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N Availability (n + availability)
Kinds of N Availability Selected AbstractsNitrogen fertilization effects on Myzus persicae aphid dynamics on peach: vegetative growth allocation or chemical defence?ENTOMOLOGIA EXPERIMENTALIS ET APPLICATA, Issue 2 2010Marie-Hélène Sauge Abstract Plant nitrogen (N) fertilization is a common cropping practice that is expected to serve as a pest management tool. Its effects on the dynamics of the aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) were examined on young peach [Prunus persica (L.) Batsch (Rosaceae)] trees grown under five N treatments, ranging from N shortage to supra-optimal supply for growth. Aphid population increased over time at the three intermediate N levels. It remained stable at the lowest N level and decreased at the highest N level. Four weeks after the start of infestation, the number of aphids displayed a parabolic response to N level. The relationships between N status and parameters of plant vegetative growth (stem diameter) or biomass allocation (lateral-total leaf area and root-shoot ratio) were consistent with responses proposed by models of adaptive plasticity in resource allocation patterns. However, the variation in plant growth predicted aphid population dynamics only partially. Whereas aphid number was positively correlated with plant N status and vegetative growth up to the intermediate N level, it was negatively correlated with plant N status above this level, but not with vegetative growth. The concentrations of primary and secondary (plant defence-related) metabolites in the plant shoots were modified by N treatments: amino acids (main nutritional resource of aphids) and prunasin increased, whereas chlorogenic acid decreased with increasing N availability. Constitutive changes in plant chemistry in response to N fertilization could not directly explain the reduced aphid performance for the highest N level. Nevertheless, the indirect effect of N on the induction of plant defence compounds by aphid feeding warrants further investigation. The study focuses on the feasibility of handling N fertilization to control M. persicae in orchards, but findings may also be relevant for our understanding of the physiological relationships between the host's nutritional status and the requirements of the insect. [source] The influence of below-ground herbivory and defoliation of a legume on nitrogen transfer to neighbouring plantsFUNCTIONAL ECOLOGY, Issue 2 2007E. AYRES Summary 1Both foliar and root herbivory can alter the exudation of carbon from plant roots, which in turn can affect nitrogen availability in the soil. However, few studies have investigated the effects of herbivory on N fluxes from roots, which can directly increase N availability in the soil and uptake by neighbouring plants. Moreover, the combined effects of foliar and root herbivory on N fluxes remains unexplored. 2We subjected the legume white clover (Trifolium repens L.) to defoliation (through clipping) and root herbivory (by an obligate root-feeding nematode, Heterodera trifolii Goggart) to examine how these stresses individually, and simultaneously, affected the transfer of T. repens -derived N to neighbouring perennial ryegrass (Lolium perenne L.) plants using 15N stable-isotope techniques. We also examined the effects of defoliation and root herbivory on the size of the soil microbial community and the growth response of L. perenne. 3Neither defoliation nor root herbivory negatively affected T. repens biomass. On the contrary, defoliation increased root biomass (34%) and total shoot production by T. repens (100%). Furthermore, defoliation resulted in a fivefold increase in T. repens -derived 15N recovered in L. perenne roots, and increased the size of the soil microbial biomass (77%). In contrast, root herbivory by H. trifolii slightly reduced 15N transfer from T. repens to L. perenne when T. repens root 15N concentration was included as a covariate, and root herbivory did not affect microbial biomass. Growth of L. perenne was not affected by any of the treatments. 4Our findings demonstrate that defoliation of a common grassland legume can substantially increase the transfer of its N to neighbouring plants by directly affecting below-ground N fluxes. These finding require further examination under field conditions but, given the prevalence of N-limitation of plant productivity in terrestrial ecosystems, increased transfer of N from legumes to non-N-fixing species could alter competitive interactions, with implications for plant community structure. [source] CO2 and nitrogen, but not population density, alter the size and C/N ratio of Phytolacca americana seedsFUNCTIONAL ECOLOGY, Issue 3 2005J.-S. HE Summary 1Plants can provision seeds by optimizing seed size, number and nutrient content to maximize parental fitness. According to the McGinley,Charnov hypothesis, seed size should be determined by the ratio of carbon to nitrogen (C/N) available to the plant, with larger seed size correlating with larger C/N ratios and smaller absolute N content. 2This hypothesis was tested by establishing monocultures of Phytolacca americana L. (Phytolaccaceae) at three population densities under ambient and elevated CO2 environments, with two availabilities of soil N. 3Elevated CO2 reduced both seed size and N concentration while increasing the C/N ratio; high soil N availability produced the opposite result for N concentration and C/N ratio. Higher planting densities reduced plant biomass, but did not alter seed size. 4In accordance with the McGinley,Charnov hypothesis, larger seeds had both larger C/N ratios and smaller N content. However, the increase in C/N ratio caused by elevated CO2 corresponded with smaller seeds overall: elevated CO2 reduced seed size, although the seed size,C/N relationship remained positive. 5These results suggest an alternative mechanism to explain variation in seed size, and suggest that future climate conditions may alter seed quality and plant reproductive behaviour. [source] Enhanced litter input rather than changes in litter chemistry drive soil carbon and nitrogen cycles under elevated CO2: a microcosm studyGLOBAL CHANGE BIOLOGY, Issue 2 2009LINGLI LIU Abstract Elevated CO2 has been shown to stimulate plant productivity and change litter chemistry. These changes in substrate availability may then alter soil microbial processes and possibly lead to feedback effects on N availability. However, the strength of this feedback, and even its direction, remains unknown. Further, uncertainty remains whether sustained increases in net primary productivity will lead to increased long-term C storage in soil. To examine how changes in litter chemistry and productivity under elevated CO2 influence microbial activity and soil C formation, we conducted a 230-day microcosm incubation with five levels of litter addition rate that represented 0, 0.5, 1.0, 1.4 and 1.8 × litterfall rates observed in the field for aspen stand growing under control treatments at the Aspen FACE experiment in Rhinelander, WI, USA. Litter and soil samples were collected from the corresponding field control and elevated CO2 treatment after trees were exposed to elevated CO2 (560 ppm) for 7 years. We found that small decreases in litter [N] under elevated CO2 had minor effects on microbial biomass carbon, microbial biomass nitrogen and dissolved inorganic nitrogen. Increasing litter addition rates resulted in linear increase in total C and new C (C from added litter) that accumulated in whole soil as well as in the high density soil fraction (HDF), despite higher cumulative C loss by respiration. Total N retained in whole soil and in HDF also increased with litter addition rate as did accumulation of new C per unit of accumulated N. Based on our microcosm comparisons and regression models, we expected that enhanced C inputs rather than changes in litter chemistry would be the dominant factor controlling soil C levels and turnover at the current level of litter production rate (230 g C m,2 yr,1 under ambient CO2). However, our analysis also suggests that the effects of changes in biochemistry caused by elevated CO2 could become significant at a higher level of litter production rate, with a trend of decreasing total C in HDF, new C in whole soil, as well as total N in whole soil and HDF. [source] Where temperate meets tropical: multi-factorial effects of elevated CO2, nitrogen enrichment, and competition on a mangrove-salt marsh communityGLOBAL CHANGE BIOLOGY, Issue 5 2008KAREN L. McKEE Abstract Our understanding of how elevated CO2 and interactions with other factors will affect coastal plant communities is limited. Such information is particularly needed for transitional communities where major vegetation types converge. Tropical mangroves (Avicennia germinans) intergrade with temperate salt marshes (Spartina alterniflora) in the northern Gulf of Mexico, and this transitional community represents an important experimental system to test hypotheses about global change impacts on critical ecosystems. We examined the responses of A. germinans (C3) and S. alterniflora (C4), grown in monoculture and mixture in mesocosms for 18 months, to interactive effects of atmospheric CO2 and pore water nitrogen (N) concentrations typical of these marshes. A. germinans, grown without competition from S. alterniflora, increased final biomass (35%) under elevated CO2 treatment and higher N availability. Growth of A. germinans was severely curtailed, however, when grown in mixture with S. alterniflora, and enrichment with CO2 and N could not reverse this growth suppression. A field experiment using mangrove seedlings produced by CO2 - and N-enriched trees confirmed that competition from S. alterniflora suppressed growth under natural conditions and further showed that herbivory greatly reduced survival of all seedlings. Thus, mangroves will not supplant marsh vegetation due to elevated CO2 alone, but instead will require changes in climate, environmental stress, or disturbance to alter the competitive balance between these species. However, where competition and herbivory are low, elevated CO2 may accelerate mangrove transition from the seedling to sapling stage and also increase above- and belowground production of existing mangrove stands, particularly in combination with higher soil N. [source] Host-specific aphid population responses to elevated CO2 and increased N availabilityGLOBAL CHANGE BIOLOGY, Issue 11 2005Erika A. Sudderth Abstract Sap-feeding insects such as aphids are the only insect herbivores that show positive responses to elevated CO2. Recent models predict that increased nitrogen will increase aphid population size under elevated CO2, but few experiments have tested this idea empirically. To determine whether soil nitrogen (N) availability modifies aphid responses to elevated CO2, we tested the performance of Macrosiphum euphorbiae feeding on two host plants; a C3 plant (Solanum dulcamara), and a C4 plant (Amaranthus viridis). We expected aphid population size to increase on plants in elevated CO2, with the degree of increase depending on the N availability. We found a significant CO2× N interaction for the response of population size for M. euphorbiae feeding on S. dulcamara: aphids feeding on plants grown in ambient CO2, low N conditions increased in response to either high N availability or elevated CO2. No population size responses were observed for aphids infesting A. viridis. Elevated CO2 increased plant biomass, specific leaf weight, and C : N ratios of the C3 plant, S. dulcamara but did not affect the C4 plant, A. viridis. Increased N fertilization significantly increased plant biomass, leaf area, and the weight : height ratio in both experiments. Elevated CO2 decreased leaf N in S. dulcamara and had no effect on A. viridis, while higher N availability increased leaf N in A. viridis and had no effect in S. dulcamara. Aphid infestation only affected the weight : height ratio of S. dulcamara. We only observed an increase in aphid population size in response to elevated CO2 or increased N availability for aphids feeding on S. dulcamara grown under low N conditions. There appears to be a maximum population growth rate that M. euphorbiae aphids can attain, and we suggest that this response is because of intrinsic limits on development time and fecundity. [source] Nonlinear response of N2O flux to incremental fertilizer addition in a continuous maize (Zea mays L.) cropping systemGLOBAL CHANGE BIOLOGY, Issue 10 2005Claire P. McSwiney Abstract The relationship between nitrous oxide (N2O) flux and N availability in agricultural ecosystems is usually assumed to be linear, with the same proportion of nitrogen lost as N2O regardless of input level. We conducted a 3-year, high-resolution N fertilizer response study in southwest Michigan USA to test the hypothesis that N2O fluxes increase mainly in response to N additions that exceed crop N needs. We added urea ammonium nitrate or granular urea at nine levels (0,292 kg N ha,1) to four replicate plots of continuous maize. We measured N2O fluxes and available soil N biweekly following fertilization and grain yields at the end of the growing season. From 2001 to 2003 N2O fluxes were moderately low (ca. 20 g N2O-N ha,1 day,1) at levels of N addition to 101 kg N ha,1, where grain yields were maximized, after which fluxes more than doubled (to >50 g N2O-N ha,1 day,1). This threshold N2O response to N fertilization suggests that agricultural N2O fluxes could be reduced with no or little yield penalty by reducing N fertilizer inputs to levels that just satisfy crop needs. [source] Plant and microbial N acquisition under elevated atmospheric CO2 in two mesocosm experiments with annual grassesGLOBAL CHANGE BIOLOGY, Issue 2 2005Shuijin Hu Abstract The impact of elevated CO2 on terrestrial ecosystem C balance, both in sign or magnitude, is not clear because the resulting alterations in C input, plant nutrient demand and water use efficiency often have contrasting impacts on microbial decomposition processes. One major source of uncertainty stems from the impact of elevated CO2 on N availability to plants and microbes. We examined the effects of atmospheric CO2 enrichment (ambient+370 ,mol mol,1) on plant and microbial N acquisition in two different mesocosm experiments, using model plant species of annual grasses of Avena barbata and A. fatua, respectively. The A. barbata experiment was conducted in a N-poor sandy loam and the A. fatua experiment was on a N-rich clayey loam. Plant,microbial N partitioning was examined through determining the distribution of a 15N tracer. In the A. barbata experiment, 15N tracer was introduced to a field labeling experiment in the previous year so that 15N predominantly existed in nonextractable soil pools. In the A. fatua experiment, 15N was introduced in a mineral solution [(15NH4)2SO4 solution] during the growing season of A. fatua. Results of both N budget and 15N tracer analyses indicated that elevated CO2 increased plant N acquisition from the soil. In the A. barbata experiment, elevated CO2 increased plant biomass N by ca. 10% but there was no corresponding decrease in soil extractable N, suggesting that plants might have obtained N from the nonextractable organic N pool because of enhanced microbial activity. In the A. fatua experiment, however, the CO2 -led increase in plant biomass N was statistically equal to the reduction in soil extractable N. Although atmospheric CO2 enrichment enhanced microbial biomass C under A. barbata or microbial activity (respiration) under A. fatua, it had no significant effect on microbial biomass N in either experiment. Elevated CO2 increased the colonization of A. fatua roots by arbuscular mycorrhizal fungi, which coincided with the enhancement of plant competitiveness for soluble soil N. Together, these results suggest that elevated CO2 may tighten N cycling through facilitating plant N acquisition. However, it is unknown to what degree results from these short-term microcosm experiments can be extrapolated to field conditions. Long-term studies in less-disturbed soils are needed to determine whether CO2 -enhancement of plant N acquisition can significantly relieve N limitation over plant growth in an elevated CO2 environment. [source] Ten years of free-air CO2 enrichment altered the mobilization of N from soil in Lolium perenne L. swardsGLOBAL CHANGE BIOLOGY, Issue 8 2004Manuel K. Schneider Abstract Effects of free-air carbon dioxide enrichment (FACE, 60 Pa pCO2) on plant growth as compared with ambient pCO2 (36 Pa) were studied in swards of Lolium perenne L. (perennial ryegrass) at two levels of N fertilization (14 and 56 g m,2 a,1) from 1993 to 2002. The objectives were to determine how plant growth responded to the availability of C and N in the long term and how the supply of N to the plant from the two sources of N in the soil, soil organic matter (SOM) and mineral fertilizer, varied over time. In three field experiments, 15N-labelled fertilizer was used to distinguish the sources of available N. In 1993, harvestable biomass under elevated pCO2 was 7% higher than under ambient pCO2. This relative pCO2 response increased to 32% in 2002 at high N, but remained low at low N. Between 1993 and 2002, the proportions and amounts of N in harvestable biomass derived from SOM (excluding remobilized fertilizer) were, at high N, increasingly higher at elevated pCO2 than at ambient pCO2. Two factorial experiments confirmed that at high N, but not at low N, a higher proportion of N in harvestable biomass was derived from soil (including remobilized fertilizer) following 7 and 9 years of elevated pCO2, when compared with ambient pCO2. It is suggested that N availability in the soil initially limited the pCO2 response of harvestable biomass. At high N, the limitation of plant growth decreased over time as a result of the stimulated mobilization of N from soil, especially from SOM. Consequently, harvestable biomass increasingly responded to elevated pCO2. The underlying mechanisms which contributed to the increased mobilization of N from SOM under elevated pCO2 are discussed. This study demonstrated that there are feedback mechanisms in the soil which are only revealed during long-term field experiments. Such investigations are thus, a prerequisite for understanding the responses of ecosystems to elevated pCO2 and N supply. [source] Strategies to Improve the Use Efficiency of Mineral Fertilizer Nitrogen Applied to Winter WheatJOURNAL OF AGRONOMY AND CROP SCIENCE, Issue 3 2002K. Blankenau Recovery of fertilizer nitrogen (N) applied to winter wheat crops at tillering in spring is lower than that of N applied at later growth stages because of higher losses and immobilization of N. Two strategies to reduce early N losses and N immobilization and to increase N availability for winter wheat, which should result in an improved N use efficiency (= higher N uptake and/or increased yield per unit fertilizer N), were evaluated. First, 16 winter wheat trials (eight sites in each of 1996 and 1997) were conducted to investigate the effects of reduced and increased N application rates at tillering and stem elongation, respectively, on yield and N uptake of grain. In treatment 90-70-60 (90 kg N ha,1 at tillering, 70 kg N ha,1 at stem elongation and 60 kg N ha,1 at ear emergence), the average values for grain yield and grain N removal were up to 3.1 and 5.0 % higher than in treatment 120-40-60, reflecting conventional fertilizer practice. Higher grain N removal for the treatment with reduced N rates at tillering, 90-70-60, was attributed to lower N immobilization (and N losses), which increased fertilizer N availability. Secondly, as microorganisms prefer NH4+ to NO3, for N immobilization, higher net N immobilization would be expected after application of the ammonium-N form. In a pot experiment, net N immobilization was higher and dry matter yields and crop N contents at harvest were lower with ammonium (ammonium sulphate + nitrification inhibitor Dicyandiamide) than with nitrate (calcium nitrate) nutrition. Five field trials were then conducted to compare calcium nitrate (CN) and calcium ammonium nitrate (CAN) nutrition at tillering, followed by two CAN applications for both treatments. At harvest, crop N and grain yield were higher in the CN than in the CAN treatment at each N supply level. In conclusion, fertilizer N use efficiency in winter wheat can be improved if N availability to the crops is increased as a result of reduced N immobilization (and N losses) early in the growth period. N application systems could be modified towards strategies with lower N applications at tillering compensated by higher N dressing applications later. An additional advantage is expected to result from use of nitrate-N fertilizers at tillering. Strategien zur Verbesserung der Effizienz von Düngerstickstoff in Winterweizen Aus früheren Versuchen mit Winterweizen ist bekannt, daß zur Ernte die Wiederfindung von im Frühjahr zur Bestokkung gedüngtem Stickstoff (N) geringer ist, als die von N aus Spätgaben. Die Ursachen liegen in einer höheren mikrobiell-bedingten Netto-N-Immobilisation, aber auch N-Verlusten zwischen Bestockung und Schoßbeginn im Vergleich zu späteren Wachstumstadien begründet. In den vorliegenden Versuchen wurden zwei Strategien getestet, um insbesondere die früh in der Vegetation auftretende Netto-N-Immobilisation zu vermindern. Die dadurch erwartete erhöhte N-Verfügbarkeit sollte zu einer erhöhten N-Effizienz (höherer N-Entzug/Ertrag bezogen auf die N-Düngung) führen. 1996 und 1997 wurden jeweils 8 Feldversuche mit Winterweizen durchgeführt, um den Einfluß einer reduzierten Andüngung bei gleichzeitig erhöhter Schossergabe im Vergleich zur konventionellen N-Düngung zu untersuchen. Tatsächlich wurden in dem Prüfglied 90-70-60 (N-Sollwertdüngung: 90 kg N ha,1, Schossergabe: 70 kg N ha,1, Ährengabe: 60 kg N ha,1) im Mittel bis zu 3.1 % höhere Erträge und 5.0 % höhere N-Abfuhren mit dem Korn im Vergleich zur konventionellen Variante 120-40-60 (N-Sollwertdüngung: 120 kg N ha,1, Schossergabe: 40 kg N ha,1 und Ährengabe: 60 kg N ha,1) erzielt. Die höhere N-Abfuhr kann auf eine erhöhte N-Verfügbarkeit infolge geringerer mikrobieller N-Festlegung zurückgeführt werden. Da die vornehmlich heterotrophen Bodenmikroorganismen bevorzugt NH4+ gegenüber NO3, immobilisieren, kann eine höhere N-Immobilisation bei Ammonium-Düngung erwartet werden. Tatsächlich wurden in einem Gefäßversuch nach Düngung von Ammoniumsulfat (+ Nitrifikationshemmer Dicyandiamid) geringere Trokkenmasseerträge und N-Aufnahmen von Weizenpflanzen erzielt als mit Calciumnitrat. Für die Ammoniumsulfatvariante ergab sich eine höhere Netto-N-Immobilisation. Danach wurde in fünf Feldversuchen mit Winterweizen der Einfluß einer Andüngung mit Nitrat (Calciumnitrat) im Vergleich zur Verwendung des ammoniumhaltigen Kalkammonsalpeters (KAS) auf die N-Aufnahme und den Kornertrag untersucht (beide Varianten erhielten KAS als Spätgaben). In der nitratangedüngten Variante wurden zum Teil signifikant höhere Ertäge und N-Aufnahmen in Korn und Stroh ermittelt. Aus den dargestellten Versuchen kann gefolgert werden, daß die Düngerstickstoff-Effizienz verbessert werden kann, wenn vor allem die N-Immobilisation (und eventuell auch N-Verluste) in frühen Wachstumsstadien zwischen Bestockung und Schoßbeginn verringert und so die N-Verfügbarkeit erhöht wird. Es kann empfohlen werden Winterweizenbestände mit geringeren N-Mengen , als nach N-Sollwert 120 kg N ha,1 vorgesehen , anzudüngen und die Schossergabe entsprechend zu erhöhen. Die Verwendung von nitrathaltigen Düngern bei der Andüngung ist von Vorteil. [source] Fine-scale environmental variation and structure of understorey plant communities in two old-growth pine forestsJOURNAL OF ECOLOGY, Issue 2 2003Lee E. Frelich Summary 1Although it is well established that nitrogen and light play major roles in structuring plant communities across the landscape, it is not as clear how they structure communities within forest stands. Virtually nothing is known about within-stand structure of understorey communities of herbs and small shrubs in near-boreal forests. 2We tested the hypothesis that fine-scale (5,20 m) variability in N and light structure forest-floor plant communities in two old-growth mixed Pinus resinosa and Pinus strobus forests in north-eastern Minnesota, USA. 3In each forest, all trees > 1.4 m tall were mapped on a 0.75,1.0 ha area. A grid of subplots 5,10 m apart was established (total n = 147), and N mineralization (µg g,1 soil day,1), soil depth (cm), light (% canopy openness), and percentage cover of all herbs and small shrubs were measured on each subplot. 4Cluster analysis showed that the dominant understorey species fall into three groups. Group 1 is unrelated to N and light, and is negatively associated with a midstorey of the small tree Acer rubrum and the most abundant tall shrub Corylus cornuta. Group 2 reaches maximum abundance in places (mostly gaps) with relatively high light, but is unrelated to within-stand variation in N availability. Group 3 consists of a single species, Aster macrophyllus, and reaches maximum abundance in areas with low N availability and low abundance of Corylus, but higher than average abundance of P. strobus and Betula papyrifera overstorey trees. 5N and light have a moderate influence on understorey plant community structure. The plant species do arrange themselves along N and light gradients, but the gradients are likely to be too narrow to allow the degree of differentiation seen at the landscape level. Spatial patterning of the species groups is probably influenced by other factors, including disturbance history, chance and neighbourhood effects such as clonal reproduction. [source] UPTAKE AND RELEASE OF NITROGEN BY THE MACROALGAE GRACILARIA VERMICULOPHYLLA (RHODOPHYTA),JOURNAL OF PHYCOLOGY, Issue 3 2006Anna Christina Tyler Macroalgae, often the dominant primary producers in shallow estuaries, can be important regulators of nitrogen (N) cycling. Like phytoplankton, actively growing macroalgae release N to the water column; yet little is known about the quantity or nature of this release. Using 15N labeling in laboratory and field experiments, we estimated the quantity of N released relative to assimilation and gross uptake by Gracilaria vermiculophylla (Ohmi) Papenfuss (Rhodophyta, Gracilariales), a non-native macroalgae. Field experiments were carried out in Hog Island Bay, a shallow back-barrier lagoon on the Virginia coast where G. vermiculophylla makes up 85%,90% of the biomass. There was good agreement between laboratory and field measurements of N uptake and release. Daily N assimilation in field experiments (32.3±7.2 ,mol N·g dw,1·d,1) was correlated with seasonal and local N availability. The average rate of N release across all sites and dates (65.8±11.6 ,mol N·g dw,1·d,1) was 67% of gross daily uptake, and also varied among sites and seasons (range=33%,99%). Release was highest when growth rates and nutrient availability were low, possibly due to senescence during these periods. During summer biomass peaks, estimated N release from macroalgal mats was as high as 17 mmol N·m,2·d,1. Our results suggest that most estimates of macroalgal N uptake severely underestimate gross N uptake and that N is taken up, transformed, and released to the water column on short time scales (minutes,hours). [source] Understorey plant and soil responses to disturbance and increased nitrogen in boreal forestsJOURNAL OF VEGETATION SCIENCE, Issue 2 2009O.H. Manninen Abstract Question: How do N fertilization and disturbance affect the understorey vegetation, microbial properties and soil nutrient concentration in boreal forests? Location: Kuusamo (66°22,N; 29°18,E) and Oulu (65°02,N; 25°47,E) in northern Finland. Methods: We conducted a fully factorial experiment with three factors: site (two levels), N fertilization (four levels) and disturbance (two levels). We measured treatment effects on understorey biomass, vegetation structure, and plant, soil and microbial N and C concentrations. Results: The understorey biomass was not affected by fertilization either in the control or in the disturbance treatment. Fertilization reduced the biomass of deciduous Vaccinium myrtillus. Disturbance had a negative effect on the biomass of V. myrtillus and evergreen Vaccinium vitis-idaea and decreased the relative proportion of evergreen species. Fertilization and disturbance increased the biomass of grass Deschampsia flexuosa and the relative proportion of graminoids. The amount of NH4+ increased in soil after fertilization, and microbial C decreased after disturbance. Conclusions: Our results suggest that the growth of slow-growing Vaccinium species and soil microbes in boreal forests are not limited by N availability. However, significant changes in the proportion of dwarf shrubs to graminoids and a decrease in the biomass of V. myrtillus demonstrate the susceptibility of understorey vegetation to N enrichment. N enrichment and disturbance seem to have similar effects on understorey vegetation. Consequently, increasing N does not affect the rate or the direction of recovery after disturbance. Moreover, our study demonstrates the importance of understorey vegetation as a C source for soil microbes in boreal forests. [source] Consequences of shrub expansion in mesic grassland: Resource alterations and graminoid responsesJOURNAL OF VEGETATION SCIENCE, Issue 4 2003Michelle S. Lett Anon. (1986) Abstract. In the mesic grasslands of the central United States, the shrub Cornus drummondii has undergone widespread expansion in the absence of recurrent fire. We quantified alterations in light, water and N caused by C. drummondii expansion in tall-grass prairie and assessed the hypothesis that these alterations are consistent with models of resource enrichment by woody plants. Responses in graminoid species, particularly the dominant C4 grass Andropogon gerardii, were concurrently evaluated. We also removed established shrub islands to quantify their legacy effect on resource availability and assess the capability of this grassland to recover in sites formerly dominated by woody plants. The primary effect of shrub expansion on resource availability was an 87% reduction in light available to the herbaceous understorey. This reduced C uptake and N use efficiency in A. gerardii and lowered graminoid cover and ANPP at the grass-shrub ecotone relative to undisturbed grassland. Shrub removal created a pulse in light and N availability, eliciting high C gain in A. gerardii in the first year after removal. By year two, light and N availability within shrub removal areas returned to levels typical of grassland, as had graminoid cover and ANPP were similar to those in open grassland. Recovery within central areas of shrub removal sites lagged behind that at the former grass-shrub ecotone. These results indicate that the apparent alternative stable state of C. drummondii dominance in tall-grass prairie is biotically maintained and driven by reductions in light, rather than resource enrichment. Within areas of shrub removal, the legacy effect of C. drummondii dominance is manifest primarily through the loss of rhizomes of the dominant grasses, rather than any long-term changes in resource availability. C. drummondii removal facilitates grassland recovery, but the effort required to initiate this transition is a significant cost of woody plant expansion in mesic grasslands. Prevention of woody plant expansion in remnant tall-grass prairies is, therefore, a preferred management option. [source] Changes in soil nitrogen availability due to stand development and management practices on semi-arid sandy lands, in northern ChinaLAND DEGRADATION AND DEVELOPMENT, Issue 5 2009F. S. Chen Abstract Soil nitrogen (N) availability is one of the limiting factors for plant growth on sandy lands. Little is known about impacts of afforestation on soil N availability and its components in southeastern Keerqin sandy lands, China. In this study, we measured N transformation under sandy Mongolian pine (Pinus sylvestris var. mongolica Litv.) plantations of different ages (grassland, young, middle-aged, close-to-mature) and management practices (non-grazing and free-grazing) during the growing seasons using the ion exchange resin bag method. Results showed that, for all plots and growing season, soil NH -N, NO -N, mineral N, and relative nitrification index, varied from 0·18 to 1·54, 0·96 to 22·05, 1·23 to 23·58,µg,d,1,g,1 dry resin, and 0·76 to 0·97, respectively, and NO -N dominated the available N amount due to intense nitrification in these ecosystems. In general, the four indices significantly increased in the oldest plantation, with corresponding values in non-grazing sites lower than those in free-grazing sites (p,<,0·05). Our studies indicated that it is a slow, extended process to achieve improvement in soil quality after afforestation of Mongolian pine in the study area. Copyright © 2009 John Wiley & Sons, Ltd. [source] Belowground carbon allocation by trees drives seasonal patterns of extracellular enzyme activities by altering microbial community composition in a beech forest soilNEW PHYTOLOGIST, Issue 3 2010Christina Kaiser Summary ,Plant seasonal cycles alter carbon (C) and nitrogen (N) availability for soil microbes, which may affect microbial community composition and thus feed back on microbial decomposition of soil organic material and plant N availability. The temporal dynamics of these plant,soil interactions are, however, unclear. ,Here, we experimentally manipulated the C and N availability in a beech forest through N fertilization or tree girdling and conducted a detailed analysis of the seasonal pattern of microbial community composition and decomposition processes over 2 yr. ,We found a strong relationship between microbial community composition and enzyme activities over the seasonal course. Phenoloxidase and peroxidase activities were highest during late summer, whereas cellulase and protease peaked in late autumn. Girdling, and thus loss of mycorrhiza, resulted in an increase in soil organic matter-degrading enzymes and a decrease in cellulase and protease activity. ,Temporal changes in enzyme activities suggest a switch of the main substrate for decomposition between summer (soil organic matter) and autumn (plant litter). Our results indicate that ectomycorrhizal fungi are possibly involved in autumn cellulase and protease activity. Our study shows that, through belowground C allocation, trees significantly alter soil microbial communities, which may affect seasonal patterns of decomposition processes. [source] Nutrient limitation and morphological plasticity of the carnivorous pitcher plant Sarracenia purpurea in contrasting wetland environmentsNEW PHYTOLOGIST, Issue 3 2008Terry Bott Summary ,,Plasticity of leaf nutrient content and morphology, and macronutrient limitation were examined in the northern pitcher plant, Sarracenia purpurea subsp. purpurea, in relation to soil nutrient availability in an open, neutral pH fen and a shady, acidic ombrotrophic bog, over 2 yr following reciprocal transplantation of S. purpurea between the wetlands. ,,In both wetlands, plants were limited by nitrogen (N) but not phosphorus (P) (N content < 2% DW,1, N : P < 14) but photosynthetic quantum yields were high (FV/FM > 0.79). Despite carnivory, leaf N content correlated with dissolved N availability to plant roots (leaf N vs , r2 = 0.344, P < 0.0001); carnivorous N acquisition did not apparently overcome N limitation. ,,Following transplantation, N content and leaf morphological traits changed in new leaves to become more similar to plants in the new environment, reflecting wetland nutrient availability. Changes in leaf morphology were faster when plants were transplanted from fen to bog than from bog to fen, possibly reflecting a more stressful environment in the bog. ,,Morphological plasticity observed in response to changes in nutrient supply to the roots in natural habitats complements previous observations of morphological changes with experimental nutrient addition to pitchers. [source] Cross-site comparison of herbivore impact on nitrogen availability in grasslands: the role of plant nitrogen concentrationOIKOS, Issue 11 2009E. S. Bakker Herbivores may influence the nitrogen (N) recycling rates and consequently increase or decrease the productivity of grasslands. Plant N concentration emerged as a critical parameter to explain herbivore effects from several conceptual models, which predict that herbivores decrease soil N availability when plant N concentration is low whereas they increase it when plant N concentration is high (Hobbs 1996, Ritchie et al. 1998, Pastor et al. 2006). However, a broader cross-site comparison among published studies to test these predictions is hampered by the different methodologies used to measure soil N availability or a proxy thereof, and a lack of measurements of plant N concentration. Therefore it remains unclear whether these model predictions are generally valid across a range of grasslands. We tested whether there is a relationship between plant N concentration and herbivore impact on soil N availability (measured with resin bags) with a study of replicate 6,8,year old exclosures (with an unfenced control) of vertebrate herbivores (>1,kg) established at each of seven grassland sites in North America and Europe. Contrary to model predictions, we found a negative relationship between the effect of herbivores on resin bag soil N availability and plant N concentration. Our study confirms the importance of plant N concentration as a predictor of herbivore effect on soil N availability across grasslands, but contradicts the models. A possible explanation may be that the results represent a transient situation as the exclosures were relatively young whereas the models may refer to an equilibrium state. Simultaneous measurements of both plant N concentration and herbivore effect on soil N availability from more grassland sites, preferably with contrasting plant N concentrations and including exclosures of different ages, should resolve the contrast between model predictions and our field measurements. [source] pH and carbon supply control the expression of phosphoenolpyruvate carboxylase kinase genes in Arabidopsis thalianaPLANT CELL & ENVIRONMENT, Issue 12 2008ZHI-HUI CHEN ABSTRACT Phosphoenolpyruvate carboxylase (PEPC) is thought to play many roles in C3 plants including the provision of biosynthetic precursors and control of pH during N assimilation. Its activity is controlled via phosphorylation catalysed by PEPC kinases, which are encoded by PPCK genes. We examined PPCK expression in response to changes in the supply of N or C, and to changes in intracellular pH, using cultured Arabidopsis cells and seedlings. The results show that expression of both PPCK1 and PPCK2 is increased by C availability, but does not respond to N availability. Expression of the two PPCK genes and the phosphorylation state of PEPC are increased in response to increasing intracellular pH. Elevated pH also reduces the repression of PPCK gene expression by Pi. Expression of phosphoenolpyruvate carboxykinase (PEPCK), which catalyses the decarboxylation of oxaloacetate, is decreased in response to increasing intracellular pH. pH homeostasis may be mediated at least partly by reciprocal changes in the expression of PPCK genes and PEPCK. [source] Gene expression associated with N-induced shifts in resource allocation in poplarPLANT CELL & ENVIRONMENT, Issue 5 2003J. E. K. COOKE ABSTRACT Surprisingly little is known about molecular mechanisms by which nitrogen (N) availability acts to modulate the growth of forest trees. To address this issue, differential display was used in conjunction with filter-based arrays to identify 52 partial cDNA clones that were significantly regulated within days in response to limiting or luxuriant levels of NH4NO3 fertilization in Populus trichocarpa Torr. & Gray × deltoides Bartr. ex Marsh. A subset of these cDNAs also demonstrated shifts in expression patterns in stem-girdled trees, a manipulative physiology technique that disrupts phloem transport. Stem girdling also induced changes in glutamine and asparagine pools which were correlated with the observed changes in expression profiles for these genes. The identity of these genes provides insight into biochemical processes that are altered by N availability in poplar. Carbon,nitrogen interactions appear to figure prominently in the N-response. The gene expression data suggest that N availability modulates the partitioning of C and N resources into metabolic fates that have the potential to alter both wood quality and quantity, including synthesis of vegetative storage proteins, cell wall components, and terpenoids. [source] ,15N of soil N and plants in a N-saturated, subtropical forest of southern ChinaRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 17 2010K. Koba We investigated the ,15N profile of N (extractable NH, NO, and organic N (EON)) in the soil of a N-saturated subtropical forest. The order of ,15N in the soil was EON,>,NH,>,NO. Although the ,15N of EON had been expected to be similar to that of bulk soil N, it was higher than that of bulk soil N by 5,. The difference in ,15N between bulk soil N and EON (,15Nbulk-EON) was correlated significantly with the soil C/N ratio. This correlation implies that carbon availability, which determines the balance between N assimilation and dissimilation of soil microbes, is responsible for the high ,15N of EON, as in the case of soil microbial biomass ,15N. A thorough ,15N survey of available N (NH, NO, and EON) in the soil profiles from the organic layer to 100,cm depth revealed that the ,15N of the available N forms did not fully overlap with the ,15N of plants. This mismatch in ,15N between that of available N and that of plants reflects apparent isotopic fractionation during N uptake by plants, emphasizing the high N availability in this N-saturated forest. Copyright © 2010 John Wiley & Sons, Ltd. [source] Grazing and landscape controls on nitrogen availability across 330 South African savanna sitesAUSTRAL ECOLOGY, Issue 7 2009JOSEPH M. CRAINE Abstract The availability of nitrogen (N) is an important determinant of ecosystem and community dynamics for grasslands and savannas, influencing factors such as biomass productivity, plant and herbivore composition, and losses of N to waters and the atmosphere. To better understand the controls over N availability at landscape to regional scales, we quantified a range of plant and soil characteristics at each of 330 sites in three regions of South Africa: Kruger National Park (KNP), private game reserves adjacent to KNP (private protected areas , PPAs) and Hluhluwe-iMfolozi Park (HiP). In comparing regions and sites within regions, grazing appeared to have a strong influence on N availability. Sites in the PPAs adjacent to KNP as well as sodic and alluvial sites in general typically had the highest N availability. The high N availability of these sites was not generally associated with greater potential N mineralization, but instead with less grass biomass and more forb biomass that indicated greater grazing pressure. Whereas sodic sites had a long history of high N availability as evidenced by their high soil ,15N, the greater N availability in the PPAs over the two parks appeared to be relatively recent. Grazer biomass, average potential mineralization rates and grass biomass for HiP were greater than KNP, yet there were no differences in N availability as indexed by soil and foliar ,15N between sites in the two parks. Although the short-term increase in N availability in PPAs is not necessarily deleterious, it is uncertain whether current productivity levels in those ecosystems is sustainable. With differences in management causing herbivore biomass to be 150% greater in the PPAs than the adjacent KNP, changes in plant communities and nitrogen cycling might lead to long-term degradation of these ecosystems, their ability to sustain herbivore populations, and also serve as an economic resource for the region. [source] | ||||||||||||||||||||||