全文获取类型
收费全文 | 8382篇 |
免费 | 1468篇 |
国内免费 | 2321篇 |
出版年
2024年 | 76篇 |
2023年 | 339篇 |
2022年 | 251篇 |
2021年 | 243篇 |
2020年 | 479篇 |
2019年 | 506篇 |
2018年 | 571篇 |
2017年 | 536篇 |
2016年 | 528篇 |
2015年 | 506篇 |
2014年 | 527篇 |
2013年 | 632篇 |
2012年 | 453篇 |
2011年 | 510篇 |
2010年 | 382篇 |
2009年 | 466篇 |
2008年 | 457篇 |
2007年 | 467篇 |
2006年 | 452篇 |
2005年 | 379篇 |
2004年 | 334篇 |
2003年 | 322篇 |
2002年 | 323篇 |
2001年 | 273篇 |
2000年 | 249篇 |
1999年 | 211篇 |
1998年 | 199篇 |
1997年 | 166篇 |
1996年 | 161篇 |
1995年 | 137篇 |
1994年 | 138篇 |
1993年 | 106篇 |
1992年 | 121篇 |
1991年 | 73篇 |
1990年 | 75篇 |
1989年 | 70篇 |
1988年 | 61篇 |
1987年 | 43篇 |
1986年 | 48篇 |
1985年 | 53篇 |
1984年 | 39篇 |
1983年 | 19篇 |
1982年 | 46篇 |
1981年 | 31篇 |
1980年 | 24篇 |
1979年 | 24篇 |
1978年 | 19篇 |
1977年 | 8篇 |
1976年 | 16篇 |
1958年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
Urban ecosystems and the North American carbon cycle 总被引:12,自引:0,他引:12
D. E. PATAKI R. J. ALIG† A. S. FUNG‡ N. E. GOLUBIEWSKI§ C. A. KENNEDY¶ E. G. MCPHERSON D. J. NOWAK R. V. POUYAT†† P.ROMERO LANKAO§§ 《Global Change Biology》2006,12(11):2092-2102
Approximately 75–80% of the population of North America currently lives in urban areas as defined by national census bureaus, and urbanization is continuing to increase. Future trajectories of fossil fuel emissions are associated with a high degree of uncertainty; however, if the activities of urban residents and the rate of urban land conversion can be captured in urban systems models, plausible emissions scenarios from major cities may be generated. Integrated land use and transportation models that simulate energy use and traffic‐related emissions are already in place in many North American cities. To these can be added a growing dataset of carbon gains and losses in vegetation and soils following urbanization, and a number of methods of validating urban carbon balance modeling, including top down atmospheric monitoring and urban ‘metabolic’ studies of whole ecosystem mass and energy flow. Here, we review the state of our understanding of urban areas as whole ecosystems with regard to carbon balance, including both drivers of fossil fuel emissions and carbon cycling in urban plants and soils. Interdisciplinary, whole‐ecosystem studies of the socioeconomic and biophysical factors that influence urban carbon cycles in a range of cities may greatly contribute to improving scenarios of future carbon balance at both continental and global scales. 相似文献
952.
Effects of grazing intensity on soil carbon stocks following deforestation of a Hawaiian dry tropical forest 总被引:2,自引:0,他引:2
The effects of forest-to-pasture conversion on soil carbon (C) stocks depend on a combination of climatic and management factors, but factors that relate to grazing intensity are perhaps the least understood. To understand the long-term impact of grazing in converted pastures, methods are needed that accurately measure the impact of grazing on recent plant inputs to soil C in a variety of pasture management and climate settings. Here, we present an analysis from Hawai'i of changes in vegetation structure and soil organic carbon (SOC) along gradients of grazing intensity and elevation in pastures converted from dry tropical forest 100 years ago. We used hyperspectral remote sensing of photosynthetic vegetation, nonphotosynthetic vegetation (NPV) and exposed substrate to understand the effects of grazing on plant litter cover, thus, estimating recent plant inputs to soils (the NPV component). Forest-to-pasture conversion caused a shift from C3 to C4 plant physiology, thus the δ 13 C method was used in soil cores to measure the fraction of SOC accumulated from pasture vegetation sources following land conversion. SOC decreased in pasture by 5–9 kg C m−2 , depending upon grazing intensity. SOC derived from C3 (forest) sources was constant across the grazing gradient, indicating that the observed variation in SOC was attributable to changes in C inputs following deforestation. Soil C stocks were also reduced in pastures relative to forest soils. We found that long-term grazing lowers SOC following Hawaiian forest-to-pasture conversion, and that these changes are larger in magnitude that those occurring with elevation (climate). Further we demonstrate a relationship between remotely sensed measurements of surface litter and field SOC measurements, allowing for regional analysis of pasture condition and C storage where limited field data are available. 相似文献
953.
Dynamics of fine root carbon in Amazonian tropical ecosystems and the contribution of roots to soil respiration 总被引:8,自引:0,他引:8
Susan Trumbore Enir Salazar Da Costa Daniel C. Nepstad† Plínio Barbosa De Camargo‡ Luiz A. Martinelli‡ David Ray† Teresa Restom§ Whendee Silver¶ 《Global Change Biology》2006,12(2):217-229
Radiocarbon (14C) provides a measure of the mean age of carbon (C) in roots, or the time elapsed since the C making up root tissues was fixed from the atmosphere. Radiocarbon signatures of live and dead fine (<2 mm diameter) roots in two mature Amazon tropical forests are consistent with average ages of 4–11 years (ranging from <1 to >40 years). Measurements of 14C in the structural tissues of roots known to have grown during 2002 demonstrate that new roots are constructed from recent (<2‐year‐old) photosynthetic products. High Δ14C values in live roots most likely indicate the mean lifetime of the root rather than the isotopic signature of inherited C or C taken up from the soil. Estimates of the mean residence time of C in forest fine roots (inventory divided by loss rate) are substantially shorter (1–3 years) than the age of standing fine root C stocks obtained from radiocarbon (4–11 years). By assuming positively skewed distributions for root ages, we can effectively decouple the mean age of C in live fine roots (measured using 14C) from the rate of C flow through the live root pool, and resolve these apparently disparate estimates of root C dynamics. Explaining the 14C values in soil pore space CO2, in addition, requires that a portion of the decomposing roots be cycled through soil organic matter pools with decadal turnover time. 相似文献
954.
RUSSELL L. SCOTT TRAVIS E. HUXMAN† DAVID G. WILLIAMS‡ DAVID C. GOODRICH 《Global Change Biology》2006,12(2):311-324
Across many dryland regions, historically grass‐dominated ecosystems have been encroached upon by woody‐plant species. In this paper, we compare ecosystem water and carbon dioxide (CO2) fluxes over a grassland, a grassland–shrubland mosaic, and a fully developed woodland to evaluate potential consequences of woody‐plant encroachment on important ecosystem processes. All three sites were located in the riparian corridor of a river in the southwest US. As such, plants in these ecosystems may have access to moisture at the capillary fringe of the near‐surface water table. Using fluxes measured by eddy covariance in 2003 we found that ecosystem evapotranspiration (ET) and net ecosystem exchange of carbon dioxide (NEE) increased with increasing woody‐plant dominance. Growing season ET totals were 407, 450, and 639 mm in the grassland, shrubland, and woodland, respectively, and in excess of precipitation by 227, 265, and 473 mm. This excess was derived from groundwater, especially during the extremely dry premonsoon period when this was the only source of moisture available to plants. Access to groundwater by the deep‐rooted woody plants apparently decouples ecosystem ET from gross ecosystem production (GEP) with respect to precipitation. Compared with grasses, the woody plants were better able to use the stable groundwater source and had an increased net CO2 gain during the dry periods. This enhanced plant activity resulted in substantial accumulation of leaf litter on the soil surface that, during rainy periods, may lead to high microbial respiration rates that offset these photosynthetic fluxes. March–December (primary growing season) totals of NEE were ?63, ?212, and ?233 g C m?2 in the grassland, shrubland, and woodland, respectively. Thus, there was a greater disparity between ecosystem water use and the strength of the CO2 sink as woody plants increased across the encroachment gradient. Despite a higher density of woody plants and a greater plant productivity in the woodland than in the shrubland, the woodland produced a larger respiration response to rainfall that largely offset its higher photosynthetic potential. These data suggest that the capacity for woody plants to exploit water resources in riparian areas results in enhanced carbon sequestration at the expense of increased groundwater use under current climate conditions, but the potential does not scale specifically as a function of woody‐plant abundance. These results highlight the important roles of water sources and ecosystem structure on the control of water and carbon balances in dryland areas. 相似文献
955.
JONATHAN A. NEWMAN 《Global Change Biology》2006,12(9):1634-1642
Climate change will alter the abundance and distribution of species. Predicting these shifts is a challenge for ecologists and essential information for the formation of public policy. Here, I use a mechanistic mathematical model of the interaction between grass growth physiology and aphid population dynamics, coupled with the climate change projections from the UK's Hadley Centre HadCM3 global circulation model (GCM) and Canada's Center for Climate Modeling and Analysis CGCM2 GCM to predict the changes in the abundance and distribution of summer cereal aphid populations in wheat-growing regions of Canada. When used with the HadCM3 projections, the model predicts a latitudinal shift northward in abundances but there is longitudinal variation as well. However, when used with the CGCM2 projections the model predicts that continental regions will see a decline while coastal regions will see an increase in summer cereal aphid populations. These effects are stronger under the higher emissions scenarios. 相似文献
956.
Calcium Additions and Microbial Nitrogen Cycle Processes in a Northern Hardwood Forest 总被引:1,自引:0,他引:1
Peter M. Groffman Melany C. Fisk Charles T. Driscoll Gene E. Likens Timothy J. Fahey Christopher Eagar Linda H. Pardo 《Ecosystems》2006,9(8):1289-1305
Evaluating, and possibly ameliorating, the effects of base cation depletion in forest soils caused by acid deposition is an
important topic in the northeastern United States. We added 850 kg Ca ha−1 as wollastonite (CaSiO3) to an 11.8-ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA,
in fall 1999 to replace calcium (Ca) leached from the ecosystem by acid deposition over the past 6 decades. Soil microbial
biomass carbon (C) and nitrogen (N) concentrations, gross and potential net N mineralization and nitrification rates, soil
solution and stream chemistry, soil:atmosphere trace gas (CO2, N2O, CH4) fluxes, and foliar N concentrations have been monitored in the treated watershed and in reference areas at the HBEF before
and since the Ca addition. We expected that rates of microbial C and N cycle processes would increase in response to the treatment.
By 2000, soil pH was increased by a full unit in the Oie soil horizon, and by 2002 it was increased by nearly 0.5 units in
the Oa soil horizon. However, there were declines in the N content of the microbial biomass, potential net and gross N mineralization
rates, and soil inorganic N pools in the Oie horizon of the treated watershed. Stream, soil solution, and foliar concentrations
of N showed no response to treatment. The lack of stimulation of N cycling by Ca addition suggests that microbes may not be
stimulated by increased pH and Ca levels in the naturally acidic soils at the HBEF, or that other factors (for example, phosphorus,
or Ca binding of labile organic matter) may constrain the capacity of microbes to respond to increased pH in the treated watershed.
Possible fates for the approximately 10 kg N ha−1 decline in microbial and soil inorganic pools include components of the plant community that we did not measure (for example,
seedlings, understory shrubs), increased fluxes of N2 and/or N storage in soil organic matter. These results raise questions about the factors regulating microbial biomass and
activity in northern hardwood forests that should be considered in the context of proposals to mitigate the depletion of nutrient
cations in soil. 相似文献
957.
N. Kljun T. A. Black T. J. Griffis A. G. Barr D. Gaumont-Guay K. Morgenstern J. H. McCaughey Z. Nesic 《Ecosystems》2006,9(7):1128-1144
In 2001–03, continuous eddy covariance measurements of carbon dioxide (CO2) flux were made above mature boreal aspen, black spruce, and jack pine forests in Saskatchewan, Canada, prior to and during
a 3−year drought. During the 1st drought year, ecosystem respiration (R) was reduced at the aspen site due to the drying of surface soil layers. Gross ecosystem photosynthesis (GEP) increased as
a result of a warm spring and a slow decrease of deep soil moisture. These conditions resulted in the highest annual net ecosystem
productivity (NEP) in the 9 years of flux measurements at this site. During 2002 and 2003, a reduction of 6% and 34% in NEP,
respectively, compared to 2000 was observed as the result of reductions in both R and GEP, indicating a conservative response to the drought. Although the drought affected most of western Canada, there was
considerable spatial variability in summer rainfall over the 100−km extent of the study area; summer rainfalls in 2001 and
2002 at the two conifer sites minimized the impact of the drought. In 2003, however, precipitation was similarly low at all
three sites. Due to low topographic position and consequent poor drainage at the black spruce site and the coarse soil with
low water-holding capacity at the jack pine site almost no reduction in R, GEP, and NEP was observed at these two sites. This study shows that the impact of drought on carbon sequestration by boreal
forest ecosystems strongly depends on rainfall distribution, soil characteristics, topography, and the presence of vegetation
that is well adapted to these conditions. 相似文献
958.
Reconciling Carbon-cycle Concepts, Terminology, and Methods 总被引:5,自引:1,他引:4
F. S. Chapin III G. M. Woodwell J. T. Randerson E. B. Rastetter G. M. Lovett D. D. Baldocchi D. A. Clark M. E. Harmon D. S. Schimel R. Valentini C. Wirth J. D. Aber J. J. Cole M. L. Goulden J. W. Harden M. Heimann R. W. Howarth P. A. Matson A. D. McGuire J. M. Melillo H. A. Mooney J. C. Neff R. A. Houghton M. L. Pace M. G. Ryan S. W. Running O. E. Sala W. H. Schlesinger E.-D. Schulze 《Ecosystems》2006,9(7):1041-1050
Recent projections of climatic change have focused a great deal of scientific and public attention on patterns of carbon (C)
cycling as well as its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric
carbon dioxide (CO2). Net ecosystem production (NEP), a central concept in C-cycling research, has been used by scientists to represent two different
concepts. We propose that NEP be restricted to just one of its two original definitions—the imbalance between gross primary
production (GPP) and ecosystem respiration (ER). We further propose that a new term—net ecosystem carbon balance (NECB)—be
applied to the net rate of C accumulation in (or loss from [negative sign]) ecosystems. Net ecosystem carbon balance differs
from NEP when C fluxes other than C fixation and respiration occur, or when inorganic C enters or leaves in dissolved form.
These fluxes include the leaching loss or lateral transfer of C from the ecosystem; the emission of volatile organic C, methane,
and carbon monoxide; and the release of soot and CO2 from fire. Carbon fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However,
even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological
advances have led to a diversity of approaches to the measurement of C fluxes at different temporal and spatial scales. These
approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully
specifying the fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components
of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we can provide a less ambiguous framework
for understanding and communicating recent changes in the global C cycle. 相似文献
959.
Our objective was to determine how varied is the response of C cycling to temperature and irradiance in tundra vegetation. We used a large chamber to measure C exchange
at 23 locations within a small arctic catchment in Alaska during summer 2003 and 2004. At each location, we determined light
response curves of C exchange using shade cloths, twice during a growing season. We used data to fit a simple photosynthesis-irradiance,
respiration-temperature model, with four parameters. We used a maximum likelihood technique to determine the acceptable parameter
space for each light curve, given measurement uncertainty. We then explored which sites and time periods had parameter sets
in common—an indication of functional similarity. We found that seven distinct parameter sets were required to explain observed
C flux responses to temperature and light variation at all sites and time periods. The variation in estimated maximum photosynthetic
rate (Pmax) was strongly correlated with measurements of site leaf area index (LAI). The behavior of tussock tundra sites, the dominant
vegetation of arctic tundra, could largely be described with a single parameter set, with a Pmax of 9.7 μmol m−2 s−1. Tussock tundra sites had, correspondingly, similar LAI (mean = 0.66). Non-tussock sites (for example, sedge and shrub tundras)
had larger spatial and temporal variations in both C dynamic parameters (Pmax varying from 9.7–25.7 μmol m−2 s−1) and LAI (0.6–2.0). There were no clear relationships between dominant non-tussock vegetation types and a particular parameter
set. Our results suggest that C dynamics of the acidic tussock tundra slopes and hilltops in northern Alaska are relatively
simply described during the peak growing season. However, the foot-slopes and water tracks have more variable patterns of
LAI and C exchange, not simply related to the dominant vegetation type. 相似文献
960.
Plant allocation patterns may affect soil C and N storage due to differences in litter quality and the depth of plant C and
N inputs into the soil. We studied the dynamics of dual-labeled (13C/15N) Pinus ponderosa needles and fine roots placed at two soil depths (O and A horizon) in a temperate conifer forest soil during 2 y. Input of
C as fine roots resulted in much more C retained in soil (70.5 ± 2.2% of applied) compared with needle C (42.9 ± 1.3% of applied)
after 1.5 y. Needles showed faster mass loss, rates of soil 13CO2 efflux, and more 15N immobilized into microbial biomass than did fine roots. The larger proportion of labile C compounds initially present in
needles (17% more needle C was water soluble than in fine roots) likely contributed to its shorter C residence time and greater
degree of transformation in the soil. A double exponential decay function best described the rate of 13C loss, with a smaller initial pulse of C loss from fine roots (S1k1) and a slower decay rate of the recalcitrant C pool for fine roots (0.03 y−1) compared with (0.19 y−1) for needles. Soil 13C respiration, representing heterotrophic respiration of litter C, was much more seasonal from the O horizon than from the
A. However, offsetting seasonal patterns in 13C dynamics in the O horizon resulted in no net effect of soil depth on total 13C retention in the soil after 1.5 y for either litter. Almost 90% of applied litter N was retained in the soil after 1.5 y,
independent of litter quality or soil depth. Very small amounts of 13C or 15N (<3% of applied) moved to the horizon above or below the placement depth (i.e., O to A or A to O). Our results suggest that
plant allocation belowground to fine roots results in more C retained and less N mineralized compared with allocation aboveground
to needles, primarily due to litter quality differences. 相似文献