Summary 1.To increase understanding of the water table's influence on both fine-root growth and wholeplant growth of tree species in arid and semi-arid regions, we exposed Populus alba L. cuttings to contrasting soil water conditions via various water table gradients. 2. One-year-old rooted cuttings were grown individually in pots containing sandy soil in a greenhouse for 90 days in four water-table depth treatments: constant depth at 45 cm from the soil surface, constant depth at 30 cm, constant depth at 15 cm, and fluctuating depths between 45 and 30 cm. Growth responses, biomass partitioning, and structure and morphology of leaves and roots were determined for harvested trees every 30 days. 3. Fine (< 1-mm diameter) root growth was affected by water-table levels and water profiles, even in the fluctuating water-table depth treatment. Fine root proliferation was inhibited below the water table and was stimulated in the layers just above the water table and near the soil surface. 4. At the whole-plant level, with deeper water tables, P. alba allocated more biomass to roots and root morphology changed, but total root length did not increase, suggesting that trees faced with soil water deficits are not likely to increase root surface area to obtain more water. 5. Total biomass and root length in each treatment were positively correlated and the relationships were similar among the treatments, although the growth responses varied under the various watertable conditions. These results suggest that variation in root length may have an important effect on the biomass of the rooted cuttings across a gradient of water-table depths.

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Nitrogen (N) is one of the most common limiting nutrients for primary production in terrestrial ecosystems. Soil microbes transform organic N into inorganic N, which is available to plants, but soil microbe activity in drylands is sometimes critically suppressed by environmental factors, such as low soil substrate availability or high salinity. Tamarisk (Tamarix spp.) is a halophytic shrub species that is widely distributed in the drylands of China; it produces litter enriched in nutrients and salts that are thought to increase soil fertility and salinity under its crown. To elucidate the effects of tamarisks on the soil microbial community, and thus N dynamics, by creating "islands of fertility" and "islands of salinity," we collected soil samples from under tamarisk crowns and adjacent barren areas at three habitats in the summer and fall. We analyzed soil physicochemical properties, inorganic N dynamics, and prokaryotic community abundance and composition. In soils sampled beneath tamarisks, the N mineralization rate was significantly higher, and the prokaryotic community structure was significantly different, from soils sampled in barren areas, irrespective of site and season. Tamarisks provided suitable nutrient conditions for one of the important decomposers in the area, Verrucomicrobia, by creating "islands of fertility," but provided unsuitable salinity conditions for other important decomposers, Flavobacteria, Gammaproteobacteria, and Deltaproteobacteria, by mitigating salt accumulation. However, the quantity of these decomposers tended to be higher beneath tamarisks, because they were relatively unaffected by the small salinity gradient created by the tamarisks, which may explain the higher N mineralization rate beneath tamarisks.

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