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1.
JOHN LICHTER SHARON A. BILLINGS SUSAN E. ZIEGLER DEEYA GAINDH REBECCA RYALS ADRIEN C. FINZI ROBERT B. JACKSON ELIZABETH A. STEMMLER WILLIAM H. SCHLESINGER 《Global Change Biology》2008,14(12):2910-2922
The impact of anthropogenic CO2 emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO2 concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO2 concentrations, the Duke Forest Free‐Air CO2 Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO2 concentrations 200 μL L?1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO2 conditions, with significantly greater accumulations under the elevated CO2 treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ~30 g C m?2 yr?1 was sequestered in the forest floor of the elevated CO2 treatment plots relative to the control plots maintained at ambient CO2 owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO2 effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO2 suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO2 (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO2. 相似文献
2.
The mechanisms behind the 13C enrichment of organic matter with increasing soil depth in forests are unclear. To determine if 13C discrimination during respiration could contribute to this pattern, we compared δ13C signatures of respired CO2 from sieved mineral soil, litter layer and litterfall with measurements of δ13C and δ15N of mineral soil, litter layer, litterfall, roots and fungal mycelia sampled from a 68-year-old Norway spruce forest stand
planted on previously cultivated land. Because the land was subjected to ploughing before establishment of the forest stand,
shifts in δ13C in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As 13C-depleted organic matter accumulated in the upper soil, a 1.0‰ δ13C gradient from −28.5‰ in the litter layer to −27.6‰ at a depth of 2–6 cm was formed. This can be explained by the 1‰ drop
in δ13C of atmospheric CO2 since the beginning of reforestation together with the mixing of new C (forest) and old C (farmland). However, the isotopic
change of the atmospheric CO2 explains only a portion of the additional 1.0‰ increase in δ13C below a depth of 20 cm. The δ13C of the respired CO2 was similar to that of the organic matter in the upper soil layers but became increasingly 13C enriched with depth, up to 2.5‰ relative to the organic matter. We hypothesise that this 13C enrichment of the CO2 as well as the residual increase in δ13C of the organic matter below a soil depth of 20 cm results from the increased contribution of 13C-enriched microbially derived C with depth. Our results suggest that 13C discrimination during microbial respiration does not contribute to the 13C enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when
natural variations in δ13C of respired CO2 are used to separate different components of soil respiration or ecosystem respiration. 相似文献
3.
Application of tree leaves (C3 plants) on maize (Zea mays L.) (C4 plant) fields is an agroforestry management technology to restore or maintain soil fertility. The rate at which the tree
leaves decompose is crucial for the nutrient supply to the crop. We studied the in situ decomposition of Sesbania sesban (L.) Merr. leaves or C3 sugar for 4 – 8 days after application to a maize field in Kenya. By using the difference of around 10‰ in natural abundance
of 13C between the endogenous soil C (mainly C4) and the applied C (C3), we could calculate the contributions of the two C sources to soil respiration. The δ13C value of the basal respiration was from –15.9 to –16.7‰. The microbial response to the additions of leaves and sugar to
this tropical soil was immediate. Application of sesbania leaves gave an initial peak in respiration rates that lasted from
one to less than 6 days, after which it levelled off and remained about 2 – 3 times higher (230–270 mg C m-2 h-1) than the control respiration rates throughout the rest of the experiment (5 – 8 days). In the sugar treatment, there was
no initial peak in respiration rate. The respiration rate was 170 mg C m-2 h-1 after 4 days. At the end of the experiments, after 4–8 days, as much as 14–17% of the added C had been respired and about
60% of the total respiration was from the added sesbania leaves or C3 sugar. This non-destructive method allows repeated measurements of the actual rate of C mineralisation and facilitates decomposition
studies with high temporal resolution in the field.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
4.
R. B. Thompson 《Plant and Soil》1996,180(1):49-55
A preliminary study was conducted using the stable isotope 13C to pulse label the cover crop phacelia (Phacelia tanacetifolia) to examine its decomposition in soil, under field conditions. Plants were grown, in pots, in the greenhouse and after four weeks of growth were labelled with 13CO2 six times, at 1–2 week intervals. A single chamber was placed over the pots, and 13CO2 was generated, inside the chamber, by injecting lactic acid into sodium carbonate (99 atom % 13C). For calculating the quantity of Na2CO3 required, a target enrichment of 5 atom% 13C within the shoots of plants, assuming no respiration losses, was used. When harvested, at flowering, the mean enrichment of the shoot material was 3.0466 atom% 13C, or 1.9654 atom% excess 13C. To assess uniformity of labelling within plants, the shoot of a single plant was divided into leaves and stem from three sections of equal length. Ninety-three percent of this plant's dry matter had a 13C enrichment within 20 % of the weighted mean. At a field site with sandy soil, 13C labelled shoot and root material were combined and mixed with soil (0–15 cm). The soil was sampled 16 and 179 days later to determine the recovery of the added excess 13C in soil total C. The recoveries in soil (0–30 cm) were, respectively, 78 and 40 % at 16 and 179 days; there was appreciable variation associated with the recovery data from day 16, much less so at day 179. Methodological procedures for (i) enhancing the uniformity of labelling with 13C within plants, and (ii) minimising variability in the recovery of 13C from soil are suggested. ei]R Merckx 相似文献
5.
Rhizosphere priming effects on the decomposition of soil organic matter in C4 and C3 grassland soils 总被引:1,自引:0,他引:1
Using a natural abundance 13C method, soil organic matter (SOM) decomposition was studied in a C3 plant – `C4 soil' (C3 plant grown in a soil obtained from a grassland dominated by C4 grasses) system and a C4 plant – `C3 soil' (C4 plant grown in a soil taken from a pasture dominated by C3 grasses) system. In C3 plant – `C4 soil' system, cumulative soil-derived CO2–C were higher in the soils planted with soybean (5499 mg pot–1) and sunflower (4484 mg pot–1) than that in `C4 soil' control (3237 mg pot–1) without plants. In other words, the decomposition of SOM in soils planted with soybean and sunflower were 69.9% and 38.5% faster than `C4 soil' control. In C4 plant – `C3 soil' system, there was an overall negative priming effect of live roots on the decomposition of SOM. The cumulative soil-derived CO2–C were lower in the soils planted with sorghum (2308 mg pot–1) and amaranthus (2413 mg pot–1) than that in `C3 soil' control (2541 mg pot–1). The decomposition of SOM in soils planted with sorghum and amaranthus were 9.2% and 5.1% slower than `C3 soil' control. Our results also showed that rhizosphere priming effects on SOM decomposition were positive at all developmental stages in C3 plant – `C4 soil' system, but the direction of the rhizosphere priming effect changed at different developmental stages in the C4 plant – `C3 soil' system. Implications of rhizosphere priming effects on SOM decomposition were discussed. 相似文献
6.
Effects of long term CO2 enrichment on microbial community structure in calcareous grassland 总被引:1,自引:0,他引:1
Diana Ebersberger Nicola Wermbter Pascal A. Niklaus Ellen Kandeler 《Plant and Soil》2004,264(1-2):313-323
Elevated CO2 generally increases plant productivity, and has been found to alter plant community composition in many ecosystems. Because soil microbes depend on plant-derived C and are often associated with specific plant species, elevated CO2 has the potential to alter structure and functioning of soil microbial communities. We investigated soil microbial community structure of a species-rich semi-natural calcareous grassland that had been exposed to elevated CO2 (600 μL L?1) for 6 growing seasons. We analysed microbial community structure using phospholipid fatty acid (PLFA) profiles and DNA fingerprints obtained by Denaturing Gradient Gel Electrophoresis (DGGE) of 16S rDNA fragments amplified by the Polymerase Chain Reaction (PCR). PLFA profiles were not affected by CO2 enrichment and the ratio of fungal and bacterial PLFA did not change. Ordination analysis of DNA fingerprints revealed a significant relation between CO2 enrichment and variation in DNA fingerprints in summer (P=0.01), but not in spring. This variation was due to changes in low-intensity bands, while dominant bands did not differ between CO2 treatments. Diversity of the bacterial community, as assessed by number of bands in DNA fingerprints and calculation of Shannon diversity indices, was not affected by elevated CO2. Overall, only minor effects on microbial community structure were detected, corroborating earlier findings that soil carbon inputs did probably change much less than suggested by plant photosynthetic responses. 相似文献
7.
We investigated microbial responses in a late successional sedge-dominated alpine grassland to four seasons of CO2 enrichment. Part of the plots received fertilizer equivalent to 4.5g N m−2 a−1. Soil basal respiration (R
mic
), the metabolic quotient for CO2 (qCO2=R
mic
/C
mic
), microbial C and N (C
mic
and N
mic
) as well as total soil organic C and N showed no response to CO2 enrichment alone. However, when the CO2 treatment was combined with fertilizer addition R
mic
and qCO2 were statistically significantly higher under elevated CO2 than under ambient conditions (+57% and +71%, respectively). Fertilizer addition increased microbial N pools by 17%, but
this was not influenced by elevated CO2. Microbial C was neither affected by elevated CO2 nor fertilizer. The lack of a CO2-effect in unfertilized plots was suprising in the light of our evidence (based on C balance) that enhanced soil C inputs
must have occurred under elevated CO2 regardless of fertilizer treatment. Based on these data and other published work we suggest that microbial responses to elevated
CO2 in such stable, late-successional ecosystems are limited by the availability of mineral nutrients and that results obtained
with fertile or heavily disturbed substrates are unsuitable to predict future microbial responses to elevated CO2 in natural systems. However, when nutrient limitation is removed (e.g. by wet nitrogen deposition) microbes make use of the
additional carbon introduced into the soil system. We believe that the response of natural ecosystems to elevated CO2 must be studied in situ in natural, undisturbed systems. 相似文献
8.
9.
10.
The leaves of 37 grass, herb, shrub and tree species were collected from a mesotrophic grassland to assess natural variability in bulk, fatty acid and monosaccharide delta(13)C values of leaves from one plant community. The leaf tissue mean bulk delta(13)C value was -29.3 per thousand. No significant differences between tissue bulk delta(13)C values with life form were determined (P=0.40). On average, C(16:0), C(18:2) and C(18:3) constituted 89% of leaf tissue total fatty acids, whose delta(13)C values were depleted compared to whole leaf tissues. A general interspecific (between different species) trend for fatty acids delta(13)C values was observed, i.e. delta(13)C(16:0)delta(13)C(xylose)>delta(13)C(glucose)>delta(13)C(galactose), was consistently observed. Therefore, we have shown (i) diversity in compound-specific delta(13)C values contributing to leaf bulk delta(13)C values; (ii) interspecific variability between bulk and compound-specific delta(13)C values of leaves of individual grassland species, and (iii) trends between individual fatty acid and monosaccharide delta(13)C values common to leaves of all species within one plant community. 相似文献
11.
Coupling 13C natural abundance and 14C pulse labelling enabled us to investigate the dependence of 13C fractionation on assimilate partitioning between shoots, roots, exudates, and CO2 respired by maize roots. The amount of recently assimilated C in these four pools was controlled by three levels of nutrient
supply: full nutrient supply (NS), 10 times diluted nutrient supply (DNS), and deionised water (DW). After pulse labelling
of maize shoots in a 14CO2 atmosphere, 14C was traced to determine the amounts of recently assimilated C in the four pools and the δ13C values of the four pools were measured. Increasing amounts of recently assimilated C in the roots (from 8% to 10% of recovered
14C in NS and DNS treatments) led to a 0.3‰ 13C enrichment from NS to DNS treatments. A further increase of C allocation in the roots (from 10% to 13% of recovered 14C in DNS and DW treatments) resulted in an additional enrichment of the roots from DNS to DW treatments by 0.3‰. These findings
support the hypothesis that 13C enrichment in a pool increases with an increasing amount of C transferred into that pool. δ13C of CO2 evolved by root respiration was similar to that of the roots in DNS and DW treatments. However, if the amount of recently
assimilated C in root respiration was reduced (NS treatment), the respired CO2 became 0.7‰ 13C depleted compared to roots. Increasing amounts of recently assimilated C in the CO2 from NS via DNS to DW treatments resulted in a 1.6‰ δ13C increase of root respired CO2 from NS to DW treatments. Thus, for both pools, i.e. roots and root respiration, increasing amounts of recently assimilated
C in the pool led to a δ13C increase. In DW and DNS plants there was no 13C fractionation between roots and exudates. However, high nutrient supply decreased the amount of recently assimilated C in
exudates compared to the other two treatments and led to a 5.3‰ 13C enrichment in exudates compared to roots. We conclude that 13C discrimination between plant pools and within processes such as exudation and root respiration is not constant but strongly
depends on the amount of C in the respective pool and on partitioning of recently assimilated C between plant pools.
Section Editor: H. Lambers 相似文献
12.
Pascal A. Niklaus 《Global Change Biology》1998,4(4):451-458
Microbial responses to three years of CO2 enrichment (600 μL L–1) in the field were investigated in calcareous grassland. Microbial biomass carbon (C) and soil organic C and nitrogen (N) were not significantly influenced by elevated CO2. Microbial C:N ratios significantly decreased under elevated CO2 (– 15%, P = 0.01) and microbial N increased by + 18% (P = 0.04). Soil basal respiration was significantly increased on one out of 7 sampling dates (+ 14%, P = 0.03; December of the third year of treatment), whereas the metabolic quotient for CO2 (qCO2 = basal respiration/microbial C) did not exhibit any significant differences between CO2 treatments. Also no responses of microbial activity and biomass were found in a complementary greenhouse study where intact grassland turfs taken from the field site were factorially treated with elevated CO2 and phosphorus (P) fertilizer (1 g P m–2 y–1). Previously reported C balance calculations showed that in the ecosystem investigated growing season soil C inputs were strongly enhanced under elevated CO2. It is hypothesized that the absence of microbial responses to these enhanced soil C fluxes originated from mineral nutrient limitations of microbial processes. Laboratory incubations showed that short-term microbial growth (one week) was strongly limited by N availability, whereas P was not limiting in this soil. The absence of large effects of elevated CO2 on microbial activity or biomass in such nutrient-poor natural ecosystems is in marked contrast to previously published large and short-term microbial responses to CO2 enrichment which were found in fertilized or disturbed systems. It is speculated that the absence of such responses in undisturbed natural ecosystems in which mineral nutrient cycles have equilibrated over longer periods of time is caused by mineral nutrient limitations which are ineffective in disturbed or fertilized systems and that therefore microbial responses to elevated CO2 must be studied in natural, undisturbed systems. 相似文献
13.
Elevated [CO2], temperature increase and N supply effects on the turnover of below-ground carbon in a temperate grassland ecosystem 总被引:5,自引:0,他引:5
The effects of elevated [CO2] (700 μl l-1 CO2) and temperature increase (+3 °C) on carbon turnover in grassland soils were studied during 2.5 years at two N fertiliser
supplies (160 and 530 kg N ha-1 y-1) in an experiment with well-established ryegrass swards (Lolium perenne) supplied with the same amounts of irrigation water.
During the growing season, swards from the control climate (350 μl l-1 [CO2] at outdoor air temperature) were pulse labelled by the addition of 13CO2. The elevated [CO2] treatments were continuously labelled by the addition of fossil-fuel derived CO2 (13
C of -40 to -50 ‰). Prior to the start of the experimental treatments, the carbon accumulated in the plant parts and in the
soil macro-organic matter (‘old’ C) was at −32‰. During the experiment, the carbon fixed in the plant material (‘new’ C) was
at −14 and −54‰ in the ambient and elevated [CO2] treatments, respectively. During the experiment, the 13C isotopic mass balance method was used to calculate, for the top soil (0–15 cm), the carbon turnover in the stubble and roots
and in the soil macro-organic matter above 200 μ (MOM). Elevated [CO2] stimulated the turnover of organic carbon in the roots and stubble and in the MOM at N+, but not at N−. At the high N supply,
the mean replacement time of ‘old’ C by ‘new’ C declined in elevated, compared to ambient [CO2], from 18 to 7 months for the roots and stubble and from 25 to 17 months for the MOM. This resulted from increased rates
of ‘new’ C accumulation and of ‘old’ C decay. By contrast, at the low N supply, despite an increase in the rate of accumulation
of ‘new’ C, the soil C pools did not turnover faster in elevated [CO2], as the rate of ‘old’ C decomposition was reduced. A 3 °C temperature increase in elevated [CO2] decreased the input of fresh C to the roots and stubble and enhanced significantly the exponential rate for the ‘old’ C
decomposition in the roots and stubble. An increased fertiliser N supply reduced the carbon turnover in the roots and stubble
and in the MOM, in ambient but not in elevated [CO2]. The respective roles for carbon turnover in the coarse soil OM fractions, of the C:N ratio of the litter, of the inorganic
N availability and of a possible priming effect between C-substrates are discussed.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
14.
Earthworms make up the dominant fraction of the biomass of soil animals in most temperate grasslands and have important effects
on the structure and function of these ecosystems. We hypothesized that the effects of elevated atmospheric CO2 on soil moisture and plant biomass production would increase earthworm activity, expressed as surface cast production. Using
a screen-aided CO2 control facility (open top and open bottom rings), eight 1.2-m2 grassland plots in Switzerland have been maintained since March 1994 at ambient CO2 concentrations (350 μl CO2 l−1) and eight at elevated CO2 (610 μl CO2 l−1). Cumulative earthworm surface cast production measured 40 times over 1 year (April 1995–April 1996) in plots treated with
elevated CO2 (2206 g dry mass m−2 year−1) was 35% greater (P<0.05) than that measured in plant communities maintained at ambient CO2 (1633 g dry mass m−2 year−1). At these rates of surface cast production, worms would require about 100 years to egest the equivalent of the amount of
soil now found in the Ah horizon (top 15 cm) under current ambient CO2 concentrations, and 75 years under elevated CO2. Elevated atmospheric CO2 had no influence on the seasonality of earthworm activity. Cumulative surface cast production measured over the 7-week period
immediately following the 6-week summer dry period in 1995 (no surface casting) was positively correlated (P<0.05) with the mean soil water content calculated over this dry and subsequent wetter period, when viewed across all treatments.
However, no correlations were observed with soil temperature or with annual aboveground plant biomass productivity. No CO2-related differences were observed in total nitrogen (Ntot) and organic carbon (Corg) concentration of surface casts, although concentrations of both elements varied seasonally. The CO2-induced increase in earthworm surface casting activity corresponded to a 30% increase of the amount of Ntot (8.9 mg N m−2 vs. 6.9 mg N m−2) and Corg (126 mg C m−2 vs. 94 mg C m−2) egested by the worms in one year. Thus, our results demonstrate an important indirect stimulatory effect of elevated atmospheric
CO2 on earthworm activity which may have profound effects on ecosystem function and plant community structure in the long term.
Received: 3 November 1996 / Accepted: 11 January 1997 相似文献
15.
Temperate grasslands contribute about 20% to the global terrestrial carbon (C) budget with sugars contributing 10–50% to this soil C pool. Whether the observed increase of the atmospheric CO2 concentration (pCO2) leads to additional C sequestration into these ecosystems or enhanced mineralization of soil organic matter (SOM) is still unclear. Therefore, the aim of the presented study was to investigate the impact of elevated atmospheric pCO2 on C sequestration and turnover of plant‐ (arabinose and xylose) and microbially derived (fucose, rhamnose, galactose, mannose) sugars in soil, representing a labile SOM pool. The study was carried out at the Swiss Free Air Carbon Dioxide Enrichment (FACE) experiment near Zurich. For 7 years, Lolium perenne swards were exposed to ambient and elevated pCO2 (36 and 60 Pa, respectively). The additional CO2 in the FACE plots was depleted in 13C compared with ambient plots, so that ‘new’ (<7 years) C inputs could be determined by means of compound‐specific stable isotope analysis (13C : 12C). Samples were fractionated into clay, silt, fine sand and coarse sand, which yielded relatively stable and labile SOM pools with different turnover rates. Total sugar sequestration into bulk soil after 7 years of exposure to elevated pCO2 was about 28% compared with the control plots. In both ambient and elevated plots, total sugar concentrations in particle size fractions increased in the order sand2 for coarse sand, fine sand and silt (about 274%, 17% and 96%, respectively) but about 14% lower for clay compared with the control plots, corroborating that sugars belong to the labile SOM pool. The fraction of newly produced sugars gradually increased by up to 50% in bulk soil samples after 7 years under elevated pCO2. In the ambient plots, sugars were enriched in 13C by up to 10‰ when compared with bulk soil samples from the same plots. The enrichment of 13C in plant‐derived sugars was up to 13.4‰ when compared with parent plant material. After 7 years, the δ13C values of individual sugars decreased under elevated (13C‐depleted) CO2 in bulk soil and particle size fractions, varying between −13.7‰ and −37.8‰ under elevated pCO2. In coarse and fine sand, silt and clay fractions newly produced sugars made up 106%, 63%, 60% and 45%, respectively, of the total sugars present after 7 years. Mean residence time (MRT) of the sugars were calculated according to two models revealing a few decades, mean values increasing in the order coarse sand2 led to a net sequestration of about 30% of labile SOM (sugars) while no increase of total organic C was observed at the same plots. The additional labile SOM is gradually incorporated into more stable SOM pools such as silt and clay fractions in the medium term (<7 years). MRT of labile (sugar) SOM under elevated pCO2 is in the same order of magnitude when compared with studies under ambient pCO2 though no direct comparison of elevated and ambient plots was possible. 相似文献
16.
Review of elevated atmospheric CO2 effects on agro-ecosystems: residue decomposition processes and soil C storage 总被引:15,自引:0,他引:15
A series of studies using major crops (cotton [Gossypium hirsutum L.], wheat [Triticum aestivum L.], grain sorghum [Sorghum bicolor (L.) Moench.] and soybean [Glycine max (L.) Merr.]) were reviewed to examine the impact of elevated atmospheric CO2 on crop residue decomposition within agro-ecosystems. Experiments evaluated utilized plant and soil material collected from
CO2 study sites using Free Air CO2 Enrichment (FACE) and open top chambers (OTC). A incubation study of FACE residue revealed that CO2-induced changes in cotton residue composition could alter decomposition processes, with a decrease in N mineralization observed
with FACE, which was dependent on plant organ and soil series. Incubation studies utilizing plant material grown in OTC considered
CO2-induced changes in relation to quantity and quality of crop residue for two species, soybean and grain sorghum. As with cotton,
N mineralization was reduced with elevated CO2 in both species, however, difference in both quantity and quality of residue impacted patterns of C mineralization. Over
the short-term (14 d), little difference was observed for CO2 treatments in soybean, but C mineralization was reduced with elevated CO2 in grain sorghum. For longer incubation periods (60 d), a significant reduction in CO2-C mineralized per g of residue added was observed with the elevated atmospheric CO2 treatment in both crop species. Results from incubation studies agreed with those from the OTC field observations for both
measurements of short-term CO2 efflux following spring tillage and the cumulative effect of elevated CO2 (> 2 years) in this study. Observations from field and laboratory studies indicate that with elevated atmospheric CO2, the rate of plant residue decomposition may be limited by N and the release of N from decomposing plant material may be
slowed. This indicates that understanding N cycling as affected by elevated CO2 is fundamental to understanding the potential for soil C storage on a global scale.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
17.
18.
D. F. Ferretti E. Pendall J. A. Morgan J. A. Nelson D. LeCain A. R. Mosier 《Plant and Soil》2003,254(2):291-303
The stable isotopic composition of soil water is controlled by precipitation inputs, antecedent conditions, and evaporative losses. Because transpiration does not fractionate soil water isotopes, the relative proportions of evaporation and transpiration can be estimated using a simple isotopic mass balance approach. At our site in the shortgrass steppe in semi-arid northeastern Colorado, 18O values of soil water were almost always more enriched than those of precipitation inputs, owing to evaporative losses. The proportion of water lost by evaporation (E/ET) during the growing season ranged from nil to about 40% (to >90% in the dormant season), and was related to the timing of precipitation inputs. The sum of transpiration plus evaporation losses estimated by isotopic mass balance were similar to actual evapotranspiration measured from a nearby Bowen ratio system. We also investigated the evapotranspiration response of this mixed C3/C4 grassland to doubled atmospheric [CO2] using Open-Top Chambers (OTC). Elevated atmospheric [CO2] led to increased soil-water conservation via reduced stomatal conductance, despite greater biomass growth. We used a non-invasive method to measure the 18O of soil CO2 as a proxy for soil water, after establishing a strong relationship between 18O of soil CO2 from non-chambered control (NC) plots and 18O of soil–water from an adjacent area of native grassland. Soil–CO2 18O values showed significant treatment effects, particularly during a dry summer: values in ambient chambers (AC) were more enriched than in NC and elevated chamber (EC) plots. During the dry growing season of 2000, transpiration from the EC treatment was higher than from AC and lower than from NC treatments, but during 2001, transpiration was similar on all three treatments. Slightly higher evaporation rates from AC than either EC or NC treatments in 2000 may have resulted from increased convection across the soil surface from the OTC blowers, combined with lower biomass and litter cover on the AC treatment. Transpiration-use efficiency, or the amount of above-ground biomass produced per mm water transpired, was always greatest on EC and lowest on NC treatments. 相似文献
19.
Summary The growth and photosynethetic responses to atmospheric CO2 enrichment of 4 species of C4 grasses grown at two levels of irradiance were studied. We sought to determine whether CO2 enrichment would yield proportionally greater growth enhancement in the C4 grasses when they were grown at low irradiance than when grown at high irradiance. The species studied were Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350, 675 and 1,000 l 1-1 CO2 and 1,000 or 150 mol m-2 s-1 photosynthetic photon flux density (PPFD). An increase in CO2 concentration and PPFD significantly affected net photosynthesis and total biomass production of all plants. Plants grown at low PPFD had significantly lower rates of photosynthesis, produced less biomass, and had reduced responses to increases in CO2. Plants grown in CO2-enriched atmosphere had lower photosynthetic capacity relative to the low CO2 grown plants when exposed to lower CO2 concentration at the time of measurement, but had greater rate of photosynthesis when exposed to increasing PPFD. The light level under which the plants were growing did not influence the CO2 compensation point for photosynthesis. 相似文献
20.
MAXIMILIAN H. O. M. WITTMER KARL AUERSWALD YONGFEI BAI RUDI SCHÄUFELE HANS SCHNYDER 《Global Change Biology》2010,16(2):605-616
Global warming, increasing CO2 concentration, and environmental disturbances affect grassland communities throughout the world. Here, we report on variations in the C3/C4 pattern of Inner Mongolian grassland derived from soil and vegetation. Soil samples from 149 sites covering an area of approximately 250 000 km2 within Inner Mongolia, People's Republic of China were analyzed for the isotopic composition (δ13C) of soil organic carbon (SOC). The contrast in δ13C between C3 and C4 plants allowed for calculation of the C3/C4 ratio from δ13C of SOC with a two‐member mixing model, which accounted for influences of aridity and altitude on δ13C of the C3 end‐member and for changes in δ13C of atmospheric CO2. Maps were created geostatistically, and showed a substantially lower C4 abundance in soil than in recent vegetation (?10%). The difference between soil and vegetation varied regionally and was most pronounced within an E–W belt along 44°N and in a mountainous area, suggesting a spread of C4 plants toward northern latitudes (about 1°) and higher altitudes. The areas of high C4 abundance for present vegetation and SOC were well delineated by the isotherms of crossover temperature based on the climatic conditions of the respective time periods. Our study indicates that change in the patterns of C3/C4 composition in the Inner Mongolia grassland was mainly triggered by increasing temperature, which overrode the antagonistic effect of rising CO2 concentrations. 相似文献