水、肥添加对内蒙古农牧交错区不同土地利用历史的草地群落结构和稳定性的影响

草地是我国陆地面积最大的生态系统类型，它在维持我国自然生态系统格 局、功能以及过程方面具有特殊的生态学意义。内蒙古草原是我国北方最重要的自然群落类型，对于维持当地的生态平衡和社会经济的可持续发展起着重要的作用。由于历史上人类的过度放牧以及开垦农田，加上气候的变化使得内蒙古草地大面积退化，并引发了许多生态、环境问题。研究土地利用历史对草地植物群落结构、组成和稳定性的影响对于退化草地的恢复和管理具有重要的意义。通过适当的水、肥管理措施加快退化草地的植被恢复，改善其生态服务功能是当前的紧迫任务，同时也将对理解未来气候变化对草地群落的影响具有重要作用。 本试验进行了内蒙古农牧交错区不同土地利用历史的草地（围封的天然草地和弃耕的草地）植物群落结构和稳定性的对比研究，并且讨论了模拟降水增加和氮、磷添加及其互作效应对群落结构、组成和稳定性的影响。结果表明： 1. 土地利用历史对草地植物群落的结构和稳定性产生了显著的影响。围封的天然草地比农田弃耕地具有更高的物种丰富度、均匀度和植被盖度。历史上草地的垦殖使群落结构发生极大的变化，造成物种多样性和均匀度的明显降低，同时，也降低了群落的稳定性，使弃耕地在环境的变化下更加脆弱。 2. 水分的添加显著地改善了群落的结构和功能。增水的处理增加了物种丰富度、密度、植被盖度和生产力。同时，添加水分也提高了群落的稳定性，对于加快内蒙古退化草地的恢复具有明显的作用。 3. 添加氮肥增加了植被盖度和群落的生产力，同时改变了群落的物种组成，增加了禾本科草的比例，尤其是增加了围封样地中克氏针茅（Stipa krylovii Roshev.）的盖度，在弃耕地主要增加了冰草（Agropyron cristatum (Linn.) Gaertn.）的盖度。不同的氮肥梯度之间只对植被盖度的影响存在差异，在功能群水平上，这种差异则存在于对禾本科草盖度的影响中。添加氮肥增大了物种丰富度在时间上的变异，在一定程度上降低了群落的稳定性。添加磷肥只增加了群落地上生物量，对群落结构和稳定性都没有显著影响。氮、磷肥同时添加增加了草地群落的生产力和植被盖度，在功能群水平上增加了禾草的盖度。二者的共同作用也增大了物种丰富度在时间上的变异，降低了群落的稳定性。 4. 同时添加水分和氮肥增加了植被盖度，在围封样地同时增加了禾草和杂类草的盖度，在弃耕地只增加了禾草的盖度；水分和氮肥结合也降低了群落盖度在时间上的变异。同时添加水分和磷肥改善了群落结构，增加了植被盖度、密度及地上生物量。在功能群水平上，增加了围封样地杂类草的盖度。同时添加水分和磷肥减小了植被盖度的时间变异，有利于群落的稳定。水分、氮肥和磷肥共同添加增加了植被盖度和群落的生产力，并且比其他处理组合效果更显著。在水、氮、磷共同作用下，禾草和杂类草盖度同时增加。同时添加水分、氮肥和磷肥也降低了群落盖度在时间上的变异。 由此可见，水分是决定内蒙古农牧交错区草地植物群落结构和生产力的最主要的环境因子，氮素对群落的结构和生产力起到调节的作用，而磷素只有在水分较为充足的条件下才能对群落产生显著影响。水分和养分的结合能够有效地改善群落的结构和功能，增加群落的稳定性，进而加快退化草地的恢复进程。

Cementing mechanism of algal crusts from desert area

34-, 17-, 4-, 1.5-year old natural algal crusts were collected from Shapotou Scientific Station of the Chinese Academy of Sciences, 40-day old field and greenhouse artificial algal crusts were in situ developed in the same sandy soil and the same place (37degrees27'N, 104degrees57'E). Their different cohesions both against wind force and pressure were measured respectively by a sandy wind-tunnel experiment and a penetrometer. On the basis of these algal crusts, the cementing mechanism was revealed from many subjects and different levels. The results showed that in the indoor artificial crusts with the weakest cohesion bunchy algal filaments were distributed in the surface of the crusts, produced few extracellular polymers (EPS), the binding capacity of the crusts just accomplished by mechanical bundle of algal filaments. For field crusts, most filaments grew toward the deeper layers of algal crusts, secreted much more EPS, and when organic matter content was more than 2.4 times of chlorophyll a, overmuch organic matter (primarily is EPS) began to gather onto the surface of the crusts and formed an organic layer in the relatively lower micro-area, and this made the crust cohesion increase 2.5 times. When the organic layer adsorbed and intercepted amounts of dusts, soil particles and sand grains scattered down from wind, it changed gradually into an inorganic layer in which inorganic matter dominated, and this made the crusts cohesion further enhanced 2-6 times. For crust-building species Microcoleus vaginatus, 88.5% of EPS were the acidic components, 78% were the acidic proteglycan of 380 kD. The uronic acid content accounted for 8% of proteglycan, and their free carboxyls were important sites of binding with metal cations from surrounding matrix.

Effects of Prenatal Exposure to a 50-Hz Magnetic Field on One-Trial Passive Avoidance Learning in 1-Day-Old Chicks

We investigated memory impairment in newly hatched chicks following in ovo exposure to a 50-Hz magnetic field (MF) of 2 mT (60 min/day) on embryonic days 12-18. Isolated and paired chicks were used to test the effect of stress during training, and memory retention was tested at 10, 30, and 120 min, following exposure to a bitter-tasting bead (100% methylanthranilate). Results showed that memory was intact at 10 min in both isolated and paired chicks with or without MF exposure. However, while isolated chicks had good memory retention levels at 30 and 120 min, those exposed to MF did not. The results suggest a potential disruption of memory formation following in ovo exposure to MF, with this effect only evident in the more stressed, isolated chicks. Bioelectromagnetics 31:150-155, 2010. (C) 2009 Wiley-Liss. Inc.