关键词: 荒漠草原, 人工灌丛, 土壤种子库, 土壤水分

Abstract: This study explored the responses of the soil seed bank (0-5 and 5-10 cm soil depths sampled) to variation in precipitation and to encroachment by anthropogenically introduced shrubs (Caragana) in desert steppe grassland in Yanchi County, Ningxia Province. Simulated rainfall levels relative to mean annual precipitation of 290 mm (-60%, -40%, -20%, 0, 20%, 40%) were generated for eight otherwise similar sites with different vegetation categories: grassland fenced to preclude grazing (FY), grazed grassland (FM), and grassland with introduced shrubs established for 3, 12, or 22 years (3 sites, PY3, PY12, PY22) or at planting distances of 2, 6, or 40 m (3 sites, PS2, PS6, PS40). Species represented in the seedbank and their numbers were determined using germination counts after incubation of sampled soil in the field and in the greenhouse. A major finding was that in grassland with introduced shrubs, soil moisture for 0-200 cm soil depth was 8.14% when averaged across seasons and over PY sites, and 8.83% when averaged over PS sites, which was significantly lower (P<0.05) than corresponding values for FY and FM sites (16.00% and 13.00%, respectively). PS and PY sites also did not show higher soil moisture levels in spring than in autumn that were seen in FY and FM plots, and the soil moisture reduction at 40 m spacing was about 50% of that at 6 and 2 m shrub spacing. The number of species detected in the soil seed bank in field germination tests at the eight sites ranged between 4 and 7, comprising annual herbs, perennial herbs and shrubs, with total counts of 2965·m^-2 and 2065·m^-2, respectively, for FY and FM sites; 6198, 6465 and 5897·m^-2, respectively, for PY3, PY12 and PY22 sites; and 3898, 5897 and 7698·m^-2, respectively, for PS40, PS6 and PS2 sites. Of the total seed bank germination counts across the eight sites, 89.6% (range 87.12% to 96.55%) belonged to the annual herb Artemisia scoparia, and 70.04% came from the 0-5 cm soil layer. The number of species detected in glasshouse pot germination tests at different simulated rainfall levels, for samples from the eight sites, ranged between 1 and 5. For soil samples from all eight sites, the highest numbers of germinated seeds occurred in the +20% rainfall treatments and the lowest in the -60% treatment, while the +40% treatment was usually second lowest. For FY and FM sites the germinated seed density of the +20% treatment was 533 and 233·m^-2, respectively. For sites with introduced shrubs, peak germination was 2266·m^-2 and 1600·m^-2 averaged over PY and PS sites, respectively. In summary, at these sites, introduced Caragana shrubs depleted soil moisture at depth, and the soil moisture reduction increased the dominance in the seedbank of the annual herb Artemisia scoparia, and tended to reduce species richness of the seedbank.

Key words: desert grassland, anthropogenic introduced shrub, soil seed bank, soil moisture