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River ecosystems have been disturbed and aggravated by increasing human activities including those activities wherein forest area is changed to agricultural area in riparian zones and intensive fertilizer usage in agriculture. These anthropogenic activities lead to an increase in the light received by the surface of the river and nutrient (nitrogen and phosphorus) enrichment in the river. Denitrification driven by microorganisms in sediments is the most important path for the removal of nitrogen from aquatic ecosystems. The alteration of light intensity influences the denitrification process owing to the change in the amount of available organic carbon produced via the metabolism of algae and photosynthetic bacteria. We added slow-releasing fertilizer into the river to simulate nutrient enrichment similar to the effect of fertilization from the agricultural industry. We also covered a part of the stream surface to achieve conditions similar to those of rivers in headwaters with thick forest coverage. We selected six streams to conduct manipulated experiments at the core area in the natural reserve in the upper Jinshui River, Hanjiang, to assess the responses of denitrification to the different treatments-nutrient addition and coverage. We also analyzed subsequent changes in community structure and diversity of bacteria using the encoding nirS gene by MiSeq high-throughput sequencing technology. The results showed that artificial shading above streams can reduce the denitrification activity of sediments, relative abundances of dominant bacteria (Dechloromonas) and the diversity of bacteria as indicated by the encoding nirS gene. Furthermore, nutrient addition into the stream increased the denitrification activity of sediments, whereas there was a decrease in relative abundance of dominant bacteria and the diversity of bacteria. The preliminary results confirmed the increase in denitrification activity of stream sediments as a result of increasing light intensity and nutrient content. This study has allowed us to understand the denitrifying capacity of rivers and provides scientific primary data for river management.

Issues around the loss of diversity caused by fast-growing tree plantations such as Eucalyptus have aroused controversy for many years.It is generally believed that Eucaluptus plantations bring about a decrease of plant′s biodiversity by influencing resource competitions,allelopathy,or the level of soil fertility.Above and below-ground of forest ecosystems interact implicitly.Complex interactions between above-belowground biodiversity may provide important feedbacks regulating ecosystem.However,seldom information was available on the soil organism′s diversity in Eucalyptus plantations.Furthermore,most of the previous studies were being conducted in the short-term rotation Eucalyptus plantations with a certain plantation age,which might limit our understanding of the actual plantation ecosystem process.Therefore,the abundance and diversity of plants and soil biodiversity were simultaneously measured across a range of eucalypt plantation ages(1—10 years) in southwest China to determine how Eucalyptus grandis afforestation of agricultural lands affected the native biodiversity.The present results showed that afforestation of E.grandis caused changes in plant and soil biodiversity with soil depth and was dependent on the stand age.The species richness,abundance and the diversity of understory plants and soil organisms showed a parallel development during the process of the succession,viz.both of them increased significantly with time but with a 'dip' around 4 years old.CCA analysis showed that the plant′s life forms transited from liana,hemicryptophytes and therophytes into perennial herbaceous-phanerophytes and phanerophytes.Bacteria quantitatively dominated the soil microbe in E.grandis regardless of plantation age.The individual proportion of saprozoic group quantitatively dominated in the soils of the all the study sites,followed by omnivorous group.The proportion of saprozoic groups decreased in 1—7 year old stands and then increased significantly with time.The proportion of omnivorous group increased as S shape with the plantation age.The proportion of predators and phytophage groups decreased significantly with time.The individual percentage of nematode decreased with the plantation age,but that of acarina and the number ratio of the acarina and collembola present the S shape increasing trend over time.The multiple comparisons test showed that plant diversity of the understory vegetation in 1,2 and 4—7 years-old plantations were significantly lower than that in the control forests(Pinus massoniana).Plant diversity in 1—10 year-old E.grandis plantations were significantly higher than that of the control arable land.Soil biodiversity in the plantations before the rotation period has no significant differences compared with agricultural lands,but were significantly higher in the plantations on or after the rotation period than the crop land.There were significant correlations between the plant′ s and soil organism′s diversity index and the correlation coefficient decreased with soil depth.

Eucalyptus grandis,a fast-growing timber species with outstanding economic efficiency,has been extensively planted as short-term industrial material in southern China.But what′s the difference of features in relation to water consumption between E.grandis and common or native tree species of Sichuan Province,whether or not the amount of water consumption of E.grandis would change the original water balance in its cultivation area remains to be answered.In this paper,the net photosynthetic rate(Pn),the transpiration rate(Tr) and the water use efficiency(WUE) of seedlings of E.grandis and other five tree species under different light intensities,air temperatures and atmosphere humidities were determined with the help of LI-6400 photosynthesis meter,the plant total leaf weight,biomass and the water consumption were measured using weighing method,then the water consumption characteristics of these species were evaluated and compared based on the indicators mentioned above.The results were showed as follows:(1) Of all the tested species,E.grandis possessed the highest Tr and the lowest WUE under the same condition,in addition the maximum water consumption per unit biomass,so the water consumption per plant of which was significantly larger than the others,its largest total leaf area was probably one of the important reasons.Its Pn was lower than that of P.× R270,but its biomass accumulation was the largest.We drew a conclusion that E.grandis was a tree species with strong photosynthesis,high transpiration,and low water use efficiency.(2) The Pn and the Tr of the broad-leaved tree species were obviously higher than those of the coniferous tree species,while the WUE of the former was lower than that of the latter.(3) Environmental factors(the light intensity,the air temperature and the atmosphere humidity) influenced the Pn,the Tr and the WUE of plants to a large extent.Particularly,the Pn was mainly affected by the light intensity,the Tr was most sensitive to the atmosphere humidity,and generally the WUE rose with the increase of the atmosphere humidity.In the range of variation temperature(24—32℃),which was set in the experiment,the photosynthesis of these tree species did not changed to a large extent.The light intensity of 800 μmol · m-2 · s-1 and the temperature of 28 ℃ in this study were observed most conducive to plant photosynthesis.(4) Fast-growing tree species such as E.grandis,grew more rapidly and were more potential to fix considerable carbon because of its comparatively stronger capability of photosynthesis,thus higher economic benefit could be obtained.However,in terms of the higher transpiration and lower water use efficiency,replacing the former vegetation(mainly composed by indigenous tree species) with large-scale plantations of E.grandis was more likely to be adverse to the environment of introduction areas because of more water consumption.Therefore,the area for E.grandis plantation should be in a good water condition,especially where abundant annual rainfall and uniform rainfall distribution were seen.Moreover,adequate attention should be paid to the scientific and rational water management.