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1.
We examined the effects of forest clearfelling on the fluxes of soil CO2, CH4, and N2O in a Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation on an organic-rich peaty gley soil, in Northern England. Soil CO2, CH4, N2O as well as environmental factors such as soil temperature, soil water content, and depth to the water table were recorded in two mature stands for one growing season, at the end of which one of the two stands was felled and one was left as control. Monitoring of the same parameters continued thereafter for a second growing season. For the first 10 months after clearfelling, there was a significant decrease in soil CO2 efflux, with an average efflux rate of 4.0 g m−2 d−1 in the mature stand (40-year) and 2.7 g m−2 d−1 in clearfelled site (CF). Clearfelling turned the soil from a sink (−0.37 mg m−2 d−1) for CH4 to a net source (2.01 mg m−2 d−1). For the same period, soil N2O fluxes averaged 0.57 mg m−2 d−1 in the CF and 0.23 mg m−2 d−1 in the 40-year stand. Clearfelling affected environmental factors and lead to higher daily soil temperatures during the summer period, while it caused an increase in the soil water content and a rise in the water table depth. Despite clearfelling, CO2 remained the dominant greenhouse gas in terms of its greenhouse warming potential.  相似文献   

2.
We evaluated the spatial structures of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) fluxes in an Acacia mangium plantation stand in Sumatra, Indonesia, in drier (August) and wetter (March) seasons. A 60 × 100-m plot was established in an A. mangium plantation that included different topographical elements of the upper plateau, lower plateau, upper slope and foot slope. The plot was divided into 10 × 10-m grids and gas fluxes and soil properties were measured at 77 grid points at 10-m intervals within the plot. Spatial structures of the gas fluxes and soil properties were identified using geostatistical analyses. Averaged N2O and CO2 fluxes in the wetter season (1.85 mg N m−2 d−1 and 4.29 g C m−2 d−1, respectively) were significantly higher than those in the drier season (0.55 mg N m−2 d−1 and 2.73 g C m−2 d−1, respectively) and averaged CH4 uptake rates in the drier season (−0.62 mg C m−2 d−1) were higher than those in the wetter season (−0.24 mg C m−2 d−1). These values of N2O fluxes in A. mangium soils were higher than those reported for natural forest soils in Sumatra, while CO2 and CH4 fluxes were in the range of fluxes reported for natural forest soils. Seasonal differences in these gas fluxes appears to be controlled by soil water content and substrate availability due to differing precipitation and mineralization of litter between seasons. N2O fluxes had strong spatial dependence with a range of about 18 m in both the drier and wetter seasons. Topography was associated with the N2O fluxes in the wetter season with higher and lower fluxes on the foot slope and on the upper plateau, respectively, via controlling the anaerobic-aerobic conditions in the soils. In the drier season, however, we could not find obvious topographic influences on the spatial patterns of N2O fluxes and they may have depended on litter amount distribution. CO2 fluxes had no spatial dependence in both seasons, but the topographic influence was significant in the drier season with lowest fluxes on the foot slope, while there was no significant difference between topographic positions in the wetter season. The distributions of litter amount and soil organic matter were possibly associated with CO2 fluxes through their effects on microbial activities and fine root distribution in this A. mangium plantation.  相似文献   

3.
Northern wetlands are critically important to global change because of their role in modulating atmospheric concentrations of greenhouse gases, especially CO2 and CH4. At present, continuous observations for CO2 and CH4 fluxes from northern wetlands in Asia are still very limited. In this paper, two growing season measurements for CO2 flux by eddy covariance technique and CH4 flux by static chamber technique were conducted in 2004 and 2005, at a permanently inundated marsh in the Sanjiang Plain, northeastern China. The seasonal variations of CO2 exchange and CH4 flux and the environmental controls on them were investigated. During the growing seasons, large variations in net ecosystem CO2 exchange (NEE) and gross ecosystem productivity (GEP) were observed with the range of −4.0 to 2.2 (where negative exchange is a gain of carbon from the atmosphere) and 0-7.6 g C m−2 d−1, respectively. Ecosystem respiration (RE) displayed relatively smooth seasonal pattern with the range of 0.8-4.2 g C m−2 d−1. More than 70% of the total GEP was consumed by respiration, which resulted in a net CO2 uptake of 143 ± 9.8 and 100 ± 9.2 g C m−2 for the marsh over the growing seasons of 2004 and 2005, respectively. A significant portion of the accumulated NEE-C was lost by CH4 emission during the growing seasons, indicating the great potential of CH4 emission from the inundated marsh. Air temperature and leaf area index jointly affected the seasonal variation of GEP and the seasonal dynamic of RE was mainly controlled by soil temperature and leaf area index. Soil temperature also exerted the dominant influence over variation of CH4 flux while no significant relationship was found between CH4 emission and water table level. The close relationships between carbon fluxes and temperature can provide insights into the response of marsh carbon exchange to a changing climate. Future long term flux measurements over the freshwater marsh ecosystems are undoubtedly necessary.  相似文献   

4.
Abstract

Nitrous oxide (N2O) emissions were measured monthly over 1 year in three ecosystems on tropical peatland of Sarawak, Malaysia, using a closed-chamber technique. The three ecosystems investigated were mixed peat swamp forest, sago (Metroxylon sagu) and oil palm (Elaeis guineensis) plantations. The highest annual N2O emissions were observed in the sago ecosystem with a production rate of 3.3 kg N ha?1 year?1, followed by the oil palm ecosystem at 1.2 kg N ha?1 year?1 and the forest ecosystem at 0.7 kg N ha?1 year?1. The N2O emissions ranged from –3.4 to 19.7 µg N m?2 h?1 for the forest ecosystem, from 1.0 to 176.3 µg N m?2 h?1 for the sago ecosystem and from 0.9 to 58.4 µg N m?2 h?1 for the oil palm ecosystem. Multiple regression analysis showed that N2O production in each ecosystem was regulated by different variables. The key factors influencing N2O emissions in the forest ecosystem were the water table and the NH+ 4 concentration at 25–50 cm, soil temperature at 5 cm and nitrate concentration at 0–25 cm in the sago ecosystem, and water-filled pore space, soil temperature at 5 cm and NH+ 4 concentrations at 0–25 cm in the oil palm ecosystem. R2 values for the above regression equations were 0.57, 0.63 and 0.48 for forest, sago and oil palm, respectively. The results suggest that the conversion of tropical peat swamp forest to agricultural crops, which causes substantial changes to the environment and soil properties, will significantly affect the exchange of N2O between the tropical peatland and the atmosphere. Thus, the estimation of net N2O production from tropical peatland for the global N2O budget should take into consideration ecosystem type.  相似文献   

5.
In boreal forests, canopy-scale emissions of biogenic volatile organic compounds (BVOCs) are rather well characterised, but knowledge of ecosystem-scale BVOC emissions is still inadequate. We used adsorbent tubes to measure BVOCs from a boreal Scots pine (Pinus sylvestris L.) forest floor in southern Finland and analysed the compounds with a gas chromatograph-mass spectrometer. The most abundant compound group was the monoterpenes (averaging 5.04 μg m−2 h−1), in which α-pinene, Δ3-carene and camphene contributed over 90% of the emissions. Emissions of other terpenoids (isoprene and sesquiterpenes) were low (averaging 0.05 and 0.04 μg m−2 h−1, respectively). BVOC emissions from the forest floor varied seasonally, peaking in early summer and autumn, with most of the compounds following similar patterns. The emission pattern was sustained throughout the measurement period, suggesting that the main sources of the emissions remained more or less stable. We compared the BVOC fluxes with environmental parameters such as temperature, precipitation and PAR, and with fluxes of other trace gases (CO2, CH4, N2O), as well as with ground vegetation photosynthesis and with litter input. Several of these parameters were correlated with the presence of BVOCs. The sources of soil BVOC emissions are very poorly understood, but our results suggest, that changes in litter quantity and quality, soil microbial activity and the physiological stages of plants are linked with changes in BVOC fluxes.  相似文献   

6.
To assess the impacts of yak excreta patches on greenhouse gas (GHG) fluxes in the alpine meadow of the Qinghai-Tibetan plateau, methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) fluxes were measured for the first time from experimental excreta patches placed on the meadow during the summer grazing seasons in 2005 and 2006. Dung patches were CH4 sources (average 586 μg m−2 h−1 in 2005 and 199 μg m−2 h−1 in 2006) during the investigation period of two years, while urine patches (average −31 μg m−2 h−1 in 2005 and −33 μg m−2 h−1 in 2006) and control plots (average −28 μg m−2 h−1 in 2005 and −30 μg m−2 h−1 in 2006) consumed CH4. The cumulative CO2 emission for dung patches was about 36-50% higher than control plots during the experimental period in 2005 and 2006. The cumulative N2O emissions for both urine and dung patches were 2.1-3.7 and 1.8-3.5 times greater than control plots in 2005 and 2006, respectively. Soil water-filled pore space (WFPS) explained 35% and 36% of CH4 flux variation for urine patches and control plots, respectively. Soil temperature explained 40-75% of temporal variation of CO2 emissions for all treatments. Temporal N2O flux variation in urine patches (34%), dung patches (48%), and control (56%) plots was mainly driven by the simultaneous effect of soil temperature and WFPS. Although yak excreta patches significantly affected GHG fluxes, their contributions to the whole grazing alpine meadow in terms of CO2 equivalents are limited under the moderate grazing intensity (1.45 yak ha−1). However, the contributions of excreta patches to N2O emissions are not negligible when estimating N2O emissions in the grazing meadow. In this study, the N2O emission factor of yak excreta patches varied with year (about 0.9-1.0%, and 0.1-0.2% in 2005 and 2006, respectively), which was lower than IPCC default value of 2%.  相似文献   

7.
The effects of elevated CO2 supply on N2O and CH4 fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m−2. In two of the greenhouses, CO2 concentration was kept at atmospheric concentration (360 μmol mol−1) and in the other two at doubled concentration (720 μmol mol−1). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH4 production and CH4 oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N2O and CH4 exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N2O fluxes (on average, 255 μg N2O m−2 h−1 during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N2O m−2 h−1 during period I) when the competition of vegetation for N was low. CH4 fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH4 were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO2 increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N2O fluxes tended to be higher under doubled CO2 concentrations, indicating that increasing atmospheric CO2 concentration may affect N and C dynamics in farmed peat soil.  相似文献   

8.
Quantifying the net carbon (C) storage of forest plantations is required to assess their potential to offset fossil fuel emissions. In this study, a biometric approach was used to estimate net ecosystem productivity (NEP) for two monoculture plantations in South China: Acacia crassicarpa and Eucalyptus urophylla. This approach was based on stand-level net primary productivity (NPP, based on direct biometric inventory) and heterotrophic respiration (Rh). In comparisons of Rh determination based on trenching vs. tree girdling, both trenching and tree girdling changed soil temperature and soil moisture relative to undisturbed control plots, and we assess the effects of corrections for disturbances of soil moisture and soil moisture on the estimation of soil CO2 efflux partitioning. Soil microbial biomass and dissolved organic carbon were significantly lower in trenched plots than in tree girdled plots for both plantations. Annual soil CO2 flux in trenched plots (Rh-t) was significantly lower than in tree-girdled plots (Rh-g) in both plantations. The estimates of Rh-t and Rh-g, expressed as a percentage of total soil respiration, were 58 ± 4% and 74 ± 6%, respectively, for A. crassicarpa, and 64 ± 3% and 78 ± 5%, respectively, for E. urophylla. By the end of experiment, the difference in soil CO2 efflux between the trenched plots and tree-girdled plots had become small for both plantations. Annual Rh (mean of the annual Rh-t and Rh-g) and net primary production (NPP) were 470 ± 25 and 800 ± 118 g C m−2 yr−1, respectively, for A. crassicarpa, and 420 ± 35 and 2380 ± 187 g C m−2 yr−2, respectively, for E. urophylla. The two plantations in the developmental stage were large carbon sinks: NEP was 330 ± 76 C m−2 yr−1 for A. crassicarpa and 1960 ± 178 g C m−2 yr−1 for E. urophylla.  相似文献   

9.
Here we present results from a field experiment in a sub-arctic wetland near Abisko, northern Sweden, where the permafrost is currently disintegrating with significant vegetation changes as a result. During one growing season we investigated the fluxes of CO2 and CH4 and how they were affected by ecosystem properties, i.e., composition of species that are currently expanding in the area (Carex rotundata, Eriophorum vaginatum and Eriophorum angustifolium), dissolved CH4 in the pore water, substrate availability for methane producing bacteria, water table depth, active layer, temperature, etc. We found that the measured gas fluxes over the season ranged between: CH4 0.2 and 36.1 mg CH4 m−2 h−1, Net Ecosystem Exchange (NEE) −1000 and 1250 mg CO2 m−2 h−1 (negative values meaning a sink of atmospheric CO2) and dark respiration 110 and 1700 mg CO2 m−2 h−1. We found that NEE, photosynthetic rate and CH4 emission were affected by the species composition. Multiple stepwise regressions indicated that the primary explanatory variables for NEE was photosynthetic rate and for respiration and photosynthesis biomass of green leaves. The primary explanatory variables for CH4 emissions were depth of the water table, concentration of organic acid carbon and biomass of green leaves. The negative correlations between pore water concentration and emission of CH4 and the concentrations of organic acid, amino acid and carbohydrate carbon indicated that these compounds or their fermentation by-products were substrates for CH4 formation. Furthermore, calculation of the radiative forcing of the species expanding in the area as a direct result of permafrost degradation and a change in hydrology indicate that the studied mire may act as an increasing source of radiative forcing in future.  相似文献   

10.
We measured methane (CH4) emissions from the stem surfaces of mature Fraxinus mandshurica var. japonica trees in a floodplain forest. Flux measurements were conducted almost monthly from May to October 2005, and positive CH4 fluxes were detected throughout the study period, including the leafless season. The mean CH4 flux was 176 and 97 μg CH4 m−2 h−1 at the lower (15 cm above the ground) and upper (70 cm above the ground) stem positions, respectively. The CH4 concentration was lower in soil gas than in ambient air to a depth of at least 40 cm. One possible source of CH4 emitted from the stems might be the dissolved CH4 in groundwater; maximum concentrations were 10,000 times higher than atmospheric CH4 concentrations. Our results suggest that CH4 transport from the submerged soil layer to the atmosphere may occur through internal air spaces in tree bodies.  相似文献   

11.
A long-term field experiment was conducted to examine the influence of mineral fertilizer and organic manure on the equilibrium dynamics of soil organic C in an intensively cultivated fluvo-aquic soil in the Fengqiu State Key Agro-Ecological Experimental Station (Fengqiu county, Henan province, China) since September 1989. Soil CO2 flux was measured during the maize and wheat growing seasons in 2002-2003 and 2004 to evaluate the response of soil respiration to additions and/or alterations in mineral fertilizer, organic manure and various environmental factors. The study included seven treatments: organic manure (OM), half-organic manure plus half-fertilizer N (NOM), fertilizer NPK (NPK), fertilizer NP (NP), fertilizer NK (NK), fertilizer PK (PK) and control (CK). Organic C in soil and the soil heavy fraction (organo-mineral complex) was increased from 4.47 to 8.61 mg C g−1 and from 3.32 to 5.68 mg C g−1, respectively, after the 13 yr application of organic manure. In contrast, organic C and the soil heavy fraction increased in NPK soil to only 5.41 and 4.38 mg C g−1, respectively. In the CK treatment, these parameters actually decreased from the initial C concentrations (4.47 and 3.32 mg C g−1) to 3.77 and 3.11 mg C g−1, respectively. Therefore, organic manure efficiently elevated soil organic C. However, only 66% of the increased soil organic C was combined with clay minerals in the OM treatment. Cumulative soil CO2 emissions from inter-row soil in the OM and NPK treatments were 228 and 188 g C m−2 during the 2002 maize growing season, 132 and 123 g C m−2 during the 2002/2003 wheat growing season, and 401 and 346 g C m−2 yr−1 in 2002-2003, respectively. However, during the 2004 maize growing season, cumulative soil CO2 emissions were as high as 617 and 556 g C m−2, respectively, due to the contribution of rhizosphere respiration. The addition of organic manure contributed to a 16% increase in soil CO2 emission in 2002-2003 (compared to NPK), where only 27%, 36% and 24% of applied organic C was released as CO2 during the 2002 and 2004 maize growing seasons and in 2002-2003, respectively. During the 2002/2003 wheat growing season, soil CO2 flux was significantly affected by soil temperature below 20 °C, but by soil moisture (WFPS) during the 2004 maize growing season at soil temperatures above 18 °C. Optimum soil WFPS for soil CO2 flux was approximately 70%. When WFPS was below 50%, it no longer had a significant impact on soil CO2 flux during the 2002 maize growing season. This study indicates the application of organic manure composted with wheat straw may be a preferred strategy for increasing soil organic C and sequestering C in soil.  相似文献   

12.
We studied the effects of soil management and changes of land use on soils of three adjacent plots of cropland, pasture and oak (Quercus robur) forest. The pasture and the forest were established in part of the cropland, respectively, 20 and 40 yr before the study began. Soil organic matter (SOM) dynamics, water-filled pore space (WFPS), soil temperature, inorganic N and microbial C, as well as fluxes of CO2, CH4 and N2O were measured in the plots over 25 months. The transformation of the cropland to mowed pasture slightly increased the soil organic and microbial C contents, whereas afforestation significantly increased these variables. The cropland and pasture soils showed low CH4 uptake rates (<1 kg C ha−1 yr−1) and, coinciding with WFPS values >70%, episodes of CH4 emission, which could be favoured by soil compaction. In the forest site, possibly because of the changes in soil structure and microbial activity, the soil always acted as a sink for CH4 (4.7 kg C ha−1 yr−1). The N2O releases at the cropland and pasture sites (2.7 and 4.8 kg N2O-N ha−1 yr−1) were, respectively, 3 and 6 times higher than at the forest site (0.8 kg N2O-N ha−1 yr−1). The highest N2O emissions in the cultivated soils were related to fertilisation and slurry application, and always occurred when the WFPS >60%. These results show that the changes in soil properties as a consequence of the transformation of cropfield to intensive grassland do not imply substantial changes in SOM or in the dynamics of CH4 and N2O. On the contrary, afforestation resulted in increases in SOM content and CH4 uptake, as well as decreases in N2O emissions.  相似文献   

13.
The self-heating correction is known to modify open-path eddy covariance estimates of net ecosystem CO2 exchange, typically towards reduced uptake or enhanced emissions, but with a magnitude heretofore not generally documented. We assess the magnitude of this correction to be of order 1 μmol m−2 s−1 (daytime) for half-hourly fluxes and consistently over 100 g C m−2 for annual integrations, across a tower network (CARBORED-ES) spanning climate zones from Mediterranean temperate to cool alpine. We furthermore examine the sensitivity of the correction to its determining factors. Due to significant diurnal variation, the means of discriminating day versus night can lead to differences of up to several tens of g C m−2 year−1. Since its principal determinants - temperature and wind speed - do not include gas flux data, the annual correction can be estimated using only meteorological data so as to avoid uncertainties introduced when filling gaps in flux data. For fast retro-correction of annual integrations published prior to the recognition of this instrument surface heating effect, the annual impact can be roughly approximated to within 12 g C m−2 year−1 by a linear function of mean annual temperature. These determinations highlight the need for the flux community to reach a consensus regarding the need for and the specific form of this correction.  相似文献   

14.
We quantified spatial and temporal variations of the fluxes of nitrous oxide (N2O) and methane (CH4) and associated abiotic sediment parameters across a subtropical river estuary sediment dominated by grey mangrove (Avicennia marina). N2O and CH4 fluxes from sediment were measured adjacent to the river (“fringe”) and in the mangrove forest (“forest”) at 3-h intervals throughout the day during autumn, winter and summer. N2O fluxes from sediment ranged from an average of −4 μg to 65 μg N2O m−2 h−1 representing N2O sink and emission. CH4 emissions varied by several orders of magnitude from 3 μg to 17.4 mg CH4 m−2 h−1. Fluxes of N2O and CH4 differed significantly between sampling seasons, as well as between fringe and forest positions. In addition, N2O flux differed significantly between time of day of sampling. Higher bulk density and total carbon content in sediment were significant contributors towards decreasing N2O emission; rates of N2O emission increased with less negative sediment redox potential (Eh). Porewater profiles of nitrate plus nitrite (NOx−) suggest that denitrification was the major process of nitrogen transformation in the sediment and possible contributor to N2O production. A significant decrease in CH4 emission was observed with increasing Eh, but higher sediment temperature was the most significant variable contributing to CH4 emission. From April 2004 to July 2005, sediment levels of dissolved ammonium, nitrate, and total carbon content declined, most likely from decreased input of diffuse nutrient and carbon sources upstream from the study site; concomitantly average CH4 emissions decreased significantly. On the basis of their global warming potentials, N2O and CH4 fluxes, expressed as CO2-equivalent (CO2-e) emissions, showed that CH4 emissions dominated in summer and autumn seasons (82-98% CO2-e emissions), whereas N2O emissions dominated in winter (67-95% of CO2-e emissions) when overall CO2-e emissions were low. Our study highlights the importance of seasonal N2O contributions, particularly when conditions driving CH4 emissions may be less favourable. For the accurate upscaling of N2O and CH4 flux to annual rates, we need to assess relative contributions of individual trace gases to net CO2-e emissions, and the influence of elevated nutrient inputs and mitigation options across a number of mangrove sites or across regional scales. This requires a careful sampling design at site-level that captures the potentially considerable temporal and spatial variation of N2O and CH4 emissions.  相似文献   

15.
The accumulation and transformation of organic matter during soil development is rarely investigated although such processes are relevant when discussing about carbon sequestration in soil. Here, we investigated soils under grassland and forest close to the North Sea that began its genesis under terrestrial conditions 30 years ago after dikes were closed. Organic C contents of up to 99 mg g−1 soil were found until 6 cm soil depth. The humus consisted mainly of the fraction lighter than 1.6 g cm−3 which refers to poorly degraded organic carbon. High microbial respiratory activity was determined with values between 1.57 and 1.17 μg CO2-C g−1 soil h−1 at 22 °C and 40 to 70% water-holding capacity for the grassland and forest topsoils, respectively. The microbial C to organic C ratio showed values up to 20 mg Cmic g−1 Corg. Although up to 2.69 kg C m−2 were estimated to be sequestered during 30 years, the microbial indicators showed intensive colonisation and high transformation rates under both forest and grassland which were higher than those determined in agricultural and forest topsoils in Northern Germany.  相似文献   

16.
The aim of this study was to investigate the effects of increased N deposition on new and old pools of soil organic matter (SOM). We made use of a 4-yr experiment, where spruce and beech growing on an acidic loam and a calcareous sand were exposed to increased N deposition (7 vs. 70 kg N ha−1 yr−1) and to elevated atmospheric CO2. The added CO2 was depleted in 13C, which enabled us to distinguish between old and new C in SOM-pools fractionated into particle sizes. Elevated N deposition for 4 yr increased significantly the contents of total SOM in 0-10 cm depth of the acidic loam (+9%), but not in the calcareous sand. Down to 25 cm soil depth, C storage in the acidic loam was between 100 and 300 g C m−2 larger under high than under low N additions. However, this increase was small as compared with the SOM losses of 600-700 g C g C 0.25 m−1 m−2 from the calcareous sand resulting from the disturbance of soils during setting up of the experiment. The amounts of new, less than 4 yr old SOM in the sand fractions of both soils were greater under high N deposition, showing that C inputs from trees into soils increased. Root biomass in the acidic loam was larger under N additions (+25%). Contents of old, more than 4 yr old C in the clay and silt fractions of both soils were significantly greater under high than under low N deposition. Since clay- and silt-bound SOM consists of humified compounds, this indicates that N additions retarded mineralization of old and humified SOM. The retardation of C mineralization in the clay and silt fraction accounted for 60-80 g C m−2 4 yr−1, which corresponds to about 40% of the old SOM mineralized in these fraction. As a consequence, preservation of old and humified SOM under elevated N deposition might be a process that could lead to an increased soil C storage in the long-term.  相似文献   

17.
Afforestation and reforestation of pastures are key land-use changes in New Zealand that help sequester carbon (C) to offset its carbon dioxide (CO2) emissions under the Kyoto Protocol. However, relatively little attention has been given so far to associated changes in trace gas fluxes. Here, we measure methane (CH4) fluxes and CO2 production, as well as microbial C, nitrogen (N) and mineral-N, in intact, gradually dried (ca. 2 months at 20 °C) cores of a volcanic soil and a heavier textured, non-volcanic soil collected within plantations of Pinus radiata D. Don (pine) and adjacent permanent pastures. CH4 fluxes and CO2 production were also measured in cores of another volcanic soil under reverting shrubland (mainly Kunzea var. ericoides (A. Rich) J. Thompson) and an adjacent pasture. CH4 uptake in the pine and shrubland cores of the volcanic soils at field capacity averaged about 35 and 14 μg CH4-C m−2 h−1, respectively, and was significantly higher than in the pasture cores (about 21 and 6 μg CH4-C m−2 h−1, respectively). In the non-volcanic soil, however, CH4-C uptake was similar in most cores of the pine and pasture soils, averaging about 7-9 μg m−2 h−1, except in very wet samples. In contrast, rates of CO2 production and microbial C and N concentrations were significantly lower under pine than under pasture. In the air-dry cores, microbial C and N had declined in the volcanic soil, but not in the non-volcanic soil; ammonium-N, and especially nitrate-N, had increased significantly in all samples. CH4 uptake was, with few exceptions, not significantly influenced by initial concentrations of ammonium-N or nitrate-N, nor by their changes on air-drying. A combination of phospholipid fatty acid (PLFA) and stable isotope probing (SIP) analyses of only the pine and pasture soils showed that different methanotrophic communities were probably active in soils under the different vegetations. The C18 PLFAs (type II methanotrophs) predominated under pine and C16 PLFAs (type I methanotrophs) predominated under pasture. Overall, vegetation, soil texture, and water-filled pore space influenced CH4-C uptake more than did soil mineral-N concentrations.  相似文献   

18.
We examined net greenhouse gas exchange at the soil surface in deciduous forests on soils with high organic contents. Fluxes of CO2, CH4 and N2O were measured using dark static chambers for two consecutive years in three different forest types; (i) a drained and medium productivity site dominated by birch, (ii) a drained and highly productive site dominated by alder and (iii) an undrained and highly productive site dominated by alder. Although the drained sites had shallow mean groundwater tables (15 and 18 cm, respectively) their average annual rates of forest floor CO2 release were almost twice as high compared to the undrained site (1.9±0.4 and 1.7±0.3, compared to 1.0±0.2 kg CO2 m−2 yr−1). The average annual CH4 emission was almost 10 times larger at the undrained site (7.6±3.1 compared to 0.9±0.5 g CH4 m−2 yr−1 for the two drained sites). The average annual N2O emissions at the undrained site (0.1±0.05 g N2O m−2 yr−1) were lower than at the drained sites, and the emissions were almost five times higher at the drained alder site than at the drained birch site (0.9±0.35 compared to 0.2±0.11 g N2O m−2 yr−1). The temporal variation in forest floor CO2 release could be explained to a large extent by differences in groundwater table and air temperature, but little of the variation in the CH4 and N2O fluxes could be explained by these variables. The measured soil variables were only significant to explain for the within-site spatial variation in CH4 and N2O fluxes at the undrained swamp, and dark forest floor CO2 release was not explained by these variables at any site. The between-site spatial variation was attributed to variations in drainage, groundwater level position, productivity and tree species for all three gases. The results indicate that N2O emissions are of greater importance for the net greenhouse gas exchange at deciduous drained forest sites than at coniferous drained forest sites.  相似文献   

19.
The exchange of CO2 between the atmosphere and a beech forest near Sorø, Denmark, was measured continuously over 14 years (1996-2009). The simultaneous measurement of many parameters that influence CO2 uptake makes it possible to relate the CO2 exchange to recent changes in e.g. temperature and atmospheric CO2 concentration. The net CO2 exchange (NEE) was measured by the eddy covariance method. Ecosystem respiration (RE) was estimated from nighttime values and gross ecosystem exchange (GEE) was calculated as the sum of RE and NEE. Over the years the beech forest acted as a sink of on average of 157 g C m−2 yr−1. In one of the years only, the forest acted as a small source. During 1996-2009 a significant increase in annual NEE was observed. A significant increase in GEE and a smaller and not significant increase in RE was also found. Thus the increased NEE was mainly attributed to an increase in GEE. The overall trend in NEE was significant with an average increase in uptake of 23 g C m−2 yr−2. The carbon uptake period (i.e. the period with daily net CO2 gain) increased by 1.9 days per year, whereas there was a non significant tendency of increase of the leafed period. This means that the leaves stayed active longer. The analysis of CO2 uptake by the forest by use of light response curves, revealed that the maximum rate of photosynthetic assimilation increased by 15% during the 14-year period. We conclude that the increase in the overall CO2 uptake of the forest is due to a combination of increased growing season length and increased uptake capacity. We also conclude that long time series of flux measurements are necessary to reveal trends in the data because of the substantial inter-annual variation in the flux.  相似文献   

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