六盘山四种森林生态系统的碳氮储量、组成及分布特征

Carbon and nitrogen storage and distribution in four forest ecosystems in Liupan Mountains,northwestern China

碳和氮是森林生态系统的重要组成元素,其含量有很大时空差异,并和立地及森林特征关系很大,需做大量的积累性调查才能得到其变化规律,尤其是加强在过去较少研究的西北地区的调查。在宁夏六盘山区选择华北落叶松(Larix principisrupprechtii)人工林、华山松(Pinus armandii)次生林、桦木(Betula platyphylla)次生林和野李子(Prunus salicina)灌丛4种典型森林,测定了乔木层(分不同器官)、灌木层、草本层、枯落物层、根系层(0—100 cm土壤)的碳、氮含量,分析了生态系统的碳、氮储量及成分组成和层次分布特征。结果表明,碳含量在不同乔木树种及其不同器官之间的差异不明显;但氮含量存在显著的树种差别和器官差异,以树叶的最高、树干的最低。灌木层和草本层的碳氮含量均表现为地上部分>地下部分。各森林样地的乔木层、灌木层、草本层的碳含量依次降低,但氮含量依次增高;枯落物层的碳含量低于各植被层,但氮含量高于各植被层;根系层土壤的碳、氮含量则随土层增深而递减。包括活植被层、枯落物层和根系层土壤在内的华北落叶松人工林、华山松次生林、桦木次生林、野李子灌丛的生态系统碳储量依次为364.56、450.98、640.02、196.55 t/hm2,氮储量依次为27.86、36.19、47.02、15.99 t/hm2。所有4种森林生态系统的根系层土壤的碳氮储量均占整个生态系统总储量的绝大部分,其比例对碳储量为84.69%—93.92%,氮储量为98.09%—98.64%。从乔木层、灌木层、草本层、枯落物层到根系层(土壤),呈现出C/N比依次减小的趋势;根系层土壤和整个生态系统的C/N比分别为华北落叶松林的11.84和13.12、华山松林的10.76和12.56、桦木林的12.48和13.52、野李子灌丛的11.70和12.29。

Carbon （C） and nitrogen （N） are important elements of forest ecosystems, and quantifying C and N storage and distribution is a critical step for better understanding of C and N cycles in forest ecosystems. C and N pools in forests vary greatly, depending on the forest type, forest growth stage, and site quality. The mechanisms controlling the spatial and temporal variation of C and N accumulation in forest ecosystems, especially in the northwestern China, have been sporadically investigated in the past. In this paper, the content, storage and distribution of C and N were investigated in four typical forests at the Liupan Mountains in Ningxia, northwestern China. The four forest types included Larix principis-rupprechtii plantation, Pinus armandii secondary forest, Betula platyphylla secondary forest, and Prunus salicina shrubs. The concentrations of C and N were measured in different organs of the trees, and the contents of C and N in the vegetation layers of trees, shrubs, grasses, and layers of humus, and soil （0-100 cm depth） were estimated. Storage, composition, and distribution of C and N in these ecosystems were examined. The results showed that the concentrations of C did not significantly differ among the different organs and tree species, but the concentration of N varied. The highest N concentration was found in the leaves and the lowest in the trunk. The contents of C and N were higher in the above-ground biomass than in the below-ground biomass for both shrubs and grasses. The C contents decreased in the order of tree layer〉 shrub layer〉 grass layer, but N contents increased in the order of tree layer〈 shrub layer〈 grass layer in the studied forests. The C content was lower in the humus layer than in each of the vegetation layers, but the situation was opposite for the N content. The contents of both C and N decreased with increasing soil depth. The total C storage in the four forest ecosystems was in the following order： B. platyphylla secondary forest （640.02 t/hm^2）〉 P. armandii secondary forest （450.98 t/hm^2）〉 L. principis-rupprechtii plantation （364.56 t/hm^2）〉 P. salicina shrub （196.55 t/hm^2）. The total N storage decreased from 47.02 t/hm^2 in B. platyphylla secondary forest to 36.19 t/hm^2 in P. armandii secondary forest, 27.86 t/hm^2 in L. principis-rupprechtii plantation and 15.99 t/hm^2 in P. salicina shrub ecosystems in the study sites. The amount of C and N in the soil （0-100 cm depth） accounted for the major proportion of the total C and N storage in the studied forest ecosystems, with the range of 84.7%-93.9% for C and 98.1%-98.6% for N for the four forest types. On average, the C/N ratio in the soil and in the entire ecosystem was 12.48 and 13.52 in B. platyphylla secondary forest, 11.84 and 13.12 in L. principis-rupprechtii plantation, 11.70 and 12.29 in P. salicina shrub, and 10.76 and 12.56 in P. armandii secondary forest, respectively. In addition, the C/N ratio decreased gradually from the tree layer to shrub layer, grass layer, humus layer, and mineral soil layer. Our study demonstrated that various forest types had different ability for the sequestration and accumulation of C and N in the studied area.