Land Use/Cover Change And Its Impact On Carbon Storage In The Coastal Zone Of Hainan Island

Coastal zone is a special zone with two different attributes of land and sea.Under the interference of land-sea interaction and human activies,the ecological environment is fragile and it could be aflect easily.Exploring the correlation between land use/cover change and carbon,nitrogen and phosphorus storage change in coastal zones is conducive to understanding the impact of human activities on land resources.Since 1988,the land uses in coastal zone of the Hainan Province have undergone great changes.Understanding the influence of natural factors and human activities on the coastal zones of Hainan Island could provide scientific basis for the sustainable development of the coastal zone.In order to reveal the spatial and temporal variation characteristics of land use pattern in the coastal zone of Hainan Island in recent 30 years and the influences of spatial and temporal variation of coastal shelter forests and mangroves on carbon reserves in the coastal zone,the coastal zone of Hainan Island was taken as the research area.Based on the RS,GIS technology and land use change related methods,Lansat satellite remote sensing images of 1988,1998,2008 and 2017 were selected to analyze the land use/cover change and its driving forces in nearly 30 years of land use/cover change.Combining with the carbon and nitrogen and phosphorus content of soil and plant leaf from the 24 plots of casuarina coastal shelter forest and the literature data of the mangrove soil and vegetation data,the carbon,nitrogen and phosphorus storage were analyzed in the Hainan Island coastal zone.The results showed that:For the land use and areas of Hainan Island coastal zone in 2017,the cultivated land area were 2,523.66 km2,coastal shelter forest area was 454.34 kn2,other forest land area was 381.62 km2,land used for building area was 558.10 km2,water area was 245.56 km2,mangrove area was 9.57 km2,and other land area was 64.39 km2.During 1988-2017,the land use change of Hainan Island coastal zone was as follows:the cultivated land area decreased by 129.91 km2,the proportion decreased by 5%;the coastal shelter forest area decreased by 327.16 km2,the proportion decreased by 42%;the other forest land area decreased by 108.78 km2,the proportion decreased by 22%;the land used for building area increased by 447.74 km2,the proportion increased by 406%;the other land area decreased by 92.54 km2 the proportion decreased by 59%;the mangrove area decreased by 32.16 km2,the proportion decreased by 77%.The main driving forces of land use/cover change were natural and social factors.Natural factors were extremely events(such as typhoons).Social factors include population growth,mudflat breeding,tourism development,real estate development and policy planning.The average contents of SOC,TN and TP were 4.33,0.38 and 0.26 g/kg in 0-10 cm sandy soil layer of coastal shelter forest in Hainan Island,with an average C:N ratio of 12.44.The average contents of SOC,TN and TP in lo-20 cm soil layer were 3.05,0.3 and 0.18 g/kg,with an average C:N of 8.79.According to the national soil nutrient classification standards,the SOC,TN and TP contents and C:N were at a low level.There was a significant positive correlation between soil TN and mean annual temperature at both under forest and bare beach.The soil TP content was significantly correlated with mean annual precipitation,but it was opposite between under forest and bare beach.The contents of TN and TPat the depth of 0-10 cm and SOC and TN at the depth of 10-20 cm under the forest were significantly higher than those in bare beach,indicating that the plant casuarina shelter forest in coastal zone had a "fertile island" effect,which was conducive to improving soil C,N and P contents.The average contents of TC,TN and TP in leaves of Casuarina were 399.06,12.55 and 1.04 g/kg,which were lower than the global average contents of terrestrial plants.The average ratios of C:N,C:P and N:P were 32.02,420.65 and 13.10 in leaves of Casuarina,which were higher than the global average values of terrestrial plants.Casuarina shelter forest in Hainan Island coastal zone may be mainly limited by soil N.Soil pH at the depth of 0-10 cm was significantly positively correlated with TN content in leaves of casuarina,and annual precipitation was significantly positively correlated with TN and TP in leaves of Casuarina.From 1988 to 2017,the carbon storacges of the coastal vegetation were reduced by 2543913.33～2557134.63 t(2.54～2.56 Pg)due to the land use type changes in the coastal zone of Hainan Island.Nitrogen and phosphorus storages(excluding mangroves)decreased by 75090.56～75506.36 t and 16612.21～16646.67 t,respectively.The reduction of mangrove forests and Casuarina forests in the coastal zone of Hainan Island has resulted in the decreasing the carbon storage of soil and coastal vegetation by 1886228.7～1896150.89 t(1.88～13.89 Pg).The nitrogen and phosphorus storages(excluding mangroves)were decreased by 47390.16～47702.2 t and 7472.55～7498.41 t,respectively.