Feedback from soil inorganic nitrogen on soil organic matter mineralisation and growth in a boreal forest ecosystem

Current nitrogen (N) deposition rates are considerably higher than during pre-industrial times and the growing interest in forest fertilisation requires better understanding of how the N and carbon (C) cycles interact. This study is based on experimental data showing how Scots pine (Pinus sylvestris L.) forests respond to single or consecutive pulse doses of N. The data were used to support the implementation of a dynamic feedback mechanism in the Q model, allowing for changes in soil N availability to regulate the rate of decomposer efficiency. Simulations of the long-term effects of slowly increasing N deposition with and without dynamic decomposer efficiency were then compared. Both versions of the model accurately predicted the response of tree growth to N fertilisation. Slowly increasing inputs of N over a century in the modified version acted on the inputs and outputs of soil C in opposing ways: (a) rate of litter input slowed down because more N was retained in the soil and thus not available for tree growth; (b) rate of C output, through soil heterotrophic respiration, was also gradually reduced due to increasing decomposer efficiency, although not enough to sufficiently balance the reduced litter input. Accurate prediction of the amount of added N retained in the ecosystem seems to be one of the key issues for estimating enhanced C sequestration.     

Pine Forest Floor Carbon Accumulation in Response to N and PK Additions: Bomb <Superscript>14</Superscript>C Modelling and Respiration Studies

The addition of nitrogen via deposition alters the carbon balance of temperate forest ecosystems by affecting both production and decomposition rates. The effects of 20 years of nitrogen (N) and phosphorus and potassium (PK) additions were studied in a 40-year-old pine stand in northern Sweden. Carbon fluxes of the forest floor were reconstructed using a combination of data on soil ¹⁴C, tree growth, and litter decomposition. N-only additions caused an increase in needle litterfall, whereas both N and PK additions reduced long-term decomposition rates. Soil respiration measurements showed a 40% reduction in soil respiration for treated compared to control plots. The average age of forest floor carbon was 17 years. Predictions of future soil carbon storage indicate an increase of around 100% in the next 100 years for the N plots and 200% for the NPK plots. As much as 70% of the increase in soil carbon was attributed to the decreased decomposition rate, whereas only 20% was attributable to increased litter production. A reduction in decomposition was observed at a rate of N addition of 30 kg C ha^–1 y^–1, which is not an uncommon rate of N deposition in central Europe. A model based on the continuous-quality decomposition theory was applied to interpret decomposer and substrate parameters. The most likely explanations for the decreased decomposition rate were a fertilizer-induced increase in decomposer efficiency (production-to-assimilation ratio), a more rapid rate of decrease in litter quality, and a decrease in decomposer basic growth rate.     

湿地枯落物分解及其对全球变化的响应

综述了当前湿地枯落物分解及其对全球变化响应的研究动态。湿地枯落物分解研究已随研究方法的改进而不断深化;当前湿地枯落物分解过程研究主要集中在有机质组分和元素含量变化特征的探讨上;湿地枯落物分解同时受生物因素（即枯落物性质以及参与分解的异养微生物和土壤动物的种类、数量和活性等）和非生物因素（即枯落物分解过程的外部环境条件,包括气候条件、水分条件、酸碱度与盐分条件以及湿地沉积的行为与特征等）的制约;模型已成为湿地枯落物分解研究的重要手段,对其研究也在不断深化。还讨论了湿地枯落物分解对于全球变化的响应,指出全球变暖、大气CO2浓度上升、干湿沉降及其化学组成改变可能对枯落物分解产生的直接、间接和综合影响。最后,指出了当前该领域研究尚存在的问题以及今后亟需加强的几个研究方面。     

An approach to the biometeorology of decomposer organisms

A search for surrogate variables of weather's control over rate of decay by decomposer organisms has revealed that Actual Evapotranspiration (AE), a water budget term, correlates well (r = 0.976) with measured values of litter decomposition rate. Using data from many biomes of the earth, a curve-fit of AE with measured decomposition rate has been formulated. This curve-fit has been used to prepare a map which displays the geography of predicted decay rate for North America. The physical properties of the litter also controls decomposition rates. Work is in progress to refine the AE to decomposer relationship by considering the lignin content of decomposing litter. Preliminary results suggest that control of decomposition rates by lignin increases with AE so that in high AE environments small changes in lignin concentration result in large changes in litter decay rates. This relationship perhaps explains the great variability in decay rates reported in tropical ecosystems.     

Role of light, carbon dioxide and nitrogen in regulation of buoyancy, growth and bloom formation of Anabaena flos-aquae

The availability of light, CO₂ and NH₄-N interacted to controlbuoyancy and growth of the gas vacuolate blue-green alga, Anabaenaflos-aquae. At high light intensities algal growth rates werehigh; however, the alga was non-buoyant regardless of the availabilityof CO₂ or NH₄-N. The mechanism for buoyancy loss involved increasedcell turgor pressures at higher light intensities which resultedin collapse of gas vacuoles. At lower light intensities algalgrowth rates and cell turgor pressures were reduced and buoyancywas controlled by the availability of CO₂ and inorganic nitrogen.Carbon dioxide limitation increased buoyancy, while reducedinorganic nitrogen availability reduced buoyancy. Mechanismsfor buoyancy regulation at low light intensities involved changesin cellular C/N ratios which appeared to affect the rate ofsynthesis and accumulation of protein-rich gas vacuoles. Algalspecific growth rates were combined with buoyancy data to forma single index (µ_bloom) to the rate of surface bloom formationof A.flos-aquae as a function of the availability of light,CO₂ and NH₄-N. The bloom formation index was enhanced with decreasedavailability of light and CO₂, and increased availability ofNH₄-N.     

Modelling carbon dynamics in coniferous forest soils in a temperature gradient

Climate change and changes in land use will alter the stores of carbon and turnover of soil organic matter. We have used a theory for carbon cycles in terrestrial ecosystems to analyse changes in soil organic matter turnover in coniferous forests. The central concepts of the theory are a continuously changing substrate quality, a constant decomposer efficiency and a climatically controlled decomposer growth rate. Measurements on litter production and soil carbon stores from field experiments have been used to successfully validate the model predictions. Measured litter production increased with increasing temperature but the response was not identical for forests of different vegetation types which reflect variations in productivity. The temperature response of needle-litter production and decomposition rate were strongest in the most productive forests and weakest for the low productive forests. Initial decay rates of soil C store from steady state showed the same trend in temperature response as decay of a single litter cohort did, but the absolute values are 16% of the decay rates of a single litter cohort. Predicted soil C ranged from 5 to 9 kg C m^–2. There exists a remarkable variation in forest soil C store response to temperature; the magnitude and even the sign depends on productivity as defined by vegetation type. The assumption that, in general, decomposition rates increase more than NPP with temperature, and consequently, soil C stores should decrease in response to a climate warming, seems therefore too simplistic.     

Elevated CO<Subscript>2</Subscript>, litter chemistry,and decomposition: a synthesis

The results of published and unpublished experiments investigating the impacts of elevated [CO₂] on the chemistry of leaf litter and decomposition of plant tissues are summarized. The data do not support the hypothesis that changes in leaf litter chemistry often associated with growing plants under elevated [CO₂] have an impact on decomposition processes. A meta-analysis of data from naturally senesced leaves in field experiments showed that the nitrogen (N) concentration in leaf litter was 7.1% lower in elevated [CO₂] compared to that in ambient [CO₂]. This statistically significant difference was: (1) usually not significant in individual experiments, (2) much less than that often observed in green leaves, and (3) less in leaves with an N concentration indicative of complete N resorption. Under ideal conditions, the efficiency with which N is resorbed during leaf senescence was found not to be altered by CO₂ enrichment, but other environmental influences on resorption inevitably increase the variability in litter N concentration. Nevertheless, the small but consistent decline in leaf litter N concentration in many experiments, coupled with a 6.5% increase in lignin concentration, would be predicted to result in a slower decomposition rate in CO₂-enriched litter. However, across the assembled data base, neither mass loss nor respiration rates from litter produced in elevated [CO₂] showed any consistent pattern or differences from litter grown in ambient [CO₂]. The effects of [CO₂] on litter chemistry or decomposition were usually smallest under experimental conditions similar to natural field conditions, including open-field exposure, plants free-rooted in the ground, and complete senescence. It is concluded that any changes in decomposition rates resulting from exposure of plants to elevated [CO₂] are small when compared to other potential impacts of elevated [CO₂] on carbon and N cycling. Reasons for experimental differences are considered, and recommendations for the design and execution of decomposition experiments using materials from CO₂-enrichment experiments are outlined.     

Nitrogen uptake and use by competing individuals in a Xanthium canadense stand

We studied differences in nitrogen uptake and use for plant growth among individuals competing in a natural dense stand of an annual herb, Xanthium canadense. Larger individuals took up more nitrogen than proportionately to their size, indicating that the competition for soil nitrogen was asymmetric among individuals, although it was more symmetric than the competition for light. The rate of nitrogen loss of individuals also increased with plant size. While smaller individuals shared smaller fractions of total plant nitrogen in the stand, they had higher nitrogen concentrations per unit mass. "Turnover" rates of nitrogen influx (r_in) and outflux (r_out) were defined as the rates of nitrogen uptake and loss per unit aboveground nitrogen, respectively. r_in was higher in larger individuals, whereas r_out was higher in smaller individuals. Consequently, the relative rate of nitrogen increment (r_in-r_out) was higher in larger individuals, whereas it was around zero in the smallest individuals. The mean residence time of nitrogen (MRT), defined as the inverse of r_out, was longer in larger individuals. Nitrogen productivity (NP), i.e. the growth rate per unit aboveground nitrogen, was higher in larger individuals. As the product of lifetime MRT and NP gives the nitrogen use efficiency (NUE), defined as biomass production per unit flux of nitrogen, higher MRT and NP observed in larger individuals would have contributed to their higher lifetime NUE. Shorter MRT in smaller individuals was caused by the abscission of leaves which contained relatively large fractions of total plant nitrogen. Xanthium canadense, as a competitive ruderal, tended to produce leaves at higher positions to acquire higher light levels at the expense of older leaves rather than to modify their productive structure to efficiently use low light levels as observed in shade-tolerant species.     

When will litter mixtures decompose faster or slower than individual litters? A model for two litters

Litter mixtures often decompose at a different rate than the average of the individual litters, but, so far, the underlying mechanisms are not understood. We propose here an explanation based on a model with two litters. The model describes the carbon and nitrogen mineralisation of the litters. The decomposition rates of the litters become linked because the growth efficiency (production‐to‐assimilation ratio) of the decomposers responds to the amount of inorganic nitrogen (initial plus mineralised) in the surrounding environment. The model shows that, when in a mixture, one litter decomposes always faster and the other one always slower compared to when they decompose on their own. The relative changes in decomposition rates are also equal and consequently the decomposition rate for the whole mixture can be expected to lie between the rates of the two individual litters. The mixture decomposes faster than the average of the two litters separately when the litter of the higher quality also mineralises nitrogen fastest. If the litter of the higher quality instead has the smallest nitrogen mineralisation rate, the mixture decomposes slower. The model predictions are consistent with observations from 23 published experimental litter‐mixture studies.     

Root system adjustments: regulation of plant nutrient uptake and growth responses to elevated CO2

Nutrients such as nitrogen (N) and phosphorus (P) often limit plant growth rate and production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO₂ levels, i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO₂ can elicit compensatory adjustments such that acquisition capacity for minerals increases in concert with carbon (C) uptake. Compensatory adjustments such as increases in (a) root mycorrhizal infection, (b) root-to-shoot ratio and changes in root morphology and architecture, (c) root nutrient absorption capacity, and (d) nutrient-use efficiency can enable plants to meet an increased nutrient demand under high CO₂. Here we examine the literature to assess the extent to which these mechanisms have been shown to respond to high CO₂. The literature survey reveals no consistent pattern either in direction or magnitude of responses of these mechanisms to high CO₂. This apparent lack of a pattern may represent variations in experimental protocol and/or interspecific differences. We found that in addressing nutrient uptake responses to high CO₂ most investigators have examined these mechanisms in isolation. Because such mechanisms can potentially counterbalance one another, a more reliable prediction of elevated CO₂ responses requires experimental designs that integrate all mechanisms simultaneously. Finally, we present a functional balance (FB) model as an example of how root system adjustments and nitrogen-use efficiency can be integrated to assess growth responses to high CO₂. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights in determining overall plant responses to high CO₂. For example, while changes in root-to-shoot biomass allocation, r, have a small effect on growth, adjustments in uptake rate per unit root mass, [`(n)]\bar \nu , and photosynthetic N use efficiency, p*, have a significantly greater leverage on growth responses to elevated CO₂ except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGR_max).     

Short-term effects of nitrogen availability on wood formation and fibre properties in hybrid poplar

The application of nitrogen-containing fertilisers is one approach used to increase growth rates and productivity of forest tree plantations. However, the effects of nitrogen fertilisation on wood properties have not been systematically assessed. The aim of this work was to document the impacts of nitrogen fertilisation on wood formation and secondary xylem fibre properties. We used three fertilisation treatments in which the level of ammonium nitrate was adjusted to 0, 1 and 10 mM in a complete nutrient solution applied daily over a period of 28 days in standardised greenhouse experiments with clonal material of Populus trichocarpa (Torr and Gray) × deltoides (Bartr. ex Marsh). We showed that there was a short-term and repeatable response in which xylem fibre morphology and secondary cell wall structure adapt to a shift in N availability. Under high-nitrogen exposure, xylem fibres were 17% wider and 18% shorter compared to the adequate nitrogen treatment. A very significant thickening of the fibre cell walls was also observed throughout the stem of trees receiving the high-N treatment. It appeared that cell wall structure was greatly affected by the high-N treatment as fibres developed a modified inner cell wall layer. Histological observations indicated that the internal cell wall layer was enriched in cellulose and chemical determinations showed that wood contained more holocellulose. Together, these results indicate that the response of poplar to nitrogen availability may involve marked effects on secondary xylem formation.     

Decomposition of 13C-labelled standard plant material in a latitudinal transect of European coniferous forests: Differential impact of climate on the decomposition of soil organic matter compartments

¹³C labelled plant material was incubated in situ over 2 to 3 years in 8 conifer forest soils located on acid and limestone parent material along a north-south climatic transect from boreal to dry Mediterranean regions in western Europe. The objectives of the experiment were to evaluate the effects of climate and the soil environment on decomposition and soil organic matter dynamics. Changes in climate were simulated using a north-to-south cascade procedure involving the relocation of labelled soil columns to the next warmer site along the transect.Double exponential, decay-rate functions (for labile and recalcitrant SOM compartments) vs time showed that the thermosensitivity of microbial processes depended on the latitude from which the soil was translocated. Cumulative response functions for air temperature, and for combined temperature and moisture were used as independent variables in first order kinetic models fitted to the decomposition data. In the situations where climatic response functions explained most of the variations in decomposition rates when the soils were translocated, the climate optimised decomposition rates for the local and the translocated soil should be similar. Differences between these two rates indicated that there was either no single climatic response function for one or both compartments, and/or other edaphic factors influenced the translocation effect. The most northern boreal soil showed a high thermosensitivity for recalcitrant organic matter compartment, whereas the labile fraction was less sensitive to climate changes for soils from more southern locations. Hence there was no single climatic function which describe the decay rates for all compartments. At the end of the incubation period it was found that the heat sum to achieve the same carbon losses was lower for soils in the north of the transect than in the south. In the long term, therefore, for a given heat input, decomposition rates would show larger increases in boreal northern sites than in warm temperate regions.The changes in climate produced by soil translocation were more clearly reflected by decomposition rates in the acid soils than for calcareous soils. This indicates that the physicochemical environment can have important differential effects on microbial decomposition of the labile and recalcitrant components of SOM.     

Analysing temperature response of decomposition of organic matter

In order to analyse temperature effects on decomposition of organic matter, we tested the following hypothesis: Can an Arrhenius type of equation with constant parameter values for the temperature response of decomposer growth rate adequately describe decomposition of organic matter or must some additional properties be made functions of temperature? Possible temperature effects were analysed by aggregating data in different ways from an experiment with ¹⁴C‐labelled wheat material incubated in the laboratory at different temperatures and with soil materials collected from seven coniferous forest stands in Europe. Our analysis shows that it is possible to let all the temperature dependence reside in the decomposer growth rate. The analysis also supports the use of an Arrhenius type of equation for the temperature response of decomposer growth rate but with the parameters specific for each soil, or at least a distinction between organic and mineral horizons seems necessary.     

Nitrogen Uptake and Plant Growth I. Effect of Nitrogen Removal on Growth of Polygonum cuspidatum

Polygonun cuspidatum was grown hydroponically to examine theeffect of nitrogen removal from the nutrient solution upon plantgrowth and the partitioning of dry matter and nitrogen amongorgans. Nitrogen removal reduced the growth rate mainly dueto the reduced growth of leaf area. Accelerated root growthwas observed only in plants which earlier had received highlevels of nitrogen. Nitrogen removal caused almost exclusiveallocation of available nitrogen to root growth. Nitrogen fluxfrom the shoot to the root occurred in plants which had receivedlow nitrogen. Not only was net assimilation rate (NAR) littleaffected by nitrogen removal, but it also was not correlatedwith the concentration of leaf nitrogen on an area basis. Light-saturatedCO₂ exchange rate (CER) was highly correlated with the concentrationof leaf nitrogen. Nitrogen use efficiency (NUE) in CER (CERdivided by leaf nitrogen) remained constant against leaf nitrogen,indicating efficient use of nitrogen under light saturation,while NUE in terms of NAR decreased with higher concentrationof leaf nitrogen. Polygonum cuspidatum Sieb. et Zuce., CO₂ exchange rate, growth analysis, leaf nitrogen, net assimilation rate, nitrogen use efficiency, partitioning of dry matter and nitrogen     

Enhancing the early performance of the leguminous shrub Retama sphaerocarpa (L.) Boiss.: fertilisation versus Rhizobium inoculation

We have investigated the effect on growth of fertilisation versus biological nitrogen fixation by rhizobial nodules in Retama sphaerocarpa(L.) Boiss, a leafless leguminous shrub native to the Iberian Peninsula and North-West Africa that has generated interest for revegation of dry Mediterranean habitats. Our main objective was to optimise the formation of root nodules under nursery conditions and to evaluate their influence on the first year of seedling growth in comparison with standard fertilisation. Seedlings of R. sphaerocarpa from two Spanish localities were grown under two levels of fertilisation, and half of each were inoculated with rhizobia isolated from adult Retama, Cytisus and Adenocarpusplants in the field. Although some promiscuity was observed, nodulation was significantly successful with specific rhizobia. At the end of the experiment, highly fertilised plants were taller and heavier and exhibited larger photosynthetic rates than either nodulated or non-nodulated plants under low fertilisation. High fertilisation enhanced seedling growth but inhibited both the nodulation and the nitrogenase activity of the nodules. Thus, physiological differences between nodulated and non-nodulated plants were observed in the low but not in the high fertilisation treatment. Nitrogen uptake and use was enhanced by root nodules, which translated into enhanced photosynthesis and growth. Since inoculation is simple, environmentally friendly and cheap, and nodulated plants are more likely to overcome transplant stress than non-nodulated ones, our results suggest that inoculation together with low, background fertilisation (instead of high fertilisation) should be used when producing high quality seedlings of this autochthonous Mediterranean shrub.     

Nutrient-limited growth rates: quantitative benefits of stress responses and some aspects of regulation

Young sunflower plants (Helianthus annuus L.) under stress oflow nitrate or phosphate availability exhibited increases inroot: shoot ratio and in kinetic parameters for uptake. Theyshowed no significant changes in photosynthetic utilizationof either nutrient. Increases in root: shoot ratio were achievedby early and persistent suppression of shoot growth, but notroot growth. Affinity for phosphate uptake, 1/K_m(P), increasedwith phosphate stress, as did affinity for nitrate uptake, 1/K_m(N),with nitrate stress. Maximal uptake rate, V_max, for phosphateuptake increased with phosphorus stress; V_max for nitrate didnot increase with nitrogen stress. Phosphate V_max was relatedstrongly to root nutrient status. Decreases in V_max with plantage were not well explained by changes in age structure of roots.Estimated benefits of acclimatory changes in root: shoot ratioand uptake kinetics ranged up to 2-fold increases in relativegrowth rate, RGR. The relation of RGR to uptake physiology followedpredictions of functional balance moderately well, with somesystematic deviations. Analyses of RGR using growth models implyno significant growth benefit from regulating Vmax, specifically,not from down-regulating it at high nutrient availability. Quantitativebenefits of increases in root: shoot ratio and uptake parametersare predicted to be quite small under common conditions whereinnutrient concentrations are significantly depleted by uptake.The root: shoot response is estimated to confer the smallestbenefit under non-depleting conditions and the largest benefitunder depleting conditions. Even then, the absolute benefitis predicted to be small, possibly excepting the case of heterogeneoussoils. Depleting and non-depleting conditions are addressedwith very different experimental techniques. We note that atheoretical framework is lacking that spans both these cases,other than purely numerical formulations that are not readilyinterpreted. Key words: Nutrient stress, nutrient uptake, nutrient use efficiency, relative growth rate, Helianthus annuus     

Simple three‐pool model accurately describes patterns of long‐term litter decomposition in diverse climates

As atmospheric CO₂ increases, ecosystem carbon sequestration will largely depend on how global changes in climate will alter the balance between net primary production and decomposition. The response of primary production to climatic change has been examined using well‐validated mechanistic models, but the same is not true for decomposition, a primary source of atmospheric CO₂. We used the Long‐term Intersite Decomposition Experiment Team (LIDET) dataset and model‐selection techniques to choose and parameterize a model that describes global patterns of litter decomposition. Mass loss was best represented by a three‐pool negative exponential model, with a rapidly decomposing labile pool, an intermediate pool representing cellulose, and a recalcitrant pool. The initial litter lignin/nitrogen ratio defined the size of labile and intermediate pools. Lignin content determined the size of the recalcitrant pool. The decomposition rate of all pools was modified by climate, but the intermediate pool's decomposition rate was also controlled by relative amounts of litter cellulose and lignin (indicative of lignin‐encrusted cellulose). The effect of climate on decomposition was best represented by a composite variable that multiplied a water‐stress function by the Lloyd and Taylor variable Q₁₀ temperature function. Although our model explained nearly 70% of the variation in LIDET data, we observed systematic deviations from model predictions. Below‐ and aboveground material decomposed at notably different rates, depending on the decomposition stage. Decomposition in certain ecosystem‐specific environmental conditions was not well represented by our model; this included roots in very wet and cold soils, and aboveground litter in N‐rich and arid sites. Despite these limitations, our model may still be extremely useful for global modeling efforts, because it accurately (R²=0.6804) described general patterns of long‐term global decomposition for a wide array of litter types, using relatively minimal climatic and litter quality data.     

Medicago sativa - Sinorhizobium meliloti Symbiosis Promotes the Bioaccumulation of Zinc in Nodulated Roots

In this study we investigated effects of Zn supply on germination, growth, inorganic solutes (Zn, Ca, Fe, and Mg) partitioning and nodulation of Medicago sativa This plant was cultivated with and without Zn (2 mM). Treatments were plants without (control) and with Zn tolerant strain (S532), Zn intolerant strain (S112) and 2 mM urea nitrogen fertilisation. Results showed that M. sativa germinates at rates of 50% at 2 mM Zn. For plants given nitrogen fertilisation, Zn increased plant biomass production. When grown with symbionts, Zn supply had no effect on nodulation. Moreover, plants with S112 showed a decrease of shoot and roots biomasses. However, in symbiosis with S532, an increase of roots biomass was observed. Plants in symbiosis with S. meliloti accumulated more Zn in their roots than nitrogen fertilised plants. Zn supply results in an increase of Ca concentration in roots of fertilised nitrogen plants. However, under Zn supply, Fe concentration decreased in roots and increased in nodules of plants with S112. Zn supply showed contrasting effects on Mg concentrations for plants with nitrogen fertilisation (increase) and plants with S112 (decrease). The capacity of M. sativa to accumulate Zn in their nodulated roots encouraged its use in phytostabilisation processes.     

Effects of soil nutrients on the sequestration of plant defence chemicals by the specialist insect herbivore,Danaus plexippus

1. Although anthropogenic nitrogen (N) enrichment has significantly changed the growth, survival and reproduction of herbivorous insects, its effects on the defensive sequestration of secondary chemicals by insect herbivores are less well understood. Previous studies have shown that soil nutrient availability can affect sequestration directly through changing concentrations of plant defence chemicals, or indirectly through altering growth rates of herbivores. There has been less exploration of how nutrient deposition affects the consumption of secondary chemicals and subsequent sequestration efficiency. In the current study, the overall effect of soil N availability on cardenolide sequestration by the monarch caterpillar Danaus plexippus was examined. Specifically, the effects of soil nutrient availability on growth, consumption, excretion and sequestration efficiency of cardenolides by D. plexippus larvae fed on the tropical milkweed Asclepias curassavica were measured. 2. The results showed that soil N and phosphorus (P) fertilisation significantly reduced caterpillar growth rate and the sequestration efficiency of cardenolides by monarch caterpillars feeding on A. curassavica. The lowered sequestration efficiency was accompanied by higher concentrations of cardenolides in frass. Although the total cardenolide contents of caterpillars were lower under high N or P fertilisation levels, caterpillar cardenolide concentrations were constant across fertilisation treatments because of lower growth rates (and therefore lower body mass) under high fertilisation. It is concluded that anthropogenic N deposition may have multiple effects on insect herbivores, including their ability to defend themselves from predators with sequestered plant defences.     

Effects of Atmospheric NO2 on Azolla-Anabaena Symbiosis

Cultures of the water fern Azolla pinnata R, Br. exposed for1 week to atmospheric NO₂ (50, 100 or 200 nl l^-1) induced additionallevels of nitrate reductase (NaR) protein and nitrite reductase(NiR) activity. At low concentrations of NO₂ (50 nl l^-1), nitratederived from NO₂ provides an alternative N source for Azollabut does not affect rates of acetylene reduction. However, thesymbiotic relationship between Azolla and its endosymbiont,Anabaena azollae is only affected adversely by high concentrations(100 and 200 nl l^-1) of atmospheric NO₂. The resultant decreasesin rate of growth, nitrogen fixation, heterocyst formation,and overall nitrogen cycling are probably due to the additionalaccumulation of N products derived from higher levels of atmosphericNO₂. Parallel increases in levels of polyamines suggest thatAzolla partially alleviates these harmful effects by incorporatingsome of the extra NO₂-induced N into polyamines.Copyright 1994,1999 Academic Press Azolla-Anabaena symbiosis, nitrogen dioxide pollution, nitrogen metabolism, polyamines