Variations of δ13C AND δ15N in Pinus Densiflora Tree-Rings and their Relationship to Environmental Changes in Eastern Korea

Natural abundances of carbon-13 and nitrogen-15 were analyzed in 3-year bands of annual rings of three red pine (Pinus densiflora Sieb. et Zucc.) trees in eastern Korea to elucidate their variations in relation to changing environmental conditions, particularly air pollution. Tree ring width had a trend to decrease with time (r = ?0.79, P < 0.001); however, tree-ring indices did not show any consistent pattern of change over time. Tree ring indices were correlated neither with the respective precipitation nor temperature. The δ¹³C (range: ?25.7 to ?24.4ä) of tree rings became less negative as tree ring indices increase (r = 0.43, P < 0.05), suggesting that radial growth of trees might have been affected by environmental factors such as nutrient deficiency and acid rain that affect carboxylation efficiency. Increasing N concentration (range: 0.40 to 0.68 g N kg^?1) with decreasing δ¹⁵N (range: +4.2 to ?0.6ä) of tree rings (r = ?0.84, P < 0.01) during the period (since 1980s) of increasing NO _x emission in Korea was consistent with the hypothesis that increasing deposition of N depleted in ¹⁵N may lead to ¹⁵N depletion in tree tissues. However, quantitative information on inter-ring translocation of N which may cause N isotopic fractionation is necessary to use the δ¹⁵N signal as a reliable indicator of air pollution.     

Symbiotic N nutrition, C assimilation, and plant water use efficiency in Bambara groundnut (Vigna subterranea L. Verdc) grown in farmers’ fields in South Africa, measured using 15N and 13C natural abundance

Bambara groundnut (Vigna subterranea L. Verdc) is the second most important indigenous food legume in Africa. The aim of this study was to evaluate plant growth, N₂ fixation, N contribution, C accumulation, and plant water relations of Bambara groundnut grown in 26 farmers’ fields in Mpumalanga Province of South Africa. The data revealed marked (p?≤?0.05) differences in plant dry matter (DM) yield, N concentration and content, δ¹⁵N, the proportion of N derived from symbiotic fixation (%Ndfa), and actual amounts of N-fixed between and among the 26 farms surveyed. Bambara groundnut plants obtained 33–98 % (mean?=?72 %) of their N nutrition from symbiotic fixation and contributed 4–200 kg N-fixed ha^?1 (mean?=?102 kg N-fixed ha^?1). Plant density correlated positively with %N (r?=?0.31***), δ¹⁵N (r?=?0.126***), and amount of N-fixed (r?=?0.15*), indicating that the high %Ndfa values obtained for Bambara groundnut in this study and the low symbiotic N yield associated with some farms were due to low plant density rather than poor symbiotic functioning. Bambara groundnut obtained more N from soil (e.g., 173 kg N ha^?1) than from symbiosis (e.g., 135 kg N-fixed ha^?1) in some fields, implying that the N₂-fixing efficacy of the microsymbionts nodulating Bambara groundnut was low at some locations in South Africa. The data from this study showed δ¹³C values ranging from ?28.01 to ?26.20?‰, which indicates differences in plant water use efficiency on the different fields studied. Furthermore, the positive correlations between δ¹³C and N-fixed (r?=?0.15*) and between δ¹³C and N content (r?=?0.14*) suggest a functional relationship between water use efficiency and N₂ fixation, just as the positively significant correlations between δ¹⁵N and DM yield (r?=?0.24***), N-fixed and DM weight (r?=?0.76**), and N content and DM yield (r?=?0.99*), as well as N-fixed and C content (r?=?0.76**) also indicate a functional relationship between N₂ fixation and photosynthesis. In the same way, the positive correlation between δ¹³C and DM weight (r?=?0.14*), or δ¹³C and C content (r?=?0.15*), also implies a functional link between water use efficiency and plant growth. Thus, an increase in water use efficiency in Bambara groundnut, whenever it occurs, seems to functionally enhance plant growth, symbiotic N₂ fixation, and photosynthetic activity, just as N₂ fixation in nodules also seems to stimulate leaf photosynthesis.     

The influence of trophic level as measured by δ15N on mercury concentrations in freshwater organisms

The relationship between mercury (Hg) concentrations in freshwater biota and trophic position, as defined by stable nitrogen isotope ratios (δ¹⁵N), was examined in 6 lakes in northwestern Ontario. The heavier isotope of nitrogen (¹⁵N) increases an average of 3 parts per thousand (‰) from prey to predator and is used as a measure of an organism's trophic position. Dorsal muscle from lake trout, burbot, walleye, northern pike, white sucker, lake cisco, lake whitefish, and yellow perch was analyzed for Hg and δ¹⁵N using flameless atomic absorption and mass spectrometry respectively. Within each lake, log Hg was significantly related to δ¹⁵N (r ² ranged from 0.47 to 0.91,P<0.01). For four species, yellow perch, northern pike, lake cisco, and lake trout, log Hg was positively related to δ¹⁵N (r ² ranged from 0.37 to 0.47,P≤0.09) across all lakes. We also used δ¹⁵N measurements (assuming a 3‰ shift between an organism and its diet) and the developed within-lake regression equations to calculate a prey Hg for each individual fish. These food Hg values were then used to predict predator Hg using Norstromet al's bioenergetics model. Predicted results were strongly correlated to measured Hg concentrations (r=0.91,P<0.001), indicating that δ¹⁵N has potential to be used in modeling.     

Effects of weed control and fertilization at early establishment on tree nitrogen and water use in an exotic F1 hybrid pine of subtropical Australia

Purpose

We investigated the effects of weed control and fertilization at early establishment on foliar stable carbon (δ¹³C) and nitrogen (N) isotope (δ¹⁵N) compositions, foliar N concentration, tree growth and biomass, relative weed cover and other physiological traits in a 2-year old F₁ hybrid (Pinus elliottii var. elliottii (Engelm) × Pinus caribaea var. hondurensis (Barr. ex Golf.)) plantation grown on a yellow earth in southeast Queensland of subtropical Australia.

Materials and methods

Treatments included routine weed control, luxury weed control, intermediate weed control, mechanical weed control, nil weed control, and routine and luxury fertilization in a randomised complete block design. Initial soil nutrition and soil fertility parameters included (hot water extractable organic carbon (C) and total nitrogen (N), total C and N, C/N ratio, labile N pools (nitrate (NO₃ ^?) and ammonium (NH₄ ⁺)), extractable potassium (K⁺)), soil δ¹⁵N and δ¹³C. Relative weed cover, foliar N concentrations, tree growth rate and physiological parameters including photosynthesis, stomatal conductance, photosynthetic nitrogen use efficiency, foliar δ¹⁵N and foliar δ¹³C were also measured at early establishment.

Results and discussion

Foliar N concentration at 1.25 years was significantly different amongst the weed control treatments and was negatively correlated to the relative weed cover at 1.1 years. Foliar N concentration was also positively correlated to foliar δ¹⁵N and foliar δ¹³C, tree height, height growth rates and tree biomass. Foliar δ¹⁵N was negatively correlated to the relative weed cover at 0.8 and 1.1 years. The physiological measurements indicated that luxury fertilization and increasing weed competition on these soils decreased leaf xylem pressure potential (Ψ_xpp) when compared to the other treatments.

Conclusions

These results indicate how increasing N resources and weed competition have implications for tree N and water use at establishment in F₁ hybrid plantations of southeast Queensland, Australia. These results suggest the desirability of weed control, in the inter-planting row, in the first year to maximise site N and water resources available for seedling growth. It also showed the need to avoid over-fertilisation, which interfered with the balance between available N and water on these soils.     

Effects of warming and increased precipitation on soil carbon mineralization in an Inner Mongolian grassland after 6?years of treatments

Understanding the responses of soil C mineralization to climate change is critical for evaluating soil C cycling in future climatic scenarios. Here, we took advantage of a multifactor experiment to investigate the individual and combined effects of experimental warming and increased precipitation on soil C mineralization and ¹³C and ¹⁵N natural abundances at two soil depths (0–10 and 10–20?cm) in a semiarid Inner Mongolian grassland since April 2005. For each soil sample, we calculated potentially mineralizable organic C (C ₀) from cumulative CO₂-C evolved as indicators for labile organic C. The experimental warming significantly decreased soil C mineralization and C ₀ at the 10–20-cm depth (P?<?0.05). Increased precipitation, however, significantly increased soil pH, NO ₃ ^? -N content, soil C mineralization, and C ₀ at the 0–10-cm depth and moisture and NO ₃ ^? -N content at the 10–20-cm depth (all P?<?0.05), while significantly decreased exchangeable NH ₄ ⁺ -N content and ¹³C natural abundances at the two depths (both P?<?0.05). There were significant warming and increased precipitation interactions on soil C mineralization and C ₀, indicating that multifactor interactions should be taken into account in future climatic scenarios. Significantly negative correlations were found between soil C mineralization, C ₀, and ¹³C natural abundances across the treatments (both P?<?0.05), implying more plant-derived C input into the soils under increased precipitation. Overall, our results showed that experimental warming and increased precipitation exerted different influences on soil C mineralization, which may have significant implications for C cycling in response to climate change in semiarid and arid regions.     

Patterns of natural N in soils and plants from chemically and organically fertilized uplands

Diagnostic tests for organic production of crops would be useful. In this study, the difference in natural ¹⁵N abundances (δ¹⁵N) of soils and plants between fertilizer-applied upland (FU) and compost-applied upland (CU) fields was investigated to study using δ¹⁵N as a marker of organic produce. Twenty samples each of soils and plants were collected from each field in early summer after applying fertilizer or compost. The δ¹⁵N of fertilizers and composts was −1.6±1.5‰ (n=8) and 17.4±1.2‰ (n=10), respectively. The δ¹⁵N of total soil-N was significantly (P<0.05) higher in CU fields (8.8±2.0‰) than in FU fields (5.9±0.7‰) due to long-term continuous application of ¹⁵N-enriched compost, as indicated by a positive correlation (r=0.62) between N content and δ¹⁵N of total soil-N. The NO₃⁻ pool of CU soils (11.6±4.5‰) was also significantly (P<0.05) enriched in ¹⁵N compared to FU soils (4.7±1.1‰), while the ¹⁵N contents of NH₄⁺ pool were not different between both soils. Compost application resulted in ¹⁵N enrichment of plants; the δ¹⁵N values were 14.6±3.3‰ for CU and 4.1±1.7‰ for FU fields. These results showed that long-term application of compost resulted in a significant ¹⁵N-enrichment of soils and plants relative to fertilizer. Therefore, this study suggested that δ¹⁵N could serve as promising indicators of organic fertilizers application when used with other independent evidence. However, further studies under many conditions should be conducted to prepare reliable δ¹⁵N guidelines for organic produce, since the δ¹⁵N of inorganic soil-N and plant-N are influenced by various factors such as soil type, plant species, the rate of N application, and processes such as mineralization, nitrification, and denitrifcation.     

Soil organic carbon stocks in Veracruz State (Mexico) estimated using the 1:250,000 soil database of INEGI: biophysical contributions

Purpose

In this study, we quantified soil organic carbon (SOC) stocks and analyzed their relationship with biophysical factors and soil properties.

Materials and methods

The study region was Veracruz State, located in the eastern part of Mexico, covering an area of 72,410 km². A soil database that contains physicochemical analyses of soil horizons such as carbon concentration data was the source of information used in this study. The database consisted of 163 soil profiles representing 464 genetic horizons. Statistical analysis was used to investigate the effect of each factor (climate, altitude, slope) on SOC stock to 0.50 m depth and to assess differences in the distribution of SOC stock in terms of soil depth (0.0–0.20, 0.20–0.40, 0.40–0.60, 0.60–0.80, 0.80–1.0 m) and land use. In order to compute the spatial distribution of SOC stock to 0.50 m depth based on the soil sampling location, the kriging method was used.

Results and discussion

Results indicated that SOC stock (0.50 m depth) ranged between 0.44 and 41.2 kg C m^?2. Regression analysis showed that SOC stocks (0.50 m depth) are negatively correlated with temperature (r?=??0.38; P?<?0.001) and positively correlated with altitude (r?=?0.40; P?<?0.001) and slope (r?=?0.40; P?<?0.001). In addition, by multiple regression, temperature combined with precipitation explained more SOC stock variations (r?=?0.43; P?<?0.001) than the regression model with precipitation (r?=?0.13; P?=?0.16) alone. Also, slope combined with temperature and precipitation explained more SOC stock variations (r?=?0.46; P?<?0.001) than the regression model with slope alone. Forest lands, grasslands, and croplands have higher SOC stocks in the 0.0–0.20-m soil layer than in deeper layers. On average, forest lands, grasslands, croplands, and other lands (wetland and dunes) had a SOC stock of 13.6, 14.6, 15.1, and 8.5 kg C m^?2 at 1 m depth, respectively. Soil color correlated (?0.25 ≤ r ≤ ?0.89) with SOC content.

Conclusions

Overall, these results indicate the influence of major interactions between biophysical factors and SOC stocks. This research indicated that SOC stock decreased with soil depth, but with slight variations depending on land use. Thus, there remains a need for more SOC data that include an improved distribution of soil sampling points in order to entirely understand the contributions of biophysical factors to SOC stocks in Veracruz State.     

用树轮δ13C重建过去300年大气CO2浓度

The annual series of δ13C were measured in tree rings of three Cryptomeria fortunei disks （OF-1, OF-2, and OF- 3） collected from West Tianmu Mountain, Zhejiang Province, China, according to cross-dating tree ring ages. There was no obvious decreasing trend of the δ13C annual time series of CF-2 before 1835. However, from 1835 to 1982 the three tree ring δ13C annual series exhibited similar decreasing trends that were significantly （P ≤ 0.001） correlated. The distribution characteristics of a scatter diagram between estimated δ13C series of CF-2 from modeling and the atmospheric CO2 concentration extracted from the Law Dome ice core from 1840 to 1978 were analyzed and a curvilinear regression equation for reconstructing atmospheric CO2 concentration was established with R2 = 0.98. Also, a test of independent samples indicated that between 1685 and 1839 the reconstructed atmospheric CO2 concentration .using the δ13C series of CF-2 had a close relationship with the Law Dome and Siple ice cores, with a standard deviation of 1.98. The general increasing trend of the reconstructed atmospheric CO2 concentration closely reflected the 10ng-term variation of atmospheric CO2 concentration recorded both before and after the Industrial Revolution. Between 1685 and 1840 the evaluated atmospheric CO2 concentration was stable, but after 1840 it exhibited a rapid increase. Given a longer δ13C annual time series of tree rings, it was feasible to rebuild a representative time series to describe the atmospheric CO2 concentration for an earlier period and for years that were not in the ice core record.     

Canopy leaching of cations in Central European forest ecosystems – a regional assessment

The leaching of Ca, Mg, and K from canopies is a major pathway of these cations into forest soils. Our aim was to quantify rates of canopy leaching and to identify driving factors at the regional scale using annual fluxes of bulk precipitation and throughfall from 37 coniferous and deciduous forests of North and Central Europe. Total deposition of Ca, Mg, K, and H⁺ was estimated with Na as an index cation. The median canopy leaching increased in the order: Mg (0.11 kmol_c ha^–1 a^–1) < Ca (0.31 kmol_c ha^–1 a^–1) < K (0.39 kmol_c ha^–1 a^–1). Canopy leaching of Ca and K was positively correlated with the calculated total H⁺ deposition and H⁺ buffered in the canopy, whereas canopy leaching of Mg was not. With contrasting effects, fluxes of SO₄‐S and NH₄‐N in throughfall explained to 64 % (P<0.001) of the Ca canopy leaching. Fluxes of NH₄‐N and Ca were negatively correlated, suggesting that buffering of H⁺ by NH₃ deposition reduced canopy leaching of Ca. Amount of bulk precipitation and SO₄‐S in throughfall were identified as much weaker driving factors for canopy leaching of K (r²=0.28, P<0.01). Our results show that Ca is the dominant cation in buffering the H⁺ input in the canopy. At the regional and annual scale, canopy leaching of Mg appears to be unaffected by H⁺ deposition and H⁺ buffering in the canopy.     

Mechanisms behind the stimulation of nitrification by N input in subtropical acid forest soil

Purpose

Input of N as NH₄ ⁺ is known to stimulate nitrification and to enhance the risk of N losses through NO₃ ^? leaching in humid subtropical soils. However, the mechanisms responsible for this stimulation effect have not been fully addressed.

Materials and methods

In this study, an acid subtropical forest soil amended with urea at rates of 0, 20, 50, 100 mg N kg^?1 was pre-incubated at 25 °C and 60 % water-holding capacity (WHC) for 60 days. Gross N transformation rates were then measured using a ¹⁵N tracing methodology.

Results and discussion

Gross rates of mineralization and nitrification of NH₄ ⁺-N increased (P?<?0.05), while gross rate of NO₃ ^? immobilization significantly decreased with increasing N input rates (P?<?0.001). A significant relationship was established between the gross nitrification rate of NH₄ ⁺ and the gross mineralization rate (R ²?=?0.991, P?<?0.01), so was between net nitrification rate of NH₄ ⁺ and the net mineralization rate (R ²?=?0.973, P?<?0.05).

Conclusions

Stimulation effect of N input on the gross rate of nitrification of NH₄ ⁺-N in the acid soil, partially, resulted from stimulation effect of N input on organic N mineralization, which provides pH-favorable microsites for the nitrification of NH₄ ⁺ in acid soils (De Boer et al., Soil Biol Biochem 20:845–850, 1988; Prosser, Advan Microb Physiol 30:125–181, 1989). The stimulated gross nitrification rate with the decreased gross NO₃ ^? immobilization rate under the elevated N inputs could lead to accumulation of NO₃ ^? and to enhance the risk of NO₃ ^? loss from humid forest soils.

     

Bacterivore nematodes stimulate soil gross N transformation rates depending on their species

We conducted a microcosm experiment with soil being sterilized, reinoculated with native microbial community and subsequently manipulated the bacterivorous nematodes, including three treatments: without (CK) or with introducing one species of the two bacterivores characterized with different body size but similar c-p (colonizer-persister) value (Rhabditis intermedia and Protorhabditis oxyuroides, accounted for 6 and 59% of bacterivores in initially undisturbed soil, respectively). We monitored the N₂O and CO₂ emissions, soil properties, and especially quantified gross N transformation rates using ¹⁵N tracing technique after the 50 days incubation. No significant differences were observed on soil NH₄ ⁺ and NO₃ ^? concentrations between the CK and two bacterivores, but this was not the case for gross N transformation rates. In comparison to CK, R. intermedia did not affect soil N transformation rates, while P. oxyuroides significantly increased the rates of mineralization of organic N to NH₄ ⁺, oxidation of NH₄ ⁺ to NO₃ ^?, immobilization of NO₃ ^? to organic N and dissimilatory NO₃ ^? reduction to NH₄ ⁺. Furthermore, the mean residence time of NH₄ ⁺ and NO₃ ^? pool was greatly lowered by P. oxyuroides, suggesting it stimulated soil N turnover. Such stimulatory effect was unrelated to the changes in abundance of bacteria and ammonia-oxidizing bacteria (AOB). In contrast to CK, only P. oxyuroides significantly promoted soil N₂O and CO₂ emissions. Noticeably, bacterivores increased the mineralization of recalcitrant organic N but decreased soil δ¹³C_-TOC and δ¹⁵N_-TN values, in particular for P. oxyuroides. Combining trait-based approach and isotope-based analysis showed high potential in moving forward to a mechanistic understanding of bacterivore-mediated N cycling.     

The potential of using 15N natural abundance in changing ammonium-N and nitrate-N pools for studying in situ soil N transformations

Purpose

This study examined the usefulness of ¹⁵N natural abundance (δ¹⁵N) with in situ core incubation to quantify the predominant N transformation processes in a natural suburban forest of subtropical Australia, which was subjected to prescribed burning.

Materials and methods

In situ core incubation for 3 days with 20 ml water, or 160.79 ml of 60 mg L^?1 NO₃^?-N surface application, and in situ core with 160.79 ml water but without incubation were set up in Toohey forest for sampling three times as before (once) and after (twice) a prescribed burning. The δ¹⁵N of NH₄⁺-N and NO₃^?-N in the top 5 cm soil before and after the incubation, and δ¹⁵N of NO₃^?-N in the 5–10 cm soil before incubation were compared with each other to examine the soil N mineralisation, nitrification, denitrification, and nitrate leaching processes.

Results and discussion

The significant decrease in δ¹⁵N of NH₄⁺-N after incubation under 20 ml water treatment was ascribed to soil N mineralisation, and the significant decrease in δ¹⁵N of NH₄⁺-N and significant increase in δ¹⁵N of NO₃^?-N after incubation with elevated water and nitrate inputs were associated with N mineralisation and nitrification, respectively, 2 months after the burning. The 160.79 ml water treatment also triggered nitrification in the baseline soil cores in both samplings after the burning. Water was crucial to stimulate soil N mineralisation and nitrification, but excessive water depleted labile N pools and reduced N mineralisation and nitrification. Burning effects were hard to separate from the seasonal impacts on soil N cycling processes.

Conclusions

The δ¹⁵N in soil mineral N pools was sensitive to indicate soil N mineralisation and nitrification processes. Soil water and labile N were determining factors for N transformations in the soil. It is suggested that δ¹⁵N combined with soil inorganic N concentrations and net N transformation rates could be used to identify primary N transformation processes. More frequent samplings would be needed to differentiate burning impacts from the seasonal impacts on soil N cycling processes.

     

Atmospheric monitoring network operations and results in Canada

Atmospheric monitoring activities in Canada relevant to the long-range transport of atmospheric pollutants and the ‘acid rain’ problem are reviewed. Particular aspects examined are network objectives, station density and location, sampling protocol, and quality assurance. Results from a number of these networks are presented for the purpose of outlining the nature and extent of air and precipitation contamination by pollution released in eastern North America. Examples discussed include: the spatial distribution of acidic wet deposition, the temporal variation of acid-related substances in both air and precipitation, an episode of long-range transport, and the impact of acidic emissions on the Arctic atmosphere. Acidic wet deposition is greatest in Canada east of the Manitoba-Ontario border. In 1978, it ranged from 18 to 46 mmol H⁺ m^?2 yr^?1 in the southern half of eastern Canada, with maxima in southern Ontario (44 mmol H⁺ m^?2 yr^?1) and southwestern Quebec (46 mmol H⁺ m^?2 yr^?1). Western Canada receives less acidity in precipitation, but areas of some concern are the Pacific Coast (10 mmol H⁺ m^?2 yr^?1) and to a lesser extent northern Alberta and Saskatchewan (3 to 5 mmol H⁺ m^?2 yr^?1). Acidic emissions from mid-latitude sources which reach the Arctic in winter cause an increase in the acidity of snow from a pH of approximately 5.6 in the summer to values of 4.9 to 5.1 in January through March.     

15N foliar dilution of western red cedar in response to seed inoculation with diazotrophic Paenibacillus polymyxa

Diazotrophic bacteria previously isolated from internal tissues of naturally regenerating lodgepole pine (Pinus contorta var. latifolia (Dougl.) Engelm.) seedlings were tested for their ability to fix N in association with western red cedar (Thuja plicata Donn.). Surface-sterilized cedar seed was sown in glass tubes containing an autoclaved sand/montmorillonite clay mixture that contained a N-free nutrient solution labeled with ¹⁵N as 0.35?mM Ca(¹⁵NO₃)₂ (5?% ¹⁵N label). After sowing, seed was inoculated with one of three bacterial strains: Paenibacillus polymyxa P2b-2R, P. polymyxa P18b-2R, or Dyadobacter fermentans P19a-2R. At the end of the 27-week plant growth period, P2b-2R was the only strain detected in the cedar rhizosphere. No bacteria were found inside plant tissues. Cedar foliar N concentrations were significantly enhanced (26–33?%, P?<?0.05) in response to inoculation with all three bacterial strains but ¹⁵N dilution (23?%, P?<?0.05) was observed only in seedlings treated with strain P2b-2R. This strain also reduced seedling dry weight (27?%, P?<?0.05). We observed similar trends in a second experiment with slight modifications to the protocol, but the magnitude of foliar ¹⁵N dilution was greater (56?%, P?<?0.05). Based on our results, we conclude that cedar seedlings inoculated with strain P2b-2R derived 23 and 56?% of their foliar N from bacterial N fixation in two seedling growth experiments.     

Primary production in a eutrophic acid lake

Total P concentrations, chlorophyll concentrations, and phytoplankton production were investigated bi-weekly in Tibbs Run Lake, Monongalia County, West Virginia, from March 1977 to March 1978. Mean H⁺ concentration in the lake was 25.1 μeq 1^?1 (pH 4.6). The acidic condition of the lake is attributed to inputs of acid via precipitation (mean H⁺ concentration of the bulk precipitation was 79 μeq 1^?1, pH 4.1), and the low buffering capacity of the watershed (bedrock composition of sandstone). Effect of the watershed is shown by the net retention of imput of P (ca. 26%) and H⁺ (ca. 68%). Total P loading to the lake was 0.495 g P m^?2 yr^?1. The single inflow accounted for 95% of the total loading while bulk precipitation accounted for the remainder. Mean summer chlorophyll concentration was 22.2 mg m^?2. Phytoplankton production expressed volumetrically as aP-vol-x value was 9.78 mg C m^?3 h^?1. Regression analysis indicated that H⁺ do not affect chlorophyll concentrations or phytoplankton production but rather that P limits algal biomass. Trophic status of Tibbs Run Lake based on a P budget model, chlorophyll concentration, and volumetric production all indicate that the lake is meso-eutrophic.     

Evidence for nitrification ability controlling nitrogen use efficiency and N losses via denitrification in paddy soils

For understanding the effects of nitrification ability on nitrogen (N) use efficiency and N losses via denitrification in paddy soils under flooding conditions, six paddy soils with different nitrification activities were sampled from various sites of China and a pot experiment was conducted. Rice plants at tillering stage were transplanted into pots and harvested 7.5 days after transplanting, ¹⁵N-(NH₄)₂SO₄ was applied 2.5 days after rice transplanting under continuously flooding conditions. The N losses by denitrification were determined by the unrecovered ¹⁵N applied as ¹⁵NH₄ ⁺ and the N use efficiency (NUE) was calculated by ¹⁵N taken up by rice plants. Plant height (from 33.8 to 37.3 cm) and biomass (from 1.07 g pot^?1 to 1.52 g pot^?1) increased significantly with the native NH₄ ⁺ concentration in the studied soils (P < 0.01). The NUE decreased, whereas the N losses via denitrification increased due to the increase in the nitrification rate of soils determined at 60% water holding capacity (P < 0.05). The results implied that the nitrification activity of paddy soils is a key factor in controlling NUE and N losses via denitrification.     

Acidic precipitation: Considerations for an air quality standard

Acidic precipitation, wet or frozen precipitation with a H⁺ concentration greater than 2.5 μeq l^?1, is a significant air pollution problem in the United States. The chief anions accounting for the H⁺ in rainfall are nitrate and sulfate. Agricultural systems may derive greater net nutritional benefits from increasing inputs of acidic rain than do forest systems when soils alone are considered. Agricultural soils may benefit because of the high N and S requirements of agricultural plants. Detrimental effects to forest soils may result if atmospheric H⁺ inputs significantly add to or exceed H⁺ production by soils. Acidification of fresh waters of southern Scandinavia, southwestern Scotland, southeastern Canada, and northeastern United States is caused by acid deposition. Areas of these regions in which this acidification occurs have in common, highly acidic precipitation with volume weighted mean annual H⁺ concentrations of 25 μeq l^?1 or higher and slow weathering of granitic or precambrian bedrock with thin soils deficient in minerals which would provide buffer capacity. Biological effects of acidification of fresh waters are detectable below pH 6.0. As lake and stream pH levels decrease below pH 6.0, many species of plants, invertebrates, and vertebrates are progressively eliminated. Generally, fisheries are severely impacted below pH 5.0 and are completely destroyed below pH 4.8. At the present time studies documenting effects of acidic precipitation on terrestrial vegetation are insufficient to establish an air quality standard. It must be demonstrated that current levels of precipitation acidity alone significantly injure terrestrial vegetation. For aquatic ecosystems, current research indicates that establishing a maximum permissible value for the volume weighted annual H⁺ concentration of precipitation at 25 μeq l^?1 may protect the most sensitive areas from permanent lake acidification. Such a standard would probably protect other systems as well.     

Atmospheric Deposition in a Rural Area in India - Net and Potential Acidity

Atmospheric deposition in India is generally described as alkaline and well buffered against an increase of acidifying components. Such conclusion has been based on measurements usually performed in urban or suburban areas. Our data on deposition in NE India (in the countryside N of Bhubaneswar) obtained with wet-only and bulk collectors show that: 1) the weighted mean concentrations (and wet deposition) of H⁺ and HCO₃ ^? are almost equal, with dustfall contributing a negligible amount of HCO₃ ^?, 2) the deposition of potential acidity, defined as the acidity that would give the same acidity contribution to the receiving surface as the actual deposition provided that all ammonium was converted to nitrate in the soil, could be as high as 40 mmol H⁺ m^?2 a^?1 corresponding to a pH of 4.3 in precipitation. The low buffer capacity against acidification and high potential acidity were discovered from long- term measurements in a vegetation covered rural area. Similar measurements in a nearby suburban area gave a much higher input of HCO₃ ^? both in wet deposition and dustfall. Further increase of the emission of acidifying components in the region will increase the acid deposition.     

Impact of pea growth and arbuscular mycorrhizal fungi on the decomposition of 15N-labeled maize residues

A pot experiment was carried out (1) to compare C and N yield of different plant parts, nutrient concentrations, and root colonization between the non-mycorrhizal mutant P2 (myc ^?) and the symbiotic isoline Frisson (myc ⁺), (2) to investigate the effects of arbuscular mycorrhizal fungi and growing pea plants on microbial decomposition of ¹⁵N-labeled maize residues, and (3) to follow the distribution of the added substrate over different soil fractions, such as particulate organic matter, soil microbial biomass, and microbial residues. Yields of C in straw, grain, and roots of myc ⁺ peas were significantly higher by 27%, 11%, and 92%, respectively, compared with those of myc ^? peas. The δ¹³ C values in the different plant parts were significantly higher in myc ⁺ than in myc ^? tissue with and without maize. Application of labeled maize residues generally resulted in ¹⁵N enrichment of pea plants. At the end of the experiment, the ergosterol concentration in roots of mature peas did not differ between the two isolines, indicating similar colonization by saprotrophic fungi. The decomposition of added maize residues was significantly reduced by the myc ^? peas, but especially by myc ⁺ peas. The formation of microbial residue C was increased and that of microbial residue N was reduced in the presence of plants. The insufficient N supply to soil microorganisms reduced decomposition of maize residues in the presence of peas, especially myc ⁺ peas.     

Seasonal Changes of Shoot Nitrogen Concentrations and 15N/14N Ratios in Common Reed in a Constructed Wetland

Abstract

Nitrogen (N) concentrations and stable N isotope abundances (δ¹⁵N) of common reed (Phragmites australis) planted in a constructed wetland were measured periodically between July 2001 and May 2002 to examine their seasonal variations in relation to N uptake and N translocation within common reed. Nitrogen concentrations in P. australis shoots were higher in the growing stage (7.5 to 24.8 g N kg^?1) than in the senescence stage (4.2 to 6.8 g N kg^?1), indicating N translocation from shoots to rhizomes. Meanwhile, the corresponding δ¹⁵N values were higher in the senescence stage (+12.2 to +22.4‰) than in the growing stage (+5.1 to +11.3‰). Coupled with the negative correlation (R²=0.24, P<0.05, n=18) between N concentrations and δ¹⁵N values of shoots in the senescence stage, our results suggested that shoot N became enriched in ¹⁵N due to N isotopic fractionation (with an isotopic fractionation factor, α_s/p, of 1.012) during N translocation to rhizomes. However, the positive correlation between N concentrations and δ¹⁵N values in the growing stage (R²=0.19, P<0.001, n=54) suggested that P. australis relies on N re‐translocated from rhizome in the early growing stage and on mineral N in the sediment during the active growing stage. Therefore, seasonal δ¹⁵N variations provide N‐isotopic evidence of N translocation within and N uptake from external N sources by common reed.