Changes in microbial activity and composition in a pasture ecosystem exposed to elevated atmospheric carbon dioxide

Elevated atmospheric CO₂ increases aboveground plant growth and productivity. However, carbon dioxide-induced alterations in plant growth are also likely to affect belowground processes, including the composition of soil biota. We investigated the influence of increased atmospheric CO₂on bacterial numbers and activity, and on soil microbial community composition in a pasture ecosystem under Free-Air Carbon Dioxide Enrichment (FACE). Composition of the soil microbial communities, in rhizosphere and bulk soil, under two atmospheric CO₂ levels was evaluated by using phospholipid fatty acid analysis (PLFA), and total and respiring bacteria counts were determined by epifluorescence microscopy. While populations increased with elevated atmospheric CO₂ in bulk soil of white clover (Trifolium repens L.), a higher atmospheric CO₂ concentration did not affect total or metabolically active bacteria in bulk soil of perennial ryegrass (Lolium perenne L.). There was no effect of atmospheric CO₂ on total bacteria populations per gram of rhizosphere soil. The combined effect of elevated CO₂ on total root length of each species and the bacterial population in these rhizospheres, however, resulted in an 85% increase in total rhizosphere bacteria and a 170% increase in respiring rhizosphere bacteria for the two plant species, when assessed on a per unit land area basis. Differences in microbial community composition between rhizosphere and bulk soil were evident in samples from white clover, and these communities changed in response to CO₂ enrichment. Results of this study indicate that changes in soil microbial activity, numbers, and community composition are likely to occur under elevated atmospheric CO₂, but the extent of those changes depend on plant species and the distance that microbes are from the immediate vicinity of the plant root surface.     

A stable isotope study of linkages between stream and terrestrial food webs through spider predation

SUMMARY 1. Transfer of carbon from freshwater to terrestrial ecosystems can occur through predation on adult aquatic insects, but the significance of this trophic pathway to the energetics of riparian communities is poorly understood. We used stable isotopes of carbon and nitrogen to explore linkages between aquatic insect production and the nutrition of web‐building and free‐living spiders alongside two streams in the North Island of New Zealand. 2. δ¹³C values for riparian tree leaves (means for each site = ?32.2 and ?30.3‰) were distinct from those of lichens collected from stream channel rocks and instream algae, both of which were similar (?23.4 to ?22.4‰). δ¹⁵N values for leaves were similar at both sites (?3.4 and ?2.7‰), but algae were considerably more depleted in δ¹⁵N atonesite suggesting significant differences in instream nitrogen sources between the twostreams. 3. Isotope values for potential aquatic prey of spiders indicated that aquatic algal production was their primary carbon source at both sites. Terrestrial invertebrates collected and assumed to be potential prey reflected a range of carbon sources and represented several trophic levels. 4. At one site, δ¹³C values indicated a primarily algae‐aquatic insect pathway of carbon transfer to both web‐building and free‐living spider guilds. The other site appeared to have a primarily terrestrial carbon pathway for the free‐living spider guild, and a mixed aquatic‐terrestrial pathway for the web‐building guild. 5. Overall, web‐building spiders were estimated to obtain around 61% of their body carbon from aquatic production compared with 55% for free‐living spiders. Our findings suggest that consumption of prey derived from aquatic sources can provide significant nutrition for spiders living along some stream channels. This pathway may represent an important feedback mechanism contributing to the energetics of riparian communities at sites where aquatic insect production is high.     

Aquatic invertebrates in riverine landscapes

1. Riverine systems consist of a mosaic of patches and habitats linked by diverse processes and supporting highly complex communities. Invertebrates show a high taxonomic and functional diversity in riverine systems and are in several ways important components of these systems. Their distribution patterns, movements and effects on ecological flows, testify to their importance in various landscape ecological processes. This paper reviews the invertebrate literature with respect to patterns and processes in the riverine landscape. 2. The distribution of invertebrates in riverine habitats is governed by a number of factors that typically act at different scales. Hence, the local community structure can be seen as the result of a continuous sorting process through environmental filters ranging from regional or catchment‐wide processes, involving speciation, geological history and climate, to the small‐scale characteristics of individual patches, such as local predation risk, substratum porosity and current velocity. 3. Dispersal is an important process driving invertebrate distribution, linking different ecological systems across boundaries. Dispersal occurs within the aquatic habitat as well as into the terrestrial surrounding, and also over land to other waterbodies. New genetic techniques have contributed significantly to the understanding of aquatic invertebrate dispersal and revealed the importance of factors such as physical barriers, synchrony of emergence and taxonomic affiliation. 4. Invertebrates affect the cycling of nutrients and carbon by being a crucial intermediate link between primary producers, detritus pools or primary consumers, and predators higher up in the trophic hierarchy. Suspension feeders increase the retention of carbon. The subsidies of aquatic invertebrates to the terrestrial ecosystem have been shown to be important, as are reciprocal processes such as the supply of terrestrial invertebrates that fall into the water. 5. Future studies are needed both to advance theoretical aspects of landscape ecology pertaining to the invertebrates in riverine systems and to intensify the experimental testing of hypotheses, for example with respect to the scaling of processes and to linkages between the terrestrial and aquatic systems. Another promising avenue is to take advantage of naturally steep environmental gradients, and of systems disturbed by humans, such as regulated rivers. By comparison with unimpaired reference sites, the mechanisms involved might be identified. The use of `natural' experiments, especially where environmental gradients are steep, is another technique with great potential.     

Effects of Pre-Planting Soaking on Growth and Survival of Black Willow Cuttings

Black willow (Salix nigra) uses periodic flood events for dispersal of vegetative propagules, subjecting them to periods of soaking before their deposition along the streambank. It was hypothesized that this life history trait results in optimal conditions for willow growth and survival. To test this hypothesis, a greenhouse experiment was conducted using 1.2‐m‐long black willow cuttings (posts) with a basal diameter of 5 cm. Cuttings were subjected to three soaking treatments (0, 3, and 10 days) and then grown under four soil moisture regimes (control, drought, permanently flooded, and intermittently flooded). Growth, biomass, and survival were recorded. Results showed that soaking posts for 10 days was most beneficial in the control soil moisture regime, enhancing root, shoot, leaf, and total biomass production. Shoot abundance and growth were also enhanced by 10 days of soaking in the control and permanently flooded moisture regimes. Finally, survival increased significantly in control and drought moisture regimes in response to the 10‐day soaking treatment. Results clearly demonstrated that soaking had significant effects on willow post success when evaluated across all soil moisture regimes. Posts subjected to the 10‐day soaking treatment consistently had greatest growth and biomass and displayed a doubling of the survival rate. Therefore, soaking willow cuttings before planting is a simple, inexpensive technique that may be used to bolster streambank restoration success.     

Pacific Salmon,Nutrients, and the Dynamics of Freshwater and Riparian Ecosystems

Pacific salmon (Oncorhynchus spp.) accumulate substantial nutrients in their bodies as they grow to adulthood at sea. These nutrients are carried to predominantly oligotrophic lakes and streams, where they are released during and after spawning. Research over more than 3 decades has shown that the annual deposition of salmon-borne marine-derived nutrients (MD-nutrients) is important for the productivity of freshwater communities throughout the Pacific coastal region. However, the pathways and mechanisms for MD-nutrient transfer and accumulation in freshwater and riparian ecosystems remain virtually unexplored, consequently, there are many uncertainties in this area. This article addresses three related topics. First, we summarize recent advances in our understanding of the linkages among MD-nutrients, freshwater (including riparian) ecosystems, and community dynamics by addressing the importance of MD-nutrients to lakes and streams and by then reviewing large-scale and long-term processes in the atmosphere and ocean that govern variability in salmon populations. Second, we evaluate the validity of the discoveries and their implications for active ecosystem management, noting areas where extrapolation from these results still requires great caution. Finally, we outline five key research issues where additional discoveries could greatly augment our understanding of the processes shaping the structure and dynamics of salmon populations and the characteristics of their freshwater habitat and associated riparian zones. Collectively, the data suggest that the freshwater portion of the salmon production system is intimately linked to the ocean. Moreover, for the system to be sustainable, a holistic approach to management will be required. This holistic approach will need to treat climate cycles, salmon, riparian vegetation, predators, and MD-nutrient flowpaths and feedbacks as an integrated system. Received 3 July 2001; accepted 14 December 2001.     

Adult aquatic insects: Potential contributors to riparian food webs in Australia's wet–dry tropics

Abstract Changes in the abundance and biomass of aquatic and terrestrial aerial insects with distance (mid‐stream, 0, 10–15 and 160 m) from lowland streams were examined across the dry season landscape in Kakadu National Park, northern Australia. Malaise traps and sticky intercept traps were used to sample the insects at four streams, spaced over an area of 1650 km². Malaise and intercept catches were dominated by Diptera (flies and midges), both numerically and by biomass. Chironomid midges were the most abundant taxon, making up 43.4 and 51.0% of the malaise and intercept trap catches, respectively. However, most chironomids were small (less than 3 mm body length), contributing 34.9% to intercept trap biomass, but only 5.2% in malaise traps. Ceratopogonid midges and caddisflies (Trichoptera) accounted for most of the remaining adult aquatic insects. Major terrestrial components were Diptera and Hymenoptera in malaise traps and Coleoptera and Diptera in intercept traps. The total abundance and biomass of insects were much greater over streams and along the water's edge than in riparian (10–15 m) and savanna (160 m) habitats primarily because of the presence of large numbers of adult aquatic insects. The abundance and biomass of terrestrial insects in malaise traps showed no relationship with distance, but intercept trap catches suggested slightly greater abundances over the water and at the water's edge. The great abundance of aquatic insects relative to terrestrial insects close to streams suggests that they have the potential to be an important component of the diets of riparian insectivores, and predation may be an important pathway by which aquatic nutrients and energy are moved into terrestrial food webs.     

Effects of hydrology on spatial patterns of soil development in created riparian wetlands

Surficial soil development was studied in four wetland basins created on the floodplain of the Des Plaines River near Chicago, Illinois, USA. These studies determined changes in the spatial distribution of plant-available nutrients as a result of establishing two different wetland hydrologic regimes. Three wetland basins had mineral soils and one an organic soil. A geostatistical analysis including kriging of collected data indicated that all soil parameters showed significant changes in their spatial structure as a result of the water inputs and unidirectional flows. The degree of spatial variability as indicated by autocorrelation in the soil data (i.e., points closer to one another are more similar than points further apart due to the influence of landscape processes) declined for all parameters except Mg⁺². Temporal changes in the spatial patterns of extractable phosphorus (P) and percent organic carbon (OC) tended to be inverse; P declined in areas where OC increased and vice versa. The spatial pattern of these changes was dissimilar in the mineral soils as compared to the organic soil and was related to patterns of primary productivity. Zones of P uptake and OC accumulation were also related to wetland hydrology and primary productivity. Changes in the distribution of nutrients, particularly P, may be viewed as a result of nutrient spirals within the wetlands. By comparison, the reorganization in the concentrations of K⁺ and Ca⁺² appear to have been mediated by cation exchange processes. The formation of new concentration gradients was strongly related to both flow pathways and the different water inflow rates. The formation of concentration gradients in exchangeable cations was not reflected in the average concentrations within each basin. Mean values changed significantly in only a few instances. Reducing data in this way missed important biogeochemical changes occurring within the experimental wetland basins. This revised version was published online in July 2006 with corrections to the Cover Date.     

Arundo micrantha,a new reed species for Italy,threatened in the freshwater habitat by the congeneric invasive A. donax

Arundo micrantha Lam., a reed species from southern Mediterranean area, has found in Sardinia. Its presence represents the first record for Italy and the third one for western Europe. Investigations on distribution and ecology of the Sardinian population have been carried out, with special focusing on synecology of the species. A. micrantha has found in 40 localities mainly distributed along temporary streams and permanent rivers in the central and southern part of the island. Most of Sardinian populations occurs on alluvial soils in Thermomediterranean bioclimatic context. The autonomy of Sardinian phytocoenoses was characterized with the definition of exclusive floristic taxa, biological and chorological elements, and syndinamic relationships with other assemblages. Comparing the community with other reed beds from the Mediterranean basin, a marked floristic differentiation arose between the phytocoenoses characterized by autochthonous Arundo species and those dominated by the invasive Arundo donax. Finally, management activities are proposed to ensure the conservation of this threatened autochthonous plant community in freshwater habitat.