Two-dimensional simulation of airflow and carbon dioxide transport over a forested mountain: Part II. Carbon dioxide budget analysis and advection effects

We apply a high-resolution atmospheric model to assess the influence of mesoscale advection of CO₂ on the estimation of net ecosystem exchange (NEE) using eddy-covariance CO₂ flux measurements at a Fluxnet-Canada forest site located on sloping terrain on Vancouver Island, Canada. The numerical simulation is performed for fair-weather conditions over an idealized two-dimensional mountain bounded by water. The model is enhanced to include a CO₂ budget with a treatment of canopy photosynthesis and soil respiration.The simulation captures the transport of CO₂ by nocturnal drainage flows and weak land breezes. The resulting vertical profiles and time evolution of CO₂ concentration show a significant variation near the ground, associated with stability changes in the atmospheric boundary layer. The simulated vertical CO₂ gradients are found to be large around sunset and sunrise. The decrease of CO₂ concentration over land after midnight and the CO₂ accumulation over the neighboring water surface indicate CO₂ advection.A CO₂ budget analysis of the numerical-model output shows that the mean horizontal and vertical advection have significant fluctuations and opposite signs during daytime, with the net result that they largely counteract each other. At night, mean advection results in the underestimation by 20% of the nocturnal respiration. The estimated NEE at night is dominated by sub-grid-scale vertical flux in this simulation. Further evaluation using 3D simulations with higher resolution is needed to see if our results hold where vertical fluxes are much better resolved.     

Soil carbon dioxide fluxes in established switchgrass land managed for biomass production

Switchgrass (Panicum virgatum L.) grown for biomass feedstock production has the potential to increase soil C sequestration, and soil CO₂ flux in grassland is an important component in the global C budget. The objectives of this study were to: (1) determine the effects of N fertilization and harvest frequency on soil CO₂ flux, soil microbial biomass carbon (SMBC), and potentially mineralizable carbon (PMC); and (2) evaluate the relationship of soil CO₂ flux with soil temperature, soil moisture, SMBC, and PMC. Two N rates (0 and 224 kg ha⁻¹) were applied as NH₄NO₃ and cattle (Bos Taurus L.) manure. Switchgrass was harvested every year at anthesis or alternate years at anthesis. The data were collected during growing season (May-October) 2001-2004 on switchgrass-dominated Conservation Reserve Program (CRP) land in east-central South Dakota, USA. Manure application increased soil CO₂ flux, SMBC, and PMC during the early portion of the growing season compared with the control, but NH₄NO₃ application did not affect soil CO₂ flux, SMBC, and PMC. However, seasonal variability of soil CO₂ flux was not related to SMBC and PMC. Estimated average soil CO₂ fluxes during the growing periods were 472, 488, and 706 g CO₂-C m⁻² for control, NH₄NO₃-N, and manure-N plots, respectively. Switchgrass land with manure application emitted more CO₂, and approximately 45% of the C added with manure was respired to the atmosphere. Switchgrass harvested at anthesis decreased soil CO₂ flux during the latter part of the growing season, and flux was lower under every year harvest treatment than under alternate years harvest. Soil temperature was the most significant single variable to explain the variability in soil CO₂ flux. Soil water content was not a limiting factor in controlling seasonal CO₂ flux.     

Carbon dioxide and energy fluxes from a boreal mixedwood forest ecosystem in Ontario, Canada

A long-term flux measurement station has been established in a 74-year-old mixedwood forest ecosystem, located approximately 80 km west of Timmins in northern Ontario, as part of the Fluxnet-Canada Research Network (FCRN). Measurements of energy, water vapour, and carbon dioxide fluxes have been made continuously since August 2003 using the eddy covariance technique, along with ancillary meteorological variables. The spatial structure of the site was evaluated using a variety of sources and techniques, including remote sensing, showing that this forest is mixed but relatively homogeneous. The canopy top height is remarkably constant at between 30 and 32 m. The basal area varies from 18 to 27 m² ha⁻¹, and the aboveground biomass ranges from 82 to 122 Mg ha⁻¹. In this paper, we summarize the diurnal and seasonal patters of carbon dioxide exchange and water loss from September 1, 2003 to August 31, 2004. Net ecosystem productivity (NEP) is strongly related to temperature. Atmospheric vapour pressure deficit (VPD) in this ecosystem exerted strong biophysical control on the daily gross ecosystem productivity (GEP) and evapotranspiration. Seasonal change in shortwave albedo, as a result of the presence of mixed deciduous and coniferous species, was clearly evident. Albedo changes were comparable to the seasonal pattern of NEP. The dormant season lasts more than 6 months of the year at this station. This forest was a moderate sink of carbon over the measurement period. Annual values of GEP, ecosystem respiration (R), and NEP were 1075, 919, and 156 ± 35 g C m⁻², respectively.     

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.     

Subtropical plantations are large carbon sinks: Evidence from two monoculture plantations in South China

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 (R_h). In comparisons of R_h 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 (R_h-t) was significantly lower than in tree-girdled plots (R_h-g) in both plantations. The estimates of R_h-t and R_h-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 R_h (mean of the annual R_h-t and R_h-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.     

Long-term effects of seasonal and diurnal temperature fluctuations on carbon dioxide efflux from a forest soil

Temperature fluctuations are a fundamental entity of the soil environment in the temperate zone and show fast (diurnal) and slow (seasonal) dynamics. Responses of soil respiration to temperature fluctuations were investigated in a root-free soil of a mid-European beech-oak forest. First, in laboratory we analysed the efflux of CO₂ from soil microcosms exposed to seasonal (±5 °C of the annual mean) and diurnal fluctuations (±5 °C of the seasonal levels) in a two-factorial design. Second, in field microcosms we investigated effects of smoothing diurnal temperature fluctuations in soil (simulating a possible global trend) on CO₂ efflux. Third, the natural temperature regime was simulated in laboratory microcosms and their CO₂ efflux was compared to the one in the field. The experiments lasted for 1 year to differentiate seasonal and annual responses.Dynamics of CO₂ efflux, microbial basal respiration, biomass and qO₂ varied with seasonal temperature regime. However, in the laboratory the annual cumulative CO₂-C production did not differ between treatments and varied between 10.9% and 11.7% of the total microcosm C, disregarding seasonal and/or diurnal fluctuations. The similarity of cumulative C production suggests that the availability of microbially mobilisable carbon pools rather than the temperature regime limited soil respiration. Diurnal fluctuations generally did not affect CO₂ efflux and microbial activity, though winter Q₁₀ values were increased in their absence. Simulation of the natural temperature regime in the laboratory resulted in CO₂ efflux similar to field microcosms. In the field, rates of CO₂ efflux and microbial activity, seasonal and annual cumulative CO₂-C production were significantly higher at smoothed than at natural temperature conditions (annually 13.1% and 11.0% of total C was respired, respectively). Facing global climate changes the mechanisms regulating responses of soil respiration to temperature fluctuations need further investigation.     

Effect of elevated carbon dioxide and water stress on gas exchange and water use efficiency in corn

CO₂ has been predicted to increase in the future, and thus leading to possible changes in precipitation patterns. The objectives of this study were to investigate water use and canopy level photosynthesis of corn plants, and to quantify water use efficiency in corn plants under two different CO₂ levels combined with four different water stress levels. Corn plants were planted in sunlit plant growth chambers and a day/night temperature of (28/18 °C) was applied. From 21 days after emergence (DAE), the eight treatments including two levels of carbon dioxide concentrations (400 and 800 μmol mol⁻¹) and four levels of water stress (well-watered control, “mild”, “moderate”, and “severe” water stress) treatments at each CO₂ level were imposed. Height, number of leaves, leaf lengths, and growth stages of corn plants were monitored from nine plants twice a week. Corn plants were separately collected, dried, and analyzed for the biomass accumulation at 21 and 60 DAE. Soil water contents were monitored by a time domain reflectometry (TDR) system (15 probes per chamber). The “breaking points” (changes from high to low rates of soil water uptake) were observed in the bottom of soil depth for the water stressed conditions, and the “breaking points” under ambient CO₂ appeared 6-9 days earlier than under elevated CO₂. Although approximately 20-49% less water was applied for the elevated CO₂ treatments than for ambient CO₂ from 21 DAE, higher soil water contents were recorded under elevated CO₂ than under ambient CO₂. However, corn growth variables such as height, leaf area, and biomass accumulation were not significantly different in CO₂ or water stressed treatments. This result may be explained by considering that significant differences in canopy level gross photosynthesis among the water stress treatments was observed only toward the end of the experiment. The higher soil water contents observed under elevated CO₂ resulted mainly from less water use than under ambient CO_2. WUE (above ground biomass per water use since 21 DAE) at the final harvest was consistently higher and varied with a smaller range under elevated CO₂ than under ambient CO₂. This study suggests that less water will be required for corn under high-CO₂ environment in the future than at present.     

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.     

Field measurement of carbon dioxide evolution from soil by a flow-through chamber method using a portable photosynthesis meter

Extract

Since a rise in atmospheric carbon dioxide (CO₂) concentration is expected to lead to global warming, it is important to quantify the global carbon circulation. The CO₂ evolution rate from soil has usually been measured by one of three methods: 1) CO₂ absorption (Anderson 1982), where the evolved CO₂ is absorbed in an alkali solution and the content subsequently determined, 2) closed chamber (Rolston 1986) in which the CO₂ evolution rate is calculated from the increase of the CO₂ concentration in a closed chamber covering the soil surface, and 3) flow-through chamber (Rolston 1986) in which a fixed rate of ambient air is pumped through an open chamber and the difference in the. CO₂ concentration between the inlet and the outlet is measured. Although the CO₂ absorption method is very simple in terms of apparatus and procedure, the determined CO₂ evolution rate tends to be underestimated in cases where the evolved CO₂ is not fully absorbed in the alkali solution (Ewel et al. 1987; Sakamoto and Yoshida 1988), or overestimated in cases where the CO₂ concentration in the chamber is too low to stimulate microbial activity (Koizumi et al. 1991; Nakadai et al. 1993), In the closed chamber method, when the gas concentration in the chamber is higher than that of the ambient air, gas diffusion from the soil to the atmosphere is restricted (Denmead 1978). At this point, the flow-through chamber method seems to be most suitable for measuring the CO₂ evolution rate, because the rate is determined under nearly natural conditions. However, this method has a disadvantage in that the apparatus is composed of an infra-red CO₂ analyzer, air pumps, mass flow meters, a recorder, and other items, which are too large, heavy, and complex to use in the field (Freijer and Bouten 1991). Hence, in spite of the above limitations, most of the studies on CO₂ evolution in situ have been carried out using the CO₂ absorption method (Kowalenko et al. 1978; Seto et al. 1978a, b; Ewel et al 1981, 1987; Gupta and Singh 1981; Reinke et al. 1981; Edwards and Ros-Todd 1983; Grahammer et al. 1991) or the closed chamber method (Naganawa et al. 1989; Mariko et al. 1994). The flow-through chamber method has been used only at sites where electric power supply and other types of equipment were available (Mathes and Schriefer 1985; Ewel et al. 1987; Nakadai et al. 1993). In the present report a flow-through chamber method using a portable CO₂ analyzer system was examined, for the determination of CO₂ evolution from soil without an electric power supply or other special equipment.     

Carbon budget and seasonal carbon dioxide emission from a central Ohio Luvisol as influenced by wheat residue amendment

Enhancement of soil organic carbon (SOC) stocks through mulching has been proposed, and although this practice can alter several soil properties, its impact on the temporal variability of carbon dioxide (CO₂) emission from soils has not been widely investigated. To that end, we monitored CO₂ fluxes from a central Ohio Luvisol (fine, mixed, mesic Aeric Ochraqualf) amended with wheat (Triticum aestivum L.) straw applied at rates of 0 (M0), 8 (M8) and 16 (M16) Mg dry matter ha⁻¹ per year and supplemented with fertilizer (244 kg N ha⁻¹ per year) or without. The experimental design was a randomized complete block design with three replications. The intensity of CO₂ emission was higher in the late winter (mean: 2.79 g CO₂-C m⁻² per day) and summer seasons (2.45 g CO₂-C m⁻² per day) and lowest in the autumn (1.34 g CO₂-C m⁻² per day). While no significant effect of N fertilization on CO₂ emission was detected, soil mulching had a significant effect on the seasonal variation of CO₂ fluxes. The percentage of annual CO₂ emitted during the winter and spring was similar across treatments (17–22%); however, 43% of the annual CO₂ loss in the M0 plots occurred during the summer as opposed to 26% in the mulch treatments. A close relationship (F=0.47X+4.45, R²=0.97, P<0.001) was found between annual CO₂ flux (F, Mg CO₂-C ha⁻¹) and residue-C input (X, Mg C ha⁻¹). Litter and undecomposed residue amounted to 0.32 and 0.67 Mg C ha⁻¹ per year in the M8 and M16 plots, respectively. After 4 years of straw application, SOC stocks (0–10 cm) were 19.6, 25.6 and 26.5 Mg C ha⁻¹ in the M0, M8 and M16 treatments, respectively. The results show that soil mulching has beneficial effect on SOC sequestration and strongly influence the temporal pattern of CO₂ emission from soils.     

A comparison of regression methods for estimating soil-atmosphere diffusion gas fluxes by a closed-chamber technique

Continuous changes in methane (CH₄) and carbon dioxide (CO₂) concentrations inside a closed chamber were measured on the forest floor at three sites: a deciduous forest and a coniferous forest in Hokkaido, Japan, and a birch forest in West Siberia, Russian Federation. Flux estimations by three types of regression methods, exponential, nonlinear, and linear, were examined using field-collected concentration data. The pattern of change with time of the gas concentration in the headspace differed, mainly according to site but also, to a lesser extent, according to the gas. This was a function of both the chamber height and surface soil property relating to soil gas diffusion and the gas concentration profile. Flux estimations did not differ statistically between the exponential and nonlinear methods for either gas at any site, because both of those regression methods were based on diffusion theory. However, the flux values estimated by linear regression were significantly different from those estimated by the other two methods for both CH₄ and CO₂ at the deciduous forest site and for CO₂ at the coniferous forest site. Shortening the chamber deployment period improved the linearity of the curve, but did not completely eliminate the error. Our results suggest that linear regression is not a good model of the change in headspace concentration with time.     

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.     

Seed priming to mitigate the impact of elevated carbon dioxide associated temperature stress on germination in rice (Oryza sativa L.)

ABSTRACT

Climate change has a negative impact on crop production by inducing several stresses throughout plant growth. As the germination is one of the critical stages, this study was to assess the impact of elevated carbon dioxide (eCO₂) and associated temperature stress on germination of rice. The study was conducted using the rice genotype CO 51 in artificially induced eCO₂ condition using open-top chamber (OTC). The efficiency of seed priming treatment with salicylic acid (SA), citric acid (CA), ascorbic acid (AsA) and distilled water (hydropriming) to alleviate stress due to eCO₂ and temperature were also studied. SA 25 mg l^?1 and AsA 100 mg l^?1 enhanced germination, other seed quality parameters along with α- amylase activity. Also, the activity of antioxidant enzymes like catalase, peroxidase and superoxide dismutase was increased. These parameters positively affected the germination and growth by mitigating the effect of oxidative stress induced under eCO₂ conditions. So, SA, CA and AsA can be effectively used for maintaining seed quality parameters and seedling growth during the stresses.     

Uncoupling of microbial CO2 production and release in frozen soil and its implications for field studies of arctic C cycling

Knowledge of seasonal trends and controls of soil CO₂ emissions to the atmosphere is important for simulating atmospheric CO₂ concentrations and for understanding and predicting the global carbon cycle. This is particularly the case for high arctic soils subject to extreme fluctuating environmental conditions. Based on field measurements of soil CO₂ efflux, temperature, water content, pore gas composition in soil and frozen cores as well as detailed temperature experiments performed in the laboratory, we evaluated seasonal controls of CO₂ effluxes from a well-drained tundra heath site in NE-Greenland. During the growing season, near-surface temperatures correlated well with observed CO₂ effluxes (r²>0.9). However, during intensive thawing of near-surface layers we observed up to 1.5-fold higher effluxes than expected due to temperature alone. These high rates were consistent with high CO₂ concentrations in frozen soil (>10% CO₂) and suggested a spring burst event during soil thawing and a corresponding trapping of produced CO₂ during winter. Laboratory experiments revealed that microbial soil respiration continued down to a least −18 °C and that up to 80% of the produced CO₂ was trapped in soil at temperatures between 0 and −9 °C. The trapping of CO₂ in frozen soil was positively correlated with soil moisture (r²=0.85) and led to an abrupt change of the temperature sensitivity (Q₁₀) observed for soil CO₂ release at 0 °C with Q₁₀ values below 0 °C being up to 100-fold higher than above 0 °C. The results of sub-zero CO₂ production allowed us to predict the microbial soil respiration throughout the year and to evaluate to what extent burst events during thawing can be explained by the release of CO₂ being produced and trapped during winter. Taking only the upper 20 cm of the soil into account, winter soil respiration accounted for about 40% of the annual soil respiration. At least 14% of the winter CO₂ production was trapped during the winter 2000-2001 and observed to be released upon thawing. Thus, the site-specific winter soil respiration is an important part of the annual C cycle and CO₂ trapping should be accounted for in future field and modelling studies of soil respiration dynamics in arctic ecosystems. In conclusion, we have discovered a soil moisture dependent uncoupling of CO₂ production and release in frozen soils with important implications for future field studies of Arctic C cycling.     

Long-term record and analysis of soil temperatures and soil heat fluxes in a grassland area, The Netherlands

A 27-year soil temperature record at five depths and soil heat flux record at one depth were analyzed for a grassland area in The Netherlands. The annual mean soil temperature of the last 23 years of soil measurements (no data gaps) showed a statistically significant increase of about 1.0 °C, consistent with the observed air temperature increase of about 1.3 °C for the same period. This positive trend correlates well with global brightening of 5.3 W m⁻² per decade. The 10-day mean soil temperature varied smoothly throughout the year with relatively small inter-annual variability. The deeper the measurement depth, the smoother the annual cycle and the smaller the variability. In February and at the end of the year the variability appears to be somewhat larger. A Fast Fourier Transform was applied to the measurements and revealed an annual and daily damping depth of 1.80 m and 0.10 m, respectively. An example of the usefulness of this data is provided for agriculture, where an aggressive root-knot nematode is affected by the mean soil temperature increase. It appears that the 600 degree day life-cycle threshold is reached 16 days earlier, which may lead to a potential serious increase in agriculture crop damage.     

Glucose additions to aggregates subjected to drying/wetting cycles promote carbon sequestration and aggregate stability

Biogeochemical mechanisms at microscale regions within soil macroaggregates strengthen aggregates during repeated DW cycles. Knowledge of additional biogeochemical processes that promote the movement of dissolved organic carbon (DOC) into and throughout soil aggregates and soil aggregate stabilization are essential before we can more accurately predict maximum carbon (C) sequestration by soils subjected to best management practices. We investigated the spatial distribution of ¹³C-glucose supplied to individual soil macroaggregate surfaces and subjected to multiple drying and wetting (DW) cycles. Subsequent distribution of added glucose-C, CO₂ respiration, increased microbial community activity and concomitant changes in soil aggregate stabilization were monitored. Moist macroaggregates were treated with no DW cycles and zero glucose C (Control), 5 DW cycles and zero glucose (DW0G), and 5 DW cycles with additions of 250 μg glucose-¹³C/g soil during each cycle (DW+G). Repeated additions of glucose-C to aggregate surfaces reduced the mineralization of pre-existing soil C by an average of 45% and established concentric gradients of glucose-derived C. It is concluded these increasing gradients promoted the diffusion of soluble C into interior regions and became less available to microbial respiration. Spatial gradients of glucose-derived C within aggregates influenced a shift in the abundance of unique ribotypes spatially distributed within aggregates. Rapid decreases in the mineralization rates of glucose-C during repeated DW cycles suggested greater C sequestration by either physical restriction of microbes or chemical sorption of new C that diffused into aggregates. Aggregate stability decreased significantly following 2-3 DW cycles, when glucose-C was not added. Additions of glucose-C with each DW cycle maintained soil aggregate stability equal to the moist but not cycled control throughout the 5 DW cycles of this study. These data simulate the strengthening of soil aggregates in no tillage agroecosystems which provides continuous additions of DOC compounds generated by decomposing plant residues on the soil surface, and root exudates and decomposition, as well as the mineralization of POM materials within nondisturbed soil profiles.     

Responses of net ecosystem CO2 exchange to nitrogen fertilization in experimentally manipulated grassland ecosystems

Nitrogen (N) addition enhances primary productivity of terrestrial ecosystems. However, the effects of N fertilization and/or deposition on net ecosystem CO₂ exchange (NEE) are not fully understood. The effects of N on NEE were investigated in two experimental cheatgrass ecosystems in Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs), Reno, Nevada. In this experiment, no N fertilization was added to the two EcoCELLs in the first year and two different N fertilization regimes were applied in the second year. N fertilizer was applied once to one EcoCELL (pulse fertilization, PF), and the same total amount of N in biweekly increments to the other EcoCell (gradual fertilization, GF). NEE, photosynthetically active radiation (PAR) and canopy green leaf area index (LAI) were continuously measured in the two EcoCELLs during the pretreatment and N-fertilized years. Plant N content and biomass were measured at the end of the growing season in each year. Radiation-use efficiency (RUE_CO2) was calculated as the ratio of gross ecosystem photosynthesis (GEP) to the intercepted photosynthetically active radiation (IPAR). The responses of NEE to IPAR were used to estimate the maximum ecosystem photosynthetic capacity (F_max). N fertilization stimulated canopy LAI, plant N content, F_max, RUE_CO2, NEE and biomass in both methods of N supply applications. PF led to higher LAI, F_max and NEE than GF, but both had a similar RUE_CO2 during the early growing season. GF maintained higher LAI, F_max, RUE_CO2 and NEE than PF during the late growing season. At the ecosystem level, N fertilization stimulated daily NEE directly by increasing canopy LAI, plant N content, shoot/root ratio and the maximum ecosystem photosynthetic capacity, and increased the seasonally accumulated NEE indirectly by extending the growing season. PF differed significantly from GF in its effects on NEE and RUE_CO2, possibly due to differential rates and timing of N availability. Our study suggested that these changes in the canopy RUE_CO2 and growing season under N fertilization or N deposition regimes should be considered in modeling studies of ecosystem C sequestration.     

Variation of soil respiration at three spatial scales: Components within measurements, intra-site variation and patterns on the landscape

Soil respiration is an important component of terrestrial carbon cycling and can be influenced by many factors that vary spatially. This research aims to determine the extent and causes of spatial variation of soil respiration, and to quantify the importance of scale on measuring and modeling soil respiration within and among common forests of Northern Wisconsin. The potential sources of variation were examined at three scales: [1] variation among the litter, root, and bulk soil respiration components within individual 0.1 m measurement collars, [2] variation between individual soil respiration measurements within a site (<1 m to 10 m), and [3] variation on the landscape caused by topographic influence (100 m to 1000 m). Soil respiration was measured over a two-year period at 12 plots that included four forest types. Root exclusion collars were installed at a subset of the sites, and periodic removal of the litter layer allowed litter and bulk soil contributions to be estimated by subtraction. Soil respiration was also measured at fixed locations in six northern hardwood sites and two aspen sites to examine the stability of variation between individual measurements. These study sites were added to an existing data set where soil respiration was measured in a random, rotating, systematic clustering which allowed the examination of spatial variability from scales of <1 m to 100+ m. The combined data set for this area was also used to examine the influence of topography on soil respiration at scales of over 1000 m by using a temperature and moisture driven soil respiration model and a 4 km² digital elevation model (DEM) to model soil moisture. Results indicate that, although variation of soil respiration and soil moisture is greatest at scales of 100 m or more, variation from locations 1 m or less can be large (standard deviation during summer period of 1.58 and 1.28 μmol CO₂ m⁻² s⁻¹, respectively). At the smallest of scales, the individual contributions of the bulk soil, the roots, and the litter mat changed greatly throughout the season and between forest types, although the data were highly variable within any given site. For scales of 1-10 m, variation between individual measurements could be explained by positive relationships between forest floor mass, root mass, carbon and nitrogen pools, or root nitrogen concentration. Lastly, topography strongly influenced soil moisture and soil properties, and created spatial patterns of soil respiration which changed greatly during a drought event. Integrating soil fluxes over a 4 km² region using an elevation dependent soil respiration model resulted in a drought induced reduction of peak summer flux rates by 37.5%, versus a 31.3% when only plot level data was used. The trends at these important scales may help explain some inter-annual and spatial variability of the net ecosystem exchange of carbon.     

Influences of recovery from clear-cut, climate variability, and thinning on the carbon balance of a young ponderosa pine plantation

From 1999 to 2002, the variations in carbon flux due to management practices (shrub removal, thinning) and climate variability were observed in a young ponderosa pine forest originated from clear-cutting and plantation in 1990. These measurements were done at the Blodgett Forest Ameriflux site located in the Sierra Nevada Mountains of California. Thinning in spring 2000 decreased the leaf area index (LAI) by 34% and added 496 g C m⁻² of wood and leaf debris at the soil surface. Total ecosystem respiration was not significantly affected by thinning (1261 g C m⁻² in 1999 and 1273 g C m⁻² in 2000), while canopy photosynthesis decreased by 202 g C m⁻². As a result the ecosystem shifted from a net sink of CO₂ in 1999 (−201 g C m⁻²) to a small net source in 2000 (13 g C m⁻²). Woody and leaf debris resulting from thinning only accounted for maximum 1% and 7% of the total respiration flux, respectively. Thinning did not affect the relative proportion of the different components of respiration to an observable degree. Low soil water availability in summer 2001 and 2002 decreased the proportion of soil respiration to the total respiration. It also imposed limitations on canopy photosynthesis: as a result the ecosystem shifted from a sink to a source of carbon 1 month earlier than in a wetter year (1999). The leaf area index and biomass of the stand increased rapidly after the thinning. The ecosystem was again a sink of carbon in 2001 (−97 g C m⁻²) and 2002 (−172 g C m⁻²). The net carbon uptake outside the traditionally-defined growing season can be important in this ecosystem (NEE = −50 g C m⁻² in 2000), but interannual variations are significant due to differences in winter temperatures.     

Sulfur dioxide inhibition of photosynthesis at different CO2 and O2 concentrations in relation to photorespiration

The mechanism of SO₂ inhibition of photosynthesis in intact leaves of tomato and maze was studied to evaluate SO₂ inhibition of photorespiration. Leaf tissues were fumigated with SO₂ under photorespiratory (low CO, and/or high O, concentrations) and non-photo-respiratory conditions. When tomato leaf disks were fumigated with 10 ppm SO₂ at 2, 21 and 100° o O., SO₂ inhibited photosynthesis at 2% O₂ in the same degrees as at 21% O₂. SO₂ inhibition of photosynthesis was depressed at higher CO₂ concentrations when the disks were fumigated with SO₂ at different CO₂ concentrations. High CO₂ concentrations also reduced the photosynthesis inhibition of maize leaf disks. These results suggest that SO₂ inhibits photosynthesis through other mechanisms than photorespiration inhibition and confirm the view that SO₂ competes with CO₂ for the carboxylating enzymes in photosynthesis