| The loess plateau is one of the most severe soil and water loss area in China. Thus, soil and water loss has seriously depleted soil erosion and vegetation degeneration. Vegetation restoration and reconstruction were essential to solving such problems and improving the ecological environment. Natural recovery and artificial reconstruction(i.e., plantation) were the main strategies on vegetation reconstruction. Therefore, a serious of ecological engineering’s were implemented such as "three north" shelterbelt project, natural forest protection project, and returning farmland to forest and grassland at the national scale. Consequently, ecological environment and carbon sequestration capacities in this region have been improved. However, carbon and nitrogen storage, rate and their potential of different forest ecosystems in the loess hilly region are poorly understood. With the long-term monitoring data from permanent sample plot and field investigation, this research conducted analysis on the biomass, C and N pools, sequestration rates of C and N, and C balance(Source and Sink) of two typical forest ecosystems of semi-arid area of Loess Plateau, i.e. the oak forest and the black locust plantation, using the method of sample plot inventory in combination of secondary analysis. Results showed that:(1) The biomass of the oak natural secondary forest and the black locust plantation were 183.58 and 163.51 t ha-1, respectively, of which the biomass of arborous layer accounts for 84.74% for the oak forest and 86.27% for the black locust plantation of the total vegetable layer. The richer undergrowth of the oak forest led to a higher percentage of biomass. The biomass in each layer was in the order of arborous layer > litter layer >shrub layer >fine root >herbaceous layer for the oak forest, and an order of arborous layer > litter layer >shrub layer >herbaceous layer >fine root for the black locust.(2)In the oak forest and black locust plantation ecosystems,vegetation carbon densities were 78.8 and 67.6 t ha-1, and nitrogen densities were 1.47 and 2.79 t ha-1 respectively. The tree layer has the highest percentage of total vegetation carbon densities, in which carbon densities for the oak forest and black locust plantation were 88.6% and 89.7% respectively. In the black locust plantation roots nitrogen density accounted for the proportion of tree layer is higher(24.7~26.0%). It associated with better nitrogen fixation ability of black locust root. Between two forests the carbon and nitrogen densities difference of vegetation layer were not significant(P < 0.05).(3)In the Loess Plateau, deep soil layers contribute considerable amounts to carbon and nitrogen storage and should be taken into account in the estimates. In the upper 100 cm, the SOC densities in the oak and black locust stands were 88.97 and 71.41 t ha-1, and STN averaged 8.14 t ha-1 in the oak forest and 7.04 tha-1 in the black locust plantation. The densities of SOC over area were greater in the oak forest(128.85 ± 9.74 t ha-1 than in the black locust plantation(117.83 ± 11.84 t ha-1), within 0–200 cm, and the differences were statistically significant(P <0.05) at most of the calculation depths. The change trend of STN density with soil depth is consistent with SOC in the two types of forest. The densities of STN within 0–200 cm depth were 12.08 ± 0.73 t ha-1 in the oak forest and 11.59 ± 0.74 t ha-1 in the black locust plantation. Within 100–200 cm, the contents of SOC and STN did not vary with depth and were nearly constant at 4 g kg-1and 0.4 g kg-1, respectively, in the oak forest and the SOC and STN were at 3 g kg-1and0.3 g kg-1in the black locust plantation. The values may be used to estimate the SOC and STN of the 100 ~ 200 cm layers for similar vegetation types in this region.(4) Most of the forest ecosystem carbon and nitrogen stored in soil. Ecosystem carbon densities(0-100 cm depth of soil layer) in the oak and black locust stands were 167.8 and 167.8 t ha-1,and nitrogen densities were 13.5 and 14.4 t ha–1, respectively. Soil carbon and nitrogen densities accounted for 51.0~53.0% of ecosystem, and nitrogen densities were 80.6% to 89.1%. If the soil densities calculated on a scale of 0-200 cm, organic carbon density of ecosystem in the oak and black locust were 207.7 and 186.3 t ha- 1, total carbon densities were respectively 601.0 and 612.6 t ha-1. Soil total nitrogen densities(0-200 cm) were 89.1% in the oak forest and 80.6% in black locust plantation. The black locust vegetation densities were obviously higher than the oak forest. However, in soil layer, especially in the topsoil nitrogen, densities of black locust were significantly lower than the oak forest. Between the two ecosystems, nitrogen densities differences were not significant.(5)The NPP in the oak forest and the black locust plantation were 18.21 t ha-1 and 15.20 t ha-1. Annual carbon and nitrogen sequestration rate were 8.91 t C ha-1 a-1and 0.136 t N ha-1a-1in the oak forest, and 7.57 t C ha-1 a-1and 0.193 t N ha-1a-1in the black locust plantation. Annual carbon sequestration of oak forest was significantly higher than that of black locust plantation. But nitrogen sequestration rate of the black locust plantation were significantly higher than that of the oak forest. The annual increment of carbon and nitrogen mainly came from litterfall.(6)The black locust plantation ecosystems in the research area are important carbon sinks. The oak forest ecosystem was also carbon sinks. The results show that Green for Grain Project and Tianbao engineering to slow the rise in CO2 concentration to have certain contribution. |
PDF Full Text Request