The Changes Of Soil Carbon And Nitrogen Under Different Reclamation Histories And Land Use Types In The Coastal Area

Coastal tidal flats are important land resources.Tidal flats reclamation has become a significant means to obtain land resources in coastal areas in China,which can promote economic development.But land reclamation will also cause drastic changes in soil conditions and affect the material circulation processes.On a large scale,it will alter the carbon and nitrogen cycles and budget,and then affect the global climate change.On a small scale,the soil physicochemical properties will also be changed with reclamation,which may has an important impaction on regional food production,farmers’behavior preferences and polices decision-making.To study the temporal and spatial changes of soil carbon,nitrogen and physicochemical properties,we can learn the evolution of soil quality and soil carbon and nitrogen cycling after reclamation,and which may guide the utilization of new tidal flat and subsequent land management in Dongtai.In this study,the surface（0～20 cm）soil samples were collected in Dongtai with different reclamation duration history（0,50,100,300,500,700,1000a）and land use types（nursery land,upland,paddy field）,and the distribution of soil organic carbon,nitrogen and correlation among them were analyzed.The results are as follows:Soil pH,electrical conductivity,Na⁺,Ca²⁺content and bulk density markedly declined in the initial 100 a and thereafter decreased slowly with reclamation durations,and those were from8.65（0 a）to 7.94（1000 a）,3.50 dS·m^-1（0 a）to 0.21 dS·m^-1（1000 a）,31.33 mg·kg^-1（0 a）to108.04 mg·kg^-1（1000 a）,631.90 mg·kg^-1（0 a）to 318.99 mg·kg^-1（1000 a）,1.32 g·cm^-3（0 a）to0.91 g·cm^-3（1000 a）,respectively.The Mg²⁺content decreased first from 434.7 mg·kg^-1（0 a）to147.03 mg·kg^-1（100 a）,and then increased to 454.89 mg·kg^-1（1000 a）.Compared with unreclaimed soil,the soil K⁺content decreased sharply after reclamation,from 250.80 mg·kg^-11 to100.40 mg·kg^-1,but there was no significant change during 700 a,and then increased to 120.557mg·kg^-11 after 1000 a.Total and available P content increased first and then decreased with time increasing,reaching the maximum in 300 a,with the average content of 1079.63 mg·kg^-1 and39.89 mg·kg^-1.Under different land use types,pH and bulk density of paddy field were significantly lower than those of nursery land and upland（p<0.05）,but the electrical conductivity and Na⁺content of paddy field were significantly higher than nursery land and upland（p<0.05）,while the effect of land use on total P and available P was not significant.The contents of soil organic carbon,easily oxidized carbon,total nitrogen and alkali hydrolyzed nitrogen showed a rising trend with time,from 0.74 g·kg^-1（0 a）to 8.28 g·kg^-1（1000 a）,from 0.61 g·kg^-1（0 a）to 1.28 g·kg^-1（1000 a）,from 0.27 g·kg^-1（0 a）to 1.16 g·kg^-1（1000 a）,from0.03 g·kg^-1（0 a）to 0.13 g·kg^-1（1000 a）,respectively,but the accumulation rates were found to be high in the initial reclamation stages（0～300 a）,gradually slowing down with time.The total carbon showed the trend of increasing first and then decreasing,reaching maximum(15.00 g·kg^-1)after 100a.Inorganic carbon sharply decreased with the time increasing,from 9.49 g·kg^-1（0 a）to 1.41 g·kg^-1（1000 a）.The distribution proportion of carbon active components decreased significantly,and tended to be stable after 100 a,while the ratio of soil carbon to nitrogen increased year by year.Under different land use types,the contents of total nitrogen,soil organic carbon,dissolved organic carbon and easily oxidized carbon in paddy soil were higher than those in nursery land,upland and bare tidal flat.In addition to total P,available P and K⁺content,most soil physicochemical properties and Soil C and N had significant correlation with reclamation history,which showed that with reclamation time increasing,the reclamation soil will gradually change from the natural state of high salt and pH to the state suitable for agricultural cultivation,and soil organic carbon and nitrogen content would be significantly increased.Soil organic carbon had strong correlations with soil basic physicochemical properties,which was shown as a very significant negative correlation with pH,electrical conductivity,bulk density and Na⁺,Ca²⁺,K⁺content,and a significant positive correlation with total P and available P.The correlation between soil physicochemical properties and total nitrogen（easily oxidized carbon,alkali hydrolyzed nitrogen）were the same as that of organic carbon.The results showed that with the decrease of pH,electrical conductivity and Na⁺,Ca²⁺content,soil organic carbon and total nitrogen content increased gradually.The increase of organic matter improved the soil conditions,caused the decrease of bulk density.To conclude,the results showed that reclamation was beneficial to the improvement of soil conditions and the accumulation of organic carbon and nitrogen.Paddy field had the best desalination and dealkalization effects and great potential for carbon and nitrogen accumulation among the four land use types.