| The'Grain for Green'program (GGP, i.e., conversion of farmland to florets) was not only a radical measure of elimnating soil erosion and improving ecological environment, but also a concrete measure of regulating lands-use composition in China. Eco-environmental benefit of'Grain for Green'program were investigated by studing biomass, effects on soil environments, nutrient accumulation and carbon storage in young stands within five afforestation patterns (Pinus massoniana pure forests, Cinnamomum camphora pure forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests) in Huitong County, Hunan Province, which could provide the significant reference and technical support of further perfecting conversion of farmland to florets policy for the relating administrative department, it was significant to consolidate achievement of the 'Grain for Green'program and sustained spread out the'Grain for Green'program. The main results are as follows:The biomass of per plant aboveground was absolute superiority in the main tree species, accounting for 66% of total plant biomass, the biomass of organs was essentially in the order as:trunk>root>leaf>branch>bark, there were some differences in biomass of per plant. The sub-biomass of Elaeocarpus sylvestris and Cinnamomum camphora mixed forest was the highest, up to 3.916 t.hm-2, the biomass of Cinnamomum camphora pure forests was the lowest,1.213 t.hm-2. Under four kinds of model for conversion of farmland to forestland, the relative distribution of percentage of each component biomass of young plantation was as follows:trunk (skin) accounted for the largest proportion of forest biomass, for more than 32.64%, the proportion of branch biomass was the smallest, only between 12.65% and 17.98%, the average net productivity of Elaeocarpus sylvestris and Cinnamomum camphora mixed forest was the highest, up to 0.4680 t.hm-2.a-1, Cinnamomum camphora pure was followed, Elaeocarpus sylvestris and Michelia chapensis mixed forest was the lowest, only 0.0754t.hm-2.a-1, the difference between the highest and the lowest was 0.3926t.hm-2.a-1.Compared with pre-converted farmland in soil layer (0-60cm), the average bulk density was declined, total porosity and natural water content were increased in Cinnamomum camphora pure forests, Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests, while the average bulk density in Pinus massoniana pure forests was increased 14.64%, the average total porosity and average natural water content were decreased by 13.21%,3.48%, respectively; the cosmid pecent (<0.01mm) was respectively increased 1.81%,14.99%,13.85%,31.26%,32.38% and the sand pecent (1～0.05mm) was respectively decreased 1.48%,13.88%,17.30%,39.78%,22.33% in Pinus massoniana pure forests, Cinnamomum camphora pure forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests, Michelia chapensis and Manglietia insignis mixed forests. Soil physical conditions had been improved in different degrees, in which the modification of Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests showed the most significant, followed by Cinnamomum camphora pure forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, that of Pinus massoniana pure forests was the worst.Compared with pre-converted farmland in soil layer (0-60cm), pH respectively was increased 4.30%,3.44%,1.51%,1.29% in Elaeocarpus sylvestris and Michelia chapensis mixed forests, Michelia chapensis and Manglietia insignis mixed forests, Cinnamomum camphora pure forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, while unchanged in Pinus massoniana pure forests. Average content of soil organic matter respectively was increased 53.60%,39.24%,38.63%,24.83% in Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, Cinnamomum camphora pure forests, but was decreased by 18.66% in Pinus massoniana pure forests. Average content of N respectively was increased 61.18%,37.28% in Michelia chapensis and Manglietia insignis mixed forests and Elaeocarpus sylvestris and Michelia chapensis mixed forests, but respectively was decreased 10.38%,14.78%,47.66% in Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, Cinnamomum camphora pure forests, Pinus massoniana pure forests. Average content of P improved in five model forests, of which the highest rate of Elaeocarpus sylvestris and Cinnamomum camphora mixed forests was 57%, and Pinus massoniana pure forests was the lowest,7.94%; the average content of Mg increased more than 33.10%, the average content of K was decreased significantly 3.44% to 61.93%, the average content of Ca was decreased more than 63.04%. Average content of available N respectively was increased 21.14%,7.53% in Elaeocarpus sylvestris and Michelia chapensis mixed forests, Michelia chapensis and Manglietia insignis mixed forests, while was decreased 4.41%,9.39%,11.39% in Cinnamomum camphora pure forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, Pinus massoniana pure forests. The average content of available P and available K obviously was increased 26.61% to 86.85% and 120.67% to 219.51% respectively in five model forest. Soil microelement content's coefficient of variation of the five forests were in 0.40% to 16.40%, and the average content of Zn, Pb, Ni, Cu, Cd were both in the third rank of GB15618-1995. Compared with pre-converted farmland in soil layer (0-60cm), in the five forests, Fe obviously was increased (p<0.05), but Cd was decreased in same level, Cu, Zn, Ni was increased in different level. Expect in Pinus massoniana pure forests, Mn obviously was increased (p<0.05) in another afforestation model, Pb, Co was increased in some level, three years after the conversion of farmland to forests, pH, soil organic matter and part of nutrition elements were increased. Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests had the most obviously change, the second were Cinnamomum camphora pure forests, Elaeocarpus sylvestrisand Cinnamomum camphora mixed forests, the last was Pinus massoniana pure forests.The significant positive correlation appears between the cosmid pecent(<0.01mm) and soil natural water content rate, pH, N, P, Mg; while the significant negative correlation appears between soil bulk density and soil organic matter, P, N, available N, available P, available K. Three years after the conversion of farmland to forests, the cosmid pecent (<0.01mm) increased and soil bulk density decreased, the ability of water conservation, pH, nutrient storage and absorption were increased. The ralationship among each chemical factor was very complex, some showed promote each other, but some showed inhibition each other. Assume that farmland was the standard, composite degradation index of Pinus massoniana pure forests, Cinnamomum camphora pure forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests and Michelia chapensis and Manglietia insignis mixed forests were-9.33%, 18.45%,18.86%,29.33%,21.86%, respectively. All of them except Pinus massoniana pure forests could improve soil properties, Elaeocarpus sylvestris and Michelia chapensis mixed forests did it best in the four.The order of the average content of ash in different organs of the same tree was as follow:Michelia chapensis> Manglietia insignis> Cinnamomum camphora> Elaeocarpus sylvestris> Pinus massoniana and the coefficient of variation is between 38.36% and 55.00%. The average content of ash in the barks of different trees was the highest, the second was roots, the lowest was trunks, and the coefficient of variation was from 37.49% to72.31%. N concent was the highest in different organs of 5 kind's trees, K and Ca was the second, the lowest was P. The order of same nutrient concent in same tree's different organs from high to low roughly was as follows:leaf> branch> bark> root> stem. The highest nutrient average content of the same nutrient in 5 kind's trees'same organ was leaves, the second was branches and the smallest was stem. Coefficient of variation was from 24.73% to 69.16%. The result showed that the highest leaves and lowest stem content was a basic feature of major afforestation trees. The average nutrient concent of different organs in the same tree was different; the nutrient concents of Pinus massoniana were generally low. The coefficients of variation were between 31.35% and 110.92%. N was the highest in Shrubs and herbs, K was the second, P was generally the lowest in shrub and Mg was generally the lowest in herb. The order of average content of nutrients between different species life ranked as:herbs>shrubs>frees. The different of nutrient content of living sort under the same stand from high to low was as follow:N> K>Ca>Mg>P, that of the dead litter layer:N>Ca>K>Mg>P.The total amount of nutrient elements of N, P, K, Ca, Mg were 38.090,22.265,37.045, 5.974 and 3.888 kg-hm-2 in five different forests of Pinus massoniana pure forests, Cinnamomum camphora pure forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests. The amount of accumulation presents was N>Ca>K>Mg>P in Pinus massoniana pure forests and N>K>Ca>Mg>P in another four different forests, which the proportion of leaf accumulation in tree accumulation were farther outweight than the proportion of biomass in tree biomass. The total amount of N, P, K, Ca, Mg in accumulation of 0-60cm soil layer of Elaeocarpus sylvestris and Michelia chapensis mixed forests was the highest. Michelia chapensis and Manglietia insignis mixed forests, Cinnamomum camphora pure forests took the second place, Pinus massoniana pure forests was the end. Compared with pre-converted farmland, the total amount of N, P, K, Ca, and Mg in soil decreased in five different forests. At the same time, the accumulation P, Mg increased and K, Ca decreased, but N presented different in different forests. In these five modle of young ecosystems, the nutrient elements capacity in 0-60cm soil was the highest, which accounted for more than 99% of the total capacity in nutrient elements of ecological system. Vegetation layer took the second place, less than 0.30%, and the dead vegetation layer was the least.Content of Fe, Mn was the firstly higher in different organs of the 5 species trees, Cu was the secondly, Zn was the lower, the order of the same microelements concent in the same organs of different species showed different with different microelements, coefficients of variation of microelements in different species were in range of 11.57% to 67.73%, concent of the same microelements of different organ in the same species were also different, coefficients of variation were in range of 35.63% to 89.09%. The results showed that it was more different for the same organs in different species to have different absorptive capacity to the same trace elements, even though different organs in the same species to have different absorptive capacity to the same trace element. Concent of Fe, Mn was higher than that of Cu and Zn in bush and herbal, the order of concent of the same trace elements in different life form species ranked as follow:herbal>bush>arbor. Content of different trace elements of the ground cover in same forest type was in the order as:Fe>Mn>Cu>Zn.The total amount of trace elements were 1490.679,679.277,1088.789,202.772 and 332.781 g·hm-2 in Pinus massoniana pure forests, Cinnamomum camphora pure forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests, Michelia chapensis and Manglietia insignis mixed forests, respectively. The amount of accumulation presents was Mn>Fe>Zn>Cu in Pinus massoniana pure forests and other four different forests were Mn>Fe>Cu>Zn, the proportionment of trace elements accumulation in different organs was positively related to the biomass of these respective organs. In 0-60cm layer soil, Pinus massoniana pure forests showed the highest content of Cu, Fe, Zn, Mn, Cinnamomum camphora pure forests was the second, the last was Elaeocarpus sylvestris and Cinnamomum camphora mixed forests. Compared with the control, Cu, Fe, Zn, Mn total accumulation were all decreased. The order of accumulation of trace elements was in Fe> Mn> Zn> Cu. Accumulation were increased with Cu, Zn and decreased with Fe. Mn was incresed except in Pinus massoniana pure forests. In these five modle of young ecosystems, the trace elements capacity in 0-60cm soil was the highest, accounted for more than 99% of the total capacity in trace elements of ecological system, vegetation layer took the second place, was up to 0.002%, and dead vegetation layer was the least.The mean carbon densities of all organs were 0.5296,0.5188,0.5178,0.5376, and 0.5355 gC·g-1 for the tree species Pinus massoniana, Cinnamomum camphora, Elaeocarpus sylvestris, Michelia chapensis, and Manglietia insignis, respectively. The average carbon densities in different organs of the five tree species ranked in the order as:trunk>root>leaf >branch>bark. Carbon densities ranged from 0.4380 to 0.5380gC·g-1,0.5060 to0.5200gC·g-1 and 0.00786 to 0.01485 gC·g-1 in living-understory layer, litterfall layer and soil layer (0～60 cm depth), respectively within the five afforestation patterns. Three years later after the conversion of farmland to forests, soil organic carbon density was increased 53.6%,39.2%, 38.6%, and 24.8% in Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests and Cinnamomum camphora forests, but decreased 18.7% in Pinus massoniana forest when compared with pre-converted farmland. Carbon storage in different organs was positively related to the biomass of these respective organs. It was found that Elaeocarpus sylvestris and Cinnamomum camphora mixed forest had the highest carbon stocks in the tree biomass pools (1.22 tC·hm-2), while Elaeocarpus sylvestris and Michelia chapensis mixed forest had the lowest value (0.20 tC·hm-2). Trunk and root stored more carbon than other organs, and the two organs accounted for 57.5% of the total carbon storage in all tree organs. At the early stage of the five forested ecosystems, the soil layer (0～60 cm) had the largest carbon storage, ranging from 74.52 to119.31 tC-hm-2, and accounted for more than 96.2% of the total carbon storage in the ecosystems. Carbon storage in the plant layer ranged from 0.63 to 2.96 tC-hm-2, which accounted for 0.64 to 3.8% of the total carbon storage of the ecosystems. The litter fall layer had the smallest proportion of the carbon storage within the ecosystems. After conversion of farmland to forestland, the carbon storage was increased 19.48,27.72,41.64,26.82 tC·hm-2 in Cinnamomum camphora pure forests, Cinnamomum camphora and Elaeocarpus sylvestris mixed forests, Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests, respectively, but decreased 1.68 tC·hm-2 in Pinus massoniana pure forests. Our results indicated that'Grain for Green' program, especially in the patterns of farmland to mixed forests, had great potential to enhance carbon sequestration in terrestrial ecosystems of southern China.Cinnamomum camphora pure forests was the most effective in the protection of water resources effectiveness, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests was the worst; Michelia chapensis and Manglietia insignis mixed forests was the most effective for the release of O2 in solid C's effectiveness, Pinus massoniana pure forests was the worst, even showed negative. Michelia chapensis and Manglietia insignis mixed forests was the highest for ecology efficiency, was up to 90794.32 yuan-hm-2·a-1, the second was Elaeocarpus sylvestris and Michelia chapensis mixed forests, up to 71471.14 yuan-hm-2·a-1; the last was Pinus massoniana pure forests, only 9791.88 yuan-hm-2·a-1. It showed that different modle of 'Grain for Green' had different characteristic and advantages, mix forests were better than singleness forest and the mix forests with hardwood and evergreen was the best.
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