克氏针茅草原的土壤种子库特征与植被恢复

近几十年来，由于过度放牧和不合理的农业开垦，内蒙古的天然草原退化严重。自2000年起，中国政府制定了“退耕还林还草”政策，并在草原地区实行了“禁牧、轮牧、休牧”等草原管理措施，期望依靠自然力来恢复退化的草原。然而，这些政策究竟能够带来什么样的后果还是一个未知数。土壤种子库和种子雨的资料对于全面理解植物种群和群落的动态具有重要价值。退化生态系统的恢复评价应该包括土壤种子库和种子雨的资料。因此，我调查了不同利用方式下克氏针茅（Stipa krylovii）草原的土壤种子库、种子雨和地上植被组成，来探讨政府的恢复政策对于该地区草原生态系统的影响。 研究地点位于中国科学院多伦恢复生态学试验示范研究站的永久实验样地，该实验样地在内蒙古自治区锡林郭勒盟的多伦县境内，该地区属于典型的农牧交错区。 采用幼苗萌发法，本研究测定了三种具有不同土地利用历史的样地的土壤种子库密度和物种组成。它们分别是：放牧样地、围封样地和弃耕样地。主要的研究结果有： （1）弃耕样地的土壤种子库密度最大，且主要是一年生和杂草类植物的种子，弃耕地由于其开垦的历史导致种子库的物种组成与放牧样地和围封样地的种子库有明显区别，其种子库缺少合适的多年生植物的种子是限制弃耕地自然恢复为天然草原的重要因素。 （2）放牧虽然会降低土壤种子库的密度，但是其中并不缺少多年生植物的种子。恢复由过度放牧引起的退化草原比恢复由农业开垦导致的退化草原要容易一些。围封对于由过度放牧引起的草原退化有明显的积极作用。 （3）采用Sorensen指数来计算地上植被与土壤种子库的相似性，这三个样地土壤种子库与地上植被的物种相似性均较高。在地上植被生物多样性的调查中，不能将草原中的一些早春植物遗漏。 采用网筛分离法，本研究测定了四种具有不同土地利用历史的样地的种子雨密度和物种组成。它们分别是：放牧样地、围封样地、割草样地和弃耕样地。主要的研究结果有： （1）不同样地间种子雨的密度差异显著（p < 0.05）。围封样地的种子雨密度最大，弃耕样地和放牧样地的种子雨密度居中，割草样地的种子雨密度最小。 （2）弃耕样地的地上植被和种子雨中均存在大量的一年生植物，这可能需要放牧或者割草等其它的一些管理措施来去除一年生植物的优势，从而加快弃耕地向天然草原的恢复速度。 （3）围封样地和割草样地的种子雨和地上植被中的物种数目均比放牧样地的要少，且围封样地中多年生非禾草类植物的比例大大增加，说明禁牧会改变草原植被的物种组成，这些究竟给草原植被的恢复带来怎样的影响，需要进一步的研究。 （4）克氏针茅草原整个取样期间种子雨的降落是连续的，因此在一些长期土壤种子库的研究中应该结合种子雨的数据，不能简单的将7月份的土壤样品作为长期土壤种子库。 采用网筛分离法，本研究测定了不同留茬高度的刈割实验样地的土壤种子库密度和物种组成。主要研究结果有： （1）无割草处理的对照样地的土壤种子库密度与其它割草处理样地的土壤种子库密度之间差异不显著。可能的原因是刈割的实验小区过小，植物种子的散布能够从不割草样地传播到割草处理的样地，或者动物的采食以及搬运降低了对照样地中种子的数量。 （2）虽然多年生禾草类植物在地上植被中占优势地位，但其在土壤种子库中的密度很小，均不到土壤种子库总量的10%。可见无性繁殖对于保持多年生禾草类植物在植被中的优势地位具有重要的意义，而相反的是非禾草类植物在土壤中保存大量的种子作为种群扩大的重要手段。

Effects of forage composition and growing season on methane emission from sheep in the Inner Mongolia steppe of China

Understanding the effects of dietary composition on methane (CH4) production of sheep can help us to understand grassland degradation resulting in an increase of CH4 emission from ruminant livestock and its resulting significance affecting CH4 source/sink in the grazing ecosystem. The objective of this study was to investigate the effect of forage composition in the diet of sheep in July and August on CH4 production by sheep in the Inner Mongolia steppe. The four diet treatments were: (1) Leymus chinensis and Cleistogenes squarrosa (LC), (2) Leymus chinensis, Cleistogenes squarrosa and concentrate supplementation (LCC), (3) Artemisia frigida and Cleistogenes squarrosa (AC), and (4) Artemisia frigida, Cleistogenes squarrosa and concentrate supplementation (ACC). CH4 production was significantly lower in July than in August (31.4 and 36.2 g per sheep-unit per day, respectively). The daily average CH4 production per unit of digestive dry matter (DM), organic matter (OM) and neutral detergent fiber (NDF) increased by 10.9, 11.2 and 42.1% for the AC diet compared with the LC diet, respectively. Although concentrate supplementation in both the AC and LC diets increased total CH4 production per sheep per day, it improved sheep productivity and decreased CH4 production by 14.8, 12.5 and 14.8% per unit of DM, OM and NDF digested by the sheep, respectively. Our results suggested that in degraded grassland CH4 emission from sheep was increased and concentrate supplementation increased diet use efficiency. Sheep-grazing ecosystem seems to be a source of CH4 when the stocking rate is over 0.5 sheep-units ha(-1) during the growing season in the Inner Mongolia steppe.

The effect of land management on plant community composition, species diversity, and productivity of alpine Kobersia steppe meadow

Grassland degradation is widespread and severe on the Tibet Plateau. To explore management approaches for sustainable development of degraded and restored ecosystems, we studied the effect of land degradation on species composition, species diversity, and vegetation productivity, and examined the relative influence of various rehabilitation practices (two seeding treatments and a non-seeded natural recovery treatment) on community structure and vegetation productivity in early secondary succession. The results showed: (1) All sedge and grass species of the natural steppe meadow had disappeared from the severely degraded land. The above-ground and root biomass of severely degraded land were only 38 and 14.7%, respectively, of those of the control. So, the original ecosystem has been dramatically altered by land degradation on alpine steppe meadow. (2) Seeding measures may promote above-ground biomass, particularly grass biomass, and ground cover. Except for the grasses seeded, however, other grass and sedge species did not occur after seeding treatments in the sixth year of seeding. Establishment of grasses during natural recovery treatment progressed slowly compared with during seeding treatments. Many annual forbs invaded and established during the 6 years of natural recovery. In addition, there was greater diversity after natural recovery treatment than after seeding treatments. (3) The above-ground biomass after seeding treatment and natural recovery treatment were 114 and 55%, respectively, of that of the control. No significant differences in root biomass occurred among the natural recovery and seeded treatments. Root biomass after rehabilitation treatment was 23-31% that of the control.