荒漠草原土壤种子库对草地管理方式的响应

以宁夏中部不同封育年限(封育1a、3a、5a和7a)、不同放牧方式(中等强度自由放牧、中等强度4区轮牧)、补播改良(2006年补播沙打旺Astragalus adsurgens和紫花苜蓿Alfalfa stiva)及未封育等管理方式下的荒漠草原为研究对象,采用空间梯度代替时间梯度法,于2012年3月下旬用样线法采集土壤样品,采用温室萌发法,统计土壤种子库物种组成和种子数,研究了不同草地管理方式下的种子库与地上植被相似性、多样性等特征。结果表明:1)宁夏荒漠草原土壤种子库中共出现14种植物,分属6科14属,除牛枝子(Lespedeza potanimii)为半灌木外其余均为草本植物。多年生植物占土壤种子库植物总数的65.3%,一年生植物占土壤种子库植物总数的35.7%;多年生植物种子数量随封育年限增加而表现为下降趋势,封育1a和未封育草地最多;2)不同管理方式下土壤种子库和地上植被的共有物种数为2—6种,封育时间延长导致土壤种子库物种与地上植被相似性下降,土壤种子库和地上植被共有物种数有减少趋势,自由放牧降低了土壤种子库和地上植被的共有物种数,补播恢复的草地土壤种子库中未出现补播物种;3)种子库物种多样性与管理方式无明显的相关性;种子库物种多样性指数4区轮牧草地最高,封育7a草地最低,物种丰富度指数封育1a草地最高,均匀度指数自由放牧草地最高,补播草地最低。若荒漠草原仅依靠土壤种子库恢复自然植被,则不建议封育时间过久,需间以适当的干扰(放牧);补播外来种未对荒漠草原土壤种子库产生贡献,反而对乡土种产生了一定限制。

Response of a soil seed bank to modes of grassland management on a desert steppe

We examined the composition of a soil seed bank in a desert steppe ecosystem under different modes of grassland management. The study was conducted on the desert steppe in the middle of the Ningxia Hui Autonomous Region (NHAR), in northern China. We collected soil seed bank samples in late March 2012, along a gradient of grazing intensities that encompassed 8 distinct treatments: no grazing in enclosures (after a duration of 1, 3, 5 and 7 years); grazing (free range at a medium intensity, and four areas of rotational grazing at a medium intensity), replanting (with Astragalus adsurgen and Alfalfa stiva, in 2006) and open grazing. Following the method of greenhouse germination, soil samples were collected, planted, and left to sprout. The species composition and abundance of the resulting plants were determined, and characteristics of the soil seed bank such as the similarity of soil seed bank and aboveground vegetation, diversity of soil seed bank were also studied. The results were as follows. 1) In total, we recorded 26 plant species belonging to 6 families and 14 genera. Lespedeza potanimii was the only subshrub, while all other species observed were herbaceous plants. The main species in the seed bank of the desert steppe soil were perennial plants, accounting for 65.3%, while 35.7% of plants were annuals. The number of seeds decreased as enclosure time increased, and was highest in the soil from the one-year enclosure and grasslands with no enclosure. 2) We found 2-6 common species in the seed bank compared to what was growing what above ground in all treatments. Long-term enclosure of the steppe grassland may lead to a decline in similarity between the soil seed bank and above-ground vegetation, as the number of species shared between the seed bank and above-ground plant community decreased with as enclosure time increased. In the free range treatment, there was a lower number of shared species. The species which had been replanted to restore the desert steppe did not appear in the seed bank. 3) There was no significant relationship between soil seed bank diversity and management modes. The highest species diversity was measured in the four-area rotation grassland, while the lowest was in the grassland enclosed for 7 years. The highest species richness was found in the one-year enclosure. The highest evenness index was in the free grazing grassland, and the lowest was in the replanted grassland. Seed banks play a central role in the regeneration of vegetation, and managers can rely on soil-stored seed banks for restoration of desert steppe with appropriate grazing disturbance, more so than long-time enclosures. Alien species that were replanted did not benefit seed banks, and the establishment of introduced species may have restricted the germination of certain indigenous pioneer species.