Soil CO2 Efflux Research Articles

Influence of streambank fencing on vegetation and soil of the Mixed prairie component in a complex corridor pasture

A 5 yr (2011–2015) field study was conducted to test the hypothesis that streambank fencing had a significant effect on selected vegetation and soil properties of the Mixed prairie component of a complex corridor pasture. The grazing treatments [ungrazed (UG) – periodic grazing (PG)] inside the corridor pasture were 11 yr (2001–2012) of cattle exclusion (UG), followed by 3 yr (2013–2015) of periodic grazing (PG) when the riparian soil was dry. A control treatment outside the fencing was continuous grazing (CG). Selected vegetation and soil properties were measured over the growing season at 10 paired locations in each treatment (nonreplicated) pasture over 5 yr (2011–2015), and rangeland health was measured in 2011. The UG–PG treatment significantly (P ≤ 0.10) increased the total biomass by 2- to 5-fold in all 5 yr compared with CG treatment and improved the rangeland health score of the UG phase of the UG–PG (63%) treatment compared with the CG treatment (50%) in 2011. It also significantly reduced surface soil temperature by 2.2–5.2 °C, significantly increased volumetric water content of the surface soil by 7%–10% in 3 of 5 yr, and significantly increased surface soil CO2 efflux (instantaneous) by 17%–60% in all 5 yr. Overall, the UG–PG treatment improved rangeland health, increased total biomass, soil water, and soil CO2 efflux of the Mixed prairie, but decreased soil temperature compared with the CG treatment. Excessive dead biomass, greater fire risk, and an increase in noxious weeds caused by cattle exclusion suggested that periodic grazing may be the preferred option.

Methane Emission from Mangrove Wetland Soils Is Marginal but Can Be Stimulated Significantly by Anthropogenic Activities

Mangrove wetland soils have been considered as important sources for atmospheric CH4, but the magnitude of CH4 efflux in mangrove wetlands and its relative contribution to climate warming compared to CO2 efflux remains controversial. In this study, we measured both CH4 and CO2 effluxes from mangrove soils during low or no tide periods at three tidal zones of two mangrove ecosystems in Southeastern China and collected CH4 efflux data from literature for 24 sites of mangrove wetlands worldwide. The CH4 efflux was highly variable among our field sites due to the heterogeneity of mangrove soil environments. On average, undisturbed mangrove sites have very low CH4 efflux rates (ranging from 0.65 to 14.18 μmol m−2 h−1; median 2.57 μmol m−2 h−1), often less than 10% of the global warming potentials (GWP) caused by the soil CO2 efflux from the same sites (ranging from 0.94 to 9.50 mmol m−2 h−1; median 3.67 mmol m−2 h−1), even after considering that CH4 has 28 times more GWP over CO2. Plant species, study site, tidal position, sampling time, and soil characteristics all had no significant effect on mangrove soil CH4 efflux. Combining our field measurement results and literature data, we demonstrated that the CH4 efflux from undisturbed mangrove soils was marginal in comparison with the CO2 efflux in most cases, but nutrient inputs from anthropogenic activities including nutrient run-off and aquaculture activities significantly increased CH4 efflux from mangrove soils. Therefore, CH4 efflux from mangrove wetlands is strongly influenced by anthropogenic activities, and future inventories of CH4 efflux from mangrove wetlands on a regional or global scale should consider this phenomenon.

Bicarbonates dissolved in irrigation water contribute to soil CO2 efflux

Understanding the processes that drive the release of carbon dioxide (CO2) from soil is essential to combat rising atmospheric greenhouse gases. Whilst significant research has focused on soil organic carbon (C) dynamics, soil inorganic C has received much less attention. In arid and semi-arid regions, crops are often irrigated with water containing inorganic (bicarbonate) C, which can be a source of CO2 emissions. CO2 released from dissolved bicarbonates is 13C enriched compared to that from organic sources in the soil (e.g., organic matter and respiring roots). Measurement of δ13CO2 at the soil surface can therefore be used to trace CO2 sources. Using on-line 13C analysis, we monitored δ13CO2 and rate of soil CO2 emissions in an apple orchard in British Columbia, Canada, irrigated with bicarbonate-containing irrigation water drawn from Okanagan Lake. In Experiment 1, we applied deionised or irrigation water to wet and dry soils; in Experiment 2, we applied irrigation water from Okanagan Lake to soils with and without a surface mulch. In both experiments, soil efflux responded within seconds to application of water. In Experiment 1, a 6‰ enrichment of 13CO2 followed the application of irrigation water, confirming the contribution of bicarbonate-C to soil surface efflux, whereas a 4‰ depletion of 13CO2 followed the application of deionised water, suggesting a stimulation of labile organic C mineralisation. Experiment 2 confirmed that bicarbonates in irrigation water contribute to soil CO2 efflux; surface mulch had no effect on the response of CO2 release following irrigation. Using an isotopic mass balance model, we calculated that bicarbonates dissolved in irrigation water accounted for between 9 and 15% of total soil surface efflux, and estimate that irrigation with Okanagan Lake water generates on average >45,000 kg bicarbonate-derived CO2 each year.

Effects of Nutrient-Limitation on Disturbance Recovery in Experimental Mangrove Wetlands

Coastal wetlands are exposed to high-energy storms that influence plant and soil structure. To understand how nutrient availability interacts with storm-induced plant stress, we tested how defoliation interacts with nutrient enrichment to affect carbon (C) and nutrient (nitrogen, N; phosphorus, P) cycling and storage within soils and plants. In outdoor experimental mesocosms, we defoliated red mangrove saplings (Rhizophora mangle), added 30 g of inorganic P to peat soils, and quantified plant [elemental stoichiometry (C:N, C:P, N:P), leaf count, and above- and below- ground biomass] and soil responses [C:N, C:P, N:P, litter breakdown rate (k), soil CO2 efflux] during a 42-d recovery period. Mangroves rapidly regrew all removed leaves and recovered nearly 30% of leaf biomass. Mangrove biomass %P increased by 50% with added P; however, soil stoichiometry remained unchanged. Defoliation reduced Soil CO2 efflux by 40% and root litter k by 30%. Phosphorus was quickly incorporated into mangrove biomass and stimulated nighttime soil CO2 efflux. This work highlights the importance of testing interactions of nutrient availability and plant stress on plant and soil biogeochemical cycling and suggests that plants quickly incorporate available nutrients into biomass and defoliation can lead to reduced soil C losses.

Impacts of forest management type and season on soil carbon fluxes in Eastern Mau Forest, Kenya

AbstractThe value of ecosystems functions performed by forests in the climate change era has prompted increasing attention towards assessment of carbon stocks and fluxes in tropical forests. The aim of this study was to understand how forest management approaches and environmental controls impacted on soil CO2 efflux in a tropical Eastern Mau forest which is one of the blocks of the greater Mau complex in Kenya. Nested experimental design approach was employed where 32 plots were nested into four blocks (disturbed natural, undisturbed natural, plantation and glades). In 10 m2 plots, data were collected on soil CO2 efflux, soil temperature and soil moisture using soda lime methods, direct measurement and proxy techniques, respectively. There was significant forest management type effect (F3,127 = 3.01, p = 0.033) and seasonality effect (t test = 3.31, df = 1, p < 0.05) on mean soil CO2 efflux. The recorded mean soil CO2 efflux levels were as follows: plantation forest (9.219 ± 3.067 g C M−2 day−1), undisturbed natural forest (8.665 ± 4.818 g C M−2 day−1), glades (8.592 ± 3.253 g C M−2 day−1) and disturbed natural forest (7.198 ± 3.457 g C M−2 day−1). The study concludes that managing a forest in plantation form is primarily responsible for forest soil CO2 efflux levels due to aspects such as increased microbial activity and root respiration. However, further studies are required to understand the role and impact of soil CO2 efflux on the greater forest carbon budget.

Short-term effects of fen peatland restoration through the moss layer transfer technique on the soil CO2 and CH4 efflux

Northern peatlands store approximately one third of the global soil carbon (C) stocks. Peat extraction for horticulture changes C fluxes turning these soils from sinks into large sources of C emitted to the atmosphere as greenhouse gases. Restoring hydrological conditions may not be enough for returning the C sink function of these ecosystems and a recovery of the original Sphagnum-dominated vegetation cover could be necessary. Here we tested the effects of the moss layer-transfer technique (MLTT) on the CO2 and CH4 fluxes three years after restoration of a minerotrophic cutover peatland in Quebec, Canada. Gas emissions were compared to nearby undisturbed plots, and plots with restored hydrology but without vegetation transfer. The main drivers for the observed emissions were inferred from the relationships with relevant environmental variables (vegetation cover, water table depth and temperature). Restoration of the extracted minerotrophic fen through the MLTT produced significant changes in the exchange of CO2, bringing these fluxes closer to undisturbed plots than plots with restored hydrology only, which remained C sources throughout the study. Methane emissions were generally low across all the restored plots and far from natural levels, with only a slight increase at the MLTT restored plots observed during the third year. Even though the MLTT failed to recover the original bryophyte-dominated vegetation, the observed changes in the emissions of CO2 and CH4 at the restored plots were strongly associated with changes in vegetation cover (i.e. increase in vascular plants), supporting the use of this restoration technique to recover the C sink function of harvested minerotrophic peatlands.

Effects of ridge–furrow mulching on soil CO2 efflux in a maize field in the Chinese Loess Plateau

Ridge–furrow mulching system (RFMS) is widely used in arid and semi–arid areas, but its effect on soil respiration (Rs) and its components, including heterotrophic (Rh) and autotrophic respiration (Ra) are still poorly understood. In this study, CO2 flux from the soil of furrows and ridges was measured across different RFMS practices (i.e., three different ridge/furrow ratios) and conventional flat planting (CK). A trenching method was used to estimate the contribution of Rh to Rs. Compared with CK, RFMS significantly increased soil temperature of the ridge, promoted soil moisture of the furrow, and enhanced microbial diversity at the early crop growth stage, resulting in increased Rs and its components. The ridge soils exhibited much higher Rs (3.43 μmol m–2 s–1) than the furrow soils (2.98 μmol m–2 s–1) under all three RFMS practices. The contribution ratios of Rh to Rs across the different practices ranged from 50.4% to 59.6%. Soil temperature rather than soil moisture explained the seasonal variation of Rs and its components for both CK and RFMS. Nonetheless, high Rs and Rh values in RFMS did not induce a decline of soil organic carbon during the two–year experimental period. Improved root growth in RFMS practices may provide more exudates to the soil, thus offsetting soil carbon decomposition. Compared with CK, RFMS with ridge/furrow ratios of 40:70 cm, 55:55 cm, and 70:40 cm, significantly increased soil CO2 emissions by 10.6%, 19.6%, and 20.4%, respectively, while increasing maize yield by 26.1%, 36.4%, and 50.3%, respectively. Carbon emission efficiency (CEE) was significantly higher in RFMS than in CK in both years. This study suggests that, due to its high CEE, RFMS with a ridge/furrow ratio of 70:40 cm could be a highly promising strategy for sustaining crop productivity while minimizing environmental impacts.

Effects of yak excreta on soil organic carbon mineralization and microbial communities in alpine wetlands of southwest of China

Improving knowledge of how soil organic carbon (SOC) mineralization responds to excreta application is essential to better understand whether wetland carbon (C) pools will react to grazing. We investigated microbial activity and community structure in the different treatments of excreta addition experiments to examine how soil C mineralization responds to the excreta input in terms of microbial activities and compositions in wetland soils. The microcosms of mineralization incubation of excreta addition were established. The structure of the microbial community was described by the fatty acid composition of the phospholipids (PLFA). The methylumbelliferyl-linked substrates (MUB) and l-dihydroxyphenylalanine (L-DOPA) substrates were used to investigate the activities of β-glucosidase (BG), N-acetyl-glucosaminidase (NAG), acid phosphatase (AP), cellobiohydrolase (CBH), and phenol oxidase (PO). Excreta addition altered the cumulative C mineralization in swamp meadow (SM) and peatland (PL) soils, but SM was lower than PL. Excreta addition increased the biomass of individual PLFA and the fungi/bacteria ratio, suggesting that microbes are stimulated by nutrients and that the soil microbial community composition is modified by excreta inputs. The hydrolytic enzyme activities were higher in the PL soils than in the SM soils, but the trend was opposite for PO activity. The changes in pH, fungi, actinomycetes (ACT), AP, and CBH after yak fecal input significantly influenced the soil CO2 efflux. Our findings suggest that yak grazing could influence the rate of C cycling in wetland soils by influencing microbial communities, enzyme activities, and soil pH. This study suggest that the yak excreta addition increased cumulative C mineralization in SM and PL soils, and the effect of dung addition was more significant than urine addition. The effect of yak excreta addition on SOC mineralization was related with the soil pH, microorganism structure, and enzyme activity which modified by the excreta addition. Soil pH, fungi, AP, and CBH were positively correlated with SOC mineralization, but ACT was negatively correlated with SOC mineralization. In addition, the changes in C and N sources with yak excreta addition play an important role in altering microbial enzyme activities. The input of yak feces into wetlands because of grazing could increase SOC mineralization and thereby promote C emission.

Forest Soil Respirations are More Sensitive to Nighttime Temperature Change in Eastern China

Soil respiration is one of the main fluxes in the global carbon cycle. The effect of temperature on soil respiration is well understood. The response of soil respiration to temperature warming is called apparent temperature sensitivity (Q10) of soil respiration, which is an important parameter in modeling soil CO2 effluxes under global climate warming. The difference of Q10 between daytime and nighttime was hardly reported although attentions are attracted by the differences of temperature change and its effects on vegetation productivity. In this study, we investigated the Q10 of soil respiration in daytime and nighttime by modeling empirical functions based on the in situ measurement of soil respiration and temperature in temperate and subtropical forests of eastern China. Our results showed that the Q10 of soil respiration is higher in nighttime with the mean value of 2.74 and 2.35 than daytime with the average of 2.49 and 2.18 in all measured months and growing season, respectively. Moreover, the explanatory rate of soil temperature to soil respiration in nighttime is also higher than in daytime in each site in both all measured and growing seasons. The Q10 and explanatory rate of soil temperature to soil respiration in nighttime is 1.08 and 1.15 times in daytime in growing season. These findings indicate that soil respiration has a bigger sensitivity to temperature in nighttime than daytime. The change of soil temperature explains more variation of soil respiration in nighttime than daytime.

Short-term effects of thinning on soil CO2, N2O and CH4 fluxes in Mediterranean forest ecosystems

In Mediterranean ecosystems an increasing demand for in situ trace gas exchange data is emerging to enhance the adaptation and mitigation strategies under forest degradation. Field-chamber green-house gas fluxes and site characteristics were analysed in two Mediterranean peri-urban pine forests showing degradation symptoms. We examined the effect of different thinning interventions on soil CO2, CH4 and N2O fluxes, addressing the relationships with the environmental variables and C and N contents along forest floor-soil layers.Soil temperature resulted as the main driving variable for CO2 efflux and CH4 uptake. Soil moisture content and organic matter availability affected CO2 emission patterns in the two sites. N2O fluxes showed a positive correlation with soil moisture under wetter climatic conditions only. GHG fluxes showed significant correlations with C and N content of both forest floor and mineral soil, especially in the deepest layers, suggesting that it should be considered, together with environmental variables when accounting GHG fluxes in degraded forests.Short-term effects of thinning on CO2 emissions were dependent on disturbance induced by logging operations and organic matter inputs. After thinning CH4 uptake increased significantly under selective treatment, independently from specific site-induced effects. N2O fluxes were characterized by low emissions in both sites and were not affected by treatments. Soil CO2 efflux was the largest component of global warming potential (GWP) from both sites (11,553 kg ha−1 y−1 on average). Although it has a large global warming potential, N2O contribution to GWP was about 131 kg CO2eq ha−1 y−1. The contribution of CH4-CO2 equivalent to total GWP showed a clear and significant CH4 sink behaviour under selective treatment (36 kg ha−1 y−1 on average). However, in the short-term both thinning approaches produced a weak effect on total GWP.

Soil respiration partitioning in afforested temperate peatlands

Understanding and quantifying soil respiration and its component fluxes are necessary to model global carbon cycling in a changing climate as small changes in soil CO2 fluxes could have important implications for future climatic conditions. A soil respiration partitioning study was conducted in eight afforested peatland sites in south-west Ireland. Using trenched points, annual soil CO2 emissions, and the contributions of root and heterotrophic respiration as components of total soil respiration, were estimated. Nonlinear regression models were evaluated to determine the best predictive soil respiration model for each component flux, using soil temperature and water table level as explanatory variables. Temporal variation in soil CO2 efflux was driven by soil temperature at 10 cm depth, with all treatment points also affected by water table level fluctuations. The effect of water table level on soil respiration was best accounted for by incorporating a water level Gaussian function into the soil-temperature–soil-respiration model. Mean root respiration was 44% of mean total soil respiration, varying between 1100 and 2049 g CO2 m−2 year−1. Heterotrophic respiration was divided between peat respiration and litter respiration, which accounted for 35 and 21% of total soil respiration, respectively. While peat respiration varied between 774 and 1492 g CO2 m−2 year−1, litter respiration varied between 514 and 1013 g CO2 m−2 year−1. Although the extrapolation of these results to other sites should be done with caution, the empirical models developed for the entire dataset in this study are a useful tool to predict and simulate CO2 emissions in similar afforested peatlands (e.g. pine and spruce plantations) in temperate maritime climate conditions.

A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere

Soil CO2 efflux (Fsoil) is commonly considered equal to soil CO2 production (Rsoil), and both terms are used interchangeably. However, a non-negligible fraction of Rsoil can be consumed in the subsurface due to a host of disparate, yet simultaneous processes. The ratio between CO2 efflux/O2 influx, known as the apparent respiratory quotient (ARQ), enables new insights into CO2 losses from Rsoil not previously captured by Fsoil. We present the first study using continuous ARQ estimates to evaluate annual CO2 losses of carbon produced from Rsoil. We found that up to 1/3 of Rsoil was emitted directly to the atmosphere, whereas 2/3 of Rsoil was removed by subsurface processes. These subsurface losses are attributable to dissolution in water, biological activities and chemical reactions. Having better estimates of Rsoil is key to understanding the true influence of ecosystem production on Rsoil, as well as the role of soil CO2 production in other connected processes within the critical zone.

Interactive priming effect of labile carbon and crop residues on SOM depends on residue decomposition stage: Three-source partitioning to evaluate mechanisms

Inputs of crop residues and labile C (e.g. root exudates) can affect the decomposition rate of soil organic matter (SOM) through the priming effect (PE). Most previous priming studies describe the addition of single labile or residue C, ignoring the interactions of labile C and fresh or decaying crop residues commonly present in field conditions. Using a dual 13C/14C labelling approach in a 62-day incubation, we investigated the effects of adding labile C (40 μg glucose-C g−1 soil) together with wheat shoot or root residues (3.1 mg C g−1 soil) on SOM priming at three residue decomposition stages: intensive (day-1), reduced (day-9) and stabilised (day-24). To estimate the PE, total soil CO2 efflux and microbial biomass were partitioned for three sources: labile C (14C-glucose), plant residues (13C-labelled) and SOM (unlabelled). Without glucose, roots were decomposed less than shoots but induced 1.4-fold stronger cumulative SOM priming (365 μg C g−1 soil) than shoots. Addition of glucose increased SOM priming, with a stronger effect in the presence of shoot than root residues. Glucose addition at the intensive stage of shoots decomposition slightly increased SOM priming. However, compared with residues alone PE, the addition of glucose during reduced residue decomposition stage, increased SOM priming by 60% (roots) to 104% (shoots). Remarkably, this SOM priming after glucose addition was followed by a decline in residue decomposition and by an increase (up to 50%) in SOM-derived C in microbial biomass. Hence, following glucose addition, microorganisms utilised more SOM rather than feeding on decaying residues during reduced decomposition stage. During stabilised residue decomposition stage, the impact of glucose on SOM priming declined again, while the residue decomposition rate remained unaffected. Furthermore, a large proportion of added glucose (up to 10%) was retained in microbial biomass and its mineralisation rate declined strongly (compared with intensive and reduced decomposition stage). Therefore, the glucose amount was not sufficient to influence microbial activities determining SOM or stabilised residue decomposition rates. Overall, SOM decomposition increased by 1- to 4-fold more than the amount of added glucose C, which resulted in a negative net soil C balance compared with residues alone. Thus, we demonstrated for the first time that 1) the interactive effects of glucose (trace amount) and residues on SOM priming depend on plant residue type (higher under shoots than roots) and 2) stage of residues decomposition (higher SOM priming when labile C was added after the end of intensive decomposition stage of plant residues).

Disentangling the effects of tree species and microclimate on heterotrophic and autotrophic soil respiration in a Mediterranean ecotone forest

Understanding how forest ecosystems influence the response of soil respiration (RS) to climate drivers is essential to accurately predict soil carbon dioxide (CO2) fluxes in a changing environment. This is particularly crucial in areas of contact between coniferous and broadleaved forests in the Mediterranean region which are already experiencing a warmer and drier climate. We present a case study in a Mediterranean ecotone forest where RS components –heterotrophic (RH) and autotrophic (RA)– were monitored beneath pure Scots pine (Pinus sylvestris L.) and Pyrenean oak (Quercus pyrenaica Willd.) stands using the root-trenching method. We used generalized linear models to predict the soil CO2 efflux based on interactions between soil water conent (SWC) and soil temperature. Regardless of the tree species, we found that the strong and inverse water availability and thermal seasonality of the Mediterranean climate intensely constrained soil microbial activity. The incorporation of the soil temperature-moisture covariation thus greatly improved the quality of the models compared to approaches that consider soil temperature alone. We also identified species-specific responses influencing both the total amount of RA and its sensitivity to environmental variables. Root respiration in the Scots pine stand showed greater vulnerability to the decline in SWC throughout the summer than in the oak stand. The RA in the Pyrenean oak stand was mainly limited by low soil temperatures in winter, indicating low maintenance rates during vegetative dormancy. Mean annual RH rates were highest in the Scots pine stand, probably driven by the larger litterfall rates and soil carbon (C) stocks; however, the apparent turnover rate of soil organic C in the oak stand was almost twice as fast as in the pine stand. While our observations are limited to a case study, our work shows that both soil moisture and forest composition can significantly control the temperature dependence of RS components under a Mediterranean climate.

From microhabitat to ecosystem: identifying the biophysical factors controlling soil CO2 dynamics in a karst shrubland

SummaryThe soil CO2 efflux (Fs) remains the least constrained component of the terrestrial carbon cycle; its estimates are still largely uncertain, mainly because of its considerable variation related to the many factors that interact over different temporal and spatial scales. Therefore, our aims were to: (i) identify the biophysical factors that control the soil CO2 molar fraction (χs) and characterize their time‐frequency patterns in a karst shrubland, (ii) explore χs variation with soil cover type (microhabitat) and (iii) estimate Fs at the ecosystem scale. Continuous measurements of aboveground variables, including net CO2 exchanges at the ecosystem level, were compared with pedoclimatic variables collected from four microhabitats (Festuca scariosa (Lag.) Asch. & Graebn., Hormathophylla spinosa (L.) P. Küpfer, Genista pumila (Hervier) Vierh. and bare soil). The microhabitat‐scale controlling factors of χs were identified by a top‐down statistical analysis, and time‐frequency patterns were analysed by wavelet spectral decomposition. Finally, Fs was upscaled from the microhabitat to the ecosystem scale by considering the spatial heterogeneity of ground cover. We determined that in addition to soil water content and temperature, which are traditionally used to predict χs or Fs, the wind (friction velocity) can also have a significant effect. Furthermore, the sensitivity of χs to the main factors identified here varied with microhabitat and season. Over a year, Fs upscaled to the ecosystem level and its uncertainty was 175 ± 13 g C m−2 compared with 155 ± 8 g C m−2 estimated from ecosystem respiration. These results provide new insight into mechanisms of soil CO2 production and transport that question and can improve models traditionally used to quantify ecosystem CO2 emissions.Highlights Explored the variation of soil CO2 in relation to its biophysical controlling factors. Used a top‐down statistical analysis and a wavelet time‐frequency decomposition. Identified controlling factors of soil CO2 dynamics: soil water content, temperature, wind, microhabitat and season. Provides new insight into soil CO2 production and transport, and improves CO2 emission modelling.

Warming exerts greater impacts on subsoil than topsoil CO2 efflux in a subtropical forest

How warming affects the magnitude of CO2 fluxes within the soil profile remains an important question, with implications for modeling the response of ecosystem carbon balance to changing climate. Information on belowground responses to warming is especially limited for the tropics and subtropics because the majority of manipulative studies have been conducted in temperate and boreal regions. We examined how artificial warming affected CO2 gas production and exchange across soil profiles in a replicated mesocosms experiment relying on heavily weathered subtropical soils and planted with Chinese fir (Cunninghamia lanceolata). Half of 2 × 2 m mesocosms (5 replications) was heated with cables buried at a 10 cm depth, which increased temperature in the whole soil profile by 4.5, 3.6 and 2.5 °C at 15, 30 and 60 cm soil depths, respectively. Using a combination of chamber-based and concentration gradient method (CGM) approaches, we found that warming increased soil CO2 efflux across the whole profile by 40%. Changes were unevenly distributed across soil depth: mean CO2 production rate decreased from 0.74 to 0.67 μmol CO2 m−2 s−1 in topsoils (0–15 cm depth) whereas it increased from 0.26 to 0.73 μmol CO2 m−2 s−1 in subsoils (15–60 cm depth). Warming reduced moisture more strongly in subsurface than surface soils and increased subsoil soluble N concentrations as well as fine root turnover, in line with previous temperate and boreal warming studies. This consistency indicates that overall responses of subtropical forests to warming may be similar to forests in higher latitudes.

Plant functional group influences arbuscular mycorrhizal fungal abundance and hyphal contribution to soil CO2 efflux in temperate grasslands

Arbuscular mycorrhizal (AM) fungi are abundant in grassland ecosystem. We assessed AM hyphal contributions to soil CO2 efflux across plant functional groups to better quantify AM fungal influences on soil carbon dynamics. We conducted a field experiment using in-growth mesocosms to partition soil CO2 efflux from roots, AM hyphae, and free-living soil microbes associated with C3 grasses, C4 grasses, forbs, and diverse plant communities from May to August in 2017. AM hyphae contributed <10% to total soil respiration in forb communities and diverse plant communities but accounted for as much as 32% in C3 grasses. Plant functional groups differed in hyphal production efficiencies (the ratio of AM hyphal length to aboveground biomass), with the lowest in C3 grasses (0.47 ± 0.15 m g−1) and the greatest in forbs (3.27 ± 0.55 m g−1). Mowing reduced hyphal production efficiency of C4 grasses and forbs but did not affect total soil respiration. AM hyphal and microbial respiration peaked at the middle of the growing season, however there was no significant seasonal variation in root respiration. AM hyphal respiration is an important pathway of carbon flux from plants to atmosphere. Shifts in plant community composition can influence soil carbon processes by regulating hyphal production and respiration.

Hot-Moments of Soil CO2 Efflux in a Water-Limited Grassland

The metabolic activity of water-limited ecosystems is strongly linked to the timing and magnitude of precipitation pulses that can trigger disproportionately high (i.e., hot-moments) ecosystem CO2 fluxes. We analyzed over 2-years of continuous measurements of soil CO2 efflux (Fs) under vegetation (Fsveg) and at bare soil (Fsbare) in a water-limited grassland. The continuous wavelet transform was used to: (a) describe the temporal variability of Fs; (b) test the performance of empirical models ranging in complexity; and (c) identify hot-moments of Fs. We used partial wavelet coherence (PWC) analysis to test the temporal correlation between Fs with temperature and soil moisture. The PWC analysis provided evidence that soil moisture overshadows the influence of soil temperature for Fs in this water limited ecosystem. Precipitation pulses triggered hot-moments that increased Fsveg (up to 9000%) and Fsbare (up to 17,000%) with respect to pre-pulse rates. Highly parameterized empirical models (using support vector machine (SVM) or an 8-day moving window) are good approaches for representing the daily temporal variability of Fs, but SVM is a promising approach to represent high temporal variability of Fs (i.e., hourly estimates). Our results have implications for the representation of hot-moments of ecosystem CO2 fluxes in these globally distributed ecosystems.

Wetland Soil Co2 Efflux Along a Latitudinal Gradient of Spatial and Temporal Complexity

Foundation plant species play an important functional role in modifying microenvironment through vegetation structure and by influencing soil properties. Mangroves and salt marsh differ in vegetation structure, yet offer similar ecosystem services. This work aimed to understand how different vegetation classes and abiotic factors along a latitudinal gradient of spatial and temporal complexity affect soil CO2 efflux. Soil CO2 efflux, biomass, soil properties, and soil and air temperature were measured in July 2015 and January 2016 in situ at 10 sites along a 342-km latitudinal gradient on the Atlantic coast of Florida. Mean CO2 flux values ranged from 0.41 ± 0.19 to 2.03 ± 0.19 μmol m−2 s−1 across vegetation classes, and efflux values in mangrove plots were not significantly different between sampling dates, whereas ecotone and salt marsh efflux values were. On average, CO2 flux was 1.11 ± 0.18 μmol m−2 s−1 greater in July than in January across plots. There was no significant trend between efflux and aboveground biomass and a positive trend between belowground biomass and CO2 efflux. Edaphic parameters (organic matter content (%), soil N, organic C, and pH) were comparable across all vegetation class soils and there was a positive trend between soil temperature and flux. Soil efflux had an inverse relationship with latitude; in January, there is an increase in efflux with a decrease in latitude, while in July, efflux increases with an increase in latitude. Consequently, we assert that differences in CO2 efflux were due to soil temperature and species productivity along the latitudinal gradient studied.

Declines in Plant Productivity Drive Carbon Loss from Brackish Coastal Wetland Mesocosms Exposed to Saltwater Intrusion

Coastal wetlands, among the most productive ecosystems, are important global reservoirs of carbon (C). Accelerated sea level rise (SLR) and saltwater intrusion in coastal wetlands increase salinity and inundation depth, causing uncertain effects on plant and soil processes that drive C storage. We exposed peat-soil monoliths with sawgrass (Cladium jamaicense) plants from a brackish marsh to continuous treatments of salinity (elevated (~ 20 ppt) vs. ambient (~ 10 ppt)) and inundation levels (submerged (water above soil surface) vs. exposed (water level 4 cm below soil surface)) for 18 months. We quantified changes in soil biogeochemistry, plant productivity, and whole-ecosystem C flux (gross ecosystem productivity, GEP; ecosystem respiration, ER). Elevated salinity had no effect on soil CO2 and CH4 efflux, but it reduced ER and GEP by 42 and 72%, respectively. Control monoliths exposed to ambient salinity had greater net ecosystem productivity (NEP), storing up to nine times more C than plants and soils exposed to elevated salinity. Submersion suppressed soil CO2 efflux but had no effect on NEP. Decreased plant productivity and soil organic C inputs with saltwater intrusion are likely mechanisms of net declines in soil C storage, which may affect the ability of coastal peat marshes to adapt to rising seas.