SOC and thus predicting the distribution of SOC are important for measuring carbon sequestration or emissions and providing useful information for global climate change.Coastal tidal flats reclamation was a common agriculture practice to gain new arable lands in eastern China.The significant alternations in soil properties caused by tidal flats reclamation may exert considerable impacts on carbon cycle in reclaimed soils.Climate conditions,soil properties,and anthropogenic activities are general factors driving SOC dynamics.However,SOC dynamics in coastal reclaimed lands is not only affected by the above factors but also by certain specific factors e.g.,salinity,pH and the changes in soil environment conditions induced by reclamation.Numerous studies have revealed the correlations between SOC and its impacting factors,the relationships between SOC and the potential driving factors of coastal reclaimed soils are still missing.In this study,a total of 746 surface(0?20 cm)soil/sediment samples were collected from three study areas with different reclamation durations and land use patterns on the coasts of Dafeng,Rudong and Cixi in eastern China.The contents of soil organic carbon,permanganate-oxidizable carbon(POxC),salinity,pH,particle sizes and some elements were determined.The major objectives of this study are:1)to reveal the dynamics of SOC,POxC,carbon pool management index(CPMI)and other soil properties with reclamation duration and land use changes;2)to investigate the relationships between SOC and the potential impacting factors using random forest(RF)model and thus predicting SOC using RF model and multiple linear regression(MLR)model;3)to identify the major drivers of POxC and Non-POxC using RF model.The main results of this study were as follows:(1)Soil/sediment properties varied significantly across study areas due to the differences in sedimentary environment,sediment sources and soil managements.Soil/sediment pH of Dafeng,Rudong and Cixi were remarkably different.The Cl,salt and CaO content of Dafeng were significant higher(p<0.05)than those of Rudong and Cixi.The total phosphorus contents of Dafeng and Cixi were significant higher(p<0.05)than that of Rudong.The clay and silt contents of Cixi were significant higher(p<0.05)than those of Dafeng and Rudong,whereas the sand content of Rudong was remarkable higher(p<0.05)than those of Dafeng and Cixi.(2)Soil/sediment properties showed strong spatial variations due to the differences in land use patterns and reclamation durations.In the three study areas,the reclaimed soils with different land use patterns had significant higher total nitrogen and phosphorus contents than those of the tidal flats,whereas the tidal flats had remarkable higher salt,Cl,pH,CaO and Na2O contents than those of the reclaimed soils.The total nitrogen,phosphorus and sand contents increased significantly with an increase in reclamation durations in the study areas,suggesting that the accumulation of soil nutrients were significantly improved by human activities.However,the contents of salt,Cl,pH,CaO and Na2O presented decreasing trends with the increase in reclamation durations,highlighting the importance of irrigation and precipitation on desalination and decalcification.(3)The results showed that mean contents and densities of SOC in Dafeng,Rudong and Cixi differed slightly.Both SOC contents and densities increased significantly with an increase in reclamation durations in all the study areas with the exception of the early stage of land conversion in Cixi and the late stage of land conversion in Dafeng.Similarly,the POxC and Non-POxC contents of Dafeng also increased significantly with the increase in reclamation duration.Those results suggest that human activities significant enhanced the SOC pool of reclaimed soils.(4)Overall,the reclaimed soils had significant higher contents and densities of SOC than those of the tidal flats,and the land use patterns with longer reclamation durations generally had much higher SOC contents and densities than those land use with shorter reclamation durations.For soils with identical reclamation durations,paddy soils showed significant higher contents and densities of SOC than those of upland soils with the exception of the 10-year old reclaimed land in Cixi,indicating that water submerged condition contributed to SOC enrichment in reclaimed soils.(5)The results of the random forest(RF)model showed differences and similarities in the importance of factors to SOC in the three study areas.In Dafeng,soil pH,Cl and CaO contents were identified as the most important drivers of SOC.However,soil CaO and Cl contents,land use patterns and reclamation durations were the most important factors to SOC dynamic in Rudong.In Cixi,soil pH?CaO and Al2O3 contents were identified as the most importance factors for the dynamic of SOC.The differences in factor importance in the three study areas were attributed to different sedimentary environments,sediment sources and soil managements.(6)The most important driving factors for POxC in Dafeng were soil total nitrogen and phosphorus contents,followed by Cl,reclamation durations,pH and land use patterns.Similarly,soil total nitrogen and phosphorus contents and pH were also identified as the most important drivers of Non-POxC.(7)The RF model performed much better than MLR in predicating SOC dynamics in reclaimed soils with the exception of Cixi,as it can reveal much lower error indices and higher R2.The RF model and MLR model involving all predictor variables produced much higher 12 and lower error indices than the RP and MLR rmodels involving independent variables.(8)In Dafeng,the CPMI of soils with different reclamation durations ranged from 95.0?176.9.The CPMI of the reclaimed soils was much higher than the tidal flats with the exception of the 10-year reclaimed soils,highlighting the importance of agricultural activities on reclaimed soil CPMI enhancement.For soils with different land use patterns,paddy soils had significant higher CPMI than upland soils,indicating the superiority of paddy management in sequestering SOC and its labile fractions.