Soil properties differently influence estimates of soil CO2 efflux from three chamber-based measurement systems

Soil CO₂ efflux is a major component of net ecosystem productivity (NEP) of forest systems. Combining data from multiple researchers for larger-scale modeling and assessment will only be valid if their methodologies provide directly comparable results. We conducted a series of laboratory and field tests to assess the presence and magnitude of soil CO₂ efflux measurement system × environment interactions. Laboratory comparisons were made with a dynamic, steady-state CO₂ flux generation apparatus, wherein gas diffusion drove flux without creating pressure differentials through three artificial soil media of varying air-filled porosity. Under these conditions, two closed systems (Li-6400-09 and SRC-1) exhibited errors that were dependent on physical properties of the artificial media. The open system (ACES) underestimated CO₂ flux. However, unlike the two other systems, the ACES results could be corrected with a single calibration equation that was unaffected by physical differences in artificial media. Both scale and rank changes occurred among the measurement systems across four sites. Our work clearly shows that soil CO₂ efflux measurement system × environment interactions do occur and can substantially impact estimates of soil CO₂ efflux. Until reliable calibration techniques are developed and applied, such interactions make direct comparison of published rates, and C budgets estimated using such rates, difficult.     

Carbon balance in tussock tundra under ambient and elevated atmospheric CO2

Summary Whole ecosystem CO₂ flux under ambient (340 l/l) and elevated (680 l/l) CO₂ was measured in situ in Eriophorum tussock tundra on the North Slope of Alaska. Elevated CO₂ resulted in greater carbon acquisition than control treatments and there was a net loss of CO₂ under ambient conditions at this upland tundra site. These measurements indicate a current loss of carbon from upland tundra, possibly the result of recent climatic changes. Elevated CO₂ for the duration of one growing season appeared to delay the onset of dormancy and resulted in approximately 10 additional days of positive ecosystem flux. Homeostatic adjustment of ecosystem CO₂ flux (sum of species' response) was apparent by the third week of exposure to elevated CO₂. Ecosystem dark respiration rates were not significantly higher at elevated CO₂ levels. Rapid homeostatic adjustment to elevated CO₂ may limit carbon uptake in upland tundra. Abiotic factors were evaluated as predictors of ecosystem CO₂ flux. For chambers exposed to ambient and elevated CO₂ levels for the duration of the growing season, seasonality (Julian day) was the best predictor of ecosystem CO₂ flux at both ambient and elevated CO₂ levels. Light (PAR), soil temperature, and air temperature were also predictive of seasonal ecosystem flux, but only at elevated CO₂ levels. At any combination of physical conditions, flux of the elevated CO₂ treatment was greater than that at ambient. In short-term manipulations of CO₂, tundra exposed to elevated CO₂ had threefold greater carbon gain, and had one half the ecosystem level, light compensation point when compared to ambient CO₂ treatments. Elevated CO₂-acclimated tundra had twofold greater carbon gain compared to ambient treatments, but there was no difference in ecosystem level, light compensation point between elevated and ambient CO₂ treatments. The predicted future increases in cloudiness could substantially decrease the effect of elevated atmospheric CO₂ on net ecosystem carbon budget. These analyses suggest little if any long-term stimulation of ecosystem carbon acquisition by increases in atmospheric CO₂.     

Mind the gap: non‐biological processes contributing to soil CO2 efflux

Widespread recognition of the importance of soil CO₂ efflux as a major source of CO₂ to the atmosphere has led to active research. A large soil respiration database and recent reviews have compiled data, methods, and current challenges. This study highlights some deficiencies for a proper understanding of soil CO₂ efflux focusing on processes of soil CO₂ production and transport that have not received enough attention in the current soil respiration literature. It has mostly been assumed that soil CO₂ efflux is the result of biological processes (i.e. soil respiration), but recent studies demonstrate that pedochemical and geological processes, such as geothermal and volcanic CO₂ degassing, are potentially important in some areas. Besides the microbial decomposition of litter, solar radiation is responsible for photodegradation or photochemical degradation of litter. Diffusion is considered to be the main mechanism of CO₂ transport in the soil, but changes in atmospheric pressure and thermal convection may also be important mechanisms driving soil CO₂ efflux greater than diffusion under certain conditions. Lateral fluxes of carbon as dissolved organic and inorganic carbon occur and may cause an underestimation of soil CO₂ efflux. Traditionally soil CO₂ efflux has been measured with accumulation chambers assuming that the main transport mechanism is diffusion. New techniques are available such as improved automated chambers, CO₂ concentration profiles and isotopic techniques that may help to elucidate the sources of carbon from soils. We need to develop specific and standardized methods for different CO₂ sources to quantify this flux on a global scale. Biogeochemical models should include biological and non‐biological CO₂ production processes before we can predict the response of soil CO₂ efflux to climate change. Improving our understanding of the processes involved in soil CO₂ efflux should be a research priority given the importance of this flux in the global carbon budget.     

Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas-fir terracosms

Although numerous studies indicate that increasing atmospheric CO₂ or temperature stimulate soil CO₂ efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO₂ and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO₂ and elevated temperature on these components of soil CO₂ efflux in Douglas-fir terracosms. We measured both soil CO₂ efflux rates and the ¹³C and ¹⁸O isotopic compositions of soil CO₂ efflux in 12 sun-lit and environmentally controlled terracosms with 4-year-old Douglas fir seedlings and reconstructed forest soils under two CO₂ concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO₂ efflux. In most cases, litter decomposition was the dominant component of soil CO₂ efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO₂ concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO₂, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual-isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.     

Comparison of soil respiration methods in a mid-latitude deciduous forest

In forest ecosystems the single largest respiratory flux influencing net ecosystem productivity (NEP) is the total soil CO₂ efflux; however, it is difficult to make measurements of this flux that are accurate at the ecosystem scale. We examined patterns of soil CO₂ efflux using five different methods: auto-chambers, portable gas analyzers, eddy covariance along and two models parameterized with the observed data. The relation between soil temperature and soil moisture with soil CO₂ effluxes are also investigated, both inter-annually and seasonally, using these observations/results. Soil respiration rates (R _soil) are greatest during the growing season when soil temperatures are between 15 and 25 °C, but some soil CO₂ efflux occurs throughout the year. Measured soil respiration was sensitive to soil temperature, particularly during the spring and fall. All measurement methods produced similar annual estimates. Depending on the time of the year, the eddy covariance (flux tower) estimate for ecosystem respiration is similar to or slightly lower than estimates of annual soil CO₂ efflux from the other methods. As the eddy covariance estimate includes foliar and stem respiration which the other methods do not; it was expected to be larger (perhaps 15–30%). The auto-chamber system continuously measuring soil CO₂ efflux rates provides a level of temporal resolution that permits investigation of short- to longer term influences of factors on these efflux rates. The expense of building and maintaining an auto chamber system may not be necessary for those researchers interested in estimating R _soil annually, but auto-chambers do allow the capture of data from all seasons needed for model parameterization.     

Mid-season gas exchange of an alpine grassland under elevated CO2

Ecosystem net CO₂ uptake, evapotranspiration (ET) and night-time CO₂ efflux were measured in an alpine grassland dominated by Carex curvula, treated with doubled ambient partial pressure of CO₂ via open-top chambers. One quarter of the plots were treated with mineral nutrients to simulate the effect of lowland nitrogen deposition rates. Depending upon fertilizer supply, ecosystem net CO₂ uptake per ground area in full sunlight (NCE_max) was 41–81% higher in open-top chambers supplied with doubled ambient partial pressure (p _a) of CO₂ than in plots receiving ambient CO₂. Short-term reversals of the CO₂ level suggest that the extent of downward adjustment of canopy photosynthesis under elevated CO₂ was 30–40%. ET tended to decline, while water use efficiency (WUE), expressed as the NCE_max:ET ratio, increased more than twofold under elevated CO₂. Night-time ecosystem CO₂ efflux did not respond to changes in CO₂ p _a. NCE_max and night-time CO₂ efflux were more responsive to mineral fertilizer than the doubling of CO₂. This suggests that in these alpine plant communities, atmospheric nutrient input may induce equal or greater effects on gas exchange than increased CO₂.     

Net uptake of atmospheric CO2 by coastal submerged aquatic vegetation

‘Blue Carbon’, which is carbon captured by marine living organisms, has recently been highlighted as a new option for climate change mitigation initiatives. In particular, coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long‐term sequestration of carbon. However, the direct contribution of Blue Carbon to the uptake of atmospheric CO₂ through air‐sea gas exchange remains unclear. We performed in situ measurements of carbon flows, including air‐sea CO₂ fluxes, dissolved inorganic carbon changes, net ecosystem production, and carbon burial rates in the boreal (Furen), temperate (Kurihama), and subtropical (Fukido) seagrass meadows of Japan from 2010 to 2013. In particular, the air‐sea CO₂ flux was measured using three methods: the bulk formula method, the floating chamber method, and the eddy covariance method. Our empirical results show that submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO_2. This finding is contrary to the conventional perception that most near‐shore ecosystems are sources of atmospheric CO₂. The key factor determining whether or not coastal ecosystems directly decrease the concentration of atmospheric CO₂ may be net ecosystem production. This study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO₂.     

Soil-atmosphere exchange of CH4, CO2, NOx,and N2O in the Colorado shortgrass steppe under elevated CO2

In late March 1997, an open-top-chamber (OTC) CO₂ enrichment study was begun in the Colorado shortgrass steppe. The main objectives of the study were to determine the effect of elevated CO₂ (720 mol mol^–1) on plant production, photosynthesis, and water use of this mixed C₃/C₄ plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by CO₂ on trace gas exchange. From this study, we report here our weekly measurements of CO₂, CH₄, NO_x and N₂O fluxes within control (unchambered), ambient CO₂ and elevated CO₂ OTCs. Soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2000. Even though both C₃ and C₄ plant biomass increased under elevated CO₂ and soil moisture content was typically higher than under ambient CO₂ conditions, none of the trace gas fluxes were significantly altered by CO₂ enrichment. Over the 43 month period of observation NO_x and N₂O flux averaged 4.3 and 1.7 in ambient and 4.1 and 1.7 g N m^–2 hr ^–1 in elevated CO₂ OTCs, respectively. NO_x flux was negatively correlated to plant biomass production. Methane oxidation rates averaged –31 and –34 g C m^–2 hr^–1 and ecosystem respiration averaged 43 and 44 mg C m^–2 hr^–1 under ambient and elevated CO₂, respectively, over the same time period.     

Base Cation Cycling in a Pristine Watershed of the Canadian Boreal Forest

In forest ecosystems the single largest respiratory flux influencing net ecosystem productivity (NEP) is the total soil CO₂ efflux; however, it is difficult to make measurements of this flux that are accurate at the ecosystem scale. We examined patterns of soil CO₂ efflux using five different methods: auto-chambers, portable gas analyzers, eddy covariance along and two models parameterized with the observed data. The relation between soil temperature and soil moisture with soil CO₂ effluxes are also investigated, both inter-annually and seasonally, using these observations/results. Soil respiration rates (R _soil) are greatest during the growing season when soil temperatures are between 15 and 25 °C, but some soil CO₂ efflux occurs throughout the year. Measured soil respiration was sensitive to soil temperature, particularly during the spring and fall. All measurement methods produced similar annual estimates. Depending on the time of the year, the eddy covariance (flux tower) estimate for ecosystem respiration is similar to or slightly lower than estimates of annual soil CO₂ efflux from the other methods. As the eddy covariance estimate includes foliar and stem respiration which the other methods do not; it was expected to be larger (perhaps 15–30%). The auto-chamber system continuously measuring soil CO₂ efflux rates provides a level of temporal resolution that permits investigation of short- to longer term influences of factors on these efflux rates. The expense of building and maintaining an auto chamber system may not be necessary for those researchers interested in estimating R _soil annually, but auto-chambers do allow the capture of data from all seasons needed for model parameterization.     

A Microscale Model for Combined CO2 Diffusion and Photosynthesis in Leaves

Transport of CO₂ in leaves was investigated by combining a 2-D, microscale CO₂ transport model with photosynthesis kinetics in wheat (Triticum aestivum L.) leaves. The biophysical microscale model for gas exchange featured an accurate geometric representation of the actual 2-D leaf tissue microstructure and accounted for diffusive mass exchange of CO_2. The resulting gas transport equations were coupled to the biochemical Farquhar-von Caemmerer-Berry model for photosynthesis. The combined model was evaluated using gas exchange and chlorophyll fluorescence measurements on wheat leaves. In general a good agreement between model predictions and measurements was obtained, but a discrepancy was observed for the mesophyll conductance at high CO₂ levels and low irradiance levels. This may indicate that some physiological processes related to photosynthesis are not incorporated in the model. The model provided detailed insight into the mechanisms of gas exchange and the effects of changes in ambient CO₂ concentration or photon flux density on stomatal and mesophyll conductance. It represents an important step forward to study CO₂ diffusion coupled to photosynthesis at the leaf tissue level, taking into account the leaf''s actual microstructure.     

Soil CO2 efflux in a beech forest: comparison of two closed dynamic systems

The aim of this study was to understand why two closed dynamic systems with a very similar design gave large differences in soil CO₂ efflux measurements (PP systems and LI-COR). Both in the field (forest beech stand) and in the laboratory, the PPsystems gave higher estimations of soil CO₂ efflux than the LI-COR system (ranging from 30% to 50%). The difference in wind speed occurring within the soil respiration chambers (0.9 m s⁻¹ within the SRC-1 and 0.4 m s⁻¹ within the LI-6000-09 chambers) may account for the discrepancy between the two systems. An excessive air movement inside the respiration chamber is thought to disrupt the high laminar boundary layer over the forest floor. This would promote an exhaust of the CO₂ accumulated into the upper soil layers into the chamber and a lateral diffusion of CO₂ in the soil towards the respiration chamber. The discrepancy between the two systems was reduced (i) by decreasing fan speed within the SRC-1, (ii) by increasing wind speed over the soil surface outside the respiration chamber, or (iii) by using an artificial soil design without high CO₂ concentration in soil pores. We show that wind speed is an important component of soil CO₂ diffusion which must be taken into account when measuring soil CO₂ efflux, even on very fine textured soil like silt-loam soil. Proper measurement can be achieved by maintaining wind speed inside the chamber below 0.4 m s⁻¹ since low wind speed conditions predominate under forest canopies. However, more accurate measurements will be obtained by regulating wind speeds within the chamber at a velocity representative of the wind speed recorded simultaneously at the floor surface. This revised version was published online in June 2006 with corrections to the Cover Date.     

A Direct Confirmation of the Standard Method of Estimating Intercellular Partial Pressure of CO(2)

The partial pressure of CO₂ inside leaves of several species was measured directly. Small gas exchange chambers were clamped above and below the same section of an amphistomatous leaf. A flowing gas stream through one chamber allowed normal CO₂ and water vapor exchange. The other chamber was in a closed circuit consisting of the chamber, an infrared gas analyzer, and a peristaltic pump. The CO₂ in the closed system rapidly reached a steady pressure which it is believed was identical to the CO₂ pressure inside the leaf, because there was no flux of CO₂ across the epidermis. This measured partial pressure was in close agreement with that estimated from a consideration of the fluxes of CO₂ and vapor at the other surface.     

The influence of elevated atmospheric CO2 on fine root dynamics in an intact temperate forest

Root dynamics are important for plant, ecosystem and global carbon cycling. Changes in root dynamics caused by rising atmospheric CO₂ not only have the potential to moderate further CO₂ increases, but will likely affect forest function. We used FACE (Free‐Air CO₂ Enrichment) to expose three 30‐m diameter plots in a 13‐year‐old loblolly pine (Pinus taeda) forest to elevated (ambient + 200 µL L^?1) atmospheric CO₂. Three identical fully instrumented plots were implemented as controls (ambient air only). We quantified root dynamics from October 1998 to October 1999 using minirhizotrons. In spite of 16% greater root lengths and 24% more roots per minirhizotron tube, the effects of elevated atmospheric CO₂ on root lengths and numbers were not statistically significant. Similarly, production and mortality were also unaffected by the CO₂ treatment, even though annual root production and mortality were 26% and 46% greater in elevated compared to ambient CO₂ plots. Average diameters of live roots present at the shallowest soil depth were, however, significantly enhanced in CO₂‐enriched plots. Mortality decreased with increasing soil depth and the slopes of linear regression lines (mortality vs. depth) differed between elevated and ambient CO₂ treatments, reflecting the significant CO₂ by depth interaction. Relative root turnover (root flux/live root pool) was unchanged by exposure to elevated atmospheric CO₂. Results from this study suggest modest, if any, increases in ecosystem‐level root productivity in CO₂‐enriched environments.     

Seasonal patterns of soil CO2 efflux and soil air CO2 concentration in a Scots pine forest: comparison of two chamber techniques

A non‐vented non‐steady state flow‐through chamber and a non‐vented non‐steady state non‐flow‐through chamber technique were used to measure CO₂ efflux of a young Scots pine forest on a fertile till soil in southern Finland. Soil temperature, soil moisture and soil CO₂ concentration were measured concurrently with CO₂ efflux for two and a half successive years. The CO₂ efflux showed a seasonal pattern, effluxes ranging from low 0.0–0.1 g CO₂ m ^{? 2} h ^{? 1} in winter to peak values of 2.3 g CO₂ m ^{? 2} h ^{? 1} occurring in late June and in July. The daily average effluxes in July measured by flow through chambers were 1.23 and 0.98 g CO₂ m ^{? 2} h ^{? 1} in 1998 and 1999, respectively. The annual accumulated CO₂ efflux was 3117 and 3326 g CO₂ m ^{? 2} in 1998 and 1999, respectively. The spatial variation in CO₂ efflux was high (CV 0.18–0.45) and increased with increasing efflux. Soil air CO₂ concentration showed similar seasonal pattern the peak concentrations occurring in July–August. The CO₂ concentrations ranged from 580 to 780 µ mol mol ^{? 1} in the humus layer to 13 620–14 470 µ mol mol ^{? 1} in the C‐horizon. In winter the soil air CO₂ concentrations were lower, especially in deeper soil layers. Drought decreased CO₂ efflux and soil air CO₂ concentration. The in situ comparison on forest soil between the chamber methods showed the non‐flow‐through chamber to give ～～50% lower efflux values than that of the flow‐through chamber. When calibrated against known CO₂ efflux ranging from 0.4 to 0.8 g CO₂ m ^{? 2} h ^{? 1} generated with a diffusion box method developed by Widén and Lindroth [Acta Universitatis Agriculturae Suecia Silvestria, 2001], the flow‐through chamber gave equal effluxes at the lower end of the calibration range, but overestimated high effluxes by 20%. Non‐flow‐through chamber underestimated the CO₂ efflux by 30%.     

Response of soil CO2 efflux to water manipulation in a tallgrass prairie ecosystem

Although CO₂ efflux plays a critical role in carbon exchange between the biosphere and atmosphere, our understanding of its regulation by soil moisture is rather limited. This study was designed to examine the relationship between soil CO₂ efflux and soil moisture in a natural ecosystem by taking advantage of the historically long drought period from 29 July to 21 September 2000 in the southern Central Great Plain, USA. At the end of August when soil moisture content at the top 50 mm was reduced to less than 50 g kg^–1 gravimetrically, we applied 8 levels of water treatments (simulated to rainfall of 0, 10, 25, 50, 100, 150, 200, and 300 mm) with three replicates to 24 plots in a Tallgrass Prairie ecosystem in Central Oklahoma, USA. In order to quantify root-free soil CO₂ efflux, we applied the same 8 levels of water treatments to 24 500-mm soil columns using soil from field adjacent to the experimental plots. We characterized dynamic patterns of soil moisture and soil CO₂ efflux over the experimental period of 21 days. Both soil moisture content and CO₂ efflux showed dramatic increases immediately after the water addition, followed by a gradual decline. The time courses in response to water treatments are well described by Y=Y₀+ate^–bt, where Y is either soil moisture or CO₂ efflux, t is time, Y ₀, a, and b are coefficients. Among the 8 water treatments, the maximal soil CO₂ efflux rate occurred at the 50 mm water level in the field and 100 mm in the root-free soil 1 day after the treatment. The maximal soil CO₂ efflux gradually shifted to higher water levels as the experiment continued. We found the relationship between soil CO₂ efflux and soil moisture using the data from the 21-day experiment was highly scattered, suggesting complex mechanisms determining soil CO₂ efflux by soil moisture.     

CO2 efflux from subterranean nests of ant communities in a seasonal tropical forest,Thailand

Many ant species construct subterranean nests. The presence of their nests may explain soil respiration “hot spots”, an important factor in the high CO₂ efflux from tropical forests. However, no studies have directly measured CO₂ efflux from ant nests. We established 61 experimental plots containing 13 subterranean ant species to evaluate the CO₂ efflux from subterranean ant nests in a tropical seasonal forest, Thailand. We examined differences in nest CO₂ efflux among ant species. We determined the effects of environmental factors on nest CO₂ efflux and calculated an index of nest structure. The mean CO₂ efflux from nests was significantly higher than those from the surrounding soil in the wet and dry seasons. The CO₂ efflux was species‐specific, showing significant differences among the 13 ant species. The soil moisture content significantly affected nest CO₂ efflux, but there was no clear relationship between nest CO₂ efflux and nest soil temperature. The diameter of the nest entrance hole affected CO₂ efflux. However, there was no significant difference in CO₂ efflux rates between single‐hole and multiple‐hole nests. Our results suggest that in a tropical forest ecosystem the increase in CO₂ efflux from subterranean ant nests is caused by species‐specific activity of ants, the nest soil environment, and nest structure.     

Soil CO2 efflux measurements in a mixed forest: impact of chamber disturbances,spatial variability and seasonal evolution

A closed‐dynamic‐chamber system (CDCS) was used to measure the spatial and seasonal variability of the soil CO₂ efflux (F_s) in beech and in Douglas fir patches of the Vielsalm forest (Belgium). First the difference between natural and measured soil CO₂ efflux induced by the presence of the CDCS was studied. The impact on the measurements of the pressure difference between the outside (natural condition) and the inside of the chamber was found to be small (0.4%). The influence of wind disturbance in the closed chamber was tested by comparison with an open‐chamber system characterized by a different wind distribution. A very good correlation between the two systems was found (r² = 0.99) but the open system yielded slightly lower fluxes than the closed one (slope = 0.88 ± 0.05). A measurement procedure has been developed to minimize the effect of the other sources of perturbation. The spatial and seasonal evolution of the soil CO₂ efflux was obtained by performing regular measurements on 29 spots in the beech patch over a period of 12 months and on 24 spots in the Douglas fir patch over 8 months. For each spot, the experimental relationship between Fs and soil temperature was compared with the fitted line for an Arrhenius relationship with a soil temperature‐dependent activation energy. Soil temperature explains 73% of the seasonal variation for all the data. The spatial average of the soil CO₂ efflux at 10 °C (Fs₁₀) in the beech patch is 2.57 ± 0.41 μmol m^?2 s^?1, approximately twice the average in the Douglas fir patch recorded at 1.42 ± 0.22 μmol m^?2 s^?1. The litter fall analysis seems to indicate that soil organic matter quality and quantity may be one the reasons for this difference. Finally the annual soil CO₂ efflux was calculated for the beech and Douglas fir patches (870 ± 140 and 438 ± 68 gC m^?2 y^?1, respectively). The beech value would represent 92 ± 15% of the annual ecosystem respiration estimated from the eddy covariance measurements.     

Precipitation variability and fire influence the temporal dynamics of soil CO2 efflux in an arid grassland

Climate models suggest that extreme rainfall events will become more common with increased atmospheric warming. Consequently, changes in the size and frequency of rainfall will influence biophysical drivers that regulate the strength and timing of soil CO₂ efflux – a major source of terrestrial carbon flux. We used a rainfall manipulation experiment during the summer monsoon season (July–September) to vary both the size and frequency of precipitation in an arid grassland 2 years before and 2 years after a lightning‐caused wildfire. Soil CO₂ efflux rates were always higher under increased rainfall event size than under increased rainfall event frequency, or ambient precipitation. Although fire reduced soil CO₂ efflux rates by nearly 70%, the overall responses to rainfall variability were consistent before and after the fire. The overall sensitivity of soil CO₂ efflux to temperature (Q₁₀) converged to 1.4, but this value differed somewhat among treatments especially before the fire. Changes in rainfall patterns resulted in differences in the periodicity of soil CO₂ efflux with strong signals at 1, 8, and 30 days. Increased rainfall event size enhanced the synchrony between photosynthetically active radiation and soil CO₂ efflux over the growing season before and after fire, suggesting a change in the temporal availability of substrate pools that regulate the temporal dynamics and magnitude of soil CO₂ efflux. We conclude that arid grasslands are capable of rapidly increasing and maintaining high soil CO₂ efflux rates in response to increased rainfall event size more than increased rainfall event frequency both before and after a fire. Therefore, the amount and pattern of multiple rain pulses over the growing season are crucial for understanding CO₂ dynamics in burned and unburned water‐limited ecosystems.     

Responses of CO2 efflux from an alpine meadow soil on the Qinghai Tibetan Plateau to multi-form and low-level N addition

Aims

To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai–Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO₂) exchange to N addition.

Methods

Based on a multi-form, low-level N addition experiment, soil CO₂ effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO₂ efflux.

Results

The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH ₄ ⁺ -N pool. N applications significantly increased aboveground biomass and soil CO₂ efflux; moreover, this effect was more significant from NH ₄ ⁺ -N than from NO ₃ ^? -N fertilizer. In addition, the soil CO₂ efflux was mainly driven by soil temperature, followed by aboveground biomass and NH ₄ ⁺ -N pool.

Conclusions

These results suggest that chronic atmospheric N deposition will stimulate soil CO₂ efflux in the alpine meadow on the Qinghai–Tibetan Plateau by increasing available N content and promoting plant growth.     

Soil CO2 Dynamics in a Tree Island Soil of the Pantanal: The Role of Soil Water Potential

The Pantanal is a biodiversity hotspot comprised of a mosaic of landforms that differ in vegetative assemblages and flooding dynamics. Tree islands provide refuge for terrestrial fauna during the flooding period and are particularly important to the regional ecosystem structure. Little soil CO₂ research has been conducted in this region. We evaluated soil CO₂ dynamics in relation to primary controlling environmental parameters (soil temperature and soil water). Soil respiration was computed using the gradient method using in situ infrared gas analyzers to directly measure CO₂ concentration within the soil profile. Due to the cost of the sensors and associated equipment, this study was unreplicated. Rather, we focus on the temporal relationships between soil CO₂ efflux and related environmental parameters. Soil CO₂ efflux during the study averaged 3.53 µmol CO₂ m⁻² s⁻¹, and was equivalent to an annual soil respiration of 1220 g C m⁻² y⁻¹. This efflux value, integrated over a year, is comparable to soil C stocks for 0–20 cm. Soil water potential was the measured parameter most strongly associated with soil CO₂ concentrations, with high CO₂ values observed only once soil water potential at the 10 cm depth approached zero. This relationship was exhibited across a spectrum of timescales and was found to be significant at a daily timescale across all seasons using conditional nonparametric spectral Granger causality analysis. Hydrology plays a significant role in controlling CO₂ efflux from the tree island soil, with soil CO₂ dynamics differing by wetting mechanism. During the wet-up period, direct precipitation infiltrates soil from above and results in pulses of CO₂ efflux from soil. The annual flood arrives later, and saturates soil from below. While CO₂ concentrations in soil grew very high under both wetting mechanisms, the change in soil CO₂ efflux was only significant when soils were wet from above.