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1.
Expansion of woody vegetation in grasslands is a worldwide phenomenon with implications for C and N cycling at local, regional and global scales. Although woody encroachment is often accompanied by increased annual net primary production (ANPP) and increased inputs of litter, mesic ecosystems may become sources for C after woody encroachment because stimulation of soil CO 2 efflux releases stored soil carbon. Our objective was to determine if young, sandy soils on a barrier island became a sink for C after encroachment of the nitrogen‐fixing shrub Morella cerifera, or if associated stimulation of soil CO 2 efflux mitigated increased litterfall. We monitored variations in litterfall in shrub thickets across a chronosequence of shrub expansion and compared those data to previous measurements of ANPP in adjacent grasslands. In the final year, we quantified standing litter C and N pools in shrub thickets and soil organic matter (SOM), soil organic carbon (SOC), soil total nitrogen (TN) and soil CO 2 efflux in shrub thickets and adjacent grasslands. Heavy litterfall resulted in a dense litter layer storing an average of 809 g C m ?2 and 36 g N m ?2. Although soil CO 2 efflux was stimulated by shrub encroachment in younger soils, soil CO 2 efflux did not vary between shrub thickets and grasslands in the oldest soils and increases in CO 2 efflux in shrub thickets did not offset contributions of increased litterfall to SOC. SOC was 3.6–9.8 times higher beneath shrub thickets than in grassland soils and soil TN was 2.5–7.7 times higher under shrub thickets. Accumulation rates of soil and litter C were highest in the youngest thicket at 101 g m ?2 yr ?1 and declined with increasing thicket age. Expansion of shrubs on barrier islands, which have low levels of soil carbon and high potential for ANPP, has the potential to significantly increase ecosystem C sequestration. 相似文献
2.
We examined the effects of root and litter exclusion on the rate of soil CO 2 efflux and microbial biomass using trenching and tent separation techniques in a secondary forest (SF) and a pine ( Pinus caribaea Morelet) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil surface CO 2 efflux was measured using the alkali trap method at 12 randomly-distributed locations in each treatment (control, root exclusion, litter exclusion, and both root and litter exclusion) in the plantation and the SF, respectively. We measured soil CO 2 efflux every two months and collected soil samples at each sampling location in different seasons to determine microbial biomass from August 1996 to July 1997. We found that soil CO 2 efflux was significantly reduced in the litter and root exclusion plots (7-year litter and/or root exclusion) in both the secondary forest and the pine plantation compared with the control. The reduction of soil CO 2 efflux was 35.6% greater in the root exclusion plots than in the litter exclusion plots in the plantation, whereas a reversed pattern was found in the secondary forest. Microbial biomass was also reduced during the litter and root exclusion period. In the root exclusion plots, total fungal biomass averaged 31.4% and 65.2% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 24% and 8.3% lower than the control plots in the plantation and the secondary forest, respectively. In the litter exclusion treatment, total fungal biomass averaged 69.2% and 69.7% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 48% and 50.1% lower than the control plots in the plantation and the secondary forest, respectively. Soil CO 2 efflux was positively correlated with both fungal and bacterial biomass in both the plantation the secondary forest. The correlation between soil CO 2 efflux and active fungal biomass was significantly higher in the plantation than in the secondary forest. However, the correlation between the soil CO 2 efflux and both the active and total bacterial biomass was significantly higher in the secondary forest than in the plantation in the day season. In addition, we found soil CO 2 efflux was highly related to the strong interactions among root, fungal and bacterial biomass by multiple regression analysis ( R2 > 0.61, P < 0.05). Our results suggest that carbon input from aboveground litterfall and roots (root litter and exudates) is critical to the soil microbial community and ecosystem carbon cycling in the wet tropical forests. 相似文献
3.
While several studies have shown that the addition of animal manures to soil can increase N 2O and CO 2 emissions, limited information is available on the effect that manure physical characteristics can have on these emissions. This study compared N 2O and CO 2 emissions from poultry litter incorporated as pellets (5.5 mm OD, 7 mm long) or fine particles (<0.83 mm) into Cecil soil samples. The soil-litter mixture was packed in acrylic plastic cylinders and adjusted to 55 or 90 % water-filled porosity (WFP). The cylinders were placed inside jars that were sealed and placed in an incubator at 25°C for 35 d, with periodic air samplings conducted for N 2O and CO 2 analyses. At 55% WFP, cumulative emission of CO 2 was similar for both litter types, but cumulative emission of N 2O was slightly higher for pelletized (6.8 % of applied N) than for fine-particle litter (5.5 %). In contrast, at 90 % WFP, cumulative emission of N 2O was larger for fine-particle litter (3.4 % of applied N) than for pelletized litter (1.5 %). These results indicate that the effect of poultry litter physical characteristics on N 2O emissions from incorporated applications can be expected to vary depending on the soil water regime. 相似文献
4.
Short rotation forests can serve as sources of renewable energy and possibly for soil C storage. However, the high frequency of management practices and the fertilisation could reduce C storage into the soil, by increasing CO 2 emissions and annulling the potential of C sequestration. The objectives of this work were to evaluate the impacts of coppicing and fertilisation on total soil CO 2 efflux, soil heterotrophic processes and consequent changes of soil C storage in a short rotation poplar plantation. Field soil CO 2 efflux, heterotrophic soil CO 2 efflux and soil organic C were compared before and after coppicing. Temporal dynamics of fine root biomass and water-soluble carbon after coppicing were also analysed. Coppicing increased total soil CO 2 efflux by more than 50%, while heterotrophic soil CO 2 efflux remained unchanged. Nevertheless, an increase in total organic carbon was observed as a result of above and belowground litter inputs, as well as root re-growth and exudation. This trend was more evident in fertilised soils due to lower heterotrophic and autotrophic soil CO 2 effluxes. Fertilisation can reduce the increase of CO 2 emissions after coppicing. Although soil organic C storage increased, the accumulation of labile fractions may trigger microbial respiration in the following years. 相似文献
5.
In an old growth coniferous forest located in the central Cascade Mountains, Oregon, we added or removed aboveground litter and terminated live root activity by trenching to determine sources of soil respiration. Annual soil efflux from control plots ranged from 727 g C m −2 year −1 in 2002 to 841 g C m −2 year −1 in 2003. We used aboveground litter inputs (149.6 g C m −2 year −1) and differences in soil CO 2 effluxes among treatment plots to calculate contributions to total soil efflux by roots and associated rhizosphere organisms and by heterotrophic decomposition of organic matter derived from aboveground and belowground litter. On average, root and rhizospheric respiration ( Rr) contributed 23%, aboveground litter decomposition contributed 19%, and belowground litter decomposition contributed 58% to total soil CO 2 efflux, respectively. These values fall within the range of values reported elsewhere, although our estimate of belowground litter contribution is higher than many published estimates, which we argue is a reflection of the high degree of mycorrhizal association and low nutrient status of this ecosystem. Additionally, we found that measured fluxes from plots with doubled needle litter led to an additional 186 g C m −2 year −1 beyond that expected based on the amount of additional carbon added; this represents a priming effect of 187%, or a 34% increase in the total carbon flux from the plots. This finding has strong implications for soil C storage, showing that it is inaccurate to assume that increases in net primary productivity will translate simply and directly into additional belowground storage. 相似文献
6.
Aboveground litter not only is an important source of nutrients to soil microbes but also regulates the microclimate in topsoil. How the changes in aboveground litter quantity would affect the microbial biogeochemical cycles is still unclear. Here we conducted a litter input manipulation experiment in a temperate mixed forest to investigate how different amounts of litter input affect soil organic carbon (SOC) and soil respiration via their regulation on soil microbes. We found that although neither SOC stock nor soil CO2 efflux was affected by litter manipulation, soil microbial characteristics had responded after four years of litter addition or removal treatments. Microbial biomass carbon (MBC) in the O horizon was higher in litter addition plots than in litter removal plots as a result of the changed availability of labile C under litter treatments. Both double litter and no litter treatments changed microbial compositions, which was probably due to the increased soil pH in no litter treatment and the increased labile C in double litter treatment. The null change in soil respiration could be attributed to the offset between the negative effect of decreased substrate and the positive effect of increased temperature on soil respiration in litter removal plots. Due to the important role of soil microbes in carbon cycling, the altered microbial properties under litter manipulation treatments suggested the inevitable changes in biogeochemical cycling in the long run and call for long-term studies on SOC dynamics in the future. 相似文献
7.
凋落物既是森林生态系统养分循环的重要构件,又是森林土壤环境和功能的关键调节因子。降雨脉冲导致的土壤碳排放变异是陆地生态系统碳汇能力评价的不确定性来源之一。凋落物在调节土壤碳排放对降雨脉冲的响应中的作用仍缺乏科学的评价。通过在暖温带栎类落叶阔叶林中设置不同凋落物处理(对照、去除凋落物和加倍凋落物)和降雨模拟实验以阐明凋落物数量变化对土壤呼吸脉冲的影响。结果表明:模拟降雨脉冲之前,不同凋落物处理下的土壤呼吸存在显著差异;与对照相比,加倍凋落物导致土壤呼吸速率显著增加57.6%,然而,去除凋落物则对土壤呼吸无显著影响。模拟降雨后52小时内,对照、去除凋落物和加倍凋落物样方的土壤累积碳排放量分别为251.69 gC/m~2,250.93 gC/m~2和409.01 gC/m~2,加倍凋落物处理下的土壤碳排放量显著高于对照和去除凋落物处理;然而,去除凋落物与对照之间无显著差异。此外,不同凋落物处理下土壤呼吸的脉冲持续时间存在显著差异;加倍凋落物显著提高降雨后土壤呼吸脉冲的持续时间,分别比对照和去除凋落物高出262%和158%。多元逐步回归分析表明,土壤总碳排放通量和土壤呼吸的脉冲持续时间与土壤理... 相似文献
8.
Wetlands have an inordinate influence on the global greenhouse gas budget, but how global changes may alter wetland contribution to future greenhouse gas fluxes is poorly understood. We determined the greenhouse gas balance of a tidal marsh exposed to nine years of experimental carbon dioxide (CO 2) and nitrogen (N) manipulation. We estimated net carbon (C) gain rates by measuring changes in plant and soil C pools over nine years. In wetland soils that accrete primarily through organic matter inputs, long-term measurements of soil elevation, along with soil C density, provide a robust estimate of net soil C gain. We used net soil C gain along with methane and nitrous oxide fluxes to determine the radiative forcing of the marsh under elevated CO 2 and N addition. Nearly all plots exhibited a net gain of C over the study period (up to 203 g C m ?2 year ?1), and C gain rates were greater with N and CO 2 addition. Treatment effects on C gain and methane emissions dominated trends in radiative forcing while nitrous oxide fluxes in all treatments were negligible. Though these soils experience salinities that typically suppress methane emissions, our results suggest that elevated CO 2 can stimulate methane emissions, overcoming positive effects of elevated CO 2 on C gain, converting brackish marshes that are typically net greenhouse gas sinks into sources. Adding resources, either CO 2 or N, will likely increase “blue carbon” accumulation rates in tidal marshes, but importantly, each resource can have distinct influences on the direction of total greenhouse forcing. 相似文献
9.
Profiles of subsurface soil CO 2 concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and
2006 in 16 locations at the free air CO 2 enrichment facility situated within a temperate loblolly pine ( Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO 2 concentration levels (ambient [CO 2] a, ambient + 200 ppmv, [CO 2] e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 g N m −2 year −1). The combination of these measurements permitted indirect estimation of belowground CO 2 production and flux profiles in the mineral soil. Adjacent to the soil CO 2 profiles, direct (chamber-based) measurements of CO 2 fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the
measured soil CO 2 profiles, neither [CO 2] e nor N fertilization had a statistically significant effect on seasonal soil CO 2, CO 2 production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects
on CO 2 concentration depending on the depth. Variations in CO 2 were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface
(within the first 10 cm), where CO 2 pulses were induced by rainfall events. The soil effluxes were equal to the CO 2 production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from
the CO 2 profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests
that a better parameterization of the soil CO 2 diffusivity is required for these soil moisture extremes. 相似文献
10.
Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N‐limited temperate forests. In N‐rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old‐growth tropical forest soil responded to 13 years of N addition at four rates: 0 (Control), 50 (Low‐N), 100 (Medium‐N), and 150 (High‐N) kg N ha ?1 year ?1. Soil organic carbon (SOC) content increased under High‐N, corresponding to a 33% decrease in CO 2 efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N 2O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes ( narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High‐N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N 2O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems. 相似文献
11.
The aim of this study is to estimate emissions of greenhouse gases CO 2, CH 4 and N 2O, and the effects of drainage and peat extraction on these processes, in Estonian transitional fens and ombrotrophic bogs. Closed-chamber-based sampling lasted from January to December 2009 in nine peatlands in Estonia, covering areas with different land-use practices: natural (four study sites), drained (six sites), abandoned peat mining (five sites) and active peat mining areas (five sites). Median values of soil CO 2 efflux were 1,509, 1,921, 2,845 and 1,741 kg CO 2-C ha ?1 year ?1 from natural, drained, abandoned and active mining areas, respectively. Emission of CH 4-C (median values) was 85.2, 23.7, 0.07 and 0.12 kg ha ?1 year ?1, and N 2O-N ?0.05, ?0.01, 0.18 and 0.19 kg ha ?1 year ?1, respectively. There were significantly higher emissions of CO 2 and N 2O from abandoned and active peat mining areas, whereas CH 4 emissions were significantly higher in natural and drained areas. Significant Spearman rank correlation was found between soil temperature and CO 2 flux at all sites, and CH 4 flux with high water level at natural and drained areas. Significant increase in CH 4 flux was detected for groundwater levels above 30 cm. 相似文献
12.
We describe the long-term effects of a CO 2 exhalation, created more than 70 years ago, on a natural C 4 dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether long-term CO 2 enrichment changes the competitive balance between plants with C 3 and C 4 photosynthetic pathways and how CO 2 enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO 2 on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO 2 alone. Species compositional data and soil δ 13C isotopic evidence suggest no detectable effect of CO 2 enrichment on C 3:C 4 plant mixtures and individual species dominance. Contrary to many general predictions C 3 grasses did not become more abundant and C 3 shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO 2 concentrations averaging 610 μmol mol −1. Leaf properties such as total N decreased in the C 3 but not C 4 grass under elevated CO 2 while total non-structural carbohydrate accumulation was not affected. Elevated CO 2 possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO 2 enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO 2 source. The conservative response of this grassland suggests that many of the reported effects of elevated CO 2 on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO 2 conditions. 相似文献
13.
Global changes that alter soil water availability may have profound effects on semiarid ecosystems. Although both elevated CO 2 (eCO 2) and warming can alter water availability, often in opposite ways, few studies have measured their combined influence on the amount, timing, and temporal variability of soil water. Here, we ask how free air CO 2 enrichment (to 600 ppmv) and infrared warming (+?1.5 °C day, +?3 °C night) effects on soil water vary within years and across wet-dry periods in North American mixed-grass prairie. We found that eCO 2 and warming interacted to influence soil water and that those interactions varied by season. In the spring, negative effects of warming on soil water largely offset positive effects of eCO 2. As the growing season progressed, however, warming reduced soil water primarily (summer) or only (autumn) in plots treated with eCO 2. These interactions constrained the combined effect of eCO 2 and warming on soil water, which ranged from neutral in spring to positive in autumn. Within seasons, eCO 2 increased soil water under drier conditions, and warming decreased soil water under wetter conditions. By increasing soil water under dry conditions, eCO 2 also reduced temporal variability in soil water. These temporal patterns explain previously observed plant responses, including reduced leaf area with warming in summer, and delayed senescence with eCO 2 plus warming in autumn. They also suggest that eCO 2 and warming may favor plant species that grow in autumn, including winter annuals and C 3 graminoids, and species able to remain active under the dry conditions moderated by eCO 2. 相似文献
14.
Soil amendment with pyrogenic organic matter (PyOM), also named biochar, is claimed to sequester carbon (C). However, possible interactions between PyOM and native soil organic carbon (SOC) may accelerate the loss of SOC, thus reducing PyOM's C sequestration potential. We combined the results of 46 studies in a meta‐analysis to investigate changes in CO 2 emission of PyOM‐amended soils and to identify the causes of these changes and the possible factors involved. Our results showed a statistically significant increase of 28% in CO 2 emission from PyOM‐amended soils. When grouped by PyOM C (PyC):SOC ratios, the group of studies with a ratio >2 showed a significant increase in CO 2 emissions, but those with a ratio <2 showed no significant effect of PyOM application on CO 2 emission. Our data are consistent with the hypothesis that increased CO 2 emission after PyOM addition is additive and mainly derived from PyOM's labile C fractions. The PyC:SOC ratio provided the best predictor of increases in CO 2 production after PyOM addition to soil. This meta‐analysis highlights the importance of taking into account the amount of applied PyC in relation to SOC for designing future decomposition experiments. 相似文献
15.
Although numerous studies indicate that increasing atmospheric CO 2 or temperature stimulate soil CO 2 efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO 2 and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO 2 and elevated temperature on these components of soil CO 2 efflux in Douglas-fir terracosms. We measured both soil CO 2 efflux rates and the 13C and 18O isotopic compositions of soil CO 2 efflux in 12 sun-lit and environmentally controlled terracosms with 4-year-old Douglas fir seedlings and reconstructed forest soils under two CO 2 concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO 2 efflux. In most cases, litter decomposition was the dominant component of soil CO 2 efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO 2 concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO 2, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual-isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes. 相似文献
16.
The contribution of leaf litter decomposition to total soil CO 2 efflux ( FL/ F) was evaluated in a beech ( Fagus sylvatica L.) forest in eastern France. The Keeling‐plot approach was applied to estimate the isotopic composition of respired soil CO 2 from soil covered with either control (?30.32‰) or 13C‐depleted leaf litter (?49.96‰). The δ13C of respired soil CO 2 ranged from ?25.50‰ to ?22.60‰ and from ?24.95‰ to ?20.77‰, respectively, with depleted or control litter above the soil. The FL/ F ratio was calculated by a single isotope linear mixing model based on mass conservation equations. It showed seasonal variations, increasing from 2.8% in early spring to about 11.4% in mid summer, and decreasing to 4.2% just after leaf fall. Between December 2001 and December 2002, cumulated F and FL reached 0.98 and 0.08 kg C m ?2, respectively. On an annual basis, decomposition of fresh leaf litter accounted for 8% of soil respiration and 80% of total C loss from fresh leaf litter. The other fraction of carbon loss during leaf litter decomposition that is assumed to have entered the soil organic matter pool (i.e. 20%) represents only 0.02 kg C m ?2. 相似文献
17.
Widespread recognition of the importance of soil CO 2 efflux as a major source of CO 2 to the atmosphere has led to active research. A large soil respiration database and recent reviews have compiled data, methods, and current challenges. This study highlights some deficiencies for a proper understanding of soil CO 2 efflux focusing on processes of soil CO 2 production and transport that have not received enough attention in the current soil respiration literature. It has mostly been assumed that soil CO 2 efflux is the result of biological processes (i.e. soil respiration), but recent studies demonstrate that pedochemical and geological processes, such as geothermal and volcanic CO 2 degassing, are potentially important in some areas. Besides the microbial decomposition of litter, solar radiation is responsible for photodegradation or photochemical degradation of litter. Diffusion is considered to be the main mechanism of CO 2 transport in the soil, but changes in atmospheric pressure and thermal convection may also be important mechanisms driving soil CO 2 efflux greater than diffusion under certain conditions. Lateral fluxes of carbon as dissolved organic and inorganic carbon occur and may cause an underestimation of soil CO 2 efflux. Traditionally soil CO 2 efflux has been measured with accumulation chambers assuming that the main transport mechanism is diffusion. New techniques are available such as improved automated chambers, CO 2 concentration profiles and isotopic techniques that may help to elucidate the sources of carbon from soils. We need to develop specific and standardized methods for different CO 2 sources to quantify this flux on a global scale. Biogeochemical models should include biological and non‐biological CO 2 production processes before we can predict the response of soil CO 2 efflux to climate change. Improving our understanding of the processes involved in soil CO 2 efflux should be a research priority given the importance of this flux in the global carbon budget. 相似文献
18.
Rates of soil respiration (CO 2 efflux) were measured for a year in a mature Eucalyptus pauciflora forest in unfertilized and phosphorus-fertilized plots. Soil CO 2 efflux showed a distinct seasonal trend, and average daily rates ranged from 124 to 574 mg CO 2 m –2 hr –1. Temperature and moisture are the main variables that cause variation in soil CO 2 efflux; hence their effects were investigated over a year so as to then differentiate the treatment effect of phosphorus (P) nutrition.Soil temperature had the greatest effect on CO 2 efflux and exhibited a highly significant logarithmic relationship (r 2 = 0.81). Periods of low soil and litter moisture occurred during summer when temperatures were greater than 10 °C, and this resulted in depression of soil CO 2 efflux. During winter, when temperatures were less than 10 °C, soil and litter moisture were consistently high and thus their variation had little effect on soil CO 2 efflux. A multiple regression model including soil temperature, and soil and litter moisture accounted for 97% of the variance in rates of CO 2 efflux, and thus can be used to predict soil CO 2 efflux at this site with high accuracy. Total annual efflux of carbon from soil was estimated to be 7.11 t C ha –1 yr –1. The model was used to predict changes in this annual flux if temperature and moisture conditions were altered. The extent to which coefficients of the model differ among sites and forest types requires testing.Increased soil P availability resulted in a large increase in stem growth of trees but a reduction in the rate of soil CO 2 efflux by approximately 8%. This reduction is suggested to be due to lower root activity resulting from reduced allocation of assimilate belowground. Root activity changed when P was added to microsites within plots, and via the whole tree root system at the plot level. These relationships of belowground carbon fluxes with temperature, moisture and nutrient availability provide essential information for understanding and predicting potential changes in forest ecosystems in response to land use management or climate change. 相似文献
19.
In nutrient impoverished landscapes in southwest Australia, terrestrial litter appears to be important in phosphorus (P) turnover and in the gradual accumulation of P in wetland systems. Little is known about the fate of P leached from litter during the wet season and the associated effects of soil microclimate on microbial activity. The effects of temperature, moisture, and litter leaching on soil microbial activity were studied on a transect across a seasonal wetland in southwestern Australia, after the onset of the wet season. Heterotrophic respiration (CO 2 efflux) was higher in the dried lakebed and riparian areas than in upland soils, and higher during the day than at night. There were significant variations in CO 2 efflux with time of sampling, largely caused by the effect of temperature. The addition of litter leachate significantly increased CO 2 efflux, more significantly in soils from upland sites, which had lower moisture and nutrient contents. There was a difference in response of microbial respiration between upland soils and wetland sediments to litter leachate and wetter, warmer conditions. In general, the litter leachate enhanced heterotrophic microbial respiration, and more significantly at warmer conditions (31 °C). The relative fungal to bacterial ratio was 2.9 – 3.2 for surface litter and 0.7–1.0 for soils, suggesting a fungal dominance in heterotrophic respiration of surface litter, but increased bacterial dominance in soils, especially in exposed sediments in the lakebed. 相似文献
20.
The results of published and unpublished experiments investigating the impacts of elevated [CO 2] on the chemistry of leaf litter and decomposition of plant tissues are summarized. The data do not support the hypothesis that changes in leaf litter chemistry often associated with growing plants under elevated [CO 2] have an impact on decomposition processes. A meta-analysis of data from naturally senesced leaves in field experiments showed that the nitrogen (N) concentration in leaf litter was 7.1% lower in elevated [CO 2] compared to that in ambient [CO 2]. This statistically significant difference was: (1) usually not significant in individual experiments, (2) much less than that often observed in green leaves, and (3) less in leaves with an N concentration indicative of complete N resorption. Under ideal conditions, the efficiency with which N is resorbed during leaf senescence was found not to be altered by CO 2 enrichment, but other environmental influences on resorption inevitably increase the variability in litter N concentration. Nevertheless, the small but consistent decline in leaf litter N concentration in many experiments, coupled with a 6.5% increase in lignin concentration, would be predicted to result in a slower decomposition rate in CO 2-enriched litter. However, across the assembled data base, neither mass loss nor respiration rates from litter produced in elevated [CO 2] showed any consistent pattern or differences from litter grown in ambient [CO 2]. The effects of [CO 2] on litter chemistry or decomposition were usually smallest under experimental conditions similar to natural field conditions, including open-field exposure, plants free-rooted in the ground, and complete senescence. It is concluded that any changes in decomposition rates resulting from exposure of plants to elevated [CO 2] are small when compared to other potential impacts of elevated [CO 2] on carbon and N cycling. Reasons for experimental differences are considered, and recommendations for the design and execution of decomposition experiments using materials from CO 2-enrichment experiments are outlined. 相似文献
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