Tibet College of Agriculture and Animal Husbandry,Nanjing Agricultural University
Northern Tibet lies within the remote regions of the Tibetan Plateau, features a dry and cold climate, and forms a broad strip of land sensitive to global warming. Meadows are the largest and most important ecosystems in northern Tibet. Alpine meadows are extensively distributed in the northern Tibetan Plateau and are typical features of the region. The alpine meadow soil type in the study area is seasonally frozen. We chose three representative plant communities as samples areas, which we identified as Potentilla spp., Kobresia pygmaea, and Kobresia littledalei based on the dominant species of each community to study soil nematode communities. The plots are situated at Dejixiangdaga village at N31°31'38″-31°32'02″, E92°04'03″-92°04'16″ in Naqu County in the Naqu region of the Tibet Autonomous Region, China. Soil nematode communities in the upper 0-30 cm soil were investigated in three typical plant communities in alpine meadows of northern Tibet from May to November 2011. The soil nematodes were collected using the shallow basin method. The following soil chemical properties were tested: pH, potentiometry, soil water content (oven drying method), and electricity conductivity. The trophic composition, c-p group structure and functional structure index, as well as the characteristic values between trophic groups, c-p groups and soil properties, were analyzed to understand the effect of cold alpine conditions on soil nematode functional diversity. A total of 33038 nematodes, which were classified to nematode phylum, 2 classes, 6 orders, 51 families and 93 genera, were collected during the surveys. Average individual density was 847 nematodes per 100g dry soil. The results were mostly within the scope of previous reports, and nematodes obviously appeared to aggregate in the upper soil layers. The research indicated soil nematode tropic composition varied from one plant community to another in alpine meadows. Herbivorous and bacterivorous nematodes were the major nematode communities in the area. The relative abundance of herbivorous to omnivorous and/or predatory nematodes obviously varied in different plant communities. An analysis of the constitution of the c-p groups showed all cp2 groups were the predominant groups in all three plant communities, and all c-p groups were mainly r-strategists. Kobresia pygmaea was the most common food source for nematodes. Kobresia littledalei had the lowest nematode density, possibly because limited food availability limits the reproduction of cp1 and cp2 nematodes. The result of analysis of the plant parasite index indicated the Potentilla community was easier for nematodes to populate when compared to the other two vegetation types, whereas the data from the maturity index (MI), PPI/MI and cp5 demonstrated nematode stability in Potentilla was highest among three vegetation types. The fungal to bacterial feeder ratio and Nematode Channel Ratio showed bacterial decomposition was the major decomposition pathway in the soil organic substance decomposition system. Correlation of nutritional groups and environmental factors showed omnivorous/predatory nematodes were affected significantly by water content during dry periods, while a close relationship was found between soil pH and fungivorous nematodes, which was obviously affected by electrical conductivity (EC) during periods of rapid growth. The abundance of bacterivorous nematodes was only correlated to pH during the dry season. The presence of different plant communities leads to effects on water content, soil formation and the distribution of nutrients. The analysis related to water content, pH and EC reveals plant communities have important influences on the functional structure of nematode communities. Differences in nematode community structure in different meadows preliminarily indicate the potential for using nematodes as environmental indicator species.