Ingestion and excretion of nitrogen by larvae of a cabbage armyworm: the effects of fertilizer application

• 1 Insect frass has significant impacts on decomposition and soil nitrogen dynamics. Although the frass contains various forms of nitrogen that may differently influence nitrogen dynamics in the decomposition process, how the nitrogen form in the insect frass is influenced by host plant quality remains poorly understood.
• 2 The present study examined the effects of application of fertilizer on leaf quality of Brassica rapa L. var. perviridis Bailey (Brassicaceae), and on the consumption, frass excretion and frass quality of its insect pest Mamestra brassicae (L.) (Lepidoptera: Noctuidae), with a particular focus on the dynamics of inorganic nitrogen.
• 3 Brassica rapa increased total nitrogen concentration, and accumulated inorganic nitrogen [i.e. leaf nitrate‐nitrogen (NO₃^?‐N) and ammonium‐nitrogen (NH₄⁺‐N)] in the leaves in response to the application of fertilizer.
• 4 Although leaf consumption and frass excreted by M. brassicae was not affected by fertilizer treatment, frass quality was influenced by host plant quality as altered by fertilizer applications. Frass contained high concentrations of total nitrogen, NO₃^?‐N, and NH₄⁺‐N under high fertilizer treatment. In particular, the larvae excreted much more NH₄⁺‐N than ingested. The relationship between host plant quality and insect frass quality, as well as the potential implications for decomposition and nutrient dynamics, are discussed.

     

Non-additive effects of leaf litter and insect frass mixture on decomposition processes

Although there is a growing body of evidence that herbivorous insects have a significant impact on decomposition and soil nutrient dynamics through frass excretion, how mixtures of leaf litter and insect frass influence such ecosystem processes remains poorly understood. We examined the effects of mixing of leaf litter and insect frass on decomposition and soil nutrient availability, using a study system consisting of a willow, Salix gilgiana Seemen, and a herbivorous insect, Parasa consocia Walker. The chemical characteristics of insect frass differed from those of leaf litter. In particular, frass had a 42-fold higher level of ammonium–nitrogen (NH₄ ⁺–N) than litter. Incubation experiments showed that the frass was decomposed and immobilized with respect to N more rapidly than the litter. Furthermore, litter and frass mixtures showed non-additive enhancement of decomposition and reduction of NH₄ ⁺–N, depending on the litter–frass mixing ratio. These indicate that, while insect frass generally accelerated decomposition, the effect of frass on soil nutrient availability was dependent largely on the relative amounts of litter and frass.     

Insect herbivores and their frass affect Quercus rubra leaf quality and initial stages of subsequent litter decomposition

Defoliation‐induced changes in plant foliage are ubiquitous, though factors mediating induction and the extent of their influence on ecosystem processes such as leaf litter decomposition are poorly understood. Soil nitrogen (N) availability, which can be affected by insect herbivore frass (feces), influences phytochemical induction. We conducted experiments to test the hypotheses that insect frass deposition would (1) reduce phytochemical induction following herbivory and (2) increase the decomposition and nutrient release of the subsequent leaf litter. During the 2002 growing season, 80 Quercus rubra saplings were subjected to a factorial experiment with herbivore and frass manipulations. Leaf samples were collected throughout the growing season to measure the effects of frass deposition on phytochemical induction. In live foliage, herbivore damage increased tannin concentrations early, reduced foliar N concentrations throughout the growing season, and lowered lignin concentrations in the late season. Frass deposition apparently reduced leaf lignin concentrations, but otherwise did not influence leaf chemistry. Following natural senescence, litter samples from the treatment groups were decomposed in replicated litterbags for 18 months at the Coweeta Hydrologic Laboratory, NC. In the dead litter samples, initial tannin concentrations were lower in the herbivore damage group and higher in the frass addition group relative to their respective controls. Tannin and N release rates in the first nine months of decomposition were also affected by both damage and frass. However, decomposition rates did not differ among treatment groups. Thus, nutrient dynamics important for some ecosystem processes may be independent from the physical loss of litter mass. Overall, while lingering effects of damage and even frass deposition can therefore carry over and affect ecosystem processes during decomposition, their effects appear short lived relative to abiotic forces that tend to homogenize the decomposition process.     

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.     

Nitrogen homeostasis in a willow leaf beetle, Plagiodera versicolora, is independent of host plant quality

While foliar nitrogen (N) content of host plants depends on environmental conditions, N content of herbivorous insects may remain relatively constant due to homeostasis. However, it is unknown to what extent insects can maintain their body elemental composition against natural variation in host plant quality. The present study examined the performance and N content of a willow leaf beetle, Plagiodera versicolora Laicharting (Coleoptera: Chrysomelidae), when fed leaves of host willow, Salix eriocarpa Franchet et Savatier (Salicaceae), with varying nutritional status. Water content, toughness, and N content of willow leaves varied seasonally, and they affected performance of the leaf beetle. The leaf beetle achieved high performance when fed young leaves. On the other hand, the N content of the leaf beetle changed little, and it was independent of that of willow leaves, indicating strong N homeostasis of the leaf beetle. We discussed the function of N homeostasis in herbivorous insects in tritrophic level interactions.     

Phytophagous insects enhance nitrogen flux in a desert creosotebush community

Summary We tested the hypothesis that herbivorous insects on desert shrubs contribute to short-term nitrogen cycling, and increase rates of nitrogen flux from nutrient rich plants. Creosotebush (Larrea tridentata) shrubs were treated with different combinations of fertilizer and water augmentations, resulting in different levels of foliage production and foliar nitrogen contents. Foliage arthropod populations, and nitrogen in canopy dry throughfall, wet throughfall and stemflow were measured to assess nitrogen flux rates relative to arthropod abundances on manipulated and unmanipulated shrubs over a one-month period during peak productivity. Numbers and biomass of foliage arthropods were significantly higher on fertilized shrubs. Sap-sucking phytophagous insects accounted for the greatest numbers of foliage arthropods, but leaf-chewing phytophagous insects represented the greatest biomass of foliage arthropods. Measured amounts of bulk frass (from leaf-chewing insects) were not significantly different among the various treatments. Amounts of nitrogen from dry and wet throughfall and stemflow were significantly greater under fertilized shrubs due to fine frass input from sap-sucking insects. Increased numbers and biomass of phytophagous insects on fertilized shrubs increased canopy to soil nitrogen flux due to increased levels of herbivory and excrement. Nitrogen excreted by foliage arthropods accounted for about 20% of the total one month canopy to soil nitrogen flux, while leaf litter accounted for about 80%.     

<Emphasis Type="Italic">Hedychium gardnerianum</Emphasis> Invasion into Hawaiian Montane Rainforest: Interactions Among Litter Quality,Decomposition Rate,and Soil Nitrogen Availability

Few studies have examined the invasion of understory species into closed-canopy forests and, despite inter-specific differences in litter quality and quantity between understory and dominant canopy trees, the influence of understory invasions on soil nitrogen (N) cycling remains unknown. This paper examines litter quality and decomposition of kahili ginger (Hedychium gardnerianum), an invasive understory herb, to determine the influence of this species on N cycling in a Hawaiian montane rainforest. To examine the potential feedback between increased soil N availability and litter decomposition, litter from the invasive ginger, a native tree, and native tree fern was collected from unfertilized and fertilized plots and decomposed in a reciprocal transplant design. Hedychium litter decomposed faster than litter from the two native species. Across species, decomposition rates were negatively correlated with litter lignin content. Despite rapid decomposition rates of Hedychium litter, soil nitrogen availability and rates of net mineralization in the soil were similar in invaded and uninvaded plots. Nitrogen cycling at this site may be more strongly influenced by native species, which contribute the most to overall stand biomass. A negative effect of fertilization on the decomposition of Hedychium litter suggests that a negative feedback between litter quality and soil N availability may exist over longer timescales.     

Aphids decelerate litter nitrogen mineralisation through changes in litter quality

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Nitrogen response efficiency increased monotonically with decreasing soil resource availability: a case study from a semiarid grassland in northern China

The concept of nutrient use efficiency is central to understanding ecosystem functioning because it is the step in which plants can influence the return of nutrients to the soil pool and the quality of the litter. Theory suggests that nutrient efficiency increases unimodally with declining soil resources, but this has not been tested empirically for N and water in grassland ecosystems, where plant growth in these ecosystems is generally thought to be limited by soil N and moisture. In this paper, we tested the N uptake and the N use efficiency (NUE) of two Stipa species (S. grandis and S. krylovii) from 20 sites in the Inner Mongolia grassland by measuring the N content of net primary productivity (NPP). NUE is defined as the total net primary production per unit N absorbed. We further distinguished NUE from N response efficiency (NRE; production per unit N available). We found that NPP increased with soil N and water availability. Efficiency of whole-plant N use, uptake, and response increased monotonically with decreasing soil N and water, being higher on infertile (dry) habitats than on fertile (wet) habitats. We further considered NUE as the product of the N productivity (NP the rate of biomass increase per unit N in the plant) and the mean residence time (MRT; the ratio between the average N pool and the annual N uptake or loss). The NP and NUE of S. grandis growing usually in dry and N-poor habitats exceeded those of S. krylovii abundant in wet and N-rich habitats. NUE differed among sites, and was often affected by the evolutionary trade-off between NP and MRT, where plants and communities had adapted in a way to maximize either NP or MRT, but not both concurrently. Soil N availability and moisture influenced the community-level N uptake efficiency and ultimately the NRE, though the response to N was dependent on the plant community examined. These results show that soil N and water had exerted a great impact on the N efficiency in Stipa species. The intraspecific differences in N efficiency within both Stipa species along soil resource availability gradient may explain the differences in plant productivity on various soils, which will be conducive to our general understanding of the N cycling and vegetation dynamics in northern Chinese grasslands.     

Evidence for temperature limitation of nitrogen mineralisation in the Drakensberg Alpine Centre

We made use of pot experiments and soil mineralisation assays to test the effect of temperature on the soil nitrogen (N) economy of the Drakensberg Alpine Centre (‘mountain site’). The approach was enhanced by the inclusion of a contrasting warm, subtropical environment on the east coast of southern Africa (‘coast site’) which presented an opportunity to test plant growth in mountain soil outside of the mountain site's natural climatic envelope. This study was further augmented by two greenhouse experiments that helped isolate the factors responsible for the growth responses in the experiments above. Plant morphology, plant nutrients and soil nutrients were used as the basis for comparing treatment effects. The primary pot experiment showed that plant growth was uniform in the mountain site regardless of whether the test species was grown in intrinsically N-rich mountain soil or intrinsically N-poor coast soil. However, we noted significant growth differences at the coast site using the aforementioned soil nutrient regimes. In terms of the soil mineralisation assay, coast soil, derived from intrinsically N-poor sandstone, predictably mineralised little soil inorganic N at the mean spring temperature of 19 °C. However against expectations, the intrinsically N-rich mountain soil mineralised < 1% of its total soil N budget into inorganic N at 12 °C, most probably because the microbes responsible for the conversion of organic soil N to inorganic soil N were severely inhibited at this mean spring temperature. However, the potential to mineralise far more N in mountain soil was apparent when using an elevated experimental temperature of 30 °C, with 369% more soil N being available under the latter regime. Our results suggest that the cooler temperatures associated with high elevations in the mountain site constrain the activity of soil microbes in mountain soil, resulting in a functionally N-poor soil economy particularly deficient in inorganic N. This also explains the similar growth responses regardless of the soil being intrinsically N-rich or N-poor. We speculate whether or not more soil inorganic N may become available under a regime of warming due to accelerated N mineralisation, to the detriment of plant taxa adapted to low soil N availability.     

Cigarette Butt Decomposition and Associated Chemical Changes Assessed by 13C CPMAS NMR

Cigarette butts (CBs) are the most common type of litter on earth, with an estimated 4.5 trillion discarded annually. Apart from being unsightly, CBs pose a serious threat to living organisms and ecosystem health when discarded in the environment because they are toxic to microbes, insects, fish and mammals. In spite of the CB toxic hazard, no studies have addressed the effects of environmental conditions on CB decomposition rate. In this study we investigate the interactive effects of substrate fertility and N transfer dynamics on CB decomposition rate and carbon quality changes. We carried out an experiment using smoked CBs and wood sticks, used as a slow decomposing standard organic substrate, incubated in both laboratory and field conditions for two years. CB carbon quality changes during decomposition was assessed by ¹³C CPMAS NMR. Our experiment confirmed the low degradation rate of CBs which, on average, lost only 37.8% of their initial mass after two years of decomposition. Although a net N transfer occurred from soil to CBs, contrary to our hypothesis, mass loss in the medium-term (two years) was unaffected by N availability in the surrounding substrate. The opposite held for wood sticks, in agreement with the model that N-rich substrates promote the decomposition of other N-poor natural organic materials with a high C/N ratio. As regards CB chemical quality, after two years of decomposition ¹³C NMR spectroscopy highlighted very small changes in C quality that are likely to reflect a limited microbial attack.     

Biomasses of Arthropod Taxa Differentially Increase on Nitrogen‐Fertilized Willows and Cottonwoods

I evaluated soil application of nitrogen fertilizer to 1‐year‐old, flood‐irrigated Salix exigua willows and Populus fremontii cottonwoods as a method for increasing arthropod abundances and biomasses (wet masses) available to insectivorous birds. Shrubs and trees, planted near the lower Colorado River in southeast California for wildlife habitat, were fertilized during April 2008. I collected spiders and insects monthly during the following May–August from unfertilized and fertilized plants by fumigating branches with insecticide. Percentages of N in leaves, and to a lesser extent percentages of water in branches, were greater on fertilized plants (averaging 2.5% N of dry mass) compared with unfertilized plants (1.6% N) in both species. Most arthropods collected were predaceous Araneae (44% of abundance, 52% of biomass) followed by phytophagous Homoptera (34%, 11%) and predaceous or phytophagous Heteroptera (10%, 11%). Abundances and biomasses of Araneae, Heteroptera, and all Arthropoda across months did not differ between unfertilized and fertilized plants in either species controlling for masses of sampled branches. In contrast, biomasses of Homoptera, mostly Cicadellidae followed by Aphididae, were 197% greater on fertilized willows and 228% greater on fertilized cottonwoods. Greater biomasses on fertilized plants were consistent across months. Biomasses of homopterans on branches of each species also increased as leaf N‐concentrations increased. Applying N‐fertilizer to willows and cottonwoods can increase leaf N‐contents and abundances and biomasses of Homoptera. Increased homopteran biomass on N‐fertilized plants may in turn diversify prey available to insectivorous birds.     

Recycling of nitrogen in herbivore feces: plant recovery,herbivore assimilation,soil retention,and leaching losses

Herbivores directly and indirectly affect ecosystem functioning in forests. Feces deposition is a direct effect that supplies ephemeral N pulses to soils. Herbivore-mediated changes in plant N allocation and uptake are indirect effects that can also influence soil N availability. These effects may interact if defoliation influences the ability of plants to recover fecal N, and this may affect subsequent generations of herbivores. We added ¹⁵N-enriched insect feces (frass) to a series of replicated red oak, Quercus rubra, mesocosms that had been damaged experimentally and then followed the frass N over the course of 2 years. In the first season, some frass N was mineralized in the soil and leached in organic form from the mesocosms within 1 week of deposition. Within 1 month, frass N had been acquired by the oaks and enriched the foliage; late-season herbivores assimilated the frass N within the same growing season. In the second season, herbivore damage from the previous year lowered total leaf N contents and ¹⁵N recovered in the foliage. A subsequent cohort of early-season herbivores fed on this foliage consequently derived less of their N from the previous year’s frass, and feral leaf rollers colonized fewer of these saplings. The 0- to 5-cm soil fraction was the largest N sink measured, and 42% of the frass N was recovered in the soil. The results demonstrate that: (1) some frass N can be recycled rapidly into foliage and assimilated by successive cohorts of herbivore within the same season; (2) damage can affect N allocation in the following year’s foliage, influencing N availability to and host selection by herbivores; and (3) leaching losses occur soon after deposition but are buffered by soil pools, which are the largest sinks for frass N.     

Ecosystem consequences of selective feeding of an insect herbivore: palatability–decomposability relationship revisited

1. The relationship between leaf palatability and litter decomposability is critical to understanding the effects of selective feeding by herbivores on decomposition processes, and several studies have reported that there is a positive relationship between them. 2. However, palatability is not always positively correlated with decomposability, because of species‐specific feeding adaptation of herbivores to host plants. Moreover, the effects of selective feeding by herbivores on soil decomposition processes should be understood in terms of the inputs of leaf litter and excrement. 3. The present study examined the relationships between leaf palatability and the decomposability of litter and frass, using Lymantria dispar Linnaeus and 15 temperate deciduous tree species. 4. Larvae of L. dispar exhibited a clear feeding preference, and subsequently the excreted frass mass differed among tree species. Litter and frass decomposability also differed among tree species, and frass was more rapidly decomposed than litter. There were no positive or negative correlations between palatability and decomposability of litter and frass. 5. These results indicate that L. dispar larvae may accelerate the decomposition process in temperate deciduous forests through selective feeding on plants with relatively low litter decomposability and the production of frass with higher decomposability than the litter.     

Neutron radiography as a tool for revealing root development in soil: capabilities and limitations

Chronic atmospheric nitrogen deposition affects the cycling of carbon (C) and nitrogen (N) in forest ecosystems, and thereby alters the stable C isotopic abundance of plant and soil. Three successional stages, disturbed, rehabilitated and mature forests were studied for their responses to different nitrogen input levels. N-addition manipulative experiments were conducted at low, medium and high N levels. To study the responses of C cycling to N addition, the C concentration and ¹³C natural abundances for leaf, litter and soil were measured. Labile organic carbon fractions in mineral soils were measured to quantify the dynamics of soil organic C (SOC). Results showed that three-year continuous N addition did not significantly increase foliar C and N concentration, but decreased C/N ratio and enriched ¹³C in N-rich forests. In addition, N addition significantly decreased microbial biomass C, and increased water soluble organic C in surface soils of N-rich forests. This study suggests that N addition enhances the water consumption per unit C assimilation of dominant plant species, restricts SOC turnover in N-poor forests at early and medium successional stages (thus favored SOC sequestration), and vice versa for N-rich mature forests.     

Time‐dependent effects of fertilization on plant biomass in floating fens

Abstract. A cross‐over fertilization experiment was carried out in Dutch floating fens to investigate effects on biomass production in the same and the following years. In total 16 fertilizer treatments were applied, combining four treatments in 1999 with four treatments in 2000 (addition of 20 g.m^?2 N, 5 g.m^?2 P, both elements and unfertilized control). The above‐ground biomass production of vascular plants was co‐limited by N and P in both years. However, in plots that were only fertilized in 1999 the effects of individual nutrients differed between the two years: N‐fertilization slightly increased the amount of biomass produced in the same year (1999), whereas P‐fertilization did so in the following year (2000). Fertilizer applied in 1999 also influenced the effects of fertilizer applied in 2000. One year after N‐fertilization vascular plant growth was still co‐limited by N and P, but one year after P‐fertilization, vascular plant growth was only limited by N. Bryophyte biomass responded weakly to fertilization. Nutrient concentrations in plant biomass, nutrient standing crops and measurements of N and P availability in the soil indicated that one year after fertilization, the N‐fertilizer had mostly ‘disappeared’ from N‐fertilized plots, whereas the availability of P remained markedly enhanced in P‐fertilized plots. In addition, P‐fertilization enhanced the uptake of N by plants the following year. The time‐dependence of fertilizer effects was probably caused by (1) higher addition of P than of N relative to the requirements of plants; (2) longer retention of P than of N in the system; (3) positive effect of P‐fertilization on the availability of N; (4) contrasting effects of N‐ and P‐fertilization on nutrient losses by plants and/or on their responses to subsequent nutrient addition; (5) changing interactions between vascular plants and mosses (mainly Sphagnum spp.); (6) nutrient export through the repeated harvest of above‐ground biomass. To determine which nutrient limits plant growth fertilization experiments should be short, avoiding that indirect effects of a non‐limiting nutrient influence results. To indicate how changed nutrient supply will affect an ecosystem longer‐term experiments are needed, so that indirect effects have time to develop and be detected.     

Insect outbreaks alter nutrient dynamics in a southern African savanna: patchy defoliation of Colophospermum mopane savanna by Imbrasia belina larvae

Invertebrate herbivore outbreaks have important impacts on system biogeochemical cycling, but these effects have been poorly documented in African savanna ecosystems. In semi‐arid African savannas, outbreaks of the lepidopteran Imbrasia belina (mopane worm) affect discrete patches of the dominant Colophospermum mopane trees; larvae may completely defoliate trees for up to six weeks during each of the early and late growing seasons. We studied the impact of mopane worm outbreaks on the availability of nitrogen (N), phosphorus (P), and potassium (K) within mopane savanna by quantifying major nutrient pools in defoliated and non‐defoliated savanna patches, including leaves, leaf litter, worm frass, and the soil beneath trees. Within an outbreak area, approximately 44 percent of trees were infested, supporting ~29,000 worms/ha, leading to ~640 kg/ha dry weight frass deposition at 1.4 g of frass/day‐individual (fourth or fifth instar), compared with an average 1645 kg/ha dry weight of leaf on trees most of which should be deposited by litterfall at the end of the growing season. Frass had twofold higher P, 10 percent higher K, but equivalent N content than litter. Taking frass and litter deposition together, the N, P, and K contents added due to the outbreak event at our study site were 0.88, 5.8, and 2.8 times those measured in non‐outbreak patches, a pattern which was reflected in the nutrient contents of soil surfaces beneath defoliated trees. Invertebrate herbivory appears to be an important driver for mopane savanna but has been largely neglected.     

Determinants of community organization of a South African mesic grassland

Abstract. Question: What is the long‐term influence of nutrient availability, productivity and soil pH on grassland community organization? Location: Ukulinga research farm, KwaZulu‐Natal, South Africa. Methods: The influence of fertilization on soil pH, nitrogen (N) and phosphorus (P) on variation in plant traits, community composition and species richness were examined in a 50‐year grassland fertilization experiment. Results: Averaged over 30 years, above‐ground net primary production (ANPP) was 337, 428 and 518 g.m^‐2 in sites not fertilized, fertilized with N, and fertilized with N plus P respectively. ANPP depended directly on N‐fertilization but not on P‐fertilization or liming, and responded positively to the interaction of N (first limiting nutrient) and P (second limiting nutrient). Short narrow‐leaved grass species —Themeda triandra, Tristachya leucothrix and Setaria nigrirostris— dominated sites of lowest ANPP where N was limiting (unfertilized, P‐fertilized or limed sites). A tall narrow‐leaved species, Eragrostis curvula, dominated sites of intermediate ANPP where P was limiting (N‐fertilized sites). By contrast, a tall broad‐leaved species, Panicum maximum, dominated the most productive sites where neither N nor P were limiting (N‐ and P‐fertilized sites). Certain species responded to liming and type of N‐fertilizer apparently because of their effects on soil pH. N‐fertilization reduced the density of herbaceous dicots (forbs) from 14 (unfertilized) to two (high N, no P, no lime) and five species per m² (high N, no P, limed). This effect was attributed to increased ANPP and a decrease in soil pH from 4.6 (KCl) in unfertilized sites to 3.49 (high N, no lime) and 4.65 (high N and lime). Soil acidification had no effect on grass species richness but influenced the abundance of certain species. Conclusions: Grassland community organization is determined not only by the influence of N availability, but also by the hierarchical interaction of N and P availability, in part through their compounded effect on ANPP, and by individualistic species responses to soil pH.     

Ecological stoichiometry and nutrient partitioning in two insect herbivores responsible for large‐scale forest disturbance in the Fennoscandian subarctic

1. Outbreaks of herbivorous insects can have large impacts on regional soil carbon (C) storage and nutrient cycling. In northernmost Europe, population outbreaks of several geometrid moth species regularly cause large‐scale defoliation in subarctic birch forests. An improved understanding is required of how leaf C and nutrients are processed after ingestion by herbivores and what this means for the quantity and quality of different materials produced (frass, bodies). 2. In this study, larvae of two geometrid species responsible for major outbreaks (Epirrita autumnata and Operophtera brumata) were raised on exclusive diets of Betula pubescens var. czerepanovii (N. I. Orlova) Hämet Ahti and two other abundant understorey species (Betula nana, Vaccinium myrtillus). The quantities of C, nitrogen (N) and phosphorus (P) ingested and allocated to frass, bodies and (in the case of C) respired were recorded. 3. Overall, 23%, 70% and 48% of ingested C, N and P were allocated to bodies, respectively, rather than frass and (in the case of C) respiration. Operophtera brumata consistently maintained more constant body stoichiometric ratios of C, N and P than did E. autumnata, across the wide variation in physico‐chemical properties of plant diet supplied. 4. These observed differences and similarities on C and nutrient processing may improve researchers' ability to predict the amount and stoichiometry of frass and bodies generated after geometrid outbreaks.     

Interactions between goldenrod (Solidago altissima L.) and its insect herbivore (Trirhabda virgata) over the course of succession

Consumers can mediate the composition of plant communities and alter ecosystem processes. Although herbivores usually increase N availability in the short term, they might decrease it in the long term. I investigated the long-term effect of insect herbivores on leaf tissue quality and soil N availability in goldenrod (Solidago altissima) fields using two approaches: (1) I compared plots from which herbivores had been excluded for 17 years with adjacent plots that had experienced normal levels of herbivory, and (2) I examined a chronosequence of nine goldenrod fields representing three successional stages: early, middle, and late. These parallel approaches showed that, in the long term, herbivores decrease the quality of leaf litter and soil N availability in goldenrod fields. These long-term effects appear to compensate for various short-term effects that increase N availability in the soil (e.g., added frass, increased light penetration). Furthermore, herbivores decrease leaf litter quality and N availability by reducing the quality of leaf tissue within the same species. This pattern may result from insect herbivores preferentially grazing on plants with a high N content thereby increasing the amount of recalcitrant litter over the course of succession. Received: 4 May 1999 / Accepted: 24 September 1999