Soil CO2 Efflux Rates Research Articles

Assessment of soil carbon dioxide efflux from contrasting land uses in a semi-arid savannah ecosystem, northeastern Ghana (West Africa)

Soil respiration (SR) emits a vast amount of atmospheric carbon dioxide (CO2) and contributes largely to the global greenhouse gas budget. The study assessed the dynamics of SR rates in the Vea catchment in northeastern Ghana, a sparsely gauged semi-arid savannah ecosystem characterized by distinct patterns of soil CO2 efflux. Through field measurements using soil chambers, the study quantified soil CO2 efflux rates in different land use types (woodland, cropland and grazeland), and assessed the influence of soil moisture, temperature, and soil organic carbon stocks on SR variability. The highest soil CO2 fluxes (12.97 ± 0.89 Mg CO2C ha−1 yr−1) were recorded in woodland, followed by grazeland (9.10 ± 0.42 Mg CO2C ha−1 yr−1) with cropland having the lowest rate (5.61 ± 0.29 Mg CO2C ha−1 yr−1). We recorded mean annual soil CO2 flux of 9.23 ± 0.53 Mg CO2C ha−1 yr−1 across the land use types and also observed significant seasonal and spatial variations in SR rates. The highest SR rate (220 mg CO2C m−2h−1) was recorded in the wet months (Jul-Sept and Mar-May) and the lowest rate (30 mg CO2C m−2h−1) in the dry months (Nov-Jan). For the wet season, the mean weekly soil CO2 fluxes ranged between 140 and 160 mg CO2C m−2 hr−1 as opposed to 60–75 mg CO2C m−2 hr−1 for the dry season. Seasonal and spatial variations in SR rates were largely driven by land use type, soil moisture and the interaction of soil temperature and moisture. The results underscore the importance of understanding the emission patterns from various land uses in West African savanna ecosystems to harnessing their potential for climate change mitigation.

Biochar Addition Increased Soil Carbon Storage but Did Not Exacerbate Soil Carbon Emission in Young Subtropical Plantation Forest

Forests play a crucial role in mitigating global warming, contributing approximately 46% of the global terrestrial carbon sink. However, it remains uncertain whether the addition of biochar to forests enhances the ecosystem’s carbon sink capacity. This study aims to address this scientific question by investigating whether biochar application increases carbon storage, potentially leading to an overall rise in carbon emissions by influencing soil respiration and identifying the underlying mechanisms. A controlled experiment was conducted in a young plantation forest that had grown for three years, where soil CO2 efflux rate and physicochemical properties, photosynthesis, and plant growth traits were measured across varying biochar addition rates (0, 5, and 10 t/ha) over five seasons. Then, statistical methods including one-way ANOVA, regression analysis, and structural equation modeling (SEM) were employed to assess differences in biological and abiotic factors among biochar addition gradients and understand the influencing mechanisms of soil CO2 efflux change. The findings revealed that biochar addition significantly increased the contents of soil organic carbon (SOC) and microbial biomass carbon (MBC), consequently promoting photosynthesis and plant growth (p < 0.05). Biochar addition accounted for 73.8% of the variation in soil CO2 efflux by affecting soil physicochemical properties, photosynthesis, and plant basal diameter growth. However, the net effect of biochar addition on soil CO2 efflux was found to be low. The positive effects of biochar addition on soil CO2 efflux via factors such as soil bulk density, total nitrogen (TN), MBC, and photosynthesis were counteracted by its negative impact through soil total phosphorus (TP), water content, pH, SOC, and plant basal diameter growth. Overall, our findings indicate that there was no significant increase in soil CO2 efflux in the short term (totaling 16 months) over the biochar addition gradient. However, we observed a substantial increase in soil carbon storage and an enhancement in the soil’s capacity to act as a carbon sink. Therefore, adding biochar to forests may be a feasible strategy to increase carbon sinks and mitigate global climate change.

Temporal patterns of fine‐root dynamics have little influence on seasonal soil CO2 efflux in a mixed, mesic forest

AbstractAmong the contributors to soil CO2 efflux, there remains uncertainty about the contribution of root activity to the overall soil efflux. Soil water and temperature frequently have been used to predict a large portion of the variation in soil CO2 efflux. We hypothesized that fine‐root dynamics explain most of the remaining variability in soil CO2 efflux that cannot be explained by soil temperature and water content. We anticipated that seasonal increases in root production, mortality via decomposition, and standing crop would result in corresponding increases in soil CO2 efflux. We tested our hypotheses by collecting and analyzing two years of minirhizotron and soil chamber CO2 flux data from plots distributed throughout the Shale Hills Catchment of the Susquehanna‐Shale Hills Critical Zone Observatory in Central Pennsylvania, USA. Here we showed that: (1) seasonal fluctuations in fine‐root dynamics yielded only a very small increase in the predictability of soil CO2 efflux; (2) fine‐root mortality effects on soil CO2 efflux were strongly tied to soil temperature; (3) fluctuations in fine‐root presence or standing mass independent of temperature and moisture had little effect on soil CO2 efflux; and (4) new fine‐root length and root length mortality had limited impacts on soil CO2 efflux rates. We conclude that, at least in temperate forests on rocky soils, characterizing fine‐root dynamics may provide only limited improvement in the estimation of soil CO2 efflux.

Understanding Soil Respiration Dynamics in Temperate Forests in Northwestern Mexico

Temperate mixed forests in Mexico are considered highly important ecosystems because of their high levels of biodiversity and capacity to store carbon. The aim of this study was to evaluate temporal and between-forest soil respiration (CO2 efflux) variability, and to assess the effect of vegetation diversity metrics on soil CO2 fluxes in mixed-uneven-aged forests in Durango, Northwestern Mexico. Soil CO2 efflux, soil moisture, and soil temperature were measured in three temperate forest types. A generalized linear model (GLM) was fitted to analyze the relationship between soil CO2 fluxes and stand variables, diversity metrics, soil moisture, and soil temperature. Furthermore, a two-way analysis of variance was used to assess the effect of forest type, month of the year, and their interaction on soil respiration. Annual average, minimum, and maximum soil CO2 efflux rate values were 3.81 (±2.94), 2.28 (±1.47), and 7.97 (±2.94) µmol m−2 s−1, respectively. Soil respiration was positively related to species richness, aboveground biomass, and quadratic mean diameter; however, forest type did not contribute to understanding the dynamics of soil CO2 fluxes. The results highlight the importance of seasonality, species diversity and aboveground biomass stocks to preserve the ecosystem processes driving soil respiration in temperate forests.

Differences in CO2 Emissions on a Bare-Drained Peat Area in Sarawak, Malaysia, Based on Different Measurement Techniques

The drainage and cultivation of peatlands will lead to subsidence and mineralisation of organic matter, increasing carbon (C) loss as more CO2 is emitted. There is little information about carbon emissions from bare peat soil. A study was undertaken to measure the CO2 emissions from a logged-over peat swamp area that was purposely vegetation-free. We aimed to report CO2 emissions from a bare, drained peatland developed for an oil palm plantation. For 12 months, we used eddy covariance (EC), closed chambers, and soil subsidence measurements to derive CO2 emissions from a logged-over peat swamp area. Significant variations in the estimated soil CO2 efflux were observed in the three tested measurement techniques. The average CO2 flux rate measured by the EC technique was 4.94 ± 0.12 µmol CO2 m−2 s−1 (or 68.55 tonnes CO2 ha−1 year−1). Meanwhile, the soil CO2 efflux rate measured by the closed chamber technique was 4.19 ± 0.22 µmol CO2 m−2 s−1 (or 58.14 tonnes CO2 ha−1 year−1). Subsidence amounted to 1.9 cm year−1, corresponding to 36.12 tonnes CO2 ha−1 year−1. The estimation of the C loss was found to be highest by the EC technique, lower by the soil chamber technique, and lowest by the peat subsidence rate technique. The higher CO2 emission rate observed in the EC technique could be attributed to soil microbial respiration and decomposing woody residues in the nearby stacking rows due to the large EC footprint. It could also be affected by CO2 advection from oil palms adjacent to the study site. Despite the large differences in the CO2 emission rates by the different techniques, this study provides valuable information on the soil heterotrophic respiration of deep peat in Sarawak. Carbon emissions from a bare peat area cover only a fraction of the soil CO2 respiration component, i.e., the soil heterotrophic respiration. Further investigations are needed to determine the CO2 emissions by soil microbial activities and plant roots from other peat areas in Sarawak.

Effects of plastic mulching on soil CO2 efflux in a cotton field in northwestern China

In Northwestern China, more and more traditional cultivation system (TC) with no mulching and flood irrigation have been replaced by modern cultivation technology (MC) combining plastic film mulching with drip irrigation. Does plastic film mulching increase or reduce soil CO2 emission in arid areas? In order to study the effects of plastic mulching on soil CO2 efflux, a field study was conducted to compare soil CO2 concentration, soil CO2 efflux, soil temperature and moisture between the TC treatment and the MC treatment during a cotton growing season in Northwestern China. The seasonal patterns of soil profile temperature and soil moisture in the TC treatment were similar to that in the MC treatment. The mean value of soil profile temperature in the MC treatment was higher than that in the TC treatment. Except for soil moisture at 15 cm depth, the mean value of soil moisture at 5 cm and 10 cm depths in the MC treatment was higher than that in the TC treatment. The variation patterns of soil CO2 concentration and soil CO2 efflux in MC treatment were different to that in the TC treatment. Although the peak of soil CO2 concentration in the TC treatment was earlier than that in the MC treatment, the duration of soil CO2 concentration with high values in TC treatment was shorter than that in the MC treatment. Based on the model of Fick’s first diffusion law, soil surface CO2 efflux in the MC and TC treatments were determined. The surface CO2 efflux in the TC treatment calculated by Fick’s first diffusion law model was in good agreement with the value measured by chamber method. The seasonal curve of soil surface CO2 efflux in the MC treatment indicate the similar pattern with that in the TC treatment, and the rate of CO2 efflux was lower in the MC system. In the MC treatment, the seasonal variation of soil surface efflux was explained more by soil moisture than by soil temperature. However, in the TC treatment, the seasonal variation of soil surface efflux was explained more by soil temperature than by soil moisture. Over the completely experimental period, accumulated rates of soil CO2 efflux were 361 g C m−2 and 474 g C m−2 for the MC and TC system, respectively. We concluded that converting agricultural practices from traditional cultivation to the plastic mulching cultivation could reduce soil CO2 efflux by approximately 110 g C m−2 year−1 in agricultural land in arid areas of Northwestern China.

The role of plant functional traits and diversity in soil carbon dynamics within riparian agroforests.

Restoration of agricultural riparian buffers with trees (agroforestry) provides an elegant solution to enhance carbon storage while also augmenting local biodiversity. Yet the scope and role of riparian plant community diversity in key soil dynamics remain unresolved. Operationalizing riparian age (young [<10 yr] and mature [>30 yr] since establishment] and forest stand type (coniferous and deciduous dominant) to capture the potential extent of plant diversity, we measured plant functional trait diversity and community weighted mean trait values, microbial composition, abiotic soil conditions, and rates of soil CO2 efflux (mg CO2 -C m-2 h-1 ). We used piecewise structural equation modeling (SEM) to further refine the role of biotic indices (leaf, root, and microbial characteristics), and abiotic factors (soil physio-chemical metrics) on soil C cycling processes in riparian systems. We found significantly lower rates of CO2 efflux (F=8.47; p<.01) over one growing season and higher total soil C (F=3.46; p=.03) in mature buffers compared with young buffers. Using SEM, we describe influences on soil C content (marginal r2 =61) and soil CO2 efflux (marginal r2 =53). Within young buffers, soil C content was significantly predicted by fungal/bacterial ratio and root length density, whereas in mature buffers, tree leaf characteristics were associated with soil C content. Soil CO2 efflux was predicted by soil moisture, soil carbon content, and herbaceous root characteristics. Evidently, leaf and root functional traits in combination with broad soil parameters significantly describe soil C dynamics in the field; however, significant pathways are not the same throughout the life cycle of a riparian agroforest.

Effects of moisture and physical disturbance on pore-scale oxygen content and anaerobic metabolisms in upland soils.

Soils are the largest dynamic stock of carbon (C) on Earth, and microbial respiration of soil organic C accounts for over 25% of global carbon dioxide (CO2) emissions. Zones of oxygen depletion in upland soils (anaerobic microsites) are increasingly recognized as an important control on soil microbial respiration rates, but the factors governing the volume and distribution of anaerobic microsites are relatively unknown. We measured the dissolved oxygen (DO) content of porewater from incubated soil cores of varying moisture contents (<80% and >80% water saturation) and degrees of disturbance (undisturbed, conventionally tilled, and physically disturbed). Porewater was extracted sequentially from pores constrained by three effective pore diameters, ≥3.0μm, 3.0-1.0μm, and 1.0-0.6μm, from cores incubated for 7, 14, or 28days, using a modified Tempe cell extraction system. We observed a parabolic pattern in mean dissolved oxygen (DO) concentrations across pore sizes, independent of soil moisture and degree of disturbance. Specifically, DO values within the largest and smallest pore domains were relatively depleted (155±10μM and 160±11μM, respectively), while DO values within medium pores were closer to saturation (214±8μM). The observed DO pattern provides insight into the balance of microbial oxygen demand versus oxygen supply across pore domains within upland soils. Additionally, we observed iron and manganese reduction in all soils except samples subjected to disturbance and incubated at <80% water saturation, suggesting that disturbance enhances aeration and diminishes anaerobic metabolisms within upland soils. Our findings highlight the influence of soil moisture and management on soil redox and CO2 efflux rates.

Larger floods reduce soil CO2 efflux during the post-flooding phase in seasonally-flooded forests of Western Amazonia

Seasonally-flooded várzea forests of Western Amazonia are one of the most productive and biodiverse wetland forests in the world. However, data on their soil CO2 emissions, soil organic matter decomposition rates, and soil C stocks are scarce. This is a concern because hydrological changes are predicted to lead to increases in the height, extent, and duration of seasonal floods, which are likely to have a significant effect on soil C stocks and fluxes. However, with no empirical data, the impact of altered flood regimes on várzea soil C cycles remains uncertain. This study quantified the effects of maximum annual flood height and soil moisture on soil CO2 efflux rate (Rs) and soil organic matter decomposition rate (k) in the várzea forests of Pacaya Samiria National Reserve, Peru. The study was conducted between May and August 2017. The results showed that Rs (10.6–182.7 mg C m−2 h–1) and k (0.016–0.078) varied between and within sites, and were considerably lower than the values reported for other tropical forests. In addition, Rs was negatively affected by flood height (P < 0.01) and soil moisture (P < 0.001), and it decreased with decreasing river levels post flooding (P < 0.001). In contrast, k was not affected by any of the above-mentioned factors. Soil moisture was the dominant factor influencing Rs, and it was significantly affected by maximum flood height, even after the floods had subsided (P < 0.001). Consequently, we concluded that larger floods will likely lead to reduced Rs, whilst k could remain unchanged but with decomposition processes becoming more anaerobic.

Carbon dioxide dynamics in a residential lawn of a tropical city

Turfgrass is an important component of the urban landscape frequently considered as an alternative land cover to offset anthropogenic CO2 emissions. However, quantitative information of the potential to directly remove CO2 from the atmosphere by turfgrass systems is lacking, especially in the tropics. Most assessments have considered the carbon accumulated by grass shoots and soil, but not the release of CO2 to the atmosphere by soil respiration (i.e., soil CO2 efflux). Here, we measured at high-temporal resolution (30-min) soil CO2 efflux, production, and storage rate for nearly three years in a residential lawn of Singapore. Furthermore, we quantified the carbon capture related to biomass production and CO2 emissions from fossil fuel consumption associated with maintenance activities (e.g., mowing equipment). Warm and humid conditions resulted in relatively constant rates of soil CO2 efflux, CO2 storage in soil, and aboveground biomass production (3370, 652, 1671 Mg CO2 km−2 yr−1; respectively), while the systematic use of mowing machinery emitted 27 Mg CO2 km−2 yr−1. Soil CO2 efflux and CO2 mowing emissions represent carbon losses to the atmosphere, while CO2 storage in soil and biomass productivity represent gains of carbon into the ecosystem. Under a steady state in which soil CO2 losses are only compensated by atmospheric CO2 uptake by photosynthesis, an ideal clipping waste disposal management, in which no CO2 molecule returns to the atmosphere (i.e., clippings are not burnt), and a 3-week mowing regime, this site can act as a sink of 2296 Mg CO2 km−2 yr−1. In the scenario of incinerating all clippings, the lawn acts as an emission source of 1046 Mg CO2 km−2 yr−1. Thus, management practices that reduce mowing frequency together with clipping disposal practices that minimize greenhouse gas emissions are needed to make urban lawns a potential natural solution to mitigate global environmental change.

Impact of an extreme monsoon on CO2 and CH4 fluxes from mangrove soils of the Ayeyarwady Delta, Myanmar

Mangrove ecosystems can be both significant sources and sinks of greenhouse gases (GHGs). Understanding variability in flux and the key factors controlling emissions in these ecosystems are therefore important in the context of accounting for GHG emissions. The current study is the first to quantify GHG emissions using static chamber measurements from soils in disused aquaculture ponds, planted mangroves, and mature mangroves from the Ayeyarwady Delta, Myanmar. Soil properties, biomass and estimated net primary productivity were also assessed. Field assessments were conducted at the same sites during the middle of the dry season in February and end of the wet season in October 2019. Rates of soil CO2 efflux were among the highest yet recorded from mangrove ecosystems, with CO2 efflux from the 8 year old site reaching 86.8 ± 17 Mg CO2 ha−1 yr−1 during February, an average of 862% more than all other sites assessed during this period. In October, all sites had significant rates of soil CO2 efflux, with rates ranging from 31.9 ± 4.4 Mg CO2 ha−1 yr−1 in a disused pond to 118.9 ± 24.3 Mg CO2 ha−1 yr−1 in the 8 year old site. High soil CO2 efflux from the 8 year old site in February is most likely attributable to high rates of primary production and belowground carbon allocation. Elevated CO2 efflux from all sites during October was likely associated with the extreme 2019 South Asian monsoon season which lowered soil pore salinity and deposited new alluvium, stimulating both autotrophic and heterotrophic activity. Methane efflux increased significantly (50–400%) during the wet season from all sites with mangrove cover, although was a small overall component of soil GHG effluxes during both measurement periods. Our results highlight the critical importance of assessing GHG flux in-situ in order to quantify variability in carbon dynamics over time.

Plant Diversity and Agroecosystem Function in Riparian Agroforests: Providing Ecosystem Services and Land-Use Transition

Achieving biologically diverse agricultural systems requires a commitment to changes in land use. While in-field agrobiodiversity is a critical route to such a transition, riparian systems remain an important, yet understudied, pathway to achieve key diversity and ecosystem services and targets. Notably, at the interface of agricultural landscapes and aquatic systems, the diversification of riparian buffers with trees reduces the non-point source pollution in waterways. However, in riparian agroforestry systems, little is known about herbaceous community patterns and, importantly, the herbaceous community’s role in governing carbon (C) and nitrogen (N) cycling. Our study investigated herbaceous community taxonomic and phylogenetic diversity patterns in riparian (i) grasslands (GRASSLAND), (ii) rehabilitated agroforests (AGROFOREST-REHAB), and (iii) remnant forests (AGROFOREST-NATURAL). We then determined the biodiversity-ecosystem function relationships between community functional diversity metrics, C and N cycling, and greenhouse gas fluxes. We observed significant differences in taxonomic and phylogenetic diversity among riparian buffer types. We found that herbaceous plant communities in riparian agroforestry systems expressed plant trait syndromes associated with fast-growing, resource acquiring strategies, while grassland buffer plants exhibited slow-growing, resource conserving strategies. Herbaceous communities with high functional diversity and resource acquiring trait syndromes, such as those in the agroforestry riparian systems, were significantly correlated with lower rates of soil CO2 efflux and N mineralization, both of which are key fluxes related to ecosystem service delivery. Our findings provide further evidence that functionally diverse, and not necessarily taxonomically diverse, plant communities are strongly correlated to positive ecosystem processes in riparian agroforestry systems, and that these communities contribute to the transition of agricultural lands toward biologically and functionally diverse landscapes.

Impacts of tree species identity and species mixing on ecosystem carbon and nitrogen stocks in a boreal forest

Forest management practices, such as selection or mixing of particular tree species, may enhance forests’ carbon (C) sinks and resilience against climate change. While a majority of research on this subject has focused on aboveground production, far less is known about how these management decisions impact belowground C storage, as well as the C and nitrogen (N) stocks of the whole ecosystem. We used a well-replicated 60-year-old experiment in boreal Sweden comparing monocultures and a mixture of the two dominant coniferous species: Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris (L.), set up at a site that was assessed as equally suitable for the growth of either species. Our aim was to evaluate the species identity and species mixing effects on ecosystem C and N stocks. We measured total standing volume, aboveground tree biomass, fine-root biomass, C and N pools in tree biomass and soil, litterfall inputs, and soil CO2 emissions. Our results show major differences in C allocation and growth patterns between spruce and pine. We found almost twice as high total standing volume and litterfall inputs in the pine stands than in the spruce stands. Higher proportion and amount of needle biomass resulted in larger amounts of N retained in the canopy and smaller accumulation of C and N in the humus in the spruce compared to pine stands. The C sinks in aboveground tree biomass and soil were larger in the pine compared to spruce stands at this site. In addition, a significantly higher soil CO2 efflux rate and fine-root biomass in the spruce compared to pine stands suggested greater tree internal allocation of C belowground to roots and ectomycorrhizal fungi in response to stronger N limitation. We found no significant mixing effect in the mixed stands, given the levels of the measured variables did not exceed levels of the most productive monoculture, with an exception of higher SOC stocks in the deeper (10–20 cm) mineral soil layer in the mixed stands. Our results do not support the idea of higher productivity and C sinks of forest mixtures compared to the best performing monoculture on the given site suggesting that these tree species are not complementary from a forest management perspective. However, in many cases the mixed stands performed equally well as the best monoculture, indicating that management for multi-species stands may not result in any loss in C uptake and storage.

Interaction of abiotic factor on soil CO2 efflux in three forest communities in tropical deciduous forest from India.

Environmental factors along with soil physico-chemical properties play a significant role on the diurnal trend of soil CO2 efflux. Soil CO2 efflux in Indian tropical forests is poorly studied. We studied the soil CO2 efflux in a representative tropical deciduous forest at Katerniaghat Wildlife Sanctuary (KWLS), Uttar Pradesh. The three forest communities namely dry mixed (DMF), Sal mixed (SMF), and Teak plantation (TPF) were selected for measuring soil CO2 efflux in the summer season during April to May 2017 using automated LI-COR 8100 soil CO2 flux system. Soil physico-chemical parameters were also studied in the three abovementioned forest communities. We also measured the different microclimatic variables at forest understorey in all three communities during the summer season. Total day time soil CO2 efflux of 826.70, 1089.24, and 828.94 (μmolCO2 m-2d-1) was observed in TPF, SMF, and DMF respectively. Soil CO2 efflux observed significant differences (P < 0.01) among the three forest communities studied for the summer season in tropical deciduous forest of Terai Himalaya. Average soil CO2 efflux rate (μmol CO2 m-2 s-1) of 4.06 ± 0.36, 5.03 ± 0.45, and 4.37 ± 0.79 was observed in TPF, SMF, and DMF, respectively, which is positively correlated with total organic carbon (TOC) and water holding capacity (WHC) among soil physico-chemical variables. Among microclimatic variables, soil temperature (ST, °C) and air temperature (AT, °C) observed strong positive correlation with day time soil CO2 efflux in all three communities. Significant increase in soil CO2 flux was observed with increasing air and soil temperature (AT and ST) in DMF and SMF. Maximum TOC of 19.23 g Kg-1 was observed in SMF among all communities in the summer season. The result showed that soil CO2 efflux is closely associated with TOC, WHC, AT, and ST for Indian deciduous forest ecosystems.

Warming Changed Soil Respiration Dynamics of Alpine Meadow Ecosystem on the Tibetan Plateau

Alpine meadow system underlain by permafrost on the Tibetan Plateau contains vast soil organic carbon and is sensitive to global warming. However, the dynamics of annual soil respiration (Rs) under long-term warming and the determined factors are still not very clear. Using opentop chambers (OTC), we assessed the effects of two-year experimental warming on the soil CO2 emission and the Q10 value (temperature sensitivity coefficient) under different warming magnitudes. Our study showed that the soil CO2 efflux rate in the warmed plots were 1.22 and 2.32 times higher compared to that of controlled plots. However, the Q10 value decreased by 45.06% and 50.34% respectively as the warming magnitude increased. These results suggested that soil moisture decreasing under global warming would enhance soil CO2 emission and lower the temperature sensitivity of soil respiration rate of the alpine meadow ecosystem in the permafrost region on the Tibetan Plateau. Thus, it is necessary to take into account the combined effect of ground surface warming and soil moisture decrease on the Rs in order to comprehensively evaluate the carbon emissions of the alpine meadow ecosystem, especially in short and medium terms.

Moth Outbreaks Reduce Decomposition in Subarctic Forest Soils

Tree mortality from insect infestations can significantly reduce carbon storage in forest soils. In subarctic birch forests (Betula pubescens), ecosystem C cycling is largely affected by recurrent outbreaks of defoliating geometrid moths (Epirrita autumnata, Operophtera brumata). Here, we show that soil C stocks in birch forests across Fennoscandia did not change up to 8 years after moth outbreaks. We found that a decrease in woody fine roots was accompanied by a lower soil CO2 efflux rate and a higher soil N availability following moth outbreaks. We suggest that a high N availability and less ectomycorrhiza likely contributed to lowered heterotrophic respiration and soil enzymatic activity. Based on proxies for decomposition (heterotrophic respiration, phenol oxidase potential activity), we conclude that a decrease in decomposition is a prime cause why soil C stocks of mountain birch forest ecosystems have not changed after moth outbreaks. Compared to disturbed temperate and boreal forests, a CO2-related positive feedback of forest disturbance on climate change might therefore be smaller in subarctic regions.

CO2 Dynamics Are Strongly Influenced by Low Frequency Atmospheric Pressure Changes in Semiarid Grasslands

AbstractDue to their large carbon storage capacity and ability to exchange subterranean CO2 with the atmosphere, soils are key components in the carbon balance in semi‐arid ecosystems. Most studies have focused on shallow (e.g., &lt;30 cm depth) soil CO2 dynamics neglecting processes in deeper soil layers where highly CO2‐enriched air can be stored or transported through soil pores and fissures. Here, we examine the relationship among variations in subterranean CO2 molar fraction, volumetric water content, soil temperature and atmospheric pressure during three years within soil profiles (0.15, 0.50, and 1.50 m depths) in two semi‐arid grasslands located in southeastern Spain. We applied a wavelet coherence analysis to study the temporal variability and temporal correlation between the CO2 molar fraction and its covariates (soil temperature, soil moisture and atmospheric pressure). Our results show that CO2 dynamics are strongly influenced by changes in atmospheric pressure from semidiurnal, diurnal and synoptic to monthly time‐scales for all soil depths. In contrast, only weak daily dependencies were found at the surface level (0.15 m) regarding soil temperature and volumetric water content. Atmospheric pressure changes substantially influence variations in the CO2 content (with daily fluctuations of up to 2000 ppm) denoting transportation through soil layers. These results provide insights into the importance of subterranean storage and non‐diffusive gas transport that could influence soil CO2 efflux rates, processes that are not considered when applying the flux‐gradient approach and, which can be especially important in ecosystems with high air permeability between the unsaturated porous media and the atmosphere.

Soil CO2 efflux in uneven-aged and even-aged Norway spruce stands in southern Finland

Abstract: Even-aged forests usually act as carbon sinks during most of their rotation. However, after clearcut they become sources of carbon for a period of several years. Applying uneven-aged forest management with selective cuttings will maintain tree cover and reduce the environmental impact on forest floor. The aim of this study was to compare the soil CO2 efflux between uneven-aged and even-aged Norway spruce stands with similar site properties, to investigate the effect of management practices on soil CO2 efflux and its possible correlation with soil environmental and chemical properties. We measured soil CO2 efflux in even- and uneven-aged Norway spruce stands (Picea abies [L.] Karst) in southern Finland during the summer of 2013 using closed chamber method on fixed measuring points. The study included two uneven-aged stands and two even-aged stands (a clearcut site and a mature even-aged stand). Soil moisture and soil temperature were measured at the same time as soil CO2 efflux. Soil cores were collected from the topsoil of each study plot to determine soil carbon and nitrogen concentrations. Mean soil CO2 efflux through the summer was highest in the clearcut plot (0.367 mg m-2 s-1) followed by the uneven-aged stands (0.298 and 0.257 mg m-2 s-1, respectively) and the smallest fluxes were measured in the mature even-aged stand (0.224 mg m-2 s-1). There was no statistically significant difference in soil CO2 efflux between the even- and uneven-aged stands of the same site fertility. Even- and uneven-aged stands did not differ significantly in soil moisture or soil temperature. Soil CO2 efflux increased steadily with soil temperature, whereas increasing soil moisture considerably increased soil CO2 efflux at lower moisture levels but only moderately at higher soil moisture levels. Soil carbon and nitrogen concentration did not differ between the study plots of the same fertility. Uneven-aged structure forestry did not prevent the increase in soil CO2 efflux after cuttings. However, the large variation in soil CO2 efflux rates within the uneven-aged stands suggests that the stand level CO2 efflux can be controlled with the intensity of the cutting.

Interactions Among Abiotic Drivers, Disturbance and Gross Ecosystem Carbon Exchange on Soil Respiration from Subtropical Pine Savannas

Globally, soil CO2 efflux rates (Fs) have been linked to changes in soil water content (SWC), rainfall and temperature and/or productivity. However, within an ecosystem, Fs can vary based on site structure and function, which can be affected by a combination of abiotic and biotic factors. This becomes particularly important when an ecosystem is faced with disturbances, such as drought or fire. Site-specific compensatory responses to disturbances may therefore alter C mineralization, as well as root respiration. Hence, single location Fs estimates may not be a representative for ecosystems across their distributional ranges. We conducted a 6-year study along an edaphic moisture gradient of longleaf pine ecosystems that were maintained with prescribed fire, using eddy covariance and soil respiration measurements to address how Fs varies with changes in ecosystem structure and function, as well as disturbances. Lower air temperatures (Tair) decreased Fs at all sites, but that response was also affected by productivity and SWC. Productivity significantly altered Fs rates at all sites, especially when we accounted for changes in temperature and SWC. Plant regrowth post-fire temporarily increased Fs (10–40%), whereas drought reduced Fs at all sites. Our results show that site productivity, Fs and the degree to which ecosystems adapt to climate variations and disturbance can be site specific. Hence, model forecasting of carbon dynamics would strongly benefit from multi-location measurements of Fs across the distributional range of an ecosystem.

Soil respiration in shrub willow (Salix x dasyclados) biomass crop increased on the third year after removal

The SWBC production system is not only a source of renewable energy, but also a carbon sink. However, one of the uncertainties in the greenhouse gas potential analysis of this system is soil CO2 efflux (Fc) rates after its termination. We examined the Fc rates for three years and determined the total CO2 production of SWBC fields following the termination and removal of SWBC fields. Four current SWBC fields of 5-, 12-, 14-, and 19-years old were used as sampling plots. The mean Fc rates did not differ statistically between ages (p = 0.66), and there was no significant interaction effect between age and year (p = 0.94). However, Fc rates significantly increased in the third year (p = 0.01). Annual cumulative Fc ratios at different seasons were 52% for summer, 26% for fall, 16% for spring, and 6% for winter.