Rhizospheric NO affects N uptake and metabolism in Scots pine (Pinus sylvestris L.) seedlings depending on soil N availability and N source

We investigated the interaction of rhizospheric nitric oxide (NO) concentration (i.e. low, ambient or high) and soil nitrogen (N) availability (i.e. low or high) with organic and inorganic N uptake by fine roots of Pinus sylvestris L. seedlings by ¹⁵N feeding experiments under controlled conditions. N metabolites in fine roots were analysed to link N uptake to N nutrition. NO affected N uptake depending on N source and soil N availability. The suppression of nitrate uptake in the presence of ammonium and glutamine was overruled by high NO. The effects of NO on N uptake with increasing N availability showed different patterns: (1) increasing N uptake regardless of NO concentration (i.e. ammonium); (2) increasing N uptake only with high NO concentration (i.e. nitrate and arginine); and (3) decreasing N uptake (i.e. glutamine). At low N availability and high NO nitrate accumulated in the roots indicating insufficient substrates for nitrate reduction or its storage in root vacuoles. Individual amino acid concentrations were negatively affected with increasing NO (i.e. asparagine and glutamine with low N availability, serine and proline with high N availability). In conclusion, this study provides first evidence that NO affects N uptake and metabolism in a conifer.     

Micro-arthropod seasonally in streams of varying pH

SUMMARY. 1. Micro-arthropods were sampled seasonally (January, May, August and October) during 1986 from ten, stony riffle sites on streams in the Ashdown Forest of southern England, using both standard benthic and interstitial samplers.
2. Total densities peaked at most sites in summer. Species richness reached a maximum at acid sites in summer but at cireumneutral sites in autumn, when Hydrachnellae and Cladocera were particularly species rich.
3. Individual species showed no obvious differences in seasonally between sites; the majority peaking in summer or autumn, regardless of pH. However, cyclopoid copepods were particularly numerous at acid sites in summer, a pattern not observed at circumneutral sites.
4. Multivariate ordination and classification of data sets from the separate seasons, and all four seasons combined, showed that mean site pH, conductivity, and aluminium and calcium concentrations were the most important variables explaining between-site variation in species composition. This clear distinction between the community structure at acidic and circumneutral sites was evident in all seasons except winter. Species composition was also more predictable throughout the year at low-pH sites.
5. A number of species were taken consistently in interstitial samples and the cyclopoids Diacydops languidus and D. languidoides were restricted to the hyporheos at circumneutral sites. The similar faunal composition of the hyporheos and the epibenthos indicated that the separation of these communities was not well defined in Ashdown Forest streams.     

Direct and indirect effects of nitrogen deposition on species composition change in calcareous grasslands

Atmospheric nitrogen (N) deposition has been identified as a major threat to biodiversity, but field surveys of its effects have rarely focussed on sites which are actively managed to maintain characteristic species. We analysed permanent quadrat data from 106 plots in nature reserves on calcareous grassland sites in the United Kingdom collected during a survey between 1990 and 1993 and compared the data with the results from resurvey of 48 of these plots between 2006 and 2009. N deposition showed no significant spatial association with species richness, species diversity, or the frequency of species adapted to low nutrient conditions in the 1990–1993 dataset. However, temporal analysis showed that N deposition was significantly associated with changes in Shannon diversity and evenness. In plots with high rates of N deposition, there was a decrease in species diversity and evenness, a decline in the frequency of characteristic calcareous grassland species, and a lower number of rare and scarce species. As all sites had active management to maintain a high diversity and characteristic species, our results imply that even focussed management on nature conservation objectives cannot prevent adverse effects of high rates of N deposition. Structural equation modelling was used to compare different causal mechanisms to explain the observed effects. For change in Shannon diversity, direct effects of N deposition were the dominant mechanism and there was an independent effect of change in grazing intensity. In contrast, for change in herb species number, indirect effects on soil acidity, linked to both N and S deposition, were more important than direct effects of N deposition.     

Contamination on sandflats and the decoupling of linked ecological functions

Benthic macrofauna can influence inputs and transformations of energy and matter in estuaries, affecting both the stocks of vital materials (e.g. carbon, oxygen) and the rates of key processes (e.g. organic matter decomposition, nutrient uptake). Although a number of studies have identified shifts in functional groups or biological traits in relation to anthropogenic stressors, there have been few field‐based assessments of changes in functioning associated with stress gradients. We used a comparative experimental approach to investigate functioning on two sandflats with differing exposures to urban contaminants. Apart from significant differences in sediment contaminant concentrations (43.2 ± 1.8 mg kg^?1 Zn and 15.6 ± 0.9 mg kg^?1 Pb at the Pollen site; 17.7 ± 0.7 mg kg^?1 Zn and 7.9 ± 0.9 mg kg^?1 Pb at the Waiheke site), the two sandflats were readily comparable: both had similar sediment grain size distributions and were dominated by the same macrofaunal species; and both were in non‐eutrophic New Zealand marine reserves with low ambient sediment organic matter content. To better understand the effects of contaminants on biologically mediated transformations of organic matter into inorganic nutrients, we manipulated sediment organic matter content and macrofaunal abundance in standardized treatments at each site. Fluxes of oxygen and ammonium, which are linked to key sandflat processes such as organic matter decomposition and benthic photosynthesis, were measured as response variables 1 week after the experimental manipulations. We predicted more efficient organic matter processing on the uncontaminated flat and thus expected to see elevated ammonium efflux in response to organic enrichment treatments at this site. Higher rates of benthic photosynthesis were predicted for plots with higher ammonium efflux, as ammonium is a readily utilizable form of limiting inorganic nitrogen. We documented significant positive relationships between ammonium uptake and benthic primary production on the uncontaminated flat, but weaker/insignificant relationships at the contaminated site. Our data were consistent with theories of increased variability and a decoupling of system processes with increasing amounts of stress.