Impact of nutrient removal through harvesting on the sustainability of the boreal forest

The cycling of base cations (K, Ca, Mg, and Na) was investigated in a boreal balsam fir forest (the Lake Laflamme Watershed) between 1999 and 2005. Base cation budgets were calculated for the soil rooting zone that included atmospheric deposition and soil leaching losses, two scenarios of tree uptake (whole-tree and stem-only harvesting), and three scenarios of mineral weathering, leading to six different scenarios. In every scenario there was a net accumulation of Mg within the soil exchangeable reservoir, while Ca accumulated in four scenarios. Potassium was lost in five of the six scenarios. Contrary to Ca and Mg, immobilization of K within tree biomass (69 mol x ha(-1) x yr(-1)) was the main pathway of K losses from the soil exchangeable reservoir, being five times higher than losses via soil leaching (14 mol x ha(-1) x yr(-1)). The amounts of K contained within the aboveground biomass and the exchangeable soil reservoir were 3.3 kmol/ha and 4.2 kmol/ha, respectively. Whole-tree harvesting may thus remove 44% of the K that is readily available for cycling in the short term, making this forest sensitive to commercial forestry operations. Similar values of annual K uptake as well as a similar distribution of K between tree biomass and soil exchangeable reservoirs at 14 other coniferous sites, distributed throughout the boreal forest of Quebec, suggest that the Lake Laflamme Watershed results can be extrapolated to a much larger area. Stem-only harvesting, which would reduce K exports due to biomass removal by 60%, should be used for these types of forest.     

Variation of salts and available nutrients in salt-affected soil during leaching process under the influence of organic ameliorators

ABSTRACT

This study investigated the potential influence of three organic ameliorators (peat, biochar and leonardite) on salts and nutrients in salt-affected soils during intermittent leaching. Results showed that nearly 90% of salt was removed from columns in the leaching process and sodium adsorption ratio (SAR) of the soil after leaching was reduced by 67.3% (control, CK), 62.9% (peat), 70.1% (biochar) and 55.0% (leonardite). Total N loss declined by 26.2% (peat), 11.7% (biochar) and 55.5% (leonardite) compared with CK in the process of leaching. The maintaining N (NH₄⁺ and NO₃^?) of soil after leaching was 8.25, 7.31, 11.31 and 14.48?mg/kg for CK, peat, biochar and leonardite treatments. Final P loss was 0.47, 0.31, 0.54, 0.27?mg/column in leaching for CK, peat, biochar and leonardite treatments. Soluble P of soil after leaching was measured as 6.95 (CK), 5.62 (peat), 8.52 (biochar) and 3.33 (leonardite) mg/kg. Leaching could remove the salt effectively but with nutrient loss in the process. The findings of this study suggest that organic ameliorators (biochar, peat or leonardite) play an important role in retaining nutrients during leaching as well as supplying nutrients after leaching to offer practical assistance for the amendment of salt-affected soil in the Yellow River Delta.     

Effects of Cadmium on Nutrient Uptake and Translocation by Indian Mustard

Plants that hyperaccumulate metals are ideal subjects for studying the mechanisms of metal and mineral nutrient uptake in the plant kingdom. Indian Mustard (Brassica juncea) has been shown to accumulate moderate levels of Cd, Pb, Cr, Ni, Zn, and Cu. In this experiment, 10 levels of Cd concentration treatments were imposed by adding 10–190 mg Cd kg^–1 to the soils as cadmium nitrate [Cd(NO₃)₂]. The effect of Cd on phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and the micronutrients iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) in B. juncea was studied. Plant growth was affected negatively by Cd, root biomass decreased significantly at 170 mg Cd kg^–1 dry weight soils treatment. Cadmium accumulation both in shoots and roots increased with increasing soil Cd treatments. The highest concentration of Cd was up to 300 mg kg^–1 d.w. in the roots and 160 mg kg^–1 d.w. in the shoots. The nutrients mainly affected by Cd were P, K, Ca, Fe, and Zn in the roots, and P, K, Ca, and Cu in the shoots. K and P concentrations in roots increased significantly when Cd was added at 170 mg kg^–1, and this was almost the same level at which root growth was inhibited. Zn concentrations in roots decreased significantly when added Cd concentration was increased from 50 to 110 mg kg^–1, then remained constant with Cd treatments from 110 to 190 mg kg^–1. However, Zn concentrations in the shoots seemed less affected by Cd. It is possible that Zn uptake was affected by the Cd but not the translocation of Zn within the plant. Ca and Mg accumulation in roots and shoots showed similar trends. This result indicates that Ca and Mg uptake is a non-specific process.     

Characteristics of mass distributions of aerosol particle and its inorganic water-soluble ions in summer over a suburb farmland in Beijing

Agricultural activity is one of the most important sources of aerosol particles. To understand the mass distribution and sources of aerosol particles and their inorganic water-soluble ions in a suburb farmland of Beijing, particle samples were collected using a microorifice uniform deposit impactor (MOUDI) in the summer of 2004 in a suburb vegetable field. The distribution of the particles and their inorganic water-soluble ions in the diameter range of 0.18–18 μm were measured. The dominant fine particle ions were SO₄ ^2?, NO₃ ^?, and NH₄ ⁺. The association of day-to-day variation of the concentration of these ions with temperature, humidity, and solar radiation suggested that they are formed by the reaction of NH₃ released from the vegetable field with the acid species produced from photochemical reactions. Fine particle K⁺ is likely from vegetation emission and biomass burning. Coarse particles like Ca²⁺, Mg²⁺, NO₃ ^?, and SO₄ ^2? are suggested to come from the mechanical process by which the soil particle entered the atmosphere, and from the reaction of the acid species at the surface of the soil particle. The results show that fertilizer and soil are important factors determining the aerosol particle over agricultural fields, and vegetable fields in suburban Beijing contribute significantly to the aerosol particle.     

Atmospheric Deposition, Mineralization and Leaching of Nitrogen in Subtropical Forested Catchments, South China

In recent years, China has conducted considerable research focusing on the emission and effects of sulphur (S) on human health and ecosystems. By contrast, there has been little emphasis on anthropogenic nitrogen (N) so far, even though studies conducted abroad indicate that long-range atmospheric transport of N and ecological effects (e.g. acidification of soil and water) may be significant. The Sino-Norwegian project IMPACTS, launched in 1999, has established monitoring sites at five forest ecosystems in the southern part of PR China to collect comprehensive data on air quality, acidification status and ecological effects. Here we present initial results about N dynamics at two of the IMPACTS sites located near Chongqing and Changsha, including estimation of atmospheric deposition fluxes of NO_x and NH_x and soil N transformations. Nitrogen deposition is high at both sites when compared with values from Europe and North America (25–38 kg ha^–1 yr^–1). About 70% of the deposited N comes as NH₄, probably derived from agriculture. Leaching of N from soils is high and nearly all as NO₃ ^–1. Transformation of N to NO₃ ^–1 in soils results in acidification rates that are high compared to rates found elsewhere. Despite considerable leaching of NO₃ ^–1 from the root zone of the soils, little NO₃ ^–1 appears in streamwater. This indicates that N retention or denitrification, both causing acid neutralization, may be important and probably occur in the groundwater and groundwater discharge zones. The soil flux density of mineral N, which is the sum of N deposition and N mineralization, and which is dominated by the N mineralization flux, may be a good indicator for leaching of NO₃ ^–1 in soils. However, this indicator seems site specific probably due to differences in land-use history and current N requirement.     

Nitrate reductase and urease enzyme activity in the marine diatom<Emphasis Type="Italic"> Thalassiosira weissflogii</Emphasis> (Bacillariophyceae): interactions among nitrogen substrates

The dissolved nitrogen pool in aquatic systems is comprised of many different nitrogen forms, both inorganic and organic. Interaction among these nitrogen forms at the level of uptake and enzyme activity is, with the exception of NH₄⁺ and NO₃^–, not completely understood. Nitrate reductase (NR) and urease (UA) activities in the marine diatom Thalassiosira weissflogii (Grunow) Fryxell et Hasle were measured in NO₃^–, NH₄⁺, and urea-sufficient cultures before and after challenge additions of NH₄⁺, NO₃^–, and urea in a factorial design. NR and UA were constitutively expressed during growth on NO₃^–, NH₄⁺, and urea. Growth on NH₄⁺ or urea resulted in NR activities that were <10% of the activity observed in the NO₃^–-grown culture, while growth on NO₃^– resulted in UA values that were ~35% of the activities during growth on either NH₄⁺ or urea. The addition of NH₄⁺ or urea to NO₃^–-grown cultures resulted in an immediate decrease in cellular NO₃^– uptake rate, which was not mirrored by an immediate repression of in vitro NR activity; however, the diel peak in NR was suppressed in these challenge experiments. The addition of NO₃^– or NH₄⁺ to urea-grown cultures resulted in non-significant decreases in the urea uptake rate. UA was not impacted by NO₃^– addition, but NH₄⁺ addition significantly decreased UA throughout the experiment. These studies demonstrate that the uptake and assimilation of NO₃^– and urea may not be subject to the same internal feedback mechanism when challenged with other nitrogen substrates.Communicated by J.P. Grassle, New Brunswick     

Model of stomatal ammonia compensation point (STAMP) in relation to the plant nitrogen and carbon metabolisms and environmental conditions

Agricultural crops can be either a source or a sink of ammonia (NH₃). Most NH₃ exchange models developed so far do not account for the plants nitrogen (N) metabolism and use prescribed compensation points. We present here a leaf-scale simplified NH₃ stomatal compensation point model related to the plants N and carbon (C) metabolisms, for C₃ plants. Five compartments are considered: xylem, cytoplasm, apoplasm, vacuole and sub-stomatal cavity. The main processes accounted for are the transport of ammonium (NH₄⁺), NH₃ and nitrate (NO₃⁻) between the different compartments, NH₄⁺ production through photorespiration and NO₃⁻ reduction, NH₄⁺ assimilation, chemical and thermodynamic equilibriums in all the compartments, and stomatal transfer of NH₃.The simulated compensation point is sensitive to paramaters related to the apoplastic compartment: pH, volume and active transport rate. Determining factors are leaf temperature, stomatal conductance and NH₄⁺ flux to the leaf. Atmospheric NH₃ concentration seem to have very little effect on the compensation point in conditions of high N fertilization. Comparison of model outputs to experimental results show that the model underestimates the NH₃ compensation point for high N fertilization and that a better parametrisation of sensitive parameters especially active trasport rate of NH₄⁺ may be required.     

Preferential leaching of nitrate,chloride and phosphate in an Australian clay soil

This paper investigates leaching of water and nutrients (NO^‐ ₃, Cl^‐, PO^3‐ ₄) from the unsaturated layer in an Australian soil using a multisegment percolation system (MPS). Large undisturbed soil cores were collected from a clay‐based, basaltic plain, agricultural soil at Grassmere, 300 km west of Melbourne, Australia. Significant heterogeneity (or preferential flow) of effluent moisture and solutes was detected (one‐way ANO VA, p < 0.001). Fifty percent of the applied nitrate and chloride leached from the soil core within three days after initial application. Hundred percent of the applied nitrate and chloride leached from the soil core within 8 days after application. These results indicate little incorporation into the soil matrix, and possible denitrification or mineralisation. In contrast, after 18 days, less than 1 % of the total applied phosphates leached from the soil, indicating strong adsorption. Our experiments indicate considerable heterogeneity in water flow patterns and solute leaching on a small spatial scale. Very rapid transport of nitrate and chloride through the soil was evident, in comparison phosphate leaching was negligible. These results have important implications for the management of nutrient schedules in agricultural soils, particularly those located in the Western District of Victoria, Australia.     

Modelling and mapping topographic variations in forest soils at high resolution: A case study

This article examines the utility of a digitally derived cartographic depth-to-water (DTW) index to model and map variations in drainage, vegetation and soil type and select soil properties within a forested area (40 ha) of the Swan Hills, Alberta, Canada. This index was derived from a LiDAR (Light Detection and Ranging) derived digital elevation model (DEM), with at least 1 ground return per m². The resulting DTW pattern was set to be zero along all DEM-derived flow channels, each with a 4 ha flow-initiation threshold. Soil type (luvisol, gleysol, mesisol), drainage type (very poor to excessive), vegetation type (hydric to xeric) and forest floor depth were determined along hillslope transects. These determinations conformed more closely to the DEM-derived log₁₀(DTW) variations (R² > 60%) than to the corresponding variations of the widely used topographic wetness index (TWI) (R² < 25%). Setting log₁₀(DTW) thresholds to represent the wet to moist to dry transitions between vegetation, drainage and soil type enabled a high-resolution mapping of these types across the study area. Also determined were soil moisture content, coarse fragment and soil particle composition (sand, silt, clay), pH, total C, N, S, P, Ca, Mg, K, Fe, Al, Mn, Zn, and available Ca, Mg, K, P and NH₄, by soil layer type and depth. Most of these variables were also more correlated with log₁₀(DTW) than with TWI, with and without soil layer depth as an additional regression variable. These variables are, therefore, subject to topographic controls to at least some extent, and can be modelled and mapped accordingly, as illustrated for soil moisture, forest floor depth and pH across the study area, from ridge tops to depressions. Further examinations revealed that the DEM-produced DTW and TWI patterns complemented one another, with DTW delineating soils in relation to local water-table influences, and with TWI delineating where the water would flow and accumulate.     

Duckweed: an effective tool for phyto-remediation

Effective wastewater treatment through conventional methods that rely on heavy aeration are expensive to install and operate. Duckweed is capable of recovering or extracting nutrients or pollutants and is an excellent candidate for bio-remediation of wastewaters. Such plants grow very fast, utilizing wastewater nutrients and also yield cost effective protein-rich biomass as a by-product. Duckweeds being tiny surface-floating plants are easy to harvest and have an appreciable amount of protein (15%–45% dry mass basis) and a lower fiber (7%–14% dry mass basis) content. Besides nutrient extraction, duckweeds has been found to reduce total suspended solids, biochemical oxygen demand (BOD), and chemical oxygen demand in wastewater significantly. Depending on the initial concentrations of nutrients, duckweed-covered systems can remove nitrate (NO₃^?) at daily rates of 120–590 mg NO₃^? m^?2 (73%–97% of initial concentration) and phosphate (PO₄^?) at 14–74 mg PO₄^? m^?2 (63%–99% of initial concentration). Removal efficiencies within 3 days of 96% and 99% have been reported for BOD and ammonia (NH₃). Besides several genera of duckweeds (Spirodela, Lemna, Wolffia), other surface-floating aquatic plants like water hyacinth (Eichhornia) are well known for their phyto-remediation qualities.     

Interlinked seasonal variation in biogenic nutrient fluxes and pore-water nutrient concentrations in intertidal sediments

The seasonal variation in biogenic fluxes of NH₄ ⁺, PO₄ ^3? and SiO₂ calculated from the nutrient excretion rates of dominant bivalves (Ruditapes philippinarum and Arcuatula [=Musculista] senhousia), and pore-water nutrient (NH₄ ⁺, PO₄ ^3?, SiO₂ and NO₃ ^? + NO₂ ^?), sedimentary acid-volatile sulfide (AVS) and benthic chlorophyll-a (Chl-a) concentrations was assessed on an intertidal sandflat in the Seto Inland Sea (Japan) from summer 1994 to autumn 1995. In spite of the large variability between sampling dates and stations, significant correlations between biogenic nutrient fluxes and pore-water nutrient concentrations were found, suggesting a seasonal linkage between bivalve-mediated biological processes and chemical features of sediments. This linkage was stronger in surface (0–0.5 cm) than subsurface (0.5–2 cm) sediments, consistent with the autoecological characteristics of R. philippinarum and A. senhousia inhabiting the uppermost sediment layer. Significant temporal variation in pore-water NO₃ ^? + NO₂ ^?, sedimentary AVS and benthic Chl-a concentrations was also found, which was related to both occasional extreme events (e.g., dystrophy) and alternating periods of production and decomposition. This study may serve twofold in (1) contributing to unravel the ecological structure and functioning of natural tidal flats, and the scale of seasonal variability in biotic and sedimentary parameters and (2) providing useful information for assessing the effectiveness of the physico-chemical and biological structure of artificial tidal flats which are growing in number and extension worldwide.     

Vermiculite addition to soil decreases N water pollution by over 30%

The large amounts of nitrogen-containing mineral fertilisers in industrial agriculture are leading to water pollution with effects such as eutrophication and death of aquatic life. There is therefore a need for techniques to keep N in the soil to prevent leaching in waters. Here, we tested the effect of the addition of vermiculite, a phyllosilicate mineral, to the soil, on fertiliser NH₄ ⁺-N retention in laboratory experiments during 14–45 days. The added amount of vermiculite was twice the amount of the nitrogen fertilizer added. Our results show that addition of vermiculite increases NH₄ ⁺-N retention by over 30%, compared to a control. Adding vermiculite is thus a promising technique to control the eutrophication of water systems as well as reducing agriculture cost.     

Evaluation of porous cup soil‐water samplers under laboratory conditions: Comparison of ceramic and PTFE cups

Three different types of soil solution samplers (two ceramic cups and PTFE) were tested in the laboratory to validate their use for collecting soil‐water samples and for monitoring the chemical composition of soil solutions. Interactions between porous materials and chemical solutions were examined under different experimental conditions for several major cations (Ca²⁺, Mg²⁺, Na⁺ and K⁺) and anions (HCO^‐ ₃, NO^‐ ₃, SO₄ ^2‐ and Cl^‐) and for several minor ions (NH⁺ ₄, NO^‐ ₄, PO₄ ^3‐and B).

The results show that ceramic cups are not inert for bicarbonate, calcium and phosphate ions. Their use is only valid for a limited number of ions such as chloride and nitrate which, however, are the most studied ions in field experiments. PTFE cups have less restrictions although their use in measuring sulphates, phosphates and ammonium presents some problems.

Choosing the appropriate type of cup depends on the objectives. In any case, laboratory experiments must be performed before installation.     

Phaeocystis blooms and eutrophication of the continental coastal zones of the North Sea

It is suggested that novel nuisance algal blooms can result from major shifts in N/P or NH₄ ⁺/NO₃ ^- ratios. Inland hydraulic engineering caused a shift from P-limitation (before 1977) towards N-limitation (after 1977) in the Marsdiep area (Dutch coastal waters). Following this shift the colonial flagellate Phaeocystis sp. became more abundant and started to bloom during the nutrient-controlled period (later spring to autumn). Competition experiments showed that the N/P ratio can influence the species composition of marine phytoplankton. In addition, the natural distribution of some species like Rhodomonas sp. and Emiliania huxleyi may be affected by the frequency of nutrient pulses in the system. Phaeocystis was a poor competitor under P-limitation and a good competitor under N-limitation. Colony formation was absent under P- and NH₄ ⁺-limitation. Colonies were formed under NO₃ ^--limitation. These preliminary results suggest that colony-forming Phaeocystic blooms may be restricted (besides light-controlled environments) to those N-controlled environments where nitrate is consumed by Phaeocystis. The distribution of Phaeocystis along the European continental coast is evaluated on the basis of its ability to compete for nutrients and to form colonies when nitrate is the major N-source.     

Urea as a nitrogen source for the phytoplankton in the Oslofjord

Ambient concentrations of urea in the inner Oslofjord, Norway, showed a pronounced yearly cycle in 1980, with values in the range 0.1 to 10.0 μg-at N l^-1; this cycle resemble that of ammonia although urea concentrations were usually lower. The uptake of urea by phytoplankton was investigated using ¹⁵N. Urea was usually a less important N source than NH ₄ ⁺ , and accounted for 0 to 53% (mean 19%) of summed NH ₄ ⁺ +NO ₃ ^- + urea uptake rates from April to October. Absolute as well as relative (specific) uptake rates of urea were higher in the summer (June–August) than at other times. Uptake of urea was inhibited by NH ₄ ⁺ concentrations higher than 1 to 2 μg-at N l^-1. The summed NH ₄ ⁺ +NO ₃ ^- + urea uptake rate was exponentially related to temperature.     

Bioturbation effects of the amphipod Corophium volutator on microbial nitrogen transformations in marine sediments

Microcosms containing different densities of Corophium volutator, ranging from 0 to 6000 ind m^-2, were incubated in a flow-through system. Benthic fluxes of CO₂, O₂, NO₃ ^- and NH₄ ⁺ were measured regularly. Thirteen days after setup the microcosms were sacrificed and sediment characteristics, pore water NO₃ ^-, NH₄ ⁺ and exchangeable NH₄ ⁺ concentrations, and potential nitrification activity were measured. The presence of C. volutator increased overall mineralization processes due to burrow construction and irrigation. The amphipods increased the ratio CO₂/O₂ fluxes from 0.73 to 0.86 in microcosms inhabited by 0 and 6000 ind m^-2, respectively. Burrow ventilation removed NH₄ ⁺ from the sediment, which was nitrified in the oxic layer and transported NO₃ ^- to the burrow sediment, where denitrification potential was enhanced. Nitrification and total denitrification rates (denitrification of NO₃ ^- coming from the overlying water and of NO₃ ^- generated within the sediment) were calculated and discussed. Bioturbation by C. volutator increased both nitrification and denitrification, but denitrification was stimulated more than nitrification. Denitrification of NO₃ ^- coming from the overlying water was stimulated 1.2- and 1.7-fold in microcosms containing 3000 and 6000 ind m^-2 relative to control microcosms. The presence of C. volutator (6000 ind m^-2) stimulated nitrogen removal from the system, as dinitrogen, 1.5-fold relative to non-bioturbated microcosms. C. volutator individuals used in our study were collected from Norsminde Fjord, Denmark, in 1990.     

土壤水溶态铜对小白菜的毒害效应及其预测模型

土壤中铜(Cu)重金属的生物毒性/有效性主要取决于它们在土壤液相中含量和土壤溶液的性质。探寻土壤有效态Cu的生物毒害效应,表征量化其与土壤溶液性质关系,可为土壤Cu的环境风险评价提供参考。选取17种典型农田土壤,探讨了有效态Cu(土壤孔隙水以及CaCl_2浸提态)对小白菜生长的毒性效应及其预测模型。结果表明:土壤孔隙水中Cu对小白菜生长10%抑制的毒性阈值值(EC_(10))和50%抑制的毒性阈值(EC_(50)),最大值与最小值相差为14.7和14.6倍;同样,对于CaCl_2提取态Cu的EC_(10)和EC_(50),最大值与最小值相差12.7和7.7倍,表明土壤溶液性质对水溶性Cu对小白菜的毒性阈值影响很大。建立了土壤溶液的重要因子(溶解性有机碳、土壤溶液pH值、电导率、全硫含量、Ca~(2+)、Mg~(2+)、K~+、Na~+)和水溶性Cu阈值之间的多元回归关系,结果显示,土壤溶液性质可以较好地预测水溶性Cu对小白菜的毒性阈值。同时,土壤溶液中Mg~(2+)、K~+和S的含量是控制孔隙水中Cu对小白菜生长毒性的最重要因子,单一的S能分别解释34%的EC_(10)变异,K~+解释26%的EC_(50)变化。本研究结果可为陆地环境中水溶性Cu的风险评价提供基础。     

Interactions between NH+4 and NO−3 uptake and assimilation: comparison of diatoms and dinoflagellates at several growth temperatures

Ammonium concentrations of ∼1 M are commonly cited as being the threshold for inhibition of NO₃ ⁻ uptake, but the applicability of this threshold to phytoplankton from different taxonomic classes has rarely been examined. Additionally, little is known about the influence of environmental variables (e.g. growth temperature) on the interaction between ambient NH₄ ⁺ and NO₃ ⁻ uptake. Four species of estuarine phytoplankton, two diatom [Chaetoceros sp., and Thalassiosira weissflogii (Grunow) Fryxell et Hasle] and two dinoflagellate [Prorocentrum minimum (Pavillard) Schiller, and Gyrodinium uncatenum Hulburt], were grown on NO₃ ⁻ at several different temperatures (4, 10, 15, or 20 °C), and the impact of NH₄ ⁺ additions on NO₃ ⁻ uptake/assimilation (non-TCA-extracted) and assimilation (TCA-extracted) was assessed. For all species at all temperatures, NO₃ ⁻ uptake/assimilation and assimilation rates decreased in a roughly exponential manner with increasing NH₄ ⁺ concentrations but were not completely inhibited even at elevated NH₄ ⁺ concentrations of 200 μM. Estimated half-inhibition concentrations (K _i) were significantly greater in the diatom species (mean ± SE; 2.70 ± 0.67 μM) than in the dinoflagellate species (1.26 ± 0.55 μM). Half-inhibition constants were positively related to temperature-limited relative growth rate although not significantly. The observed inhibition of NO₃ ⁻ uptake and assimilation, as a percentage of NO₃ ⁻ uptake in the absence of NH₄ ⁺, averaged about 80% and ranged from 49 to 100%. For all species, a significant (P < 0.001) positive correlation was found between percent inhibition of NO₃ ⁻ assimilation and temperature-limited relative growth rate. Two experiments on Chesapeake Bay phytoplankton during an April 1998 diatom bloom showed that in short-term (∼1 h) temperature manipulation experiments, percent inhibition of NO₃ ⁻ uptake/assimilation was also positively related (P = 0.05) to experimental temperature. The observed relationships between temperature-limited relative growth rate and percent inhibition of NO₃ ⁻ assimilation rates for the species tested suggest that at the enzyme level, the inhibitory mechanism of NO₃ ⁻ assimilation is similar among species, but at the whole cell level may be regulated by species-specific differences in the accumulation of internal metabolites. These findings add not only to our understanding of species-specific variability and the role of growth temperature, but also provide additional data with which to evaluate current models of NH₄ ⁺ and NO₃ ⁻ interactions. Received: 31 August 1998 / Accepted: 7 December 1998     

Effect of nutrient enrichment on growth of the eelgrass Zostera marina in the Chesapeake Bay,Virginia, USA

The addition of two commerical fertilizers, one 5% NH₄NO₃, 10% P₂O₅, 10% K₂O, and the other 10% NH₄NO₃, 10% P₂O₅, 10% K₂O, ahd a dramatic effect on the growth of Zostera marina in the Chesapeake Bay. There was a significant increase in the length, biomass and total number of turions of fertilized plots compared with controls during a 2 to 3 month period. Data from this short-term field experiment suggest that Z. marina beds in the Chesapeake Bay are nutrient-limited, that the grwoth form of Z. marina may be related to the sediment nutrient supply, and that Z. marina may competitively exclude Ruppia maritima by light-shading.     

Effect of nitrogen source on short-term light and dark CO2 uptake by a marine diatom

Based on a series of short-term incubations involving the marine diatom Chaetoceros simplex (Bbsm), precultured in NH ₄ ⁺ -, NO ₃ ^- -and urea-limited continuous cultures at several dilution rates, we found that both the short-term specific rate of ¹⁴CO₂ uptake and the amount of CO₂ fixed after 8- and 16-min incubations were unaffected by enrichment with NH ₄ ⁺ , urea, or NO ₃ ^- when NH ₄ ⁺ or urea were the preconditioning forms of N, but were slightly suppressed when the cells were first grown on NO ₃ ^- . Similar enrichments in the dark, however, led to significant CO₂ uptake under all conditions of NH ₄ ⁺ enrichment and to similarly enhanced CO₂ uptake, but only at high growth rates, when urea was the source of enrichment nitrogen. Our light results are contrary to some contemporary findings, but there does seem to be agreement that photosynthetic rates of rapidly growing phytoplankton will not be affected by exposure to pulses of nitrogen. Enhanced dark uptake, in contrast, appears to be characteristic of phytoplankton under all degrees of N limitation, and, as such, may be useful as an “all or nothing” index of the nitrogen status of natural waters. There is some indication that the index may be useful in determining both the form of and the degree of N limitation as well.