Nutrient Cycling Of Different Pure Stands Of Planted Forest Ecosystems In The Loess Plateau

Material cycle is one of the main functions of forest ecosystems, including nutrient cycling of forest ecosystems play an important role in maintaining the stability and continuity. Soil properties polarization is a result of soil properties changes caused by long-term growth of plantation, one of the fundamental reason is the difference of nutrient cycling and use process of different artificial forest ecosystems composed by different tree species. We can fully understand the process and mechanism of soil properties polarization by more in-depth research from the root causes, and can also provide valuable scientific theory for preventing soil properties polarization. In this paper, nutrient cycling characteristics of 18 different forest ecosystems in 3 regions of the Loess Plateau were analyzed. The regions included the sand-blown region in the north part of the Loess Plateau, the hilly region in the middle part of the Loess Plateau, and the gullied region in the south part of the Loess Plateau. Indices of nutrient accumulation and distribution (including biomass, litter accumulation, and nutrient accumulation), nutrient cycling flux (annual absorption, retention, and return), and nutrient cycling efficiency (recycling coefficient, utilization ratio, and nutrient productivity) were calculated. Self-organizing Feature Maps (SOFM) was used for nutrient cycling classification. from the perspective of nutrient cycling in forest ecosystem in the Loess Plateau artificial classificationstudy. That study on nutrient cycling of different pure stands of planted forest ecosystems in the Loess Plateau, revealed nutrient cycling characteristics of the forest law, and explored the function classification method of forest ecosystem. So as to provide theorical basis for plantation management, preventing and controlling soil degeneration or polarization.Though experiment research, this paper gets the following results:(1) Different artificial forest ecosystems in Jingbian, Biomass of trees is more than shrubbery, compared with biomass of various components of trees, bole> branches> roots> leaves, biomass of Pinus sylvesris var. Mongolica forest and Populus simonii forest were more, respectively: 155.64 t/hm2 and 102.17 t/hm2. Litter accumulation of 8 different forest ecosystems were low, the average: 0.104 t/hm2. Nutrient accumulation of Pinus sylvesris var. Mongolica forest, nutrient accumulation of shrubbery was is lowest, and its value: 288.833 kg/hm2. Soil nutrient storage of K was most, followed by N and P, storge in shrubbery was more than trees. The amount of biological cycles of Populus simonii forest was highest, the lowest was Salix matsudana forest, biological cycle of nutrient element N was significantly higher than K and P. Absorption coefficient was high, but cycling coefficient was lower, nutrient utilization coefficient was insignificant, nutrient productivity showed: shrubbery was significantly higher than trees, which nutrient productivity of 3 nutrient elements was in order: P> K> N. There was significant loss of nutrient, which ratio of annual nutrient loss of soil of Populus simonii forest and Pinus sylvesris var. Mongolica forest were more, turnover was short, more prone to the negative soil properties polarization, so in the forest management should provide nutrient element.(2) In study area of Huangling, biomass and nutrient accumulation of 7 forest ecosystems were higher, the average value of nutrient accumulation was: 1107.821 kg/hm2, average value of litter accumulation: 2.511 t /hm2. Soil nutrient storage of Robinia pseudoacia forest was the highest, the storage of N was: 13020 kg/hm2, the storage of P was: 5369 kg/hm2, the storage of K was: 11125 kg/hm2; the amount of the biological cycling of Robinia pseudoacia forest was highest, and the annual absorption and annual return were higher. Absorption coefficient of Populus simonii forest and Robinia pseudoacia forest were more higher, cycling coefficient of Robinia pseudoacia forest and Pinus tabulaeformis forest were highest, nutrient utilization coefficient of different stands were insignificant different, nutrient productivity of Larix principis-rupprechtii forest and Robinia pseudoacia forest were more high, which value was more than 0.60. Ratio of annual nutrient loss of Populus simonii forest was more, turnover was short, nutrient consumption was rapid, negative polarization of soil properties was more likely.(3) In study area of Chunhua, biomass of 3 different forest ecosystems were in order: Pinus tabulaeformis forest> Robinia pseudoacacia forest> Platycladus orientalis forest, litter accumulation was highest in three study areas, was: 6.533 t/hm2. The nutrient accumulation was low, and nutrient accumulation of three elements in Robinia pseudoacacia forest were more high, in order: 251.958 kg/hm2, 17.765 kg/hm2, 35.637 kg/hm2. Soil nutrient storage of three stands were all more high, Pinus tabulaeformis forest> Robinia pseudoacacia forest> Platycladus orientalis forest. Biological cycling of three stands were in order: Robinia pseudoacacia forest> Pinus tabulaeformis forest> Platycladus orientalis forest, three elements in order: N>K>P. Among of characteristic parameters of biological cycle, absorption coefficient,cycling coefficient and utilization coefficient showed: Robinia pseudoacacia forest> Platycladus orientalis forest> Pinus tabulaeformis forest, nutrient productivity was in contrary order. Ratio of annual nutrient loss of Platycladus orientalis forest was most, nutrient turnover was more shorter, and was more likely to negative polarization of soil properties than Robinia pseudoacacia forest than Pinus tabulaeformis forest. (4) Classifing forest ecosystems from the perspective of nutrient cycling is feasible, with SOFM network model, using the secondary classification, the final result was divided into first-order calsses and six secondary-order types. Classification results were better to reflect characteristics law of different ecosystems' nutrient cycling. Where I-1 type and I-2 type of forest ecosystem presented: Nutrients quick cycling, high utilization coefficien, more nutrient accumulation, trees growed fast and well, mainly including Robinia pseudoacia, Populus simoni, Pinus tabulaeformis. When building classification index system, to consider the process from various aspects of nutrient cycling in selecteing indicators, avoiding error by simply selecting a few indicators for classification, so as to make the classification results more reasonable. SOFM network model can classify comples data mathematically and effectively. From the perspective of nutrient cycling classifing ecosystems, compared with traditional mathematical methods, the operation is simple and more objective. The final result was also better to reflect the nutrient cycling of the forest ecosystems. Finally, the feasibility of using SOFM for the classification of nutrient cycling in forest ecosystems has been demonstrated through this study.