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排序方式: 共有107条查询结果,搜索用时 15 毫秒
41.
Previous research has suggested thatnitrogen-fixing lichens can play an importantrole in the nitrogen cycle of early primarysuccessional systems and other extremeenvironments. In this study, we estimaterates of nitrogen fixation by anitrogen-fixing lichen, Stereocaulonvulcani, at 1500 m on the northeast slope ofMauna Loa volcano. Using microclimatemeasurements and a climate-driven model ofnitrogen fixation, we estimate that S.vulcani fixes between 0.2 and 0.45 kg N ha–1yr–1. We calculate that S. vulcanicould have derived 40% of its nitrogencontent from biological fixation. 相似文献
42.
Introduced grass species have invaded extensive areas of Hawaii Volcanoes National Park and increased the size and frequency of fire. Following fire, grass cover is enhanced while native shrub cover is reduced; the reduction in most shrubs persists for at least 20 years even in the absence of fire. Shrub seedlings were planted in burned and unburned plots with and without grass cover. Biomass of 14 month old shrub seedlings was generally highest in recently burned/grass removed plots, intermediate in old burn/grass removed plots, and lowest in unburned/grass removed plots. In contrast, shrub biomass in plots with grass cover was low and did not differ significantly among burn treatments. Light competition is likely to be responsible for differences in shrub growth rates; grass cover reduced light to 1–10% of background levels. In addition, pool sizes of available soil N were highest in recently burned, intermediate in old burn, and lowest in unburned areas. 相似文献
43.
Georgina García-Méndez J. Manuel Maass Pamela A. Matson Peter M. Vitousek 《Oecologia》1991,88(3):362-366
Summary Emissions of nitrous oxide and soil nitrogen pools and transformations were measured over an annual cycle in two forests and one pasture in tropical deciduous forest near Chamela, México. Nitrous oxide flux was moderately high (0.5–2.5 ng cm–2 h–1) during the wet season and low (<0.3 ng cm–2 h–1) during the dry season. Annual emissions of nitrogen as nitrous oxide were calculated to be 0.5–0.7 kg ha–1 y–1, with no substantial difference between the forests and pasture. Wetting of dry soil caused a large but short-lived pulse of N2O flux that accounted for <2% of annual flux. Variation in soil water through the season was the primary controlling factor for pool sizes of ammonium and nitrate, nitrogen transformations, and N2O flux. 相似文献
44.
Metrosideros polymorpha, a dominant tree species in Hawaiian ecosystems, occupies a wide range of habitats. Complementary field and common-garden
studies of M. polymorpha populations were conducted across an altitudinal gradient at two different substrate ages to ascertain if the large phenotypic
variation of this species is determined by genetic differences or by phenotypic modifications resulting from environmental
conditions. Several characteristics, including ecophysiological behavior and anatomical features, were largely induced by
the environment. However, other characteristics, particularly leaf morphology, appeared to be mainly determined by genetic
background. Common garden plants exhibited higher average rates of net assimilation (5.8 μmol CO2 m−2 s−1) and higher average stomatal conductance (0.18 mol H2O m−2 s−1) than their field counterparts (3.0 μmol CO2 m−2 s−1, and 0.13 mol H2O m−2 s−1 respectively). Foliar δ13C of most common-garden plants was similar among sites of origin with an average value of −26.9‰. In contrast, mean values
of foliar δ13C in field plants increased substantially from −29.5‰ at low elevation to −24.8‰ at high elevation. Leaf mass per unit area
increased significantly as a function of elevation in both field and common garden plants; however, the range of values was
much narrower in common garden plants (211–308 g m−2 for common garden versus 107–407 g m−2 for field plants). Nitrogen content measured on a leaf area basis in common garden plants ranged from 1.4 g m−2 to 2.4 g m−2 and from 0.8 g m−2 to 2.5 g m−2 in field plants. Photosynthetic nitrogen use efficiency (PNUE) decreased 50% with increasing elevation in field plants and
only 20% in plants from young substrates in the common garden. This was a result of higher rates of net CO2 assimilation in the common garden plants. Leaf tissue and cell layer thickness, and degree of leaf pubescence increased significantly
with elevation in field plants, whereas in common garden plants, variation with elevation of origin was much narrower, or
was entirely absent. Morphological characteristics such as leaf size, petiole length, and internode length decreased with
increasing elevation in the field and were retained when grown in the common garden, suggesting a potential genetic basis
for these traits. The combination of environmentally induced variability in physiological and anatomical characteristics and
genetically determined variation in morphological traits allows Hawaiian M. polymorpha to attain and dominate an extremely wide ecological distribution not observed in other tree species.
Received: 12 March 1997 / Accepted: 27 August 1997 相似文献
45.
Daniela F. Cusack Oliver A. Chadwick Thegn Ladefoged Peter M. Vitousek 《Biogeochemistry》2013,112(1-3):229-243
Intensive agriculture has the potential to reduce soil carbon stocks in the years following initial cultivation, although the magnitude and direction of the effect can vary with ecosystem and management factors. Agriculture can also shift the carbon chemistry of soils via changes in crop plant chemistry, decomposition, and/or soil amendments [e.g. black carbon (i.e. charcoal)]. It is possible that soil carbon levels can recover if intensive cultivation ends, but the factors driving the extent and quality of this recovery are not well understood. Here, we examined soil carbon pool sizes and carbon chemistry >200 years after intensive cultivation by early Hawaiians. We compared soils from an extensive pre-European-contact agricultural field system with reference sites under similar modern management. Sites were selected along a climate and soil weathering gradient to investigate interactions between historic land use and ecosystem properties, such as soil mineralogy, in driving soil carbon recovery. Soil carbon content was measured from 0 to 30 cm depth, and carbon chemistry was assessed using 13C nuclear magnetic resonance spectroscopy. Overall, we found significantly lower soil carbon stocks in pre-contact agricultural sites compared to reference sites. Radiocarbon dating of bulk soil carbon showed a trend toward older carbon in agricultural versus reference soils, suggesting decreased retention of newer C in agricultural sites. Radiocarbon dating of macroscopic charcoal particles from under agricultural field walls indicated that there were black carbon inputs concurrent with pre-contact agricultural activity. Nonetheless, black carbon and carbonyl carbon levels were lower in agricultural versus reference soils, suggesting decreased retention of specific carbon groups in cultivated sites. Proteins were the only biomolecule higher in abundance in agricultural versus reference sites. Finally, there was an interacting effect of soil mineralogy and historic land use on soil carbon stocks. Whereas short range order (SRO) minerals were positively associated with total soil carbon overall, differences in soil carbon between agricultural and reference soils were largest in soils with high concentrations of SRO minerals. Our results indicate that the negative effect of agriculture on soil carbon stocks can be long-lived, may be associated with persistent changes in soil carbon chemistry, and can vary with soil mineralogical properties. 相似文献
46.
Polyphenols in litter from tropical montane forests across a wide range in soil fertility 总被引:9,自引:1,他引:8
In nutrient-poor ecosystems high polyphenol concentrations in plant litter have been proposed to influence soil nutrient availability in benefit of the plants. We addressed the question whether litter polyphenol concentrations vary across a soil chronosequence of almost identical geology, climate and plant species composition, but of a wide range in nitrogen (N) and phosphorus (P) availability in the Hawaiian Islands. Concentrations of total phenolics (TPh) and proanthocyanidins (PA) in leaf litter of the dominant tree species Metrosideros polymorpha were higher at the oldest, P-limited site compared to the youngest, N-limited site, with intermediate values at the two relatively fertile sites co-limited by N and P. Polyphenol concentrations in fine root litter differed considerably from those observed in leaf litter and varied differently across the soil age gradient. Long-term fertilization did not significantly alter polyphenol concentrations in Metrosideros litter at either site. Moreover, green leaves and leaf litter of Metrosideros showed similar relative differences among sites when compared between natural populations and plants from the same populations but grown in a common garden. These results suggest that polyphenol concentrations inherently vary among populations of the dominant tree species in Hawaiian montane forests possibly indicating an adaptation to ecosystem properties such as substrate age related differences in soil fertility. The combined above- and below-ground input rate of TPh ranged from 62.4 to 170.8 g/m2/yr and was significantly higher at the P-limited than at the N-limited site. Root-derived polyphenols contributed a much higher absolute and relative amount of phenolic input at the N-limited than at the P-limited site. The differences in amount, quality, and pathways of input might suggest specific interactions with soil processes and nutrient cycling among the Hawaiian rainforests studied here. 相似文献
47.
48.
Resource Optimization and Symbiotic Nitrogen Fixation 总被引:6,自引:1,他引:5
E. B. Rastetter P. M. Vitousek C. Field G. R. Shaver D. Herbert G. I. gren 《Ecosystems》2001,4(4):369-388
In temperate forests, symbiotic nitrogen (N) fixation is restricted to the early phases of succession despite the persistence
of N limitation on production late in succession. This paradox has yet to be explained adequately. We hypothesized that the
restriction of N fixation to early stages of succession results from the optimization of resource allocation in the vegetation.
Because of this optimization, N fixation should be restricted to periods when fixation is less costly than N uptake. Our analysis
differs from others in the way we calculate the cost of N uptake; we assess the cost of N uptake as the amount of carbon (C)
that could be assimilated if the resources necessary to acquire one gram of N from the soil were allocated instead to photosynthesis.
We then simulate N fixation as an asymptotic function of the difference in cost between N uptake and N fixation and proportional
to the abundance of host tissues for the N-fixing symbionts. The factors that contribute to conditions that favor N fixation
are (a) elevated-carbon dioxide (CO2) concentrations, (b) an open canopy, (c) low available N in the soil, and (d) a soil volume already well exploited by roots.
Our results indicate that changes in the relative cost of uptake vs fixation can explain most of the pattern in fixation through
both primary and secondary succession, but that competitive interactions with nonfixing species play a role in the final exclusion
of fixation in later stages of succession.
Received 26 September 2000; accepted 31 January 2001. 相似文献
49.
In many tropical and volcanic soils, phosphorus (P) availability is strongly influenced by geochemical sorption, which binds P to soil minerals. The aim of this study was to determine whether biological demand or soil sorption strength was the primary control over phosphate availability and retention in a wet tropical soil with high sorption capacity and low P availability. We added 32PO4 to soil from the upper two horizons and assessed the ability of soil microbes to immobilize the added phosphate in the presence of strong sorption. We added phosphate at two concentrations, one representing background turnover that adds low concentrations of P to the soil solution, and the other representing nutrient pulses that can add fairly high fluxes of P to the soil solution. Sorption and microbial immobilization were rapid for both concentrations, consuming most added P within 30 min. Thus, little P remained in the soil solution or extractable pools, which are considered more available to plants. Although soil sorption strength was almost identical for the two horizons, immobilization of tracer P was approximately three times greater in the upper horizon, where most microbial activity was located. This result suggests that microbial demand controlled how P was partitioned into biological versus geochemical sinks. Further evidence for microbial control is suggested by the movement of tracer P from the sorbed pool into the microbial pool when demand was stimulated by the addition of carbon (C). We also explored how increased nitrogen (N) and P availability changed P dynamics in this nutrient poor soil. In contrast to the unfertilized soil, long-term N and P fertilization substantially reduced biological control over inorganic P. P fertilization saturated the soils, overwhelming biological P demand, whereas N fertilization appeared to increase available P through reduced P sorption. Where biological demand for P is high and P becomes available in the soil solution, microbes may play an important role in controlling P partitioning into biological versus geochemical sinks even in soils that have high sorption capacity. 相似文献
50.
Production and Resource Use Efficiencies in N- and P-Limited Tropical Forests: A Comparison of Responses to Long-term Fertilization 总被引:5,自引:1,他引:4
At two sites at the extreme ends of a soil development chronosequence in Hawaii, we investigated whether forest responses
to fertilization on young soils were similar to those on highly weathered soils and whether the initial responses were maintained
after 6–11 years of fertilization. Aboveground net primary production (ANPP) was increased by nitrogen (N) application at
the 300-year-old site and phosphorus (P) application at the 4.1-million-year-old site, thus confirming earlier results and
their designations as N- and P-limited forests. Along with ANPP, application of the limiting element consistently increased
leaf area index (LAI), radiation conversion efficiency (RCE), and foliar and litter nutrient concentrations. Fertilization
did not consistently alter N or P retranslocation from senescent leaves at either site, but a comparison with other sites
on the chronosequence and with a common-garden study suggests that there is a genetic basis for low foliar and litter nutrients
and higher retranslocation at infertile sites vs more fertile sites. N limitation appears to be expressed as limitation to
carbon gain, with long leaf lifespans and high leaf mass per area. P limitation results in high P-use efficiency and disproportionally
large increases in P uptake after fertilization; a comparison with other studies indicates large investments in acquiring
and storing P. Although the general responses of ANPP, LAI, and RCE were similar for the two sites, other aspects of nutrient
use differ in relation to the physiological and biogeochemical roles of the two elements.
Received 2 June 2000; Accepted 4 April 2001. 相似文献