陕北固沙林恢复过程中土壤碳氮组分库特征与固存效应

选取陕北毛乌素沙地从半固定沙地到恢复2354a的灌木和乔木固沙林地,采用密度分组法分析表层土壤轻、重组分碳氮含量、C/N的演变及累积速率、固定碳氮贡献率特征。结果表明:固沙林从恢复2354a,乔木和灌木林土壤轻组碳分别增加了14.040.6倍和8.819.2倍,显著高于对应重组碳3.27.7倍和3.58.1倍的增幅;对应轻组氮分别增加了14.540.9倍和11.829.1倍,也显著高于重组氮4.68.5倍和4.412.6倍的增幅,说明轻组碳氮相对重组碳氮对固沙林恢复更加敏感。土壤轻重组碳氮含量增加使得乔木和灌木林轻组碳密度增速分别达0.57mg·hm^-2·a^-1和0.26mg·hm^-2·a^-1,重组碳密度增速则仅为0.18mg·hm^-2·a^-1和0.20mg·hm^-2·a^-1;同时,轻组氮密度增速分别达0.03mg·hm^-2·a^-1和0.02mg·hm^-2·a^-1,重组氮密度增速则分别达0.02mg·hm^-2·a^-1和0.04mg·hm^-2·a^-1。按此碳氮组分增速,到固沙林恢复54a时,乔木林和灌木林土壤轻组碳可分别贡献75.9%和59.4%的全有机碳增量;土壤重组氮则可贡献44.6%和63.9%的全氮增量。另外,恢复54a两种林地土壤重组C/N分别比半固定沙地降低11.4%和38.5%。但轻组C/N在乔木林并无显著变化,在灌木林恢复2354a土壤轻组C/N降低了21.7%31.0%,显著改变了土壤碳库性质。表明陕北固沙林恢复土壤表现出显著的固定碳氮效应,并且乔木林有更好的固碳能力,灌木林则有较好的固氮效应。

Sequestration Efficiency and Component Characteristics of Soil Carbon and Nitrogen Contents during Restoration of Sand-Fixing Forests in North Shaanxi Province

The soil samples were collected from 0^-10 cm topsoil in the semi-fixed sand land and fixed sand land under the shrubberies and arbor forests regenerated for 23-54 years in the MU US sandy land in North Shaanxi Province. Each soil sample was separated as the light and heavy fractions by density grouping method,and the evolution and accumulation rates of organic carbon and total nitrogen,C/N and the characteristics of contribution proportion for sequestrating carbon and nitrogen in these two soil fractions were analyzed. The results indicated that the organic carbon contents in soil light fraction under the shrubberies and arbor forests regenerated for 23-54 years increased by 14. 0-40. 6 times and 8. 8^-19. 2 times respectively,which were significantly higher than those in the corresponding heavy fractions (3. 2-7. 7 times and 3. 5-8. 1 times). In the same period of vegetation regeneration,the total nitrogen contents in soil light fraction under the arbor forests and shrubberies were increased by 14. 5-40. 9 times and 11. 8^-29. 1 times respectively,which were also significantly higher than those of the corresponding heavy fractions (4. 6-8. 5 times and 4. 4^-12. 6 times). These results indicated that the soil organic carbon and total nitrogen in light fraction were more sensitive to the desertification reversion than that in heavy fraction. Increase of the contents of organic carbon and total nitrogen in the light fraction and heavy fraction of soil also made the growth rates of organic carbon density of light fraction under the forests and shrubberies reach to 0. 57 mg·hm^-2·a^-1 and 0. 26 mg·hm^-2·a^-1 respectively. The growth rates of the organic carbon density in heavy fraction under the forests and shrubberies were only 0. 18 mg·hm^-2·a^-1 and 0. 20 mg·hm^-2·a^-1 respectively.Moreover,the growth rates of the total nitrogen density in light fraction reached to 0. 03 mg·hm^-2·a^-1 and 0. 02 mg·hm^-2·a-,and they in heavy fraction reached to 0. 02 mg·hm^-2·a^-1 and 0. 04 mg·hm^-2·a^-1,respectively. According to these growth rates of carbon and nitrogen fractions,the carbon of light fraction under the forests and shrubberies could contribute 75. 9% and 59. 4% of the total organic carbon increment respectively during the54-year restoration. The nitrogen in heavy fraction could contribute 44. 6% and 63. 9% of total nitrogen increment respectively. In addition,the C/N ratios of soil heavy fraction under the two woodlands regenerated for 54 years were significantly reduced by 11. 4% and 38. 5% respectively compared with those in semi-fixed sand land.However,there was no significant change of C/N in the light fraction under the forests,and the C/N in the light fraction under the shrubberies regenerated for 23-54 years was decreased by 21. 7%-31. 0%. As a result the quality of soil carbon pool was improved significantly. Therefore,the soil of restoration under the sand-fixing forests in North Shaanxi affected significantly the sequestration of carbon and nitrogen. Moreover,arbor forests has better carbon sequestration ability and shrubberies has better nitrogen sequestration effect.