Equilibrium and balanced growth of a vegetative crop

MODEL: A previously developed dynamic model, NICOLET, designed to predict growth and nitrate content of a lettuce crop, is subjected to (virtual) constant environmental conditions. For every combination of shoot and root environment, the cell sap, here assumed to reside in the "vacuole" compartment, equilibrates at a certain nitrate concentration level. This, in turn, defines the composition of the crop in terms of carbon and nitrogen content in each of the three compartments of the model. Growth under constant environmental conditions is defined as "equilibrium" growth (EG). If, in addition, the source strengths of carbon and nitrogen balance each other, as well as the sink strength of the growing crop, the growth is said to be "balanced" (BG). RESULTS: It is shown that the range of BG approximately coincides with the range of "mild" nitrogen stress, where reduction in nitrogen availability results in a mild reduction of relative growth rate (RGR). Beyond a certain low nitrate concentration in the cell sap, the N-stress becomes "severe" and the loss of growth increases considerably. CONCLUSIONS: The model is able to mimic the five central observations of many constant-environment growth-chamber experiments, namely (1) the initial exponential growth and later decline of the RGR, (2) the constant chemical composition, (3) the equality of the RGR and the relative nutrient supply rate (RNR), (4) the proportionality between the N : C ratio and the RNR, and (5) the proportionality between the water content and the reduced N content. Guidelines for the optimal combination of the shoot and root environments are suggested.     

Modelling ontogenetic changes of nitrogen and water content in lettuce

BACKGROUND AND AIMS: It is well established that the nitrogen content of plants, including lettuce, decreases with time. It has also been observed that water content of lettuce increases between planting and harvest. This paper is an attempt at modelling these observations. METHODS: An existing dynamic model (Nicolet), designed to predict growth and nitrate content of glasshouse lettuce, is modified to accommodate the ontogenetic changes of reduced-nitrogen and water contents (on a dry matter basis). The decreasing reduced-N content and the increasing water content are mimicked by dividing the originally uniform plant into 'metabolically active' tissue and 'support' tissue. The 'metabolic' tissue is assumed to contain a higher nitrogen content and a lower water content than the 'support' tissue. As the plants grow, the ratio of 'support' to 'metabolic' tissue increases, resulting in an increased mean water content and a decreased reduced-N content. Simulations with the new model are compared with experimental glasshouse data over four seasons. KEY RESULTS: The empirical linear relationship between water and reduced-N contents, matches, to a good approximation, the corresponding relationship based on the model. The agreement between the two makes it possible to effectively uncouple the estimation of the 'ontogenetic' parameters from the estimation of the other parameters. The growth and nitrate simulation results match the data rather well and are hardly affected by the new refinement. The reduced-N and water contents are predicted much better with the new model. CONCLUSION: Prediction of nitrogen uptake for the substantial nitrate pool of lettuce depends on the water content. Hence, the modified model may assist in making better fertilization decisions and better estimates of nitrogen leaching.     

Leaf age as a risk factor in contamination of lettuce with Escherichia coli O157:H7 and Salmonella enterica

Outbreaks of Escherichia coli O157:H7 infections have been linked increasingly to leafy greens, particularly to lettuce. We present here the first evidence that this enteric pathogen can multiply on the leaves of romaine lettuce plants. The increases in population size of E. coli O157:H7 in the phyllosphere of young lettuce plants ranged from 16- to 100-fold under conditions of warm temperature and the presence of free water on the leaves and varied significantly with leaf age. The population size was consistently ca. 10-fold higher on the young (inner) leaves than on the middle leaves. The growth rates of Salmonella enterica and of the natural bacterial microflora were similarly leaf age dependent. Both enteric pathogens also achieved higher population sizes on young leaves than on middle leaves harvested from mature lettuce heads, suggesting that leaf age affects preharvest as well as postharvest colonization. Elemental analysis of the exudates collected from the surfaces of leaves of different ages revealed that young-leaf exudates were 2.9 and 1.5 times richer in total nitrogen and carbon, respectively, than middle-leaf exudates. This trend mirrored the nitrogen and carbon content of the leaf tissue. Application of ammonium nitrate, but not glucose, to middle leaves enhanced the growth of E. coli O157:H7 significantly, suggesting that low nitrogen limits its growth on these leaves. Our results indicate that leaf age and nitrogen content contribute to shaping the bacterial communities of preharvest and postharvest lettuce and that young lettuce leaves may be associated with a greater risk of contamination with E. coli O157:H7.     

RELATIONSHIPS BETWEEN NITRATE REDUCTASE ACTIVITY AND RATES OF GROWTH AND NITRATE INCORPORATION UNDER STEADY-STATE LIGHT OR NITRATE LIMITATION IN THE MARINE DIATOM THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE)1

Although activity of the enzyme nitrate reductase (NR) can potentially be used to predict the rate of nitrate incorporation in field assemblages of marine phytoplankton, application of this index has met with little success because the relationship between the two rates is not well established under steady-state conditions. To provide a basis for using NR activity measurements, the relationships among NR activity, growth rate, cell composition, and nitrate incorporation rate were examined in cultures of Thalassiosira pseudonana (Hustedt)Hasle and Heimdal, growing a) under steady-state light limitation, b) during transitions between low and high irradiance (15 or 90 μmol quanta.m^?2.s^?1), and c) under steady-state nitrate limitation. Using a modified assay for NR involving additions of bovine serum albumin to stabilize enzyme activity, NR activity in light-limited cultures was positively and quantitatively related to calculated rates of nitrate incorporation, even in cultures that were apparently starved of selenium. During transitions in irradiance, growth rates acclimated to new conditions within 1 day; through the transition, the relationship between NR activity and nitrate incorporation rate remained quantitative. In nitrate-limited chemostat cultures, NR activity was positively correlated with growth rate and with nitrate incorporation rates, but the relationship was not quantitative. NR activity exceeded nitrate incorporation rates at lower growth rates (<25% of nutrient-replete growth rates), but chemostats operating at such low dilution rates may not represent ecologically relevant conditions for marine diatoms. The strong relationship between NR activity and nitrate incorporation provides support for the idea that NR is rate-limiting for nitrate incorporation or is closely coupled to the rate-limiting step. In an effort to determine a suitable variable for scaling NR activity, relationships between different cell components and growth rate were examined. These relationships differed depending on the limiting factor. For example, under light limitation, cell volume and cell carbon content increased significantly with increased growth rate, while under nitrate limitation cell volume and carbon content decreased as growth rates increased. Despite the differences found between cell composition and growth rate under light and nitrate limitation, the relationships between NR activity scaled to different compositional variables and growth rate did not differ between the limitations. In field situations where cell numbers are not easily determined, scaling NR activity to particulate nitrogen content may be the best alternative. These results establish a strong basis for pursuing NR activity measurements as indices of nitrate incorporation in the field.     

冬小麦等4种作物对铵,硝态氮的吸收能力

采用水培试验探讨了冬小麦、大豆、油菜和莴笋４种作物对硝、铵态氮的相对吸收能力以及这两种氮源对它们生长发育的影响。试验表明：（１）不同氮源对供试作物的生长发育影响极大。供给硝态氮,这些作物生长发育良好,供给等量的ＮＯ＾－３和ＮＨ＾－４（１：１）时,蔬菜作物莴笋生长量下降幅度最大;供给铵态氨,莴笋和大豆极为敏感,供给ＮＯ＾－３时莴笋吸氮量显著高于供给等氮量ＮＯ＾－３和ＮＨ＾＋４,莴上麦供给等量ＮＯ＾－     

莴笋对不同形态氮素的反应

探讨了不同形态氮素对莴笋生长发育的影响及其营养特性。结果表明,莴笋幼苗根系对NH4^+ -N的亲和力稍大于NO3^- -N的亲和力;分别供给NO3^- -N+NO3^- -N及NH4^+ -N,莴笋的生物学产量和吸N量均依次递减（分别为100：56.9：12.4,100：48.9：8.6）,因此在水培条件下,NO3^- -N是最适合莴笋生长发育的氮源,NH4^+ -N与NO3^- -N各占50％时对莴笋的生长发育已有一定的抑制作用,仅以NH4^+ -N作氮源则莴笋很难生长;NH4^+ -N与NO3^- -N各占50％时,莴笋倾向于吸收较多的NH4^+ -N,而且在培养不同阶段NH4^+/NO3^-吸收比例均大于1,莴笋表现出喜铵性,但NH4^+ -N并非莴笋很适合的氮源;营养液中NO3^- -N不足,主要影响莴笋茎叶的生长,而NH4^+ -N所占比例达50％时,莴笋根系生长受到抑制,且有明显的受害症状;以NO3^- -N作氮源预培养两周,以含微量NO3^- -N的自来水为水源,再单独以NH4^+ -N为氮源,对莴笋生长有极大的促进作用,同时还大幅度降低了体内硝酸盐的含量。尿素作氮源莴笋未出现受害症状,但莴笋的生长发育状况明显劣于其它氮源。     

局部根区水分胁迫下氮形态与供给部位对玉米幼苗生长的影响

通过向玉米幼苗分根装置一侧根室的营养液中加入聚乙二醇(PEG 6000)来模拟植物水分胁迫,并设3种供氮形态(硝态氮、铵态氮、两者各占50%的混合氮),且只加入到一侧根室(当氮加入到和PEG同侧时为水氮异区,加入到无PEG一侧时为水氮同区),测定各处理的光合、生理指标,以研究局部根区水分胁迫下氮形态与供给部位对玉米幼苗生长的影响.结果表明:同一氮形态供给下水氮同区植株的光合速率(Pn)、最大净光合速率(Pmax)、光饱和点(LSP)、CO2饱和点(CSP)、叶绿素a、b及叶绿素总含量、根系活力、氮含量和生物量高于水氮异区,光呼吸速率(Rp)、CO2补偿点(CCP)、木质部汁液脱落酸(ABA)浓度、氮利用效率、水分利用效率低于水氮异区;供混合氮和硝态氮的植株Pn、Pmax、LSP、CSP、氮含量和生物量高于供铵态氮的植株,而CCP、Rp、木质部汁液ABA浓度、氮利用效率、水分利用效率变化趋势则相反.可见,同一供氮形态下,水氮同区比水氮异区更利于植物生长,而水氮利用效率在水氮异区下较高;混合氮和硝态氮对植物生长的促进作用优于单一供给铵态氮,但铵态氮更有利于提高水氮利用效率.     

Toward a mechanistic modeling of nitrogen limitation on vegetation dynamics

Nitrogen is a dominant regulator of vegetation dynamics, net primary production, and terrestrial carbon cycles; however, most ecosystem models use a rather simplistic relationship between leaf nitrogen content and photosynthetic capacity. Such an approach does not consider how patterns of nitrogen allocation may change with differences in light intensity, growing-season temperature and CO(2) concentration. To account for this known variability in nitrogen-photosynthesis relationships, we develop a mechanistic nitrogen allocation model based on a trade-off of nitrogen allocated between growth and storage, and an optimization of nitrogen allocated among light capture, electron transport, carboxylation, and respiration. The developed model is able to predict the acclimation of photosynthetic capacity to changes in CO(2) concentration, temperature, and radiation when evaluated against published data of V(c,max) (maximum carboxylation rate) and J(max) (maximum electron transport rate). A sensitivity analysis of the model for herbaceous plants, deciduous and evergreen trees implies that elevated CO(2) concentrations lead to lower allocation of nitrogen to carboxylation but higher allocation to storage. Higher growing-season temperatures cause lower allocation of nitrogen to carboxylation, due to higher nitrogen requirements for light capture pigments and for storage. Lower levels of radiation have a much stronger effect on allocation of nitrogen to carboxylation for herbaceous plants than for trees, resulting from higher nitrogen requirements for light capture for herbaceous plants. As far as we know, this is the first model of complete nitrogen allocation that simultaneously considers nitrogen allocation to light capture, electron transport, carboxylation, respiration and storage, and the responses of each to altered environmental conditions. We expect this model could potentially improve our confidence in simulations of carbon-nitrogen interactions and the vegetation feedbacks to climate in Earth system models.     

Studies of the Relationship between the Growth Rate of Young Plants and their Total-N Concentration using Nutrient Interruption Techniques: Theory and Experiments

Alternative assumptions about the utilization of stored nitrogenare used to derive two different models for predicting how thegrowth rate of both the whole plant and its shoot vary withtheir respective total-N concentrations following interruptionof the external N supply. Model 1 predicts that plant growthshould follow monomolecular kinetics after the supply is interrupted,with the resulting relative growth rates linearly related tototal-N concentration. Model 2 predicts that plants grow logisticallyonce N is withheld, with their relative growth rates varyinglinearly with the reciprocal of total-N concentration. The versionsof the models derived for the shoot are similar to those forthe whole plant, but include an additional term to allow fortransfer of N to the roots as deficiency increases. Tests ofthe models were carried out using data from N interruption experimentswith young cabbage and lettuce plants (containing either highor low nitrate concentrations) which were grown hydroponicallyin nutrient recirculating units. The results showed that therewas little statistical difference between the fits of the twomodels to the growth data over the range tested, but that model1 was unsatisfactory because the estimates of its parameterswere inconsistent with assumptions about the physiological processescontrolling growth, and because its predictions became unrealisticwhen extrapolated to conditions of acute N deficiency. Model2 did not suffer from either of these problems and provideda better mechanistic interpretation of the data, yielding predictionsthat were in close agreement with the observed relationshipbetween relative growth rate and total-N concentration for boththe whole plant and its shoot. The curvilinear form of thisrelationship for model 2 differs from the linear form of othermodels derived from measurements in experiments where therewas a continuing but restricted supply of external N to plants.This implies that the relationship between relative growth rateand total-N concentration may vary depending on whether or nota plant has to rely entirely on its internal reserves of N intimes of shortage. The results also showed that the size ofthese reserves governed the amounts of N transferred to theroots as deficiency developed. Transfer of N was greater incabbage than lettuce because of a greater capacity to adaptby increasing root growth at the expense of the shoot.Copyright1994, 1999 Academic Press Cabbage, deficiency, dilution, hydroponics, lettuce, model, nitrogen, nitrate, nutrient recirculation units, relative growth rate, shoot, total-N concentration, whole plant     

Growth and reproduction of Arabidopsis thaliana in relation to storage of starch and nitrate in the wild-type and in starch-deficient and nitrate-uptake-deficient mutants

We have investigated the interactions between resource assimilation and storage in rosette leaves, and their impact on the growth and reproduction of the annual species Arabidopsis thaliana. The resource balance was experimentally perturbed by changing (i) the external nutrition, by varying the nitrogen supply; (ii) the assimilation and reallocation of resources from rosette leaves to reproductive organs, by cutting or covering rosette leaves at the time of early flower bud formation, and (iii) the internal carbon and nitrogen balance of the plants, by using isogenic mutants either lacking starch formation (PGM mutant) or with reduced nitrate uptake (NU mutant). When plants were grown on high nitrogen, they had higher concentrations of carbohydrates and nitrate in their leaves during the rosette phase than during flowering. However, these storage pools did not significantly contribute to the bulk flow of resources to seeds. The pool size of stored resources in rosette leaves at the onset of seed filling was very low compared to the total amount of carbon and nitrogen needed for seed formation. Instead, the rosette leaves had an important function in the continued assimilation of resources during seed ripening, as shown by the low seed yield of plants whose leaves were covered or cut off. When a key resource became limiting, such as nitrogen in the NU mutants and in plants grown on a low nitrogen supply, stored resources in the rosette leaves (e.g. nitrogen) were remobilized, and made a larger contribution to seed biomass. A change in nutrition resulted in a complete reversal of the plant response: plants shifted from high to low nutrition exhibited a seed yield similar to that of plants grown continuously on a low nitrogen supply, and vice versa. This demonstrates that resource assimilation during the reproductive phase determines seed production. The PGM mutant had a reduced growth rate and a smaller biomass during the rosette phase as a result of changes in respiration caused by a high turnover of soluble sugars ( Caspar et al. 1986 ; W. Schulze et al. 1991 ). During flowering, however, the vegetative growth rate in the PGM mutant increased, and exceeded that of the wild-type. By the end of the flowering stage, the biomass of the PGM mutant did not differ from that of the wild-type. However, in contrast to the wild-type, the PGM mutant maintained a high vegetative growth rate during seed formation, but had a low rate of seed production. These differences in allocation in the PGM mutant result in a significantly lower seed yield in the starchless mutants. This indicates that starch formation is not only an important factor during growth in the rosette phase, but is also important for whole plant allocation during seed formation. The NU mutant resembled the wild-type grown on a low nitrogen supply, except that it unexpectedly showed symptoms of carbohydrate shortage as well as nitrogen deficiency. In all genotypes and treatments, there was a striking correlation between the concentrations of nitrate and organic nitrogen and shoot growth on the one hand, and sucrose concentration and root growth on the other. In addition, nitrate reductase activity (NRA) was correlated with the total carbohydrate concentration: low carbohydrate levels in starchless mutants led to low NRA even at high nitrate supply. Thus the concentrations of stored carbohydrates and nitrate are directly or indirectly involved in regulating allocation.     

NITRATE AND PHOSPHATE UPTAKE INTERACTIONS IN A MARINE PRYMNESIOPHYTE1

The short- and long-term uptake of nitrate and phosphate ions, and their interactions, were studied as functions of the preconditioning of Pavlova lutheri (Droop) Green. Populations were preconditioned in continuous culture at a variety of growth rates and N:P supply ratios. The maximum uptake rates cell^?1 for nitrate and phosphate were of similar magnitudes, in spite of the forty-fold smaller requirement for phosphorus. Short-term phosphate uptake was independent of the nitrate concentration, but the short-term nitrate uptake rate was reduced in the presence of phosphate. The severity of inhibition of nitrate uptake by phosphate was positively correlated with the preconditioning N:P supply ratio and the preconditioning growth rate. In response to large additions of nutrients, P. lutheri was able to increase its phosphorus content sixty-fold, but was only able to take up enough nitrate to double its nitrogen content. The high rate of phosphate uptake relative to its requirement, the inhibition of nitrate uptake by phosphate, and the large capacity for phosphorus storage relative to its requirement, all of which were observed even under N limitation, may imply that even where nitrogen is limiting there can be interspecific competition for available phosphate.     

施用猪粪对油麦菜产量、硝酸盐含量及土壤养分的影响

应用盆栽试验方法,研究了施用猪粪对西南地区黄壤和紫色土中油麦菜产量、硝酸盐含量及土壤养分的影响.结果表明:施用猪粪能显著提高油麦菜产量,且黄壤中油麦菜增产幅度大于紫色土;油麦菜中硝酸盐、氮磷钾含量与土壤类型及猪粪施用量密切相关,以中国农业科学院制定的蔬菜中硝酸盐污染程度评价标准为依据,在紫色土对照(CK)及1倍猪粪(相当于施纯N 200 kg·hm^-2)处理下油麦菜硝酸盐含量较低,符合一级标准(≤432 mg·kg^-1,轻度污染);其他处理多超过二级标准(≤758 mg·kg^-1,中度污染),但均未超过三级标准(≤1440 mg·kg-¹,重度污染);黄壤中除化肥和8倍猪粪(相当于施纯N 1600 kg·hm^-2)处理下油麦菜硝酸盐含量超过二级标准外,其他各处理均符合一级标准;黄壤和紫色土中表征磷素淋失风险的有效磷临界值分别为96.3和107.7 mg·kg^-1.黄壤的猪粪环境安全容量较紫色土高.施用猪粪能显著提高两种土壤的有机碳和全氮含量.     

A Mechanistic Theory for the Relationship between Growth Rate and the Concentration of Nitrate-N or Organic-N in Young Plants derived from Nutrient Interruption Experiments

A simple assumption about nitrate assimilation (incorporatinga single parameter to represent the conversion of nitrate intoorganic-N) has been used to derive mechanistic equations todescribe the interrelationships between the concentrations ofnitrate-N and organic-N, and dry weight for both the whole plantand its shoot in nutrient interruption experiments. These equationshave been combined with a logistic growth model, which was derivedfrom initial assumptions about the way in which plants use storedN under these conditions (Burns, 1994), to quantify effectsof nitrate-N and organic-N concentrations on relative growthrate. The models were tested by fitting equations for the predictedrelationships to data for young cabbage and lettuce plants,from which estimates of the N assimilation parameter were obtained.The tests showed that predictions of relative growth rate weregenerally in good agreement with the data over the whole range,as were those for the corresponding relationships between dryweight and either nitrate-N or organic N- concentration, andfor the interrelationships between the two forms of N. The mostreliable estimates of the N assimilation parameter were obtainedfrom relationships where nitrate-N concentration was the explanatory(independent) variable, because the fits of the correspondingrelationships with organic-N were relatively insensitive tolarge changes in its values. The results showed no evidenceof any consistent variation in the size of this N assimilationparameter with the nitrate status of the plant. However, smallbetween-crop differences in its value suggest that shoot nitratemay have been assimilated slightly more efficiently in cabbagethan in lettuce. The new model predicts that dry matter production is restrictedas soon as the external N supply is withheld (irrespective ofthe plant nitrate status), producing a slow but consistent declinein relative growth rate which is maintained until nitrate isalmost depleted, whereupon it falls rapidly. This implies thatthe rate of chemical reduction of stored nitrate was not sufficientto maintain an adequate supply of organic-N for the productionof new dry matter (even when its concentration in the plantsis still high). The results show that nitrate concentrationsin excess of 0·1 mmol g^-1 are required in plants to avoidserious reductions in growth rate when N is in short supply.Copyright1994, 1999 Academic Press Cabbage, concentration, deficiency, hydroponics, lettuce, model, nitrogen, nitrate, nutrient interruption, organic-N, relative growth rate, shoot, whole plant     

Decoupling the direct and indirect effects of nitrogen deposition on ecosystem function

Elevated nitrogen (N) inputs into terrestrial ecosystems are causing major changes to the composition and functioning of ecosystems. Understanding these changes is challenging because there are complex interactions between 'direct' effects of N on plant physiology and soil biogeochemistry, and 'indirect' effects caused by changes in plant species composition. By planting high N and low N plant community compositions into high and low N deposition model terrestrial ecosystems we experimentally decoupled direct and indirect effects and quantified their contribution to changes in carbon, N and water cycling. Our results show that direct effects on plant growth dominate ecosystem response to N deposition, although long-term carbon storage is reduced under high N plant-species composition. These findings suggest that direct effects of N deposition on ecosystem function could be relatively strong in comparison with the indirect effects of plant community change.     

The effect of nitrogen limitation on lipid productivity and cell composition in Chlorella vulgaris

Chlorella vulgaris accumulates lipid under nitrogen limitation, but at the expense of biomass productivity. Due to this tradeoff, improved lipid productivity may be compromised, despite higher lipid content. To determine the optimal degree of nitrogen limitation for lipid productivity, batch cultures of C. vulgaris were grown at different nitrate concentrations. The growth rate, lipid content, lipid productivity and biochemical and elemental composition of the cultures were monitored for 20 days. A starting nitrate concentration of 170 mg L^?1 provided the optimal tradeoff between biomass and lipid production under the experimental conditions. Volumetric lipid yield (in milligram lipid per liter algal culture) was more than double that under nitrogen-replete conditions. Interpolation of the data indicated that the highest volumetric lipid concentration and lipid productivity would occur at nitrate concentrations of 305 and 241 mg L^?1, respectively. There was a strong correlation between the nitrogen content of the cells and the pigment, protein and lipid content, as well as biomass and lipid productivity. Knowledge of the relationships between cell nitrogen content, growth, and cell composition assists in the prediction of the nitrogen regime required for optimal productivity in batch or continuous culture. In addition to enhancing lipid productivity, nitrogen limitation improves the lipid profile for biodiesel production and reduces the requirement for nitrogen fertilizers, resulting in cost and energy savings and a reduction in the environmental burden of the process.     

Nutrient Biogeochemistry During the Early Stages of Delta Development in the Mississippi River Deltaic Plain

Nutrient biogeochemistry associated with the early stages of soil development in deltaic floodplains has not been well defined. Such a model should follow classic patterns of soil nutrient pools described for alluvial ecosystems that are dominated by mineral matter high in phosphorus and low in carbon and nitrogen. A contrast with classic models of soil development is the anthropogenically enriched high nitrate conditions due to agricultural fertilization in upstream watersheds. Here we determine if short-term patterns of soil chemistry and dissolved inorganic nutrient fluxes along the emerging Wax Lake delta (WLD) chronosequence are consistent with conceptual models of long-term nutrient availability described for other ecosystems. We add a low nitrate treatment more typical of historic delta development to evaluate the role of nitrate enrichment in determining the net dinitrogen (N₂) flux. Throughout the 35-year chronosequence, soil nitrogen and organic matter content significantly increased by an order of magnitude, whereas phosphorus exhibited a less pronounced increase. Under ambient nitrate concentrations (>60 μM), mean net N₂ fluxes (157.5 μmol N m^?2 h^?1) indicated greater rates of gross denitrification than gross nitrogen fixation; however, under low nitrate concentrations (<2 μM), soils switched from net denitrification to net nitrogen fixation (?74.5 μmol N m^?2 h^?1). As soils in the WLD aged, the subsequent increase in organic matter stimulated net N₂, oxygen, nitrate, and nitrite fluxes producing greater fluxes in more mature soils. In conclusion, soil nitrogen and carbon accumulation along an emerging delta chronosequence largely coincide with classic patterns of soil development described for alluvial floodplains, and substrate age together with ambient nitrogen availability can be used to predict net N₂ fluxes during early delta evolution.     

The Influence of Urea and Other Nitrogen Sources on Growth Rate of Scots Pine Seedlings

The growth rate and water content of urea-fed seedlings of Pinus silvestris L. were compared with those of nitrate-and ammonium-fed seedlings grown in continuously renewed nutrient solutions, in which the hydrolysis of urea to ammonia and carbon dioxide was minimized. The growth rate of seedlings grown in an ammonium nutrient solution, in an urea nutrient solution and in a nitrate nutrient solution was about 90 per cent, 75 per cent and 60 per cent, respectively, of that of seedlings grown in a mixture of ammonium and nitrate. Seedlings with urea as the sole nitrogen source developed very severe chlorosis of the needles, the old roots were dark-coloured, the whole root system was very fragile, and the lateral roots of the third order were missing. Urea-grown seedlings had the highest nitrogen contents, closely followed by the ammonium and the ammonium + nitrate seedlings. The lowest nitrogen level was in nitrate seedlings. The low growth rate and the chlorosis of urea-fed seedlings were suggested to be the result of a hydrolysis of urea inside the root, causing an increase in pH and an accumulation of ammonia in the root.     

Seasonal growth pattern of an Icelandic Laminaria population (section Simplices,Laminariaceae, Phaeophyta) containing solid- and hollow-stiped plants

Lamina elongation and content of mannitol, laminaran and nitrate were measured during one year in Laminaria saccharina sensu lato from Iceland. The population contained both solid- and hollow-stiped plants. Growth rate was at its minimum from October to December, and started to increase in mid-winter, slightly earlier at 3 m than at 5 m. The increase in growth rate coincided with a strong reduction in stored carbohydrates and an increase in nitrate content of the laminae, indicating that stored mannitol and laminaran provided extra energy for increased lamina growth and/or for nitrate uptake. The results showed that stored mannitol was utilised before laminaran. The growth rate was at its maximum from April to June, and was reduced from June to July. The ambient nitrate concentration at the locality was low from May to August. The nitrate content of the lamina tissue in relation to dry weight was high during spring but was reduced to low values by July, indicating that nitrate levels limited growth during summer. However, high nitrate concentration of the sea-water and high levels of storage carbohydrates in the plants during autumn indicate that the low growth rate at this time cannot be attributed to lack of nitrate or energy in the form of stored carbon. The Laminaria population in Iceland that was examined showed morphological similarity with L. longicruris populations in Canada (hollow stipe), while the growth pattern corresponds with European L. saccharina populations.     

DIFFERENCES IN GROWTH AND PHYSIOLOGY OF MARINE SYNECHOCOCCUS (CYANOBACTERIA) ON NITRATE VERSUS AMMONIUM ARE NOT DETERMINED SOLELY BY NITROGEN SOURCE REDOX STATE1

The preference of phytoplankton for ammonium over nitrate has traditionally been explained by the greater metabolic cost of reducing oxidized forms of nitrogen. This “metabolic cost hypothesis” implies that there should be a growth disadvantage on nitrate compared to ammonium or other forms of reduced nitrogen such as urea, especially when light limits growth, but in a variety of phytoplankton taxa, this predicted difference has not been observed. Our experiments with three strains of marine Synechococcus (WH7803, WH7805, and WH8112) did not reveal consistently faster growth (cell division) on ammonium or urea as compared to nitrate. Urease and glutamine synthetase (GS) activities varied with nitrogen source in a manner consistent with regulation by cellular nitrogen status via NtcA (rather than by external availability of nitrogen) in all three strains and indicated that each strain experienced some degree of nitrogen insufficiency during growth on nitrate. At light intensities that strongly limited growth, the composition (carbon, nitrogen, and pigment quotas) of WH7805 cells using nitrate was indistinguishable from that of cells using ammonium, but at saturating light intensities, cellular carbon, nitrogen, and pigment quotas were significantly lower in cells using nitrate than ammonium. These and similar results from other phytoplankton taxa suggest that a limitation in some step of nitrate uptake or assimilation, rather than the extra cost of reducing nitrate per se, may be the cause of differences in growth and physiology between cells using nitrate and ammonium.     

Modelling the ecosystem effects of nitrogen deposition: Simulation of nitrogen saturation in a Sitka spruce forest,Aber, Wales,UK

A new model for simulating nitrogen leaching fromforested ecosystems has been applied to data from anexperimentally manipulated 30-year-old Sitka sprucestand. The manipulation experiment (at Aber, in north-western Wales, UK) was part of the European NITREXproject and involved five years of additions ofinorganic nitrogen to the spruce stand. The model(MERLIN) is a catchment-scale, mass-balance model thatsimulates both biotic and abiotic processes affectingnitrogen in ecosystems.The structure of MERLIN includes representationsof the inorganic soil, one plant compartment and twosoil organic compartments. Fluxes in and out of thesimulated ecosystem and transfers between compartmentsare regulated by atmospheric deposition, hydrologicaldischarge and biological processes such as plantuptake, litter production, immobilization,mineralization, nitrification and denitrification.Rates of nitrogen uptake, cycling and release amongpools are regulated by carbon productivity, inorganicnitrogen availability and the C:N ratios of theorganic pools. Inputs to the model are temporalsequences of carbon fluxes and pools, hydrologicaldischarge and external sources of nitrogen.The NITREX experiment at Aber began in 1990 withweekly additions of ammonium nitrate(NH4NO3) at a rate of 35 kg N ha-1 yr-1.Data were collected from both control andtreatment plots within the stand. The site-intensivedata from the control plots at Aber were augmented bydata taken from a chronosequence of 20 Sitka sprucestands and data from a survey of 5 moorland catchmentsin the same region to providecalibration data for the model. The data were used toestablish current conditions at the Aber site and toreconstruct historical sequences of carbon fluxes andpools from 1900 to the present day with which to drivethe model. The reconstructed sequences included anincrease in nitrogen deposition and a vegetationchange from moorland to plantation forest in 1960. Thecalibrated model was then used to predict the effectsof the experimental nitrogen additions begun in 1990.MERLIN successfully reproduced the observedincrease in NO3 leaching from aging spruce standsthat results from forest maturation and increasednitrogen deposition (as inferred from thechronosequence and forest survey data in the region).MERLIN also correctly predicted the increases insoilwater NO3 concentrations, the changes innitrogen content of tree and soil organic matterpools, and the changes in nitrogen fluxes that occurin spruce stands in response to increased nitrogeninputs (as observed in the nitrogen additionexperiment).