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Plant nitrogen source in the soil is challenging to track. Compiling the most comprehensive global δ15N dataset, a new study shows the plant use of various available soil nitrogen forms (ammonium, nitrate, and organic nitrogen) is strongly controlled by temperature. [ABSTRACT FROM AUTHOR]

This study aimed at assessing the effect of nitrogen addition under two forms, nitrate and ammonium, on the stabilization of Zn, Pb, and Cd by Populus euramericana Dorskamp grown in contaminated soils for 35 days under controlled conditions. Temporal changes in the soil pore water (SPW) were monitored for pH, dissolved organic carbon (DOC), and total dissolved concentrations of metals in the soils rhizosphere. Rhizospheric SPW pH decreased gradually with NH addition and increased with NO addition up to one unit, while it slightly decreased initially then increased for the untreated control soil DOC increased with time up to six times, the highest increase occurring with NH fertilization. An increase in the metal concentrations in the rhizospheric SPW was observed for NH addition associated with the lowest rhizospheric SPW pH, whereas the opposite was observed for the control soil and NO fertilization. Fertilization did not affect plant shoots or roots biomass development compared to the untreated control (without N addition). Metals were mostly accumulated in the rhizosphere and N fertilization increased the accumulation for Zn and Pb while Cd accumulation was enhanced for NH addition. Collectively, our results suggest metal stabilization by P. euramericana Dorskamp rhizosphere with nitrogen fertilization and are potential for phytostabilization of contaminated technosol. [ABSTRACT FROM AUTHOR]

Nitrate (NO) and ammonium (NH) are the two major dissolved inorganic nitrogen (DIN) species available in streams. Human activities increase stream DIN concentrations and modify the NO:NH ratio. However, few studies have examined biofilm responses to enrichment of both DIN species. We examined biofilm responses to variation in ambient concentrations and enrichments in either NO or NH. We incubated nutrient diffusing substrata (NDS) bioassays with three treatments (DIN-free, +NO and +NH) in five streams. Biomass-specific uptake rates ( U) of NO and NH were then measured using in situ additions of N-labeled NO and NH. Biomass (estimated from changes in carbon content) and algal accrual rates, as well as U-NO of biofilms in DIN-free treatments varied among the streams in which the NDS had been incubated. Higher ambient DIN concentrations were only correlated with enhanced biofilm growth rates. U-NO was one order of magnitude greater and more variable than U-NH, however similar relative preference index (RPI) suggested that biofilms did not show a clear preference for either DIN species. Biofilm growth and DIN uptake in DIN-amended NDS (i.e., +NO and +NH) were consistently lower than in DIN-free NDS (i.e., control). Lower values in controls with respect to amended NDS were consistently more pronounced for algal accrual rates and U-NO and for the +NH than for the +NO treatments. In particular, enrichment with NH reduced biofilm U-NO uptake, which has important implications for N cycling in high NH streams. [ABSTRACT FROM AUTHOR]

Atmospheric deposition can be an important source of ions to terrestrial and aquatic ecosystems. Previous studies have indicated that dry deposition of ions in and near large cities is greater than in nearby rural areas. However, few studies have compared dry deposition in and near smaller cities. We measured dry deposition of ions at various distances from Greenville, a smaller city in the piedmont of northwestern South Carolina. Dry deposition was estimated by exposure of artificial surfaces (glass Petri plates and paper filters) to the atmosphere at 13 locations during June-July 2008. Petri plates were expected to collect dust particles primarily, whereas filters were expected to collect both dust and gases. Fluxes measured by filters were significantly greater than those measured by Petri plates for nitrate and ammonium, suggesting that dry deposition of nitrogen in gases exceeded dry deposition in dust. Dry deposition of ammonium and nitrate declined significantly with distance from Greenville, and rates were significantly higher at urban than at rural locations. Also, dry deposition rates of ammonium correlated positively with road densities and traffic volumes around sampling locations, suggesting that automobiles were important sources of ammonia gas. Relationships between ammonium deposition and urban land cover and roads were stronger than for nitrate deposition, perhaps reflecting the influence of automobiles using catalytic converters. Base cation concentrations in dry deposition typically were below detection, precluding flux calculations. Overall, our results provide evidence that smaller cities influence atmospheric deposition of nitrogen, though perhaps not as strongly as larger cities. [ABSTRACT FROM AUTHOR]

The coastal Chukchi Sea has a particularly productive landfast ice ecosystem but is currently lacking direct measurements of nitrogen (N) cycling within the ice. Using stable isotopic tracers, we measured uptake and regeneration of ammonium and nitrate, along with primary and secondary production at three depths within landfast sea ice in the Chukchi Sea near Barrow, Alaska, during April 2011 and January 2012. These are the first data of N uptake and regeneration rates in landfast Arctic sea ice during spring and winter. Both inorganic and organic nutrient concentrations in the ice were generally higher than the water column, with the exception of phosphate, which may have limited production in certain sections of ice. Primary production at all ice depths was higher than the water column during April, but below the detection limit in January. Bacterial production in the bottom ice (0-10 cm from the ice-water interface) was consistently higher than the water column across seasons. Absolute uptake of ammonium was highest in the bottom ice and higher than absolute uptake of nitrate at all depth horizons except in the upper ice (30-40 cm from ice-water interface) during January. While N uptake rates in the ice were higher than the water column, nitrification rates were lower. Regeneration of ammonium and nitrate far exceeded uptake within the ice. The magnitude of N uptake and regeneration rates in landfast ice highlights the importance of the biological sea ice community to the region. [ABSTRACT FROM AUTHOR]