Tao ZHANG，F(xiàn)uxu WAN，Zhengxin TAN

Nanjing Forestry University，Nanjing 210037，China

Volcanic rocks are widely distributed in various regions of China， and due to sparse vegetation，shallow soil and little surface water in the volcanic mountain，the success rate of afforestation is low and tree growth is slow．Afforestation has always been difficult for the volcanicmountain，while the plantation is considered to be one of the main media for increasing forest cover and implementing carbon emission reduction plan［1］．It plays an important role in the evaluation of the contribution of forests to the terrestrial carbon sink［2］．Therefore，with Yueliangshan Ecological Park in Xuyi County，Huai'an City，Jiangsu Province as the study area，we study the soil carbon stocksunder five kinds of plantations in order to provide a theoretical basis and technical reference for the choice ofmost appropriate tree species and ecological restoration in volcanic mountain of northern Jiangsu．

1 Overview of the study area

Xuyi County（32°43′－33°13′N，118°11′－118°54′E）is under the administration of Huai'an，Jiangsu Province，China．As the southernmost of Huai'an's county-level divisions，it borders the prefecture-level cities of Suqian to the north and Chuzhou（Anhui）to the south and west．It has a prominent monsoon climate with four distinct seasons．The annual sunshine duration averages 2224．2h，the average annual temperature is14．7℃， and the average annual precipitation is 985．3mm．Due to prominent interseasonal and inter annual variation，the drought and waterlogging disasters are frequent and serious，affecting wide areas．The soil develops from the volcanic rock， and the zonal yellow brown soil occupies the largest area，accounting for 36．2%，followed by volcanic ash soil，paddy soil and moisture soil，with poor fertility and sticky structure．The low-yielding fields account for nearly two thirds．

2 Research methods

2．1 Sample plot setting and sampling methodsOn the basis of the actual survey in the study area，we select five kinds of plantations（Cupressus lusitanica，Celtis sinensis Pers．，PopulusL．，Amygdalus persicaL． andMyrica rubra（Lour．）S．et Zucc．）， and set 20m×20m Standard plots in each plantation and control barren hill，respectively．In each sample plot，we dig 3 S-shaped soil profiles and perform the 0－10，10－20，20－40，40－60cm stratified sampling in the profiles，respectively．Then wemix the soil samples in the same plot，dry them to be sifted by 0．25mm sieve，seal them with a plastic bag and store them for chemical analysis．At the same time，we use the ring knife to take three samples in each layer and bring them back to laboratory tomeasure soil density．

2．2 Sample testing methodWe use potassium dichromate external heating method to test the soil organicmatter and use cutting ring method to test the soil density．

2．3 Calculation of soil carbon stocksThe data processing method［3］is as follows：60cm soil carbon stocksS（t·hm2）is calculated using soil densityDi（g·cm3），carbon contentCi（g·kg－1），thickness of each layer of soilEi（cm） and percentage（%）of gravelwith volume greater than 2mmGi．

whereiis the soil layer．

2．4 Calculation of soil carbon densityThe soil organic carbon density refers to the organic carbon reserve of soil per unitarea at a certain depth．The organic carbon density in soil layeri（T0，kg·m－2）is calculated as follows：whereCiis the soil organic carbon content（g·kg－1）；Diis the soil density（g·cm－3）；Eiis the soil thickness（cm）；Giis the percentage（%）of gravel with volume greater than 2mm．

2．5 Data processingExcel2010 and SPSS18．0 are used for data calculation and statistical analysis．

3 Results and analysis

3．1 The soil organic carbon content of plantation and control wastel andYueliangshan Ecological Park covers an area of 333．3 ha， and the soil is formed from lava．Due to vegetation-soilhuman activity intervention，there have been some differences in the physical and chemical composition of soil under different vegetation， and soil organic carbon content and density have showed different features．The soil carbon and soil density determination of woodl and can be shown in Table 1．Table1 indicates thatunder the effect of different vegetation，the topsoil has high organic carbon content，but it decreases with the increase of soil depth．Based on the multiple comparisons of average soil organic carbon content under different vegetation，we get the following results（Table 2）：there are no significant differences in the organic carbon between wastel and soil and soil underMyrica rubra（Lour．）S．et Zucc．andCupressus lusitanica，but there are significant differences in the organic carbon between soil underMyrica rubra（Lour．）S．et Zucc．andCupressus lusitanicaand soil under other st anding forests；there are significant differences in the soil amongCeltis sinensis Pers．，Amygdalus persicaL．， andPopulusL．Based on the actualmeasured data，the soil underCupressus lusitanicashows the highest level of organic carbon，with an average of 0．08712，significantly higher than the average carbon content of other species，while the soil underCeltis sinensis Pers．shows the lowest level of organic carbon，as can be shown in Table 2．

Table 1 The organic carbon content and soil density of soil profile of plantations and wastel and

Table 2 Duncan multiple comparison of average organic carbon content

3．2 Organic carbon density of plantations and wastel and

As can be seen from Table 3，there are significant differences in soil carbon density between the five plantations and wastel and， and the soil layer variation range is 0．8－3．04kg·m－2．For the whole soil layer，the soil carbon density of five plantations and wastel and is 4．79－6．46 kg·m－2．The soil carbon density of all plantations basically shows the same regularity as carbon content，in descending order ofwastel and＞Amygdalus persicaL．＞PopulusL．＞Cupressus lusitanica＞Myrica rubra（Lour．）S．et Zucc．＞Celtis sinensis Pers．The carbon density of wastel and is 1．34 times that ofCeltis sinensis Pers．In terms of the vertical section of soil，soil organic carbon density decreaseswith the increase of soil depth， and there are significant differences between the soil layers．Meanwhile，the analysis of contribution rate of soil organic carbon density of five plantations and wastel and indicates that for the layer（≧0－10 cm），the contribution rate of soil organic carbon density is34．12%；for the layer（≧0－40 cm），the contribution rate of soil organic carbon density averages60．12%．It indicates that for the soil layer（≧0－40cm）occupies a position that can not be ignored in the study of forest soil organic carbon density．

3．3 The organic carbon storage of plantations and wastel andBased on the above experimental data，we use the previous formula to calculate the soil organic carbon stocks under various l and usemodes，respectively， and get the following results：Cupressus lusitanica（50．264 t·hm－2）；Celtis sinensis Pers．（47．859 t·hm－2）；PopulusL．53．216（t·hm－2）；Myrica rubra（Lour．）S．et Zucc．（49．581 t·hm－2）；Amygdalus persicaL．（58．202 t·hm－2）；wastel and（60．964 t·hm－2）（Fig．1）．The vegetation soil organic carbon is mainly from plant andanimal residues and litters transformed bymicrobial decomposition and chemical leaching．In this study，the size of carbon stocks is in the order ofAmygdalus persicaL．＞PopulusL．＞Cupressus lusitanica＞Myrica rubra（Lour．）S．et Zucc．＞Celtis sinensis Pers．As the local native species，Amygdalus persicaL．has the largest carbon stocks， and there are little differences in the carbon stocks betweenPopulus L． andCupressus lusitanica．However，the soil carbon stocks of five plantations are all lower than the organic carbon stocks ofwastel and due to the thin soil and selection of biennial plants which absorb large amounts of carbon and some other nutrients for their growth．

4 Conclusions

Currently，there are many studies on the soil organic carbon in China，but the difference is large．Li Keranget al．［4］use CEVSA model to estimate the average soil organic carbon stock snation wide at91．7 t·hm－2，but the result is 115．9 t·hm－2for Xie Xianli et al．［5］and 193．6 t·hm－2for Zhou Yuronget al．［6］．The experimental results in this paper indicate that the soil organic carbon stocks of five plantations in Yueliangshan Ecological Park of Xuyi County are as follows：Cupressus lusitanica（50．264 t·hm－2）；Celtis sinensis Pers．（47．859 t·hm－2）；PopulusL．53．216（t·hm－2）；Myrica rubra（Lour．）S．et Zucc．（49．581 t·hm－2）；AmygdaluspersicaL．（58．202 t·hm－2）．The soil organic carbon stocks average 51．824 t·hm－2．It can be seen that the soil carbon stocks of plantations in the study area are lower than the nationalaverage，so it is necessary to strengthen the tending and management of plantations to enhance their ability to sequester carbon．The soil carbon density of five plantations is4．79－5．62 kg·m－2， and the soil carbon density of all plantations basically shows the same regularity as carbon content，in descending order of wastel and＞Amygdalus persicaL．＞PopulusL．＞Cupressus lusitanica＞Myrica rubra（Lour．）S．et Zucc．＞Celtis sinensis Pers．The carbon density ofwastel and is1．34 times thatofCeltissinens is Pers．In terms of the vertical section of soil，soilorganic carbon density decreaseswith the increase of soil depth， and there are significant differences between the soil layers，which is the same as the results of many studies［7－11］．Overall，Cupressus lusitanicaandAmygdalus persicaL．are better than other tree species，so it is necessary to promote the cultivation of the two species in this region．

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