Environmental control of net ecosystem CO2 exchange in a treed, moderately rich fen in northern Alberta

Peatlands cover about 21% of the landscape and contain about 80% of the soil carbon stock in western Canada. However, the current rates of carbon accumulation and the environmental controls on ecosystem photosynthesis and respiration in peatland ecosystems are poorly understood. As part of Fluxnet-Canada, we continuously measured net ecosystem carbon dioxide exchange (NEE) using the eddy covariance technique in a treed fen dominated by stunted Picea mariana and Larix laricina trees during August 2003–December 2004. The total carbon stock in the ecosystem was approximately 51,000 g C m⁻², with only 540 g C m⁻² contributed by live above ground vegetation. The NEE measurements were used to parameterize simple physiological models to assess temporal variation in maximum ecosystem photosynthesis (A_max) and ecosystem respiration rate at 10 °C (R₁₀). During mid-summer the ecosystem had a relatively high A_max (approx. 30 μmol m⁻² s⁻¹) with relatively low R₁₀ (approx. 4 μmol m⁻² s⁻¹). The peak mid-day NEE uptake rate during July and August was 10 μmol m⁻² s⁻¹. The ecosystem showed large seasonal variation in photosynthetic and respiratory activity that was correlated with shifts in temperature, with both spring increases and fall decreases in A_max well predicted by the mean daily air temperature averaged over the preceding 21 days. Leaf-level gas exchange and spectral reflectance measurements also suggested that seasonal changes in photosynthetic activity were primarily controlled by shifts in temperature. Ecosystem respiration was strongly correlated with changes in ecosystem photosynthesis during the growing season, suggesting important links between plant activity and mycorrhizae and microbial activity in the shallow layers of the peat. Only very low rates of respiration were observed during the winter months. During 2004, the peatland recorded a net annual gain of 144 g C m⁻² year⁻¹, the result of a difference between gross photosynthesis of 713 and total ecosystem respiration of 569 g C m⁻² year⁻¹.     

Net ecosystem productivity of boreal aspen forests under drought and climate change: Mathematical modelling with Ecosys

The net ecosystem productivity (NEP) of boreal aspen is strongly affected by comparative rates of annual potential evapotranspiration (E_a) and precipitation (P_a). Changes in E_a versus P_a during future climate change will likely determine changes in aspen NEP and consequently the magnitude of the carbon sink/source of a significant part of the boreal forest. We hypothesize that the effects of E_a versus P_a on aspen NEP can be modelled with a soil–root–canopy hydraulic resistance scheme coupled to a canopy energy balance closure scheme that determines canopy water status and thereby CO₂ uptake. As part of the ecosystem model ecosys, these schemes were used to model diurnal declines in CO₂ and latent heat (LE) exchange during a 3-year drought (2001–2003) at the Fluxnet-Canada Research Network (FCRN) southern old aspen site (SOA). These declines were consistent with those measured by eddy covariance (EC) at SOA, except that ecosystem CO₂ effluxes modelled during most nights were larger that those measured by EC or gap-filled from other EC measurements. Soil CO₂ effluxes in the model were close to, but sometimes smaller than, those measured by automated surface chambers at SOA. Diurnal declines in CO₂ exchange during the drought caused declines in annual NEP in the model, and in gap-filled EC measurements (model versus EC in g C m⁻²: 275 versus 367 ± 110 in 2001, 82 versus 144 ± 43 in 2002 and 23 versus 104 ± 31 in 2003). Lower modelled NEP was attributed to the larger modelled CO₂ effluxes. Ecosys was then used to predict changes in aspen net biome productivity (NBP = NEP − C lost from disturbance) caused by 6-year versus 3-year recurring droughts during 100-year fire cycles under current climate versus climate change projected under the IPCC SRES A1B scenario. Although NBP was adversely affected during recurring 6-year droughts under current climate, it recovered quickly during non-drought years so that long-term NBP was maintained at 4 g C m⁻² year⁻¹. NBP rose by 10, 108 and 126 g C m⁻² year⁻¹ during the first, second and third centuries under climate change with recurring 3-year droughts, indicating a gradual rise in sink activity by boreal aspen. However recurring 6-year droughts during climate change caused recurring negative NBP (C losses), gradually depleting aspen C reserves and eventually causing dieback of the aspen overstory during the third century of climate change. This dieback was followed by a large decline in NBP.We conclude that NBP of boreal aspen will rise gradually under current projections of climate change, except under prolonged (e.g. 6 years) recurring droughts, which would eventually cause aspen to die back and substantial amounts of C to be lost.     

Regional surface flux of CO2 inferred from changes in the advected CO2 column density

A Lagrangian experiment was conducted over Iowa during the daytime (9:00–17:30 LT) on June 19, 2007 as part of the North American Carbon Program's Mid-Continent Intensive using a light-weight and operationally flexible aircraft to measure a net drawdown of CO₂ concentration within the boundary layer. The drawdown can be related to net ecosystem exchange when anthropogenic emissions are estimated using a combination of the Vulcan fossil fuel emissions inventory coupled with a source contribution analysis using HYSPLIT. Results show a temporally and spatially averaged net CO₂ flux of −9.0 ± 2.4 μmol m⁻² s⁻¹ measured from the aircraft data. The average flux from anthropogenic emissions over the measurement area was 0.3 ± 0.1 μmol CO₂ m⁻² s⁻¹. Large-scale subsidence occurred during the experiment, entraining 1.0 ± 0.2 μmol CO₂ m⁻² s⁻¹ into the boundary layer. Thus, the CO₂ flux attributable to the vegetation and soils is −10.3 ± 2.4 μmol m⁻² s⁻¹. The magnitude of the calculated daytime biospheric flux is consistent with tower-based eddy covariance fluxes over corn and soybeans given existing land-use estimates for this agricultural region. Flux values are relatively insensitive to the choice of integration height above the boundary layer and emission footprint area. Flux uncertainties are relatively small compared to the biospheric fluxes, though the measurements were conducted at the height of the growing season.     

Soil carbon dioxide flux, carbon sequestration and crop productivity in a tropical dryland agroecosystem: Influence of organic inputs of varying resource quality

In view of the significance of agricultural soils in affecting global C balance, the impact of manipulation of the quality of exogenous inputs on soil CO₂–C flux was studied in rice–barley annual rotation tropical dryland agroecosystem. Chemical fertilizer, Sesbania shoot (high quality resources), wheat straw (low quality resource) and Sesbania + wheat straw (high + low quality), all carrying equivalent recommended dose of N, were added to soil. A distinct seasonal variation in CO₂–C flux was recorded in all treatments, flux being higher during rice period, and much reduced during barley and summer fallow periods. During rice period the mean CO₂–C flux was greater in wheat straw (161% increase over control) and Sesbania + wheat straw (+129%) treatments; however, during barley and summer fallow periods differences among treatments were small. CO₂–C flux was more influenced by seasonal variations in water-filled pore space compared to soil temperature. In contrast, the role of microbial biomass and live crop roots in regulating soil CO₂–C flux was highly limited. Wheat straw input showed smaller microbial biomass with a tendency of rapid turnover rate resulting in highest cumulative CO₂–C flux. The Sesbania input exhibited larger microbial biomass with slower turnover rate, leading to lower cumulative CO₂–C flux. Addition of Sesbania to wheat straw showed higher cumulative CO₂–C flux yet supported highest microbial biomass with lowest turnover rate indicating stabilization of microbial biomass. Although single application of wheat straw or Sesbania showed comparable net change in soil C (18% and 15% relative to control, respectively) and crop productivity (32% and 38%), yet they differed significantly in soil C balance (374 and −3 g C m⁻² y⁻¹ respectively), a response influenced by the recalcitrant and labile nature of the inputs. Combining the two inputs resulted in significant increment in net change in soil C (33% over control) and crop yield (49%) in addition to high C balance (152 g C m⁻² y⁻¹). It is suggested that appropriate mixing of high and low quality inputs may contribute to improved crop productivity and soil fertility in terms of soil C sequestration.     

Spatial structures of N2O, CO2, and CH4 fluxes from Acacia mangium plantation soils during a relatively dry season in Indonesia

We investigated spatial structures of N₂O, CO₂, and CH₄ fluxes during a relatively dry season in an Acacia mangium plantation stand in Sumatra, Indonesia. The fluxes and soil properties were measured at 1-m intervals in a 1 × 30-m plot (62 grid points) and at 10-m intervals in a 40 × 100-m plot (55 grid points) at different topographical positions of the upper plateau, slope, and valley bottom in the plantation. Spatial structures of each gas flux and soil property were identified using geostatistical analysis. The means (±SD) of N₂O, CO₂, and CH₄ fluxes in the 10-m grids were 0.54 (±0.33) mg N m⁻² d⁻¹, 2.81 (±0.71) g C m⁻² d⁻¹, and −0.84 (±0.33) mg C m⁻² d⁻¹, respectively. This suggests that A. mangium soils function as a larger source of N₂O than natural forest soils in the adjacent province on Sumatra during the relatively dry season, while CO₂ and CH₄ emissions from the A. mangium soils were less than or consistent with those in the natural forest soils. Multiple spatial dependence of N₂O fluxes within 3.2 m (1-m grids) and 35.0 m (10-m grids), and CO₂ fluxes within 1.8 m (1-m grids) and over 65 m (10-m grids) was detected. From the relationship among N₂O and CO₂ gas fluxes, soil properties, and topographic elements, we suggest that the multiple spatial structures of N₂O and CO₂ fluxes are mainly associated with soil resources such as readily mineralizable carbon and nitrogen in a relatively dry season. The soil resource distributions were probably controlled by the meso- and microtopography. Meanwhile, CH₄ fluxes were spatially independent in the A. mangium soils, and the water-filled pore space appeared to mainly control the spatial distribution of these fluxes.     

Winter soil CO2 efflux and its contribution to annual soil respiration in different ecosystems of a forest-steppe ecotone, north China

Most soil respiration measurements are conducted during the growing season. In tundra and boreal forest ecosystems, cumulative winter soil CO₂ fluxes are reported to be a significant component of their annual carbon budgets. However, little information on winter soil CO₂ efflux is known from mid-latitude ecosystems. Therefore, comparing measurements of soil respiration taken annually versus during the growing season will improve the accuracy of ecosystem carbon budgets and the response of soil CO₂ efflux to climate changes. In this study we measured winter soil CO₂ efflux and its contribution to annual soil respiration for seven ecosystems (three forests: Pinus sylvestris var. mongolica plantation, Larix principis-rupprechtii plantation and Betula platyphylla forest; two shrubs: Rosa bella and Malus baccata; and two meadow grasslands) in a forest-steppe ecotone, north China. Overall mean winter and growing season soil CO₂ effluxes were 0.15-0.26 μmol m⁻² s⁻¹ and 2.65-4.61 μmol m⁻² s⁻¹, respectively, with significant differences in the growing season among the different ecosystems. Annual Q₁₀ (increased soil respiration rate per 10 °C increase in temperature) was generally higher than the growing season Q₁₀. Soil water content accounted for 84% of the variations in growing season Q₁₀ and soil temperature range explained 88% of the variation in annual Q₁₀. Soil organic carbon density to 30 cm depth was a good surrogate for SR₁₀ (basal soil respiration at a reference temperature of 10 °C). Annual soil CO₂ efflux ranged from 394.76 g C m⁻² to 973.18 g C m⁻² using observed ecosystem-specific response equations between soil respiration and soil temperature. Estimates ranged from 424.90 g C m⁻² to 784.73 g C m⁻² by interpolating measured soil respiration between sampling dates for every day of the year and then computing the sum to obtain the annual value. The contributions of winter soil CO₂ efflux to annual soil respiration were 3.48-7.30% and 4.92-7.83% using interpolated and modeled methods, respectively. Our results indicate that in mid-latitude ecosystems, soil CO₂ efflux continues throughout the winter and winter soil respiration is an important component of annual CO₂ efflux.     

Carbon balance of a three crop succession over two cropland sites in South West France

Long term flux measurements of different crop species are necessary to improve our understanding of management and climate effects on carbon flux variability as well as cropland potential in terrestrial carbon sequestration. The main objectives of this study were to analyse the seasonal dynamics of CO₂ fluxes and to establish the effects of climate and cropland management on the annual carbon balance.CO₂ fluxes were measured by means of the eddy correlation (EC) method over two cropland sites, Auradé and Lamasquère, in South West France for a succession of three crops: rapeseed, winter wheat and sunflower at Auradé, and triticale, maize and winter wheat at Lamasquère. The net ecosystem exchange (NEE) was partitioned into gross ecosystem production (GEP) and ecosystem respiration (R_E) and was integrated over the year to compute net ecosystem production (NEP). Different methodologies tested for NEP computation are discussed and a methodology for estimating NEP uncertainty is presented.NEP values ranged between −369 ± 33 g C m⁻² y⁻¹ for winter wheat at Lamasquère in 2007 and 28 ± 18 g C m⁻² y⁻¹ for sunflower at Auradé in 2007. These values were in good agreement with NEP values reported in the literature, except for maize which exhibited a low development compared to the literature. NEP was strongly influenced by the length of the net carbon assimilation period and by interannual climate variability. The warm 2007 winter stimulated early growth of winter wheat, causing large differences in GEP, R_E and NEE dynamics for winter wheat when compared to 2006. Management had a strong impact on CO₂ flux dynamics and on NEP. Ploughing interrupted net assimilation during voluntary re-growth periods, but it had a negligible short term effect when it occurred on bare soil. Re-growth events after harvest appeared to limit carbon loss: at Lamasquère in 2005 re-growth contributed to store up to 50 g C m⁻². Differences in NEE response to climatic variables (VPD, light quality) and vegetation index were addressed and discussed.Net biome production (NBP) was calculated yearly based on NEP and considering carbon input through organic fertilizer and carbon output through harvest. For the three crops, the mean NBP at Auradé indicated a nearly carbon balanced ecosystem, whereas Lamasquère lost about 100 g C m⁻² y⁻¹; therefore, the ecosystem behaved as a carbon source despite the fact that carbon was imported through organic fertilizer. Carbon exportation through harvest was the main cause of this difference between the two sites, and it was explained by the farm production type. Lamasquère is a cattle breeding farm, exporting most of the aboveground biomass for cattle bedding and feeding, whereas Auradé is a cereal production farm, exporting only seeds.     

Influence of temperature and drought on seasonal and interannual variations of soil, bole and ecosystem respiration in a boreal aspen stand

Continuous half-hourly measurements of soil (R_s) and bole respiration (R_b), as well as whole-ecosystem CO₂ exchange, were made with a non steady-state automated chamber system and with the eddy covariance (EC) technique, respectively, in a mature trembling aspen stand between January 2001 and December 2003. Our main objective was to investigate the influence of long-term variations of environmental and biological variables on component-specific and whole-ecosystem respiration (R_e) processes. During the study period, the stand was exposed to severe drought conditions that affected much of the western plains of North America. Over the 3 years, daily mean R_s varied from a minimum of 0.1 μmol m⁻² s⁻¹ during winter to a maximum of 9.2 μmol m⁻² s⁻¹ in mid-summer. Seasonal variations of R_s were highly correlated with variations of soil temperature (T_s) and water content (θ) in the surface soil layers. Both variables explained 96, 95 and 90% of the variance in daily mean R_s from 2001 to 2003. Aspen daily mean R_b varied from negligible during winter to a maximum of 2.5 μmol m⁻² bark s⁻¹ (2.2 μmol m⁻² ground s⁻¹) during the growing season. Maximum R_b occurred at the end of the aspen radial growth increment and leaf emergence period during each year. This was 2 months before the peak in bole temperature (T_b) in 2001 and 2003. Nonetheless, R_b was highly correlated with T_b and this variable explained 77, 87 and 62% of the variance in R_b in the respective years. Partitioning of R_b between its maintenance (R_bm) and growth (R_bg) components using the mature tissue method showed that daily mean R_bg occurred at the same time as aspen radial growth increment during each growing season. This method led, however, to systematic over- and underestimations of R_bm and R_bg, respectively, during each year. Annual totals of R_s, R_b and estimated foliage respiration (R_f) from hazelnut and aspen trees were, on average, 829, 159 and 202 g C m⁻² year⁻¹, respectively, over the 3 years. These totals corresponded to 70, 14 and 16%, respectively, of scaled-up respiration estimates of R_e from chamber measurements. Scaled R_e estimates were 25% higher (1190 g C m⁻² year⁻¹) than the annual totals of R_e obtained from EC (949 g C m⁻² year⁻¹). The independent effects of temperature and drought on annual totals of R_e and its components were difficult to separate because the two variables co-varied during the 3 years. However, recalculation of annual totals of R_s to remove the limitations imposed by low θ, suggests that drought played a more important role than temperature in explaining interannual variations of R_s and R_e.     

Carbon dioxide fluxes in coastal Douglas-fir stands at different stages of development after clearcut harvesting

Forests play a significant role in the global carbon (C) cycle. Variability in weather, species, stand age, and current and past disturbances are some of the factors that control stand-level C dynamics. This study examines the relative roles of stand age and associated structural characteristics and weather variability on the exchange of carbon dioxide between the atmosphere and three different coastal Douglas-fir stands at different stages of development after clearcut harvesting. The eddy covariance technique was used to measure carbon dioxide fluxes and a portable soil chamber system was used to measure soil respiration in the three stands located within 50 km of each other on the east coast of Vancouver Island, British Columbia, Canada. In 2002, the recently clearcut harvested stand (HDF00) was a large C source, the pole/sapling aged stand (HDF88) was a moderate C source, and the rotation-aged stand (DF49) was a moderate C sink (net ecosystem production of −606, −133, and 254 g C m⁻² year⁻¹, respectively). Annual gross ecosystem production and ecosystem respiration also increased with increasing stand age. Differences in stand structural characteristics such as species composition and phenology were important in determining the timing and magnitude of maximum gross ecosystem production and net ecosystem production through the year. Both soil and ecosystem respiration were exponentially related to soil temperature in each stand with total ecosystem respiration differing more among stands than soil respiration. Between 1998 and 2003, annual net ecosystem production ranged from 254 to 424 g C m⁻² year⁻¹ over 6 years for DF49, from −623 to −564 g C m⁻² year⁻¹ over 3 years for HDF00, and from −154 to −133 g C m⁻² year⁻¹ over 2 years for HDF88. Interannual variations in C exchange of the oldest, most structurally stable stand (DF49) were related to variations in spring weather while the rapid growth of understory and pioneer species influenced variations in HDF00. The differences in net ecosystem production among stands (maximum of 1000 g C m⁻² year⁻¹ between the oldest and youngest stands) were an order of magnitude greater than the differences among years within a stand and emphasized the importance of age-related differences in stand structure on C exchange processes.     

CO2/heat fluxes in rice fields: Comparative assessment of flooded and non-flooded fields in the Philippines

The seasonal fluxes of heat, moisture and CO₂ were investigated under two different rice environments: flooded and aerobic soil conditions, using the eddy covariance technique during 2008 dry season. The fluxes were correlated with the microclimate prevalent in each location. This study was intended to monitor the environmental impact, in terms of C budget and heat exchange, of shifting from lowland rice production to aerobic rice cultivation as an alternative to maintain crop productivity under water scarcity.The aerobic rice fields had higher sensible heat flux (H) and lower latent heat flux (LE) compared to flooded fields. On seasonal average, aerobic rice fields had 48% more sensible heat flux while flooded rice fields had 20% more latent heat flux. Consequently, the aerobic rice fields had significantly higher Bowen ratio (0.25) than flooded fields (0.14), indicating that a larger proportion of the available net radiation was used for sensible heat transfer or for warming the surrounding air.The total C budget integrated over the cropping period showed that the net ecosystem exchange (NEE) in flooded rice fields was about three times higher than in aerobic fields while gross primary production (GPP) and ecosystem respiration (R_e) were 1.5 and 1.2 times higher, respectively. The high GPP of flooded rice ecosystem was evident because the photosynthetic capacity of lowland rice is naturally large. The R_e of flooded rice fields was also relatively high because it was enhanced by the high photosynthetic activities of lowland rice as manifested by larger above-ground plant biomass. The NEE, GPP, and R_e values for flooded rice fields were −258, 778, and 521 g C m⁻², respectively. For aerobic rice fields, values were −85, 515, and 430 g C m⁻² for NEE, GPP, and R_e, respectively. The ratio of R_e/GPP in flooded fields was 0.67 while it was 0.83 for aerobic rice fields.This short-term data showed significant differences in C budget and heat exchange between flooded and aerobic rice ecosystems. Further investigation is needed to clarify seasonal and inter-annual variations in microclimate, carbon and water budget of different rice production systems.     

Soil carbon dioxide and methane fluxes from long-term tillage systems in continuous corn and corn–soybean rotations

Although the Midwestern United States is one of the world's major agricultural production areas, few studies have assessed the effects of the region's predominant tillage and rotation practices on greenhouse gas emissions from the soil surface. Our objectives were to (a) assess short-term chisel (CP) and moldboard plow (MP) effects on soil CO₂ and CH₄ fluxes relative to no-till (NT) and, (b) determine how tillage and rotation interactions affect seasonal gas emissions in continuous corn and corn–soybean rotations on a poorly drained Chalmers silty clay loam (Typic Endoaquoll) in Indiana. The field experiment itself began in 1975. Short-term gas emissions were measured immediately before, and at increasing hourly intervals following primary tillage in the fall of 2004, and after secondary tillage in the spring of 2005, for up to 168 h. To quantify treatment effects on seasonal emissions, gas fluxes were measured at weekly or biweekly intervals for up to 14 sampling dates in the growing season for corn. Both CO₂ and CH₄ emissions were significantly affected by tillage but not by rotation in the short-term following tillage, and by rotation during the growing season. Soil temperature and moisture conditions in the surface 10 cm were significantly related to CO₂ emissions, although the proportion of variation explained by temperature and moisture was generally very low (never exceeded 27%) and varied with the tillage system being measured. In the short-term, CO₂ emissions were significantly higher for CP than MP and NT. Similarly, mean seasonal CO₂ emissions during the 2-year period were higher for CP (6.2 Mg CO₂-C ha⁻¹ year⁻¹) than for MP (5.9 Mg CO₂-C ha⁻¹ year⁻¹) and NT (5.7 Mg CO₂-C ha⁻¹ year⁻¹). Both CP and MP resulted in low net CH₄ uptake (7.6 and 2.4 kg CH₄-C ha⁻¹ year⁻¹, respectively) while NT resulted in net emissions of 7.7 kg CH₄-C ha⁻¹ year⁻¹. Mean emissions of CO₂ were 16% higher from continuous corn than from rotation corn during the two growing seasons. After 3 decades of consistent tillage and crop rotation management for corn and soybean producing grain yields well above average in the Midwest, continuous NT production in the corn–soybean rotation was identified as the system with the least soil-derived C emissions to the atmosphere from among those evaluated prior to and during corn production.     

Estimating transpiration and the sensitivity of carbon uptake to water availability in a subalpine forest using a simple ecosystem process model informed by measured net CO2 and H2O fluxes

Modeling how the role of forests in the carbon cycle will respond to predicted changes in water availability hinges on an understanding of the processes controlling water use in ecosystems. Recent studies in forest ecosystem modeling have employed data-assimilation techniques to generate parameter sets that conform to observations, and predict net ecosystem CO₂ exchange (NEE) and its component processes. Since the carbon and water cycles are linked, there should be additional process information available from ecosystem H₂O exchange. We coupled SIPNET (Simple Photosynthesis EvapoTranspiration), a simplified model of ecosystem function, with a data-assimilation system to estimate parameters leading to model predictions most closely matching the net CO₂ and H₂O fluxes measured by eddy covariance in a high-elevation, subalpine forest ecosystem. When optimized using measurements of CO₂ exchange, the model matched observed NEE (RMSE = 0.49 g C m⁻²) but underestimated transpiration calculated independently from sap flow measurements by a factor of 4. Consequently, the carbon-only optimization was insensitive to imposed changes in water availability. Including eddy flux data from both CO₂ and H₂O exchange to the optimization reduced the model fit to the observed NEE fluxes only slightly (RME = 0.53 g C m⁻²), however this parameterization also reproduced transpiration calculated from independent sap flow measurements (r² = 0.67, slope = 0.6). A significant amount of information can be extracted from simultaneous analysis of CO₂ and H₂O exchange, which improved the accuracy of transpiration estimates from measured evapotranspiration. Conversely, failure to include both CO₂ and H₂O data streams can generate results that mask the responses of ecosystem carbon cycling to variation in the precipitation. In applying the model conditioned on both CO₂ and H₂O fluxes to the subalpine forest at the Niwot Ridge AmeriFlux site, we observed that the onset of transpiration is coincident with warm soil temperatures. However, after snow has covered the ground in the fall, we observed significant inter-annual variability in the fraction of evapotranspiration composed of transpiration; evapotranspiration was dominated by transpiration in years when late fall air temperatures were high enough to maintain photosynthesis, but by sublimation from the surface of the snowpack in years when late fall air temperatures were colder and forest photosynthetic activity had ceased. Data-assimilation techniques and simultaneous measurements of carbon and water exchange can be used to quantify the response of net carbon uptake to changes in water availability by using an ecosystem model where the carbon and water cycles are linked.     

Modelling multi-year coupled carbon and water fluxes in a boreal aspen forest

In order to test two hypotheses: (i) that carbon (C) and energy exchanges between terrestrial ecosystems and the atmosphere are closely constrained by soil water availability, and (ii) that vegetation is able to optimize soil water uptake from different soil layers; two model simulations were conducted. The Boreal Ecosystem Productivity Simulator (BEPS) model was run to simulate an aspen forest in Saskatchewan, Canada during the period 1997–2004. In Simulation 1, the effect of soil water availability in different soil layers on stomatal conductance was weighted only by root fraction. In Simulation 2, the influence of soil water availability in different soil layers on stomatal conductance was weighted according to both the root fraction and soil water availability, in order to allow easier access of roots to soil layers containing more water.Comparison against measured fluxes showed that Simulation 2 was an improvement over Simulation 1 in predicting C, water and energy fluxes at different time scales in dry years. In Simulation 1, the daytime C and water fluxes were underestimated during the transition from adequate to insufficient soil water content in the upper layers. In this run, the model captured 92, 79 and 91% of the daily variances in gross primary productivity (GPP), net ecosystem productivity (NEP), and ecosystem respiration (R_e) during 1997–2004. In Simulation 2, the daily variances of GPP, NEP, and R_e explained by the model increased to 93, 82 and 92%, respectively. In Simulation 1, the annual NEP was considerably underestimated in the dry years and years with dry periods, with a root mean square error (RMSE) of 45 g C m⁻² year⁻¹ (n = 8) from 1997 to 2004. In Simulation 2, the RMSE value of simulated annual NEP was reduced to 14 g C m⁻² year⁻¹, a relatively small value compared with the average NEP of 157 g C m⁻² year⁻¹ during 1997–2004. This suggested that the ability of plant roots to extract water from deep soil layers is critical for the forest to maintain growth when surface layers dried out. Our model results showed that NEP was very sensitive to water conditions at this site. In wet years, heterotrophic respiration was enhanced and NEP was reduced.     

The distribution of volatile isoprenoids in the soil horizons around Pinus halepensis trees

We measured the terpene concentration in pentane and water extracts from soil horizons (litter, organic, top and low mineral) and from roots growing in top and low mineral horizons on a distance gradient from Pinus halepensis L. trees growing alone on a grassland. Terpene concentrations in pentane were higher than in water extracts, although β-caryophyllene showed relatively high solubility in water. The litter and roots were important sources of terpenes in soil. Alpha-pinene dominated in roots growing in both “top” (A1) and “low” (B) mineral horizons (123 ± 36 μg g⁻¹ or 14 ± 5 mg m⁻²) and roots in low mineral horizon (270 ± 91 μg g⁻¹ or 7 ± 2 mg m⁻²). Beta-caryophyllene dominated in litter (1469 ± 331 μg g⁻¹ or 2004 ± 481 mg m⁻²). Terpene concentration in soil decreased with increasing distance to the trunk. This is likely to be related to changes in litter and roots type on the distance gradient from pine to grass and herbs. The relative contributions of all compounds, except α-pinene, were similar in the mineral soils and litter. This suggests that litter of P. halepensis is probably the main source of major terpene compounds. However, long-term emissions of α-pinene from P. halepensis roots might also contribute to α-pinene concentrations in rhizosphere soils.     

CO2 emission in an intensively cultivated loam as affected by long-term application of organic manure and nitrogen fertilizer

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.     

Effect of heavy metals contamination on root-derived and organic matter-derived CO2 efflux from soil planted with Zea mays

The CO₂ efflux from loamy Haplic Luvisol and heavy metal (HM) uptake by Zea mays L. were studied under increased HM contamination: Cd, Cu, and Ni up to 20, 1000, and 2500 mg kg⁻¹ soil, respectively. Split-root system with contrasting HM concentrations in both soil halves was used to investigate root-mediated HM translocation in uncontaminated soil zones. To separate root-derived and soil organic matter (SOM)-derived CO₂ efflux from soil, ¹⁴CO₂ pulse labeling of 15-, 25-, and 35-days-old plants was applied. The CO₂ evolution from the bare soil was 10.6 μg C–CO₂ d⁻¹ g⁻¹ (32 kg C–CO₂ d⁻¹ ha⁻¹) and was not affected by HM (except 2500 mg Ni kg⁻¹). The average CO₂ efflux from the soil with maize was about two times higher and amounted for about 22.0 μg C–CO₂ d⁻¹ g⁻¹. Portion of assimilates respired in the rhizosphere decreased with plant development from 6.0 to 7.0% of assimilated C for 25-days-old Zea mays to 0.4–2.0% for 45-days-old maize. The effect of the HM on root-derived ¹⁴CO₂ efflux increased with rising HM content in the following order: Cd < Cu < Ni. In Cu and Ni contaminated soils, shoot and root dry matter decreased to 70% and to 50% of the uncontaminated control, respectively. Plants contained much more HM in the roots than in the shoots. A split-root system with contrasting HM concentrations allowed to trace transport of mobile forms of HM by roots from contaminated soil half into the uncontaminated soil half. The portion of mobile HM forms in the soil (1 M NH₄NO₃ extract) increased with contamination and amounted to 9–16%, 2–6% and 1.5–3.5% for Cd, Cu, and Ni, respectively. Corresponding values for the easily available HM (1 M NH₄OAc extract) were 22–52%, 1–20% and 5–8.5%. Heavy metal availability for plants decreased in the following order: Cd > Cu ≥ Ni. No increase of HM availability in the soil was found after maize cultivation.     

Elevated CO2, but not defoliation, enhances N cycling and increases short-term soil N immobilization regardless of N addition in a semiarid grassland

Elevated CO₂ and defoliation effects on nitrogen (N) cycling in rangeland soils remain poorly understood. Here we tested whether effects of elevated CO₂ (720 μl L⁻¹) and defoliation (clipping to 2.5 cm height) on N cycling depended on soil N availability (addition of 1 vs. 11 g N m⁻²) in intact mesocosms extracted from a semiarid grassland. Mesocosms were kept inside growth chambers for one growing season, and the experiment was repeated the next year. We added ¹⁵N (1 g m⁻²) to all mesocosms at the start of the growing season. We measured total N and ¹⁵N in plant, soil inorganic, microbial and soil organic pools at different times of the growing season. We combined the plant, soil inorganic, and microbial N pools into one pool (PIM-N pool) to separate biotic + inorganic from abiotic N residing in soil organic matter (SOM). With the ¹⁵N measurements we were then able to calculate transfer rates of N from the active PIM-N pool into SOM (soil N immobilization) and vice versa (soil N mobilization) throughout the growing season. We observed significant interactive effects of elevated CO₂ with N addition and defoliation with N addition on soil N mobilization and immobilization. However, no interactive effects were observed for net transfer rates. Net N transfer from the PIM-N pool into SOM increased under elevated CO₂, but was unaffected by defoliation. Elevated CO₂ and defoliation effects on the net transfer of N into SOM may not depend on soil N availability in semiarid grasslands, but may depend on the balance of root litter production affecting soil N immobilization and root exudation affecting soil N mobilization. We observed no interactive effects of elevated CO₂ with defoliation. We conclude that elevated CO₂, but not defoliation, may limit plant productivity in the long-term through increased soil N immobilization.     

Carbon exchange in a freshwater marsh in the Sanjiang Plain, northeastern China

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.     

Short-term effects of clearfelling on soil CO2, CH4, and N2O fluxes in a Sitka spruce plantation

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.     

Nitrous oxide and methane emissions from long-term tillage under a continuous corn cropping system in Ohio

Nitrous oxide (N₂O) and methane (CH₄) emitted by anthropogenic activities have been linked to the observed and predicted climate change. Conservation tillage practices such as no-tillage (NT) have potential to increase C sequestration in agricultural soils but patterns of N₂O and CH₄ emissions associated with NT practices are variable. Thus, the objective of this study was to evaluate the effects of tillage practices on N₂O and CH₄ emissions in long-term continuous corn (Zea mays) plots. The study was conducted on continuous corn experimental plots established in 1962 on a Crosby silt loam (fine, mixed, mesic Aeric Ochraqualf) in Ohio. The experimental design consisted of NT, chisel till (CT) and moldboard plow till (MT) treatments arranged in a randomized block design with four replications. The N₂O and CH₄ fluxes were measured for 1-year at 2-week intervals during growing season and at 4-week intervals during the off season. Long-term NT practice significantly decreased soil bulk density (ρ_b) and increased total N concentration of the 0–15 cm layer compared to MT and CT. Generally, NT treatment contained higher soil moisture contents and lower soil temperatures in the surface soil than CT and MT during summer, spring and autumn. Average daily fluxes and annual N₂O emissions were more in MT (0.67 mg m⁻² d⁻¹ and 1.82 kg N ha⁻¹ year⁻¹) and CT (0.74 mg m⁻² d⁻¹ and 1.96 kg N ha⁻¹ year⁻¹) than NT (0.29 mg m⁻² d⁻¹ and 0.94 kg N ha⁻¹ year⁻¹). On average, NT was a sink for CH₄, oxidizing 0.32 kg CH₄-C ha⁻¹ year⁻¹, while MT and CT were sources of CH₄ emitting 2.76 and 2.27 kg CH₄-C ha⁻¹ year⁻¹, respectively. Lower N₂O emission and increased CH₄ oxidation in the NT practice are attributed to decrease in surface ρ_b, suggesting increased gaseous exchange. The N₂O flux was strongly correlated with precipitation, air and soil temperatures, but not with gravimetric moisture content. Data from this study suggested that adoption of long-term NT under continuous corn cropping system in the U.S. Corn Belt region may reduce GWP associated with N₂O and CH₄ emissions by approximately 50% compared to MT and CT management.