Aims Elevated atmospheric CO_(2)has the potential to enhance the net primary productivity of terrestrial ecosystems.However,the role of soil microorganisms on soil C cycling following this increased available C remains ambiguous.This study was conducted to determine how quality and quantity of plant litter inputs would affect soil microorganisms and consequently C turnover.Methods Soil microbial biomass and community structure,bacterial community-level physiological profile,and CO_(2)emission caused by different substrate C decomposition were investigated using techniques of biological measurements,chemical and stable C isotope analysis,and BIOLOG-ECO microplates in a semiarid grassland ecosystem of northern China in 2006 and 2007 by mixing three contrasting types of plant materials,C_(3)shoot litter(SC_(3)),C_(3)root litter(RC_(3)),and C4 shoot litter(SC4),into the 10-to 20-cm soil layer at rates equivalent to 0(C0),60(C60),120(C120)and 240 g C m2(C240).Important Findings Litter addition significantly enriched soil microbial biomass C and N and resulted in changes in microbial structure.Principal component analysis of microbial structure clearly differentiated among zero addition,C_(3)-plant-derived litter,and C4-plant-derived litter and among shoot-and root-derived litter of C_(3)plants;soilmicroorganismsmainly utilized carbohydrates without litter addition,carboxylic acids with C_(3)-plant-derived litter addition and amino acidswith C4-plant-derived litter addition.We also detected stimulated decomposition of older substratewith C4-plant-derived litter inputs.Our results showthat both quality and quantity of belowground litter are involved in affecting soil microbial community structure in semiarid grassland ecosystem.
Combined effects of ozone （O3） and cadmium （Cd） on growth and physiology of winter wheat （Triticum aestivum L. cv. JM22） were determined. Wheat plants were grown without or with Cd and exposed to charcoal-filtered air （〈 10 ppb O3） or elevated O3 （80 ＋ 5 ppb, 7 hr/day） for 20 days. Results showed that O3 considerably depressed light saturated net photosynthetic rate （-20%）, stomatal conductance （-33%）, chlorophyll content （-33%）, and total biomass （-29%） without Cd. The corresponding decreases were further enhanced by 45%, 56%, 60% and 59%, respectively with Cd, indicating a synergistic effect of O3 and Cd on wheat. Ozone significantly increased the activity of superoxide dismutase （46%）, catalase （48%） and peroxidase （56%）. However, great increases in malondialdehyde （MDA） content （2.55 folds） and intercellular CO2 concentration （1.13 folds） were noted in O3＋Cd treatment compared to control. Our findings demonstrated that the increased anti-oxidative activities in wheat plants exposed to O3＋Cd might not be enough to overcome the adverse effects of the combination of both pollutants as evidenced by further increase in MDA content, which is an important indicator of lipid peroxidation. Precise prediction model on O3 damages to crop should be conducted to ensure agricultural production security by considering environmental constraints in an agricultural system in peri-urban regions.
Aims Precipitation pulses and different land use practices(such as grazing)play important roles in regulating soil respiration and carbon balance of semiarid steppe ecosystems in Inner Mongolia.However,the interactive effects of grazing and rain event magnitude on soil respiration of steppe ecosystems are still unknown.We conducted a manipulative experiment with simulated precipitation pulses in Inner Mongolia steppe to study the possible responses of soil respiration to different precipitation pulse sizes and to examine how grazing may affect the responses of soil respiration to precipitation pulses.Methods Six water treatments with different precipitation pulse sizes(0,5,10,25,50 and 100 mm)were conducted in the ungrazed and grazed sites,respectively.Variation patterns of soil respiration of each treatment were determined continuously after the water addition treatments.Important Findings Rapid and substantial increases in soil respiration occurred 1 day after the water treatments in both sites,and the magnitude and duration of the increase in soil respiration depended on pulse size.Significantly positive relationships between the soil respiration and soil moisture in both sites suggested that soil moisture was the most important factor responsible for soil respiration rate during rain pulse events.The ungrazed site maintained significantly higher soil moisture for a longer time,which was the reason that the soil respiration in the ungrazed site was maintained relatively higher rate and longer period than that in the grazed site after a rain event.The significant exponential relationship between soil temperature and soil respiration was found only in the plots with the high water addition treatments(50 and 100 mm).Lower capacity of soil water holding and lower temperature sensitivity of soil respiration in the grazed site indicated that degraded steppe due to grazing might release less CO_(2) to the atmosphere through soil respiration under future precipitation and temperature scenarios.
Rhus typhina, an alien species introduced from North America, was identified as a main afforestation species in Beijing municipality. However, its invasiveness is still at odds. To clarify this problem, we applied the North American Screening System and the Australian Screening System to preliminarily predict its invasion possibility. Both screening systems gave the same recommendation to ＂reject＂. The geographical distribution was surveyed, with the population features of R. typhina against the native plant communities being assessed. With anthropogenic assistance, R. typhina has been scattered on almost all habitats from downtown to mountains, including roadsides, farmlands and protected areas. As a clonal shrub, R. typhina possessed a high spreading rate, varying from 6.3 m/3 years at sterile habitats to 6.7 m/3 years at fertile ones. Significantly lower species richness, individual density and diversity were observed in the R. typhina community than those of the native Vitex negundo Linn.var. heterophylla （Franch.） Rehd. community at both sterile and fertile habitats. Continual wide plantation of R. typhina may further foster its population expansion, which helps the species to overcome spatial isolation, The fact that each root fragment can develop into a new individual makes R. typhina very difficult to be eradicated once established. From a biological point of view, we believe that R. typhina is a plant invader in Beijing. We therefore suggest the government should remove the name of R. typhina from the main tree species list in afforesUng Beijing.
Guangmei WangGaoming JiangShunli YuYinghao LiHui Liu
为了了解草原生态系统氨气挥发情况、历史放牧的影响及其影响机制,2005和2006年生长季期间,在长达17年放牧处理的样地上测定了5个放牧强度下的氨气挥发量。结果表明:氨气挥发随季节变化波动很大,春季和初秋高,晚秋和夏季低,变化范围为0.88-3.52 gN ha^(-1)d^(-1)。土壤pH值、NH_4^+-N浓度、湿度以及容重对氨气挥发控制作用较强。但是容重的影响与其他3个因子的影响作用相反,而容重的大小直接受不同放牧强度影响。虽然不同的放牧强度对氨气释放的影响在统计学上来说并不显著,但是,相对不放牧的情况而言,氨气挥发量在放牧情况下更大。而土壤氮浓度、植物生物量却在放牧的情况下更低。这些都意味着放牧草地生态系统比不放牧草地生态系统恢复起来会更慢。这种关于草原生态系统氨气挥发的研究意义重大,因为它有助于加强了解氮素丢失对草原各进程的抑制作用,同时对制订适宜的放牧及施肥措施从而达到合理利用资源的目的也具有重要的参考价值。
