Long-term tree growth rate,water use efficiency,and tree ring nitrogen isotope composition of <Emphasis Type="Italic">Pinus massoniana</Emphasis> L. in response to global climate change and local nitrogen deposition in Southern China

Purpose  

We aimed to investigate long-term tree growth rates, water use efficiencies (WUE), and tree ring nitrogen (N) isotope compositions (δ¹⁵N) of Masson pine (Pinus massoniana L.) in response to global climate change and local N deposition in Southern China.     

Effects of weed control and fertilization on soil carbon and nutrient pools in an exotic pine plantation of subtropical Australia

Purpose  

Soil carbon (C) and nutrient pools under different plantation weed control and fertilizer management treatments were assessed in a 7-year-old, F₁ hybrid (Pinus elliottii var. elliottii × Pinus caribaea var. hondurensis) plantation in southeast Queensland, Australia. This research aimed to investigate how early establishment silvicultural treatments would affect weed biomass, soil C, nitrogen (N) and other nutrient pools; and soil C (δ¹³C) and N isotope composition (δ¹⁵N) to help explain the key soil processes regulating the soil C and nutrient pools and dynamics.     

In situ mineral <Superscript>15</Superscript>N dynamics and fate of added <Superscript>15</Superscript>NH<Subscript>4</Subscript><Superscript>+</Superscript> in hoop pine plantation and adjacent native forest in subtropical Australia

Background, aim, and scope  Hoop pine (Araucaria cunninghamii) is a nitrogen (N) demanding indigenous Australia softwood species with plantations in Southeast Queensland, Australia. Soil fertility has declined with increasing rotations and comparison study of N cycling between hoop pine plantations, and adjacent native forest (NF) is required to develop effective forest management for enhancing sustainable forest production and promoting environmental benefits. Field in situ mineral ¹⁵N transformations in these two forest ecosystems have not been studied. Hence, the present study was to compare the differences in soil nutrients, N transformations, ¹⁵N fluxes, and fate between the hoop pine plantation and the adjacent native forest. Materials and methods  The study sites were in Yarraman State Forest (26°52′ S, 151°51′ E), Southeastern Queensland, Australia. The in situ core incubation method was used in the field experiments. Mineral N was determined using a LACHAT Quickchem Automated Ion Analyzer. ¹⁵N were performed using an isotope ratio mass spectrometer with a Eurovector elemental analyzer. All statistical tests were carried out by the SPSS 11.0 for Windows statistical software package. Results  Soil total C and N were significantly higher in the NF than in the 53-year-old hoop pine plantation. Concentrations of NO₃ ^– were significantly higher in the NF soil than in the plantation soil. The plantation soil had significantly higher ¹⁵N and ¹³C natural abundances than the NF soil. The NF soil had significantly lower C/N ratios than the plantation soil. NO₃ ^––N was dominated in mineral N pools in both NF and plantation soils, accounting for 91.6% and 70.3% of the total mineral N pools, respectively. Rates of net nitrification and net N mineralization were, respectively, four and three times higher in the NF soil than in the plantation soil. The ¹⁵NO₃ ^––N and mineral ¹⁵N were significantly higher in the NF soil than in the plantation soil. Significant difference in ¹⁵NH₄ ⁺–N was found in the NF soil before and after the incubation. Discussion  The NF soil had significantly higher NO₃ ^––N, mineral N, total N and C but lower δ¹⁵N, δ¹³C, and C/N ratios than the plantation soil. Moreover, the rates of soil net N mineralization and nitrification were significantly higher, but ammonification rate was lower in the NF than in the plantation. The NF soil had many more dynamic N transformations than the plantation soil due to the combination of multiple species and layers and, thus, stimulation of microbial activity and alteration of C and N pool sizes in favor of the N transformations by soil microbes. The net rate of N and ¹⁵N transformation demonstrated differences in N dynamic related to the variation in tree species between the two ecosystems. Conclusions  The change of land use and trees species had significant impacts on soil nutrients and N cycling processes. The plantation had larger losses of N than the NF. The NO₃ ^––N and ¹⁵NO₃ ^––N dominated in the mineral N and ¹⁵N pools in both forest ecosystems. Recommendations and perspectives  Native forest soil had strong N dynamic compared with the plantation soil. Composition of multiple tree species with different ecological niches in the plantation could promote the soil ecosystem sustainability. The ¹⁵N isotope dilution technique in the field can be quite useful for studying in situ mineral ¹⁵N transformations and fate to further understand actual N dynamics in natural forest soils.     

Soil carbon and nutrient pools, microbial properties and gross nitrogen transformations in adjacent natural forest and hoop pine plantations of subtropical Australia

Background, Aims, and Scope  An improved understanding of important soil carbon (C) and nutrient pools as well as microbial activities in forest ecosystems is required for developing effective forest management regimes underpinning forest productivity and sustainability. Forest types and management practices can have significant impacts on soil C and nutrient pools as well as biological properties in forest ecosystems. Soil C and nutrient pools were assessed for adjacent natural forest (NF), first rotation (1R) (50-year-old), and second rotation (2R) (1-year-old) hoop pine (Araucaria cunninghamii Ait. ex D. Don) plantations in southeast Queensland of subtropical Australia. Materials and Methods  Five transects spaced 3 m apart with 9 sampling points along each transect were selected (9.6 m × 12.0 m each site), with 45 soil cores (7.5 cm in diameter) collected and separated into 0–10 and 10–20 cm depths. These soils were analysed for total C, total nitrogen (N), C (δ¹³C) and N (δ¹⁵N) isotope composition. The 0–10 cm soils were analysed for pH, CEC, exchangeable cations, total P and total K, and assayed for microbial biomass C and N, respiration, metabolic quotient, potential mineralizable N (PMN), gross N mineralization (M) and immobilization (I). Results  Total C and N in 0–10 cm soils were higher under NF and 1R plantation than under 2R plantation, while they were highest in 10–20 cm soils under NF, followed by the 1R and then 2R plantation. δ¹³C was lower under NF than under the plantations, while δ¹⁵N was higher under NF than under the plantations. Total P was the highest under NF, followed by the 1R and then 2R plantation, while total K was higher under the 2R plantation. No significant differences were detected for pH, CEC, exchangeable cations, microbial C and N, respiration and metabolic quotient among the 3 sites. PMN and M were higher under NF, while I was the highest under the 2R plantation, followed by the NF and then 1R plantation. Discussion  Soil total C and N in 0–10 cm depth were significantly lower under 2R hoop pine plantation than those under NF and 1R hoop pine plantation. There were significant reductions in soil total C and N from NF to 1R and from 1R to 2R hoop pine plantations in 10–20 cm depth. This highlights potential N deficiency in the 2R hoop pine plantations, and application of N fertilizers may be required to improve the productivity of 2R hoop pine plantations. There were no significant differences in other soil chemical and physical properties in 0–10 cm depth among the 3 sites under NF, 1R and 2R hoop pine plantations, except for soil total P and K. Soil microbial biomass C, CO₂ respiration and metabolic quotient did not differ among the 3 sites assessed, perhaps mainly due to these biological variables being too sensitive to variations in soil chemical and physical properties and thereby being associated with a larger variability in the soil biological properties. However, soil potential mineralizable N, gross N mineralization and immobilization were rather sensitive to the conversion of NF to hoop pine plantation and forest management practices. Conclusions  Total C and N in the top 20 cm soil were highest under NF, followed by 1R and then 2R hoop pine plantations, indicating that N deficiency may become a growth-limiting factor in the 2R hoop pine plantations and subsequent rotations of hoop pine plantation. The sample size for soil δ¹³C seems to be much smaller than those for soil total C and N as well as δ¹⁵N. The significant reductions in soil total P from NF to 1R and then from 1R to 2R hoop pine plantations highlight that P deficiency might become another growth-limiting factor in the second and subsequent rotations of hoop pine plantations. Soil microbial properties may be associated with large spatial variations due to these biological properties being too sensitive to the variations in soil chemical and physical properties in these forest ecosystems. Recommendations and Perspectives  Soil potential mineralizable N, gross N mineralization and immobilization were useful indices of soil N availability in response to forest types and management practices. The sampling size for soil δ¹³C was much smaller than the other soil chemical and biological properties due to the different patterns of spatial variation in these soil properties.     

Nitrogen Cycling in Pinus banksiana and Populus tremuloides Stands in the Athabasca Oil Sands Region, Alberta, Canada

Elevated emissions of nitrogen oxides (NO_x) in the Athabasca Oil Sands Region, Alberta and higher foliar nitrogen (N) concentrations in jack pine (Pinus banksiana) needles close to major emission sources has led to concerns that the surrounding boreal forest may become N-saturated. Despite these concerns, N deposition and impacts on upland forests in the region is poorly quantified. The objective of this study was to characterize N cycling in five plots representing the two dominant upland forest types (jack pine and trembling aspen, Populus tremuloides) close (<30 km) to the largest mining operations in the region, during a 2-year period. Despite the high level of NO_x emissions, bulk throughfall and deposition measured at both study sites were surprisingly very low (<2 kg N ha⁻¹ year⁻¹). Internal N cycling was much greater in aspen stands; annual N input in litterfall was ten times greater, and net N mineralization rates were two to five times greater than in jack pine stands. Nitrogen use efficiency (NUE) was much greater in jack pine when calculated based on N litterfall indices, but not when N pools in biomass were considered. Despite differences in internal cycling among forest types, nitrate leaching from mineral soil in both forest types was negligible (<0.1 kg N ha⁻¹ year⁻¹) and patterns of ¹⁵N in roots, foliage, and mineral soil were typical of N-limited ecosystems, and both sites show no evidence of N saturation.     

Improved estimation of biological nitrogen fixation of soybean cultivars (<Emphasis Type="Italic">Glycine max</Emphasis> L. Merril) using <Superscript>15</Superscript>N natural abundance technique

Biological nitrogen fixation (BNF) of 17 soybean cultivars was comparatively estimated by the δ¹⁵N natural abundance technique using two non-nodulation soybeans (Clay and Chippewa) as reference plants. A field study was established on the experimental farm of the University of Abomey-Calavi, Benin on a typical “terre de barre” soil classified by Food and Agriculture Organization-United Nations Educational, Scientific and Cultural Organisation as Rhodic Ferralsol. A nitrogen-free pot trial was also carried out using soil substrate sampled from the Atlantic Ocean beach. In the N-free medium, N content of the whole soybean cultivars ranged from 2.6 to 8.1 mg N per plant compared with an average of 1.8 mg N per plant observed with the non-fixing soybeans. Plant δ¹⁵N of the nodulating soybeans ranged from −2.7756‰ (Jupiter) to 0.1951‰ (Conquista), while the non-nodulating cultivars Chippewa and Clay had 2.67‰ and 9.30‰, respectively. Percentage and amount of N derived from air (Ndfa) were significantly different (P < 0.01) among soybean cultivars, and values depended highly on the selected reference plants. When Clay was used as the reference plant, the average percentage Ndfa was 1.4 times higher than when Chippewa was the reference plant. Both reference plants consistently ranked promiscuous soybean cvs. TG× 1894 3F and TG× 1908 8F as the best cultivars and cv. TG× 1888 29F as the least in percentage Ndfa, suggesting that any of the reference plants could be used in δ¹⁵N method for assessing N₂-fixation. The two identified promiscuous soybean cultivars with greatest capacity to fix N could be included in a soybean extension program for West African farming systems.     

Evaluating N2 fixation by food grain legumes in farmers’ fields in three agro-ecological zones of Zambia, using 15N natural abundance

A survey of N₂ fixation in farmers’ fields of Northern (>1,000 mm rainfall), Central (800–1,000 mm rainfall), and Southern (<800 mm rainfall) Zambia revealed some significant differences in plant growth and symbiotic performance of different food grain legumes. Of the three grain legumes (i.e., Bambara groundnut (Vigna subterranea L. Verdc.), cowpea (Vigna unguiculata L. Walp.), and groundnut (Arachis hypogaea L.)) grown in Southern Zambia, cowpea showed greater shoot biomass and significantly lower shoot δ¹⁵N values than groundnut and Bambara groundnut. The lower shoot δ¹⁵N resulted in greater %Ndfa (59%) in shoots and higher amounts of N-fixed, whether per square meters (6,394.0 mg N), per plant (650.8 mg N), or per hectare (63.9 kg N) relative to groundnut and Bambara groundnut, even though the number of cowpea plants per square meter was significantly lower than that of groundnut or Bambara groundnut. Although the shoot δ¹⁵N values of cowpea, Bambara groundnut and common bean (Phaseolus vulgaris L.) were significantly lower than those of groundnut in Central Zambia and their %Ndfa values, therefore, greater, the higher number of groundnut plants per square meter resulted in significantly greater shoot N content, as well as N-fixed per square meter and per hectare relative to the other species. Despite having similar plant density as cowpea in Central Zambia, common bean could fix only 6.0 kg N ha⁻¹ compared with 35.4 kg N ha⁻¹ by cowpea. In Northern Zambia, Bambara groundnut showed the lowest mean shoot δ¹⁵N value (0.54 ± 0.3‰), followed by groundnut (1.59 ± 1.0‰), and then common bean (the three grain legumes grown in that region). As a result, %Ndfa and N-fixed were significantly greater in groundnut (69.7% and 566.0 mg N per plant) and Bambara groundnut (62.9% and 440.1 mg N per plant) than in common bean (2.6% and 2.4 mg N per plant). In Northern Zambia, groundnut, Bambara groundnut and common bean fixed 78.7, 67.6, and 0.9 kg N ha⁻¹, respectively, even though the plant density per square meter of common bean (which fixed the lowest amount of N per hectare) was twice that of groundnut and Bambara groundnut. A species × site analysis showed that cowpea fixed relatively greater amounts of N per plant, per square meter, and per hectare in Southern than Central Zambia. Bambara groundnut and common bean also had significantly lower δ¹⁵N values and higher %Ndfa in Central than Northern Zambia.     

Atmospheric Deposition of Nitrogen to a Caribbean Coastal Zone (Cayo Coco,Cuba): Temporal Trends and Relative Importance as a Nitrogen Source

Nitrogen (N) deposition to the ocean is thought to be increasing worldwide, but the amount of coastal and open ocean measurements is very limited. In this paper, we assess N deposition in the coastal zone of Cayo Coco, in central Cuba, during a multi-annual period (2005–2007). Wet and dry N depositions were estimated based on the NH₄⁺ and NO_x^– concentrations in the rain. Cold fronts and troughs, coming from the west, contributed most to rain (41%) and to N deposition, followed by tropical waves and storms coming from the east, which caused 31% of the rain. Average concentrations of NH₄⁺ and NO_x^– in the rain were 8.8 and 8.3 μM. NO_x^– presented a clearly decreasing trend (0.26 μM per month), decreasing by half during 2005–2007. Total N deposition averaged 3.23 kg N ha⁻¹ year⁻¹, similar to that found in Virgin Islands and Puerto Rico, but lower than previously measured in Cuba and in nearby areas of the USA and than model predictions for the oceanic region around Cuba. These low values and the decreasing trend found are attributed to drastic reduction of fossil fuel and fertilizer use in Cuba since 1990. Because land input has decreased even more drastically, deposition seems to be nowadays the most important N source to the coastal zone of Cayo Coco. The δ¹⁵N range of seagrass (Thalassia testudinum) and macroalgae (Penicillus dumetosus) in the area (−1.83‰ to 3.02‰ and +1.02‰ to +4.17‰, respectively) sustain that atmospheric sources (deposition and N₂ fixation) comprise 70–90% of the N budget.     

Do nitrogen isotope patterns reflect microbial colonization of soil organic matter fractions?

Different theories have been brought forward to explain the commonly observed δ¹⁵N enrichment with depth in soil profiles, including the discrimination against ¹⁵N during N decomposition and the buildup of ¹⁵N-enriched microbial residues. A combination of soil organic matter (SOM) size and density fractionations, ¹⁵N determinations, and phospholipid fatty acid (PLFA) analyses was conducted on soils from a pristine N-limited Nothofagus forest in southern Chile. The purpose of this study was to investigate which SOM fractions mostly reflect the ¹⁵N-enrichment pattern and to link ¹⁵N SOM enrichment with microbial community composition. Nitrogen-15 enrichments were greater for the microaggregate (<150 μm) than for the macroaggregate (>150 μm) size fraction, with Rayleigh isotope enrichment factors averaging −8.5‰ and −3.7‰, respectively. The macro-organic matter density fractions (>150 μm) showed intermediate enrichment factors of −5.1‰ and −7.3‰ for the light (<1.37 g cm⁻³) and heavy (>1.37 g cm⁻³) fraction, respectively. The abundance of fungal and bacterial PLFAs was significantly higher in the microaggregate compared to the macroaggregate size fraction, but their relative abundance did not change between aggregate size fractions. Our data link differential ¹⁵N enrichment of SOM fractions to “total” microbial abundance and, as such, corroborates existing theories of microbial-induced ¹⁵N enrichment. Isotopic fractionation during microbial N decomposition processes alone could not explain the large ¹⁵N enrichment in the microaggregate size fraction (−8.5‰) and the heavy density fraction (−7.3‰). We therefore suggest that microbial turnover and accretion of ¹⁵N-enriched microbial (especially fungal) compounds was an additional driver for ¹⁵N enrichment of this soil profile.     

Land use effects on microbial biomass C, β-glucosidase and β-glucosaminidase activities,and availability,storage, and age of organic C in soil

Microbial biomass, β-glucosidase and β-glucosaminidase activities, and availability, storage, and age of soil organic C were investigated after 26 years of conversion from sugarcane (Saccharum officinarum) to forest (Eucaliptus robusta or Leucaena leucocephala), pasture (mixture of tropical grasses), and to vegetable cropping (agriculture) in a vertisol in Puerto Rico. Soil organic C (SOC) at 0–100 cm was similar under Leucaena (22.8 kg C/m²), Eucalyptus (18.6 kg C/m²), and pasture (17.2 kg C/m²), which were higher than under agriculture (13.0 kg C/m²). Soil organic N (SON) at 0–100 cm was similar under the land uses evaluated which ranged from 1.70 (under agriculture) to 2.28 kg N/m² (under Leucaena forest). Microbial biomass C (MBC) and N (MBN) of the 0–15-cm soil layer could be ranked as: pasture > Leucaena = Eucalyptus > agriculture. The percentages of SOC and SON present as MBC and MBN, respectively, were nearly 1% in pasture and less than 0.50% in forest under Leucaena or Eucalyptus and agricultural soil. The activity of β-glucosidase of the 0–15-cm soil layer could be ranked as: Leucaena = Eucalyptus > pasture > agriculture; while β-glucosaminidase activity was ranked as: Eucalyptus > Leucaena = pasture > agriculture. The soil δ ¹³C changed from 1996 to 2006 in forest under Eucalyptus (18.7‰ to 21.2‰), but not under Leucaena (20.7‰ to 20.8‰). The soil under Leucaena preserved a greater proportion of old C compared to the forest under Eucalyptus; the former had an increased soil mineralizable C from the current vegetation inputs. The soil under agriculture had the lowest enzyme activities associated with C cycling, lowest percentage of SOC as MBC, highest percentage of SOC present as mineralizable C, and highest percentage of MBC present as mineralizable C compared to the other land uses.     

The effect of starvation on the planarian Arthurdendyus triangulatus (Tricladida: Terricola) as measured by stable isotopes

Many studies of the population dynamics of the planarian Arthurdendyus triangulatus have been confounded because it has not been possible to define if small flatworms are adults that have been starved or young ones being recruited into the population. Body shrinkage is a process exhibited by planarians under nutritional stress. In birds and the few invertebrate species studied, starvation leads to a ¹⁵N-enrichment of body tissues (i.e. an increased δ ¹⁵N). Under laboratory conditions, the posterior parts of flatworms were removed at day 0 and frozen and compared isotopically with the anterior parts of flatworms, which were starved for 67, 108, 171 and 243 days. With one exception, δ ¹³C 108 days after starvation, the impact of starvation on A. triangulatus %N, %C and δ ¹³C was not significant statistically. In contrast, δ ¹⁵N increased significantly (to a maximum enrichment of 2.8‰ by day 243) on all sampling dates. This is supported by significant positive correlations between the changes in δ ¹⁵N between the anterior and posterior body parts and number of days after starvation (p≤0.05) and decrease in body weight (p≤0.01), respectively. Existing δ ¹⁵N data from field populations of animals show intra-population variability of around 5.0‰, greater than that noted as a result of starvation. Thus, as a single measure, it is unlikely that δ ¹⁵N can be used to accurately characterise the proportion of starved A. triangulatus within a field population.     

Purification and isotopic signatures (<Emphasis Type="Italic">δ</Emphasis><Superscript>13</Superscript>C, <Emphasis Type="Italic">δ</Emphasis><Superscript>15</Superscript>N, Δ<Superscript>14</Superscript>C) of soil extracellular DNA

The aim of this work was to obtain pure extracellular DNA molecules so as to estimate their longevity in soil by an isotope-based approach. Extracellular DNA molecules were extracted from all horizons of a forest soil and purified by the procedure of Davis (Purification and precipitation of genomic DNA with phenol–chloroform and ethanol. In: Davis LG, Dibner MD, Battey JF (eds) Basic methods in molecular biology. Appleton & Lange, Norwalk, 16–22, 1986) without (DNA1) or with (DNA2) a successive treatment with binding resins followed by elution. The two differently purified DNA samples were compared for their A₂₆₀/A₂₈₀ ratio, polymerase chain reaction (PCR) amplification and natural abundance of stable (¹³C and ¹⁵N) and radioactive (¹⁴C) isotopes. The purity index and the PCR amplification did not differentiate the efficiency of the two purification procedures. The isotopic signature of DNA was more sensitive and was strongly affected by the purification procedures. The isotopic measurements showed that the major contaminant of extracellular DNA1 was the soil organic matter (SOM), even if it is not possible to exclude that the similar δ ¹³C, δ ¹⁵N and Δ¹⁴C values of DNA and SOM could be due to the use of SOM-deriving C and N atoms for the microbial synthesis of DNA. For extracellular DNA2, extremely low values of Δ¹⁴C were obtained, and this was ascribed to the presence of fossil fuel-derived substances used during the purification, although in amounts not revealed by gas chromatography-mass spectrometry analysis. The fact that it is not possible to obtain contaminant-free DNA molecules and the potential use of soil native organic compounds during the microbial synthesis of DNA make it not achievable to estimate the age of soil extracellular DNA by radiocarbon dating.     

The use of nitrogen-15 natural abundance in white clover (Trifolium repens L.) to determine nitrogen fixation under different management practices

 The ¹⁵N natural abundance (δ¹⁵N) of white clover (Trifolium repens L.) grown in pasture under different management practices was determined. Plants were split into leaflets, petioles and stolons and the ¹⁵N signature of each tissue was measured. The δ¹⁵N of leaflet tissue from plants of two non-N₂-fixing species (Lolium perenne L. and Ranunculus repens L.), growing in close proximity to the sampled T. repens, was also measured. By using T. repens plants grown in the absence of mineral N to provide reference material, the proportion of N derived from N₂ fixation (%Ndfa) in pasture plants was calculated. Within a plot, variation was present in the δ¹⁵N between the tissues of T. repens. Variation was also present between the same tissues under different management practices. The %Ndfa in the leaf material of T. repens varied from 34% to 100% between the plots. The use of different reference species did not affect the estimate of %Ndfa. Received: 14 December 1998     

Analysis and behavior of soluble organic nitrogen in forest soils

Background, aim, and scope  

A large proportion of soil nitrogen (N; >80%) is present in organic form. Current research on plant N uptake in terrestrial ecosystems has focused mainly on inorganic N such as ammonium (NH₄ ⁺) and nitrate (NO₃ ⁻), while soluble organic N (SON) has received little attention. In recent years, the increasing evidence showing the direct uptake of various amino acids by plants and the predominance of the organic form in N loss by leaching in many forest ecosystems has drawn attention to critically re-examine the nature and the ecological role of soil SON in terrestrial N cycling. However, little is known about the sources and dynamics, chemical nature, and ecological functions of soil SON in forest ecosystems. This paper reviews recent advances in the areas of research on current techniques for characterizing soil SON and the size, nature, and dynamics of soil SON pools in forest ecosystems.     

Nutrient limitation of alpine plants: Implications from leaf N : P stoichiometry and leaf δ15N

Nitrogen (N) deposition can affect grassland ecosystems by altering biomass production, plant species composition and abundance. Therefore, a better understanding of the response of dominant plant species to N input is a prerequisite for accurate prediction of future changes and interactions within plant communities. We evaluated the response of seven dominant plant species on the Tibetan Plateau to N input at two levels: individual species and plant functional group. This was achieved by assessing leaf N : P stoichiometry, leaf δ¹⁵N and biomass production for the plant functional groups. Seven dominant plant species—three legumes, two forbs, one grass, one sedge—were analyzed for N, P, and δ¹⁵N 2 years after fertilization with one of the three N forms: NO$ _3^- $ , NH$ _4^+ $ , or NH₄NO₃ at four application rates (0, 7.5, 30, and 150 kg N ha^–1 y^–1). On the basis of biomass production and leaf N : P ratios, we concluded that grasses were limited by available N or co‐limited by available P. Unlike for grasses, leaf N : P and biomass production were not suitable indicators of N limitation for legumes and forbs in alpine meadows. The poor performance of legumes under high N fertilization was mainly due to strong competition with grasses. The total above‐ground biomass was not increased by N fertilization. However, species composition shifted to more productive grasses. A significant negative correlation between leaf N : P and leaf δ¹⁵N indicated that the two forbs Gentiana straminea and Saussurea superba switched from N deficiency to P limitation (e.g., N excess) due to N fertilization. These findings imply that alpine meadows will be more dominated by grasses under increased atmospheric N deposition.     

Effectiveness of Air,N<Subscript>2</Subscript> (gas), Fe<Superscript>+3</Superscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanoparticles on the Sonication of Less and More Hydrophobic Polycyclic Aromatic Hydrocarbons (PAHs) and Toxicity

In this study, the effects of 1 h aeration, nitrogen gas N₂(g) sparging (15 and 30 min) and increasing ferric ions (Fe⁺³) as FeSO₄ (10, 20 and 50 mg L⁻¹) and Fe₃O₄ nanoparticles (1, 2 and 4 g L⁻¹) concentrations on three less hydrophobic and three more hydrophobic polycyclic aromatic hydrocarbons (PAHs) and toxicity removals from a petrochemical industry in Izmir (Turkey) were investigated in a sonicator with a power of 650 W and an ultrasound frequency of 35 kHz; 1 h aeration increased the yields in benzo[b]fluoranthene, benzo[k]fluoranthene and benzo[a]pyrene PAHs (less hydrophobic) from 62% to 67% to around 95–97% after 150 min sonication at 60°C. However, 1 h aeration did not contribute to the yields of more hydrophobic PAHs (indeno[1,2,3-cd]pyrene, dibenz[a,h]anthracene, benzo[g,h,i]perylene). The maximum yields were obtained at acidic and alkaline pH for more and less hydrophobic PAHs, respectively, after 60 and 120 min sonication at 30°C; 30 min N₂(g) sparging, 50 mg L⁻¹ Fe⁺³ increased the yields of less hydropobic PAHs after 150 min sonication at 60°C. Two milligrams per liter of Fe₃O₄ nanoparticles increased both less (87–88%) and more (96–98%) hydrophobic PAH yields. The Daphnia magna acute toxicity test showed that the toxicity decreased significantly with an hour aeration, 30 min N₂(g) sparging, 50 mg L⁻¹ Fe⁺³ and 2 g L⁻¹ Fe₃O₄ nanoparticles at 60°C after 120 and 150 min sonications. Vibrio fischeri was found to be more resistant to the sonicated samples than D. magna. Significant correlations were found between the physicochemical properties of sonicated PAHs and acute toxicities both organisms.     

Symbiotic nitrogen fixation in the alpine community of a lichen heath of the Northwestern Caucasus Region (the Teberda Reserve)

The symbiotic fixation of atmospheric nitrogen by leguminous plants in the alpine community of a lichen heath at the Teberda State Biosphere Reserve is well adapted to low soil temperature characteristic for the altitude of 2800 m a.s.l. For the determination of the N fixation by isotopic methods (the method of the natural ¹⁵N abundance and the method of isotopic ¹⁵N dilution), Trifolium polyphyllum was taken as the control plant. This plant was used as it does not form symbiosis with the nitrogen-fixing bacteria in the highlands of the Northern Caucasus Region. The contribution of the N fixation to the N nutrition of different leguminous plant species as determined by the natural ¹⁵N abundance method amounted to 28–73% at δ¹⁵N₀ = 0‰ and 46–117% at δ¹⁵N₀ = −1‰; for the determination of the N fixation by the method of the isotopic label’s dilution, it was 34–97%. The best correlation of the results obtained by these two isotopic methods was observed for the natural fractionation of the N isotopes in the course of the N fixation in the range of −0.5 to −0.7‰. The determination of the nitrogenase activity of the roots by the acetylene method confirmed the absence of N fixation in T. polyphyllum and its different contribution to the N nutrition of different species of leguminous plants.     

Effects of nitrogen fertilizer application rates on nitrate nitrogen distribution in saline soil in the Hai River Basin,China

Background and Objectives  

Soil nitrate nitrogen (NO₃ ⁻-N) accumulation is related closely to NO₃ ⁻ leaching, which is an important issue in groundwater pollution, especially in intensive agricultural areas with saline soils where volumes of water are used in irrigation to avoid salt accumulation in the root zone. However, in the saline environment in Hai River Basin, China, the importance of detailed research into NO₃ ⁻-N distribution in the root zone has not been adequately recognized. Considering the impacts of eco-environmental system N and crop production, the present study aimed at contributing to an understanding of the effects of N application rate on soil NO₃ ⁻-N distribution, NO₃ ⁻-N residue, N loss, and maize (Zea mays L.) yield in this region.     

Atmospheric Mercury Fluxes in a Southern Boreal Forest and Wetland

Total gaseous mercury (TGM) fluxes from the forest floor and a boreal wetland were measured by a flux chamber technique coupled with an automatic mercury vapour analyser. The fluxes were measured at three sampling sites in southern Finland, 61°14′ N, 25°04′ E in summer 2007, with additionally in situ TGM concentrations in the air at one of the sites and mercury bulk deposition at another. Most of the flux data were collected during the daytime. At one of the sites, diurnal flux behaviour was studied, and a clear cycle with an afternoon maximum and a night minimum was observed. The highest emissions (up to 3.5 ng m⁻² h⁻¹) were observed at the forest floor site having a moss and grass cover. At the wetland and litter-rich forest floor sites, the emissions were below 1 ng m⁻² h⁻¹ and sometimes negative (down to −1.0 ng m⁻² h⁻¹), indicating mercury uptake. The measured average fluxes in August were 0.9 ± 1.1 and 0.2 ± 0.3 ng m⁻² h⁻¹ for the forest floor sites and wetland sites, respectively. The flux data were compared with the mercury bulk deposition, which proved to be of the same magnitude, but opposite in sign. At the mossy forest floor site, the extrapolated TGM emissions were 130% of the Hg deposition in August 2007. Comparison with other studies showed that the fluxes in background areas are relatively uniform, regardless of measurement site location and method used. Airborne TGM remained at the background level during the study, with an average value of 1.3 ± 0.2 ng m⁻³; it frequently showed a diurnal cycle pattern.     

Nitrogen and carbon mineralization of semi-arid shrubland soil exposed to long-term atmospheric nitrogen deposition

Anthropogenic N-deposition represents a significant input of N into semi-arid chaparral and coastal sage scrub (CSS) shrublands of southern California. High levels of atmospheric N deposition have the potential to increase soil C and N mineralization, and we hypothesize that semi-arid shrubland soil exposed to long-term (decades) high N deposition will have significantly higher C and N mineralization potentials. This hypothesis was tested in a laboratory incubation where the inorganic N (NH₄+NO₃) and CO₂ production of soils maintained at a constant temperature of 25°C and a soil moisture of 0.25 g H₂O/g (65% water-filled pore space) were sampled sequentially over a 50-week period. The temporal trend in cumulative C and N mineralization was well described by a first- and zero-order model, respectively. Long-term atmospheric N deposition significantly increased potential N mineralization but not C mineralization, and both the rate and total N mineralization were significantly positively correlated with the surface (0–10 cm) soil δ ¹⁵N natural abundance and negatively correlated with the surface soil C:N ratio. While the incubation techniques used here do not provide realistic estimates of in situ C or N mineralization, these assays indicate that atmospheric N deposition has significantly altered ecosystem N storage and cycling.