Ecological effects of atmospheric reactive nitrogen deposition on semi-natural terrestrial ecosystems

Evidence that enhanced reactive nitrogen deposition is affecting semi-natural terrestrial ecosystems comes from historic increases in plant tissue N concentrations, correlations between tissue N concentrations and present-day total atmospheric N deposition, changes in plant amino-acid composition and effects on N assimilation. The ecological significance of such changes in biomarkers is uncertain. This paper explores the ecological significance of reactive atmospheric N deposition through a review of previous experimental findings and new experimental evidence from an acidic and a calcareous grassland, both showing phosphorus limitation, and a N-limited Calluna vulgaris (L.) Hull heathland in upland Britain. Nitrogen addition in the range 0–20 g N m⁻² yr⁻¹ initially (years 0–4) increased the growth of Calluna and a decline in some subordinate species. In subsequent years, shoot extension was not stimulated, but winter injury was observed from 1993 onwards, suggesting a strong interaction between N supply and climatic conditions. By contrast, the grasslands showed a small decrease in the cover of higher plants in later years (6–7) of the experimental treatments (0–14 g N m⁻² yr⁻¹) and no growth stimulation. All N treatments reduced the bryophyte cover in the acidic grassland. There were marked effects on below-ground processes, including a sustained stimulation of N mineralization in the grassland soils, and an increase in the bacterial utilization of organic substrates in the heathland, as measured in BIOLOG plates. The results strongly suggest the importance of atmospheric N deposition on microbially driven processes in soils, and are discussed in relation to the scale of potential ecosystem changes and their reversibility by pollution abatement.     

Near-zero recent carbon accumulation in a bog with high nitrogen deposition in SW Sweden

We present data on the accumulation of carbon and nitrogen into an open oceanic ombrotrophic bog, SW Sweden, with high levels of anthropogenic nitrogen deposition. The aim was to investigate if this peatland currently acts as a sink for atmospheric carbon. Peat cores were sampled from the top peat layer in five different vegetation types. Small pines were used to date the cores. The cores bulk density and carbon and nitrogen content were determined. A vegetation-classified satellite image was used to estimate the areal extent of the vegetation types and to scale up these results to bog level. The rate of current carbon input into the upper oxic acrotelm was 290 g m⁻² yr⁻¹, and there were no significant differences in accumulation rates among the vegetation types. This organic matter input to the acrotelm was almost completely decomposed before it was deposited for storage in the deeper peat layers (the catotelm) and only a small fraction (≪1%) or 0.012 g m⁻² yr⁻¹ of the carbon would be left, assuming a residence time of 100 years in the acrotelm. Nitrogen accumulation rates differed between the vegetation classes, and the average input via primary production varied from 5.33 to 16.8 g m⁻² yr⁻¹. Current nitrogen input rates into the catotelm are much lower, 0–0.059 g m⁻² yr⁻¹, with the highest accumulation rates in lawn-dominated communities. We suggest that one of the main causes of the low carbon input rates is the high level of nitrogen deposition, which enhances decomposition and changes the vegetation from peat-forming Sphagnum -dominance to dominance by dwarf shrubs and graminoids.     

Nitrogen effects on net ecosystem carbon exchange in a temperate steppe

It has widely been documented that nitrogen (N) enrichment stimulates plant growth and net primary production. However, there is still dispute on how N addition affects net ecosystem CO₂ exchange (NEE), which represents the balance between ecosystem carbon (C) uptake and release. We conducted an experimental study to examine effects of N addition on NEE in a temperate steppe in northern China from 2005 to 2008. N was added at a rate of 10 g N m⁻² yr⁻¹ with NH₄NO₃ alone or in combination with phosphorous (P, 5 g P₂O₅ m⁻² yr⁻¹) in both clipped and unclipped plots. Over the 4 years, N addition significantly stimulated growing-season NEE, on average, by 27%. Neither the main effects of P addition or clipping nor their interactions with N addition were statistically significant on NEE in any of the 4 years. However, the magnitude of N stimulation on NEE declined over time. N addition significantly increased NEE by 60% in 2005 and 21% in 2006, but its effect was not significant in 2007 and 2008. N-induced shift in species composition was primarily responsible for the declined N stimulation over time. The gradually increasing coverage of the upper canopy species ( Stipa krylovii ) and standing litter accumulation induced light limitation on the lower canopy species ( Artemisia frigida ). Thus, N-induced shifts in plant species composition strongly regulated the direct effects of N addition on C sequestration in the temperate steppe.     

A limnological investigation of the tarn Syslaktjønn, W Norway

An investigation covering hydrography, chemistry, vascular and cryptogamic plants, nitrogen fixation, phytoplankton biomass and production, and zooplankton was carried out from April to November 1976 in tarn in W Norway, The volume of the tarn was 18000 m³ and the turnover time about 30 d. Temperature ranged between 3.6 and 23.4°C and pH between 4.8 and 5.5. Nuphar luteum and Carex rostrata were the two dominating vasculars-with biomasses of 117 and 97 g m⁻², respectively The biomass of the bryophytes ( Sphagnum spp.) was about 510 g m⁻² and the production of the order 0.2–2.1 μg (mg d.w.)⁻¹h⁻¹. Nitrogen fixation in association with Sphagnum spp. was estimated at 25 g yr⁻¹ for the whole tarn. Phytoplankton was dominated by diatoms, green algae and chrysophyceans. The chlorophyll a content ranged from 2 to 20 mg m⁻³ and the carbon assimilation rates from 0.03 to 20 mg C m⁻³h⁻¹ at 0–4 m depths. Production in the period was of the magnitude 22 g C m⁻². The copepod Eudiaptomus gracilis was the most common netzooplankter. Large numbers of rotationrians were found during summer.     

Litterfall in two Canadian deciduous woods: quality, quantity and timing

Litterfall in two deciduous woods in Ontario, Canada, was caught in bag traps and by new screen traps for two years. The beech-maple wood produced 818 g m⁻² (8.2 t ha⁻¹) and the poplar wood produced only 450 g m⁻² (4.5 t ha⁻¹). Of these totals, canopy leaves alone made up 386 g m⁻² for the beech-maple wood and 270 g m⁻² for the poplar wood leaving 47% and 34%, espectively, for other litter components. Data are presented for seven types of litter in addition to canopy leaves.
Redistribution of litter on the ground causes spatial heterogeneity of substrate and of habitats for decomposers and hence of decomposition.
Temporal distributions of various types of litterfall differ between the two woods in relation to their vegetation structure. The diversity of litter types combines with extensive temporal distribution of litterfall in the beech-maple woodland to give that wood the potential for nearly continuous decomposition activity. In contrast, the poplar woodland has low diversity of litter and short duration of litterfall.     

Role of benthic insect feces in a Rocky Mountain stream: fecal production and support of consumer growth

The feces of stream insects may be a major component of fine particulate organic matter (FPOM) available to collector organisms. In Mink Creek, Idaho, winter defecation rates for 9 species ranged from 86 mg dry feces (g dry body wt)⁻¹ d⁻¹ (Ephemerella spinifera) to 154 mg g⁻¹ d⁻¹ (Paraleptophlebia heteronea) . Detailed studies of 3 species in summer revealed that rates were much greater than in winter and exhibited high between-individual variation (e.g., 695 ± 184.8 mg (g body wt)⁻¹ d⁻¹ for Baetis tricaudatus ). By combining measured and literature values, it is estimated that the benthic insect community egests approximately 3.4 kg feces m⁻² yr⁻¹. This value lies within the range of FPOM estimated to be derived from degradation of leaf litter input (0.7–9.9 kg m⁻² yr⁻¹, depending upon assimilation efficiency of the organisms).
Short-term winter growth experiments showed that, with two exceptions, collectors fed on fecal detritus will grow as well as on other food resources; relative growth rates depend upon the species. Because fecal detritus is abundant throughout the year, and is treated by many species as a source of food, it could be a major mechanism by which seasonal fluctuation in availability of allochthonous litter input is minimized and homeostasis of structure and function maintained.     

N2(C2H2)-fixation in early stages of a primary succession on a reclaimed salt marsh

Nitrogen fixation activity was determined for Lotus tenuis. Medicago lupulina and Trifolium pratense . The three species grew in clones in grassland in an area reclaimed from brackish water in the 1940s. The N₂[C₂H₂]-fixation was measured in soil cores throughout 1974 and 1975. From cores taken in dense and uniform stands of the species, the yearly N₂[C₂H₂]-fixation at maximum cover was estimated. L. tenuis fixed about 4 g N m⁻² yr⁻¹ (area with max. cover 130%), i.e. 30–56% of its requirement. Both M. lupulina and T. pratense fixed about 7 g N m⁻² yr⁻¹ (maximum cover 37% and 80%) i.e. 67% of their N-requirement. Average N₂[C₂H₂]-fixation for the whole area was 0.4 g N m⁻² yr⁻¹, considerably less than the N-addition through rainfall.     

Comparative primary productivity of algal epiphytes on three species of macrophyte in the littoral zone of Lake Ohrid, Yugoslavia

Primary productivity of algal epiphytes on the surfaces of Phragmites, Potamogeton , and Nuphar was measured seasonally from June 1978, through June 1979, in the littoral zone of Lake Ohrid, using ¹⁴C methodology. Surface areas of individual macrophytes were determined throughout the study period through the use of a non-miscible surfactant and a calibration curve of surfactant weight versus known, calculated surface areas.
Mean total surface area available for epiphytic colonization during the study period was 1.032 m² macrophyte surface per m² of littoral zone for Phragmites , 0.810 m² for Potamogeton , and 0.167 m² for Nuphar . Seasonal rates of mean primary productivity of algal epiphytes on Phragmites from the surface to the light-compensation depth ranged from 84–1406 mg C m⁻² littoral zone d⁻¹; ranges for epiphytes on Potamogeton and Nuphar were 77–586 and 69–268 mg C m⁻² littoral zone d⁻¹, respectively. Maximum rates were observed typically during June; minimum rates were observed typically during August to December. Mean daily productivity rates over the 12 month period were for epiphytes on Potamogeton 167.0, on Nuphar 100.4 and on Phragmites 671.2 mg C m⁻² littoral zone d⁻¹. Calculated annual production for epiphytes on Nuphar was 36.65, on Potamogeton 60.95 and on Phragmites 245.0 g C m⁻² littoral zone yr⁻¹. Epiphytic production data were typically considerably higher than production data obtained for littoral and pelagial planktonic algae and compare favorably with published data for epiphytic and periphytic production in Lawrence Lake, Marion Lake, and Borax Lake.     

Profundal chironomid populations during a fertilization experiment in Langvatn, Norway

Populations of Heterotrissocladius subpilosus and Phaenopsectra (Sergentia) coracina were studied from 1974 to 1978 in Langvatn, South-Trøndelag, Norway. During 1975 and 1976 the lake was fertilized respectively by 1600 kg and 1000 kg fertilizer, (20% N, 4,8% P).
Fertilization had a positive effect on primary production and zooplankton. The effect on the two chironomid species, which both need more than one year for development, was ambiguous. However, there was a positive relationship between the density of H. subpilosus larvae and the primary production.
The number of P. (S.) coracina larvae steadily increased during the investigated period. Production estimates gave values in the range 0.6–1.5 g w.w. m⁻² yr⁻¹ for H. subpilosus and ca 2.3 g w.w. m⁻² yr⁻¹ for P. (S.) coracina .     

Respiration rate in maize roots is related to concentration of reduced nitrogen and proliferation of lateral roots

The relationship between specific rate of respiration (respiration rate per unit root dry weight) and concentration of reduced nitrogen was examined for maize ( Zea mays L.) roots. Plants with 2 primary nodal root axes were grown for 8 days in a split-root hydroponic system in which NO-₃ was supplied to both axes at 1.0 mol m⁻³, to one axis at 1.0 mol m⁻³ and the other axis at 0.0 mol m⁻³ or to both axes at 0.0 mol m⁻³ Respiration rates and root characteristics were measured at 2-day intervals. Specific rate of respiration was positively correlated in a nonlinear relationship with concentration of reduced nitrogen. The lowest specific rates of respiration occurred when neither axis received exogenous NO⁻³ and the concentration of reduced nitrogen in the axes was less than 9 mg g⁻¹. The greatest rates occurred in axes that were actively absorbing NO⁻³ and contained more than 35 mg g⁻¹ of reduced nitrogen. At 23 mg g⁻¹ of reduced nitrogen, below which initiation of lateral branches was decreased by 30–50%. specific rate of respiration was 17% greater for roots actively absorbing NO⁻³ than for roots not absorbing NO⁻³ Increases in specific rate of respiration associated with concentrations of reduced nitrogen greater than 23 mg g⁻¹ were concluded to be attributable primarily to proliferation of lateral branches.     

Holocene carbon burial by lakes in SW Greenland

The role of the Arctic in future global change processes is predicted to be important because of the large carbon (C) stocks contained in frozen soils and peatlands. Lakes are an important component of arctic landscapes although their role in storing C is not well prescribed. The area around Kangerlussuaq, SW Greenland (66–68°N, 49–54°W) has extremely high lake density, with ∼20 000 lakes that cover about 14% of the land area. C accumulation rates and standing stock (kg C m⁻²), representing late- to mid-Holocene C burial, were calculated from AMS ¹⁴C-dated sediment cores from 11 lakes. Lake ages range from ∼10 000 cal yr  bp to ∼5400 cal yr  bp , and reflect the withdrawal of the ice sheet from west to east. Total standing stock of C accumulated in the studied lakes for the last ∼8000 years ranged from 28 to 71 kg C m⁻², (mean: ∼42 kg C m⁻²). These standing stock determinations yield organic C accumulation rates of 3.5–11.5 g C m⁻² yr⁻¹ (mean: ∼6 g C m⁻² yr⁻¹) for the last 4500 years. Mean C accumulation rates are not different for the periods 8–4.5 and 4.5–0 ka, despite cooling trends associated with the neoglacial period after 4.5 ka. We used the mean C standing stock to estimate the total C pool in small lakes (<100 ha) of the Kangerlussuaq region to be ∼4.9 × 10¹³ g C. This C stock is about half of that estimated for the soil pool in this region (but in 5% of the land area) and indicates the importance of incorporating lakes into models of regional C balance at high latitudes.     

Oxygen and nitrate respiration in a reed swamp sediment from a eutrophic lake

Seasonal measurements of the oxygen and nitrate uptake by a reed swamp sediment were carried out in a shallow, eutrophic Danish lake, Arreskov Sø. The oxidation of organic carbon in the sediment by aerobic and nitrate respiration was 290 and 188 g C m⁻² yr⁻¹ respectively. During winter, nitrate respiration amounted to 94% of the total carbon oxidation, whereas it was zero during summer. On an annual basis nitrate respiration constituted 39% of total respiration. Sediment nitrate uptake was correlated to nitrate concentration. In consequence of this the nitrate uptake rates varied during the year from zero in summer to 55 mg N m⁻² d⁻¹ in spring.
Oxygen uptake rates varied from 30 to 250 mg O₂ m⁻² h⁻¹ during the year, with a maximum uptake in August. The oxygen uptake per year was calculated to 860 g O₂ m⁻². The oxygen uptake rate was correlated to lake temperature and Kjeldahl nitrogen content of the sediment. The oxygen uptake rate, however, showed no correlation with loss on ignition of the sediment. A Q₁₀-value of 2.2 was found for lake measurements in the temperature interval of 5–15°C. The corresponding O₁₀-value in the laboratory was 2.6. A high microbial biomass indicated by the maximum content of Kjeldahl nitrogen and the lowest ratio of loss on ignition on Kjeldahl nitrogen appeared in late August, when the maximum oxygen uptake occurred. The oxygen uptake rate increased during the time interval from sampling to the start of the experiments.     

Productivity of a herbivorous pupfish population (Cyprinodon nevadensis) in a warm desert stream

Production and food intake by an herbivorous pupfish population ( Cyprinodon nevadensis amargosae ) living in the outflow of a thermal artesian well (Tecopa Bore) near Death Valley, California is described. Water issues from the ground at 47.5° C and cools 8.12° C before leaving the study area 300 m from the source. High stream temperatures restricted the pupfish population to some 41 % of the study area, with a resulting mean density of 89 individuals m⁻² (range = 13 to 196m⁻²). Biomass estimates ranged from 7 kcal m⁻² to 42 kcal m⁻². The mean annual standing crop of pupfishes (24 kcal m⁻²) turned over about five times annually. Growth rates were highest in juveniles (♂= 9.5% day⁻¹) and slowest in large adults (♀= 08% day⁻¹). Monthly production ranged from 22 kcal m⁻² in September to 3 kcal m⁻² in July and August. Pupfish in Tecopa Bore fed on algae and detritus, ingesting 1941 kcal m⁻² yr⁻¹ or 17.5% of the annual net primary production. 119 kcal m⁻² yr⁻¹ was deposited in growth. This latter value is approximately ten times greater than values previously reported for large carnivorous fishes but is comparable lo values reported for herbivorous fishes under pond culture.     

Effect of irradiance and plant age on the dimensions of the growing shoot of poplar

Populus euramericana (Dode) Guinier cv. Robusta plants were cultivated at irradiances of 7.5, 15 and 30 W m⁻² (32.5, 65 and 130 μmol m² s⁻¹), 400–700 nm at 22°C and a relative humidity between 40 and 60% on a gravel culture subirrigated with Hoagland's nutrient solution. The basal diameter of the growing shoot, a measure of the number of apical cells participating in growth, increased proportionally to irradiance and was correlated with mature leaf length. The development of the length of the growing shoot (L_gs) depended also on the nutritional status of the (young) shoot. L_gs was strongly correlated with the rate of height growth.     

Effects of nitrogen addition on the stress sensitivity of Calluna vulgaris

The effects of low levels of nitrogen addition (7·7 and 15·4 kg N ha⁻¹ yr⁻¹) on plant sensitivity to biotic and abiotic stress were studied at a lowland heath in the south of England that has received N treatments since 1989. Larval growth rates and adult weights of heather beetles were found to be significantly higher when insects were reared on plants that had received additional N, with implications for insect survival and reproductive success. Electrolyte leakage measurements failed to reveal any significant impact of N addition on plant sensitivity to frost episodes in early winter. In April, however, there was some evidence of slightly decreased frost hardiness in plants receiving additional N. Accelerated spring bud burst also suggested earlier physiological activity in N-treated plots. The rate of water loss from excised shoots of Calluna vulgaris (L.) Hull was significantly faster in plants receiving additional N, although no difference in plant water potential was measured in the field after a prolonged dry spell. Whilst experimental N addition had only a small effect on plant sensitivity to abiotic stress, the relationship between enhanced deposition and increased insect performance was clear, with the potential for substantially increased insect damage at deposition rates around the critical load contributing to the formation of gaps in the Calluna canopy.     

Growth pattern and distribution of biomass of Calluna vulgaris on an ombrotrophic peat bog

The distribution pattern of Calluna vulgaris on the ombrotrophic peat bog Åkhultmyren, S. Sweden, was investigated.
Calluna occurs primarily on hummocks, with its maximum biomass (ca 250 g m⁻² above ground, and ca 300 g m⁻² below ground) concentrated on hummock tops. Biomass decreased gradually along the hummock-hollow gradient to a level of ca 5 cm above the water level. This decreasing biomass was correlated with a decrease in the predominant age of the above ground shoots from 6–8 yr to 3–4 yr old shoots.
Calluna is characterized by extremely slow diameter growth and biomass increment. There was, however, a poor relation between height above water level and stem thickness, assimilating biomass and total shoot weight, respectively.     

Climate-induced changes in high elevation stream nitrate dynamics

Mountain terrestrial and aquatic ecosystems are responsive to external drivers of change, especially climate change and atmospheric deposition of nitrogen (N). We explored the consequences of a temperature-warming trend on stream nitrate in an alpine and subalpine watershed in the Colorado Front Range that has long been the recipient of elevated atmospheric N deposition. Mean annual stream nitrate concentrations since 2000 are higher by 50% than an earlier monitoring period of 1991–1999. Mean annual N export increased by 28% from 2.03 kg N ha⁻¹ yr⁻¹ before 2000 to 2.84 kg N ha⁻¹ yr⁻¹ in Loch Vale watershed since 2000. The substantial increase in N export comes as a surprise, since mean wet atmospheric N deposition from 1991 to 2006 (3.06 kg N ha⁻¹ yr⁻¹) did not increase. There has been a period of below average precipitation from 2000 to 2006 and a steady increase in summer and fall temperatures of 0.12 °C yr⁻¹ in both seasons since 1991. Nitrate concentrations, as well as the weathering products calcium and sulfate, were higher for the period 2000–2006 in rock glacier meltwater at the top of the watershed above the influence of alpine and subalpine vegetation and soils. We conclude the observed recent N increases in Loch Vale are the result of warmer summer and fall mean temperatures that are melting ice in glaciers and rock glaciers. This, in turn, has exposed sediments from which N produced by nitrification can be flushed. We suggest a water quality threshold may have been crossed around 2000. The phenomenon observed in Loch Vale may be indicative of N release from ice features such as rock glaciers worldwide as mountain glaciers retreat.     

The life cycle dynamics and production of zooplankton in Øvre Heimdalsvatn

The zooplankton in Øvre Heimdalsvatn was studied from May 1969 to Aug 1973 in order to obtain knowledge about their functioning and role in the lake ecosystem. The paper concentrates on the main species: Holopedium gibberum, Bosmina longispina, Cyclops scutifer. Heterocope saliens, Conochilus unicornis, Polyarthra vulgaris and Kellicottia longispina but Megacyclops gigus, Daphnia longispina and ciliated cells are also considered. The development of the populations was fairly synchronized and made it possible to determine life cycles, development times, growth, reproduction and production from field data. The total production of the multicellular zooplankton was 2–3 mg dry wt m⁻² yr⁻¹. The cladocerans dominated and their share of the total was 84–92% compared to the copepods 5–8% and the rotifers 3–8%. The ciliate production was not estimated, but their biomass exceeded that of the copepods and rotifers. The spring spate, June temperatures, food availability and invertebrate predation were found to be the most important regulatory factors for life cycle dynamics and production. Allochthonous organic matter was found to be a necessary food resource in addition to algae.     

Photophobic stop-response in a dinoflagellate: Modulation by preirradiation

Gyrodinium dorsum Kofoid responds photophobically to flashes of blue light. The photophobic response consists of a cessation of movement (stop-response). Without background light and after a flash fluence above 10 J m⁻², 75–85% of the cells show a stop-response, while only 50% of the cells show this response at 5 J m⁻². With a flash fluence of 5 J m⁻², background light of different wavelengths either increases (614 nm. 5.5–18.2 μmol m⁻² s⁻¹) or decreases (700 nm, 18.4–36.0 μmol m⁻² s⁻¹) the stop-response. Two hypotheses for the mechanism of the modulation by background light of the photophobic response are discussed: an effect of light on the balance of the photosynthetic system (PS I/PS II) or an effect on a phytochrome-like pigment (P_r/P_fr). This study supports the idea that a phytochrome-like pigment works in combination with a blue light-absorbing pigment. It was also found that cells of Gyrodinium dorsum cultured in red light (39.8 μmol m⁻²) had a higher absorption in the red region of the absorption spectra than those cultured in white light (92.7 μmol m⁻²).     

Phytochrome control of hypocotyl extension in light-grown Chenopodium rubrum

The regulation of hypocotyl extension in light-grown Chenopodium rubrum L. seedlings by light analogous to dense vegetation canopy shade has been monitored. Hypocotyl extension was controlled by both the quantity and quality of the actinic light. At the higher of the two background photon fluence rates which were used (10.0 μmol m⁻²s⁻¹ in the 400–700 nm waveband), increasing the proportion of phytochrome calculated to exist as Pfr resulted in greater inhibition of growth. At the lower photon fluence rate (1.0 μmol m⁻²s⁻¹ in the 400–700 nm waveband), a biphasic response was observed in which minimum inhibition was observed at intermediate photoequilibria. Although photosynthesis was not directly involved in the photomorphogenetic responses, it did play an indirect quantitative role in determining the response.