| Based on the investigation and analyses of properties of plant communities, seven different representative grassland types according to the degradation and utilizing pattern of Sanjiaocheng Sheep Breeding Farm in Qinghai Lake Region were selected to investigate their community diversities. They were severe degraded, heavily degraded, moderately degraded, lightly degraded, non-degraded, fenced and artificial grassland. The defference and variation features of soil carbon storage in different grassland, at different soil layers, and in different months were studied; the influencing factors were discussed as well. The main research results are as follows: 1. Community properties of different degraded grassland. Components, structure and species diversity property of plant communities of different degraded grade grasslands had changed greatly. The dominant plant species of plant communities successed from primary species such as Levmus secalinus, Poa crvmophila and Stipa purpurea,and companion plant species in the plant communities obviously successed because of long-term grazing on grassland.With aggravation of grassland degradation,species diversity,richness and evenness of plant communities were obviously decreased,at the same time, dominance of communities was gradually increased.However,species diversity, richness and evenness of plant communities in the enclosure grassland were apparently lower than that of moderately grazing grassland. 2. Soil characteristics The soil belongs to dark chestnut soil. The pH ranged from 8 to 9 and it increased with the depth of soil. The soil organic carbon and total nitrogen were decreased with the soil depth (p<0.05). It was also found that the content of soil organic matter increased with the increase of degradation, and the content in the enclosure and artificial grassland was lower that other grasslands. C/N was between 7~18. The soil phosphorus also differed among different grasslands and different soil layers. The similar trend was also found in available potassium, i.e. the content in top layer of the soil was more than other layers, and it was the highest in each layer of non-degraded grassland. 3. Variation of soil carbon storage of different degraded grassland It was showed that the content of soil carbon storage decreased with the increase of degradation degree. The soil carbon storage in enclosure grassland was lower than lightly degraded grassland; the total carbon storage in 0-40cm layer was close to that of heavily degraded grassland. The average storage of carbon in 0-40cm layer was as follows: severe degraded (7.76 kg/m2); heavily degraded (9.31 kg/m2); moderately degraded (9.47 kg/m2); lightly degraded (11.07 kg/m2); non-degraded (10.55 kg/m2); enclosure grassland (8.65 kg/m2), and artificial grassland (8.75 kg/m2). 4. Variation of the soil carbon storage at different soil layer Vertically distribution of soil carbon storage showed that, the deeper, the less the soil carbon, and it reached significant level (p<0.05 or p<0.01). It ranged from 2.50 to 5.16 kg/m2 within 0-10cm layer; 1.68 to 3.65 kg/m2 within 10-20cm layer; 0.82 to 2.81 kg/m2 within 20-30cm layer and 0.84 to 1.94 kg/m2 within 30-40cm layer. The total content within 0-40cm ranged from 6.60 to 12.69 kg/m2. 5. Variation of soil carbon storage in different months The soil carbon storage was high in May and low in June, and it increased after June. This tendency was similar in different degraded grassland. i.e. the lowest in June and the highest in October, and all deffereces reached significant level (p<0.05 or p<0.01). 6. Relationship between soil carbon storage and soil nitrogen storage There was a good relationship between soil carbon storage and nitrogen storage, and the variation tendency of nitrogen was similar with carbon. (1) The storage of soil carbon and nitrogen showed positive correlation. Coefficients of determination between them in each layer were that: 0.2996, 0.7544, 0.5873,0.3484 and 0.5968 respectively, and reached significant level (p<0.001). (2) The storage of soil carbon and nitrogen also showed positive correlation in different degradation. Coefficients of determination were 0.9499, 0.9383, 0.9337, 0.9201, 0.9163, 0.8413, 0.6919 respectively and reached significant level (p<0.001), and the order was: non-degraded> heavily degraded > severe degraded> moderately degraded> artificial grassland> lightly degraded> enclosure grassland. (3) It showed a positive correlation between the storage of soil carbon and the ratio of soil carbon and nitrogen (it was significant in 0-10cm and 20-30cm, p<0.01), and the total content of 0-40cm reached significant level (p<0.05). In terms of the storage of soil nitrogen, it showed a negative correlation. The value of correlation was less than that of soil carbon, and it reached significant level (p<0.05) only within 30-40cm. The correlation equation between soil carbon and nitrogen was:Y = 10.611X + 0.0294 (R2 = 0.9632,n = 210).(Y means the content of soil carbon, X means the content of soil nitrogen).7. Relationship between soil carbon storage and underground plant biomass At different degraded gradient, there was a rather positive correlation between the storage of soil carbon and underground biomass (p<0.001). Coefficients of determination were 0.8319, 0.7773, 0.7657, 0.8074, 0.8184, 0.7702, 0.7057, and the order was: severe degraded> heavily degraded > moderately degraded> lightly degraded> non-degraded> enclosure grassland >artificial grassland. The similar tendency was found among different months, i.e. positive correlation (p<0.001). Coefficients of determination were 0.7448, 0.7822, 0.7975, 0.8109, 0.8261 and 0.8418 from May to October. The correlation equation between soil carbon and underground biomass was: Y = 0.0033X + 1.1435(R2 = 0.7803,n = 210), (Y means the storage of soil carbon, X means the underground biomass). 8. Relationship between soil carbon storage and water content The influence of water content on the storage of soil carbon differed in different degraded grassland. For moderately degraded grassland and lightly degraded grassland, it showed a positive correlation (p<0.01), the effect of water on storage of soil carbon was significant, while in other grassland, it was not significant (p>0.05). Water content was not the main factor that caused the variation of the storage of soil carbon in different months and layers.
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