Rising temperature may trigger deep soil carbon loss across forest ecosystems

Significantly more carbon (C) is stored in deep soil than in shallow horizons, yet how the decomposition of deep soil organic C (SOC) will respond to rising temperature remains unexplored on large scales, leading to considerable uncertainties to predictions of the magnitude and direction of C-cycle feedbacks to climate change. Herein, short-term temperature sensitivity of SOC decomposition (expressed as Q10) from six depths within the top 1 m soil from 90 upland forest sites (540 soil samples) across China is reported. Results show that Q10 significantly increases with soil depth, suggesting that deep SOC is more vulnerable to loss with rising temperature in comparison to shallow SOC. Climate is the primary regulator of shallow soil Q10 but its relative influence declines with depth; in contrast, soil C quality has a minor influence on Q10 in shallow soil but increases its influence with depth. When considering the depth-dependent Q10 variations, results further show that using the thermal response of shallow soil layer for the whole soil profile, as is usually done in model predictions, would significantly underestimate soil C-climate feedbacks. The results highlight that Earth system models need to consider multilayer soil C dynamics and their controls to improve prediction accuracy

Early growth responses of Larix kaempferi (Lamb.) Carr. seedling to short-term extreme climate events in summer

Abstract: Extreme climate events such as heat waves, drought, and heavy rainfall are occurring more frequently and are more intense due to ongoing climate change. This study evaluated the early growth performance of one-year-old Larix kaempferi (Lamb.) Carr. seedlings under open-field extreme climate conditions including experimental warming and different precipitation regimes. We recorded the survival rate, root collar diameter, height, biomass, shoot-to-root ratio, and seedling quality index using nine treatments (three temperature levels, i.e., control, warming by 3 ◦C and by 6 ◦C, × three precipitation levels, i.e., control, drought, and heavy rainfall) in July and August 2020. The survival rate of seedlings did not differ between treatments, showing high values exceeding 94% across treatments. The measured shoot height was largest under warming by 3 ◦C and high rainfall, indicating that moderate warming increased seedling height growth in a moist environment. Heavy rainfall decreased stem volume by 21% and 25% under control and warming by 6 ◦C treatments, respectively. However, drought manipulation using rain-out shelters did not decrease the growth performance. Overall, extreme climate events did not affect the survival rate, biomass, shoot-to-root ratio, and seedling quality index of L. kaempferi. We thus conclude that, regarding growth responses, L. kaempferi seedlings may be resistant to short-term extreme warming and drought events during summer

Soil carbon flux research in the Asian region : review and future perspectives

Soil respiration (Rs ) is the largest flux of carbon dioxide (CO2) next to photosynthesis in terrestrial ecosystems. With the absorption of atmospheric methane (CH4), upland soils become a large CO2 source and CH4 sink. These soil carbon (C) fluxes are key factors in the mitigation and adaption of future climate change. The Asian region spans an extensive area from the northern boreal to tropical regions in Southeast Asia. As this region is characterised by highly diverse ecosystems, it is expected to experience the strong impact of ecosystem responses to global climate change. For the past two decades, researchers in the AsiaFlux community have meaningfully contributed to improve the current understanding of soil C dynamics, response of soil C fluxes to disturbances and climate change, and regional and global estimation based on model analysis. This review focuses on five important aspects: 1) the historical methodology for soil C flux measurement; 2) responses of soil C flux components to environmental factors; 3) soil C fluxes in typical ecosystems in Asia; 4) the influence of disturbance and climate change on soil C fluxes; and 5) model analysis and the estimation of soil C fluxes in research largely focused in Asia

Elevated CO2 does not affect stem CO2 efflux nor stem respiration in dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO2]

To quantify stem respiration (RS) under elevated CO2 (eCO2), stem CO2 efflux (EA) and CO2flux through the xylem (FT) should be accounted for, because part of respired CO2 is transported upwards with the sap solution. However, previous studies have used EA as a proxy of RS, which could lead to equivocal conclusions. Here, to test the effect of eCO2 on RS, both EA and FT were measured in a free‐air CO2 enrichment experiment located in mature Eucalyptus native forest. Drought stress substantially reduced EA and RS, which were unaffected by eCO2, likely as a consequence of its neutral effect on stem growth in this phosphorus‐limited site. However, xylem CO2 concentration measured near the stem base was higher under eCO2, and decreased along the stem resulting in a negative contribution of FT to RS, whereas the contribution of FT to RS under ambient CO2 was positive. Negative FT indicates net efflux of CO2 respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of RS on eCO2 and suggest stimulated root respiration under eCO2 that may shift vertical gradients in xylem [CO2] confounding the interpretation of EA measurements

Rising Temperature May Trigger Deep Soil Carbon Loss Across Forest Ecosystems

Significantly more carbon (C) is stored in deep soil than in shallow horizons, yet how the decomposition of deep soil organic C (SOC) will respond to rising temperature remains unexplored on large scales, leading to considerable uncertainties to predictions of the magnitude and direction of C-cycle feedbacks to climate change. Herein, short-term temperature sensitivity of SOC decomposition (expressed as Q10) from six depths within the top 1 m soil from 90 upland forest sites (540 soil samples) across China is reported. Results show that Q10 significantly increases with soil depth, suggesting that deep SOC is more vulnerable to loss with rising temperature in comparison to shallow SOC. Climate is the primary regulator of shallow soil Q10 but its relative influence declines with depth; in contrast, soil C quality has a minor influence on Q10 in shallow soil but increases its influence with depth. When considering the depth-dependent Q10 variations, results further show that using the thermal response of shallow soil layer for the whole soil profile, as is usually done in model predictions, would significantly underestimate soil C-climate feedbacks. The results highlight that Earth system models need to consider multilayer soil C dynamics and their controls to improve prediction accuracy

Water level controls on sap flux of canopy species in black ash wetlands

Black ash (Fraxinus nigra Marsh.) exhibits canopy dominance in regularly inundated wetlands, suggesting advantageous adaptation. Black ash mortality due to emerald ash borer (Agrilus planipennis Fairmaire) will alter canopy composition and site hydrology. Retention of these forested wetlands requires understanding black ash's ecohydrologic role. Our study examined the response of sap flux to water level and atmospheric drivers in three codominant species: black ash, red maple (Acer rubrum L.), and yellow birch (Betula alleghaniensis Britt.), in depressional wetlands in western Michigan, USA. The influence of water level on sap flux rates and response to vapor pressure deficit (VPD) was tested among species. Black ash had significantly greater sap flux than non-black ash at all water levels (80-160% higher). Black ash showed a significant increase (45%) in sap flux rates as water levels decreased. Black ash and red maple showed significant increases in response to VPD as water levels decreased (112% and 56%, respectively). Exploration of alternative canopy species has focused on the survival and growth of seedlings, but our findings show important differences in water use and response to hydrologic drivers among species. Understanding how a replacement species will respond to the expected altered hydrologic regimes of black ash wetlands following EAB infestation will improve species selection

Depth dependence of soil carbon temperature sensitivity across Tibetan permafrost regions

Permafrost regions with high soil organic carbon (SOC) storage are extremely vulnerable to global warming. However, our understanding of the temperature sensitivity of SOC decomposition in permafrost regions remains limited, leading to considerable uncertainties in predicting carbon-climate feedback magnitude and direction in these regions. Here, we investigate general patterns and underlying mechanisms of SOC decomposition rate and its temperature sensitivity (Q10) at different soil depths across Tibetan permafrost regions. Soils were collected at two depths (0–10 and 20–30 cm) from 91 sites across Tibetan permafrost regions. SOC decomposition rate and Q10 value were estimated using a continuous-flow incubation system. We found that the SOC decomposition rate in the upper layer (0–10 cm) was significantly greater than that in the lower layer (20–30 cm). The SOC content governed spatial variations in decomposition rates in both soil layers. However, the Q10 value in the upper layer was significantly lower than that in the lower layer. Soil pH and SOC decomposability had the greatest predictive power for spatial variations in Q10 value within the upper and lower layers, respectively. Owing to the greater temperature sensitivity in the lower layer, our results imply that subsurface soil carbon is at high risk of loss, and that soil carbon sequestration potential might decrease in these regions in a warming world

Biogeographic variation in temperature sensitivity of decomposition in forest soils

Determining soil carbon (C) responses to rising temperature is critical for projections of the feedbacks between terrestrial ecosystems, C cycle, and climate change. However, the direction and magnitude of this feedback remain highly uncertain due largely to our limited understanding of the spatial heterogeneity of soil C decomposition and its temperature sensitivity (Q10). Here, we quantified C decomposition and its response to temperature change with an incubation study of soils from 203 sites across tropical to boreal forests in China spanning a wide range of latitudes (18o16' to 50o26'N) and longitudes (81o01' to 129o28'E). Mean annual temperature (MAT) and mean annual precipitation primarily explained the biogeographic variation in the decomposition rate and temperature sensitivity of soils: soil C decomposition rate decreased from warm and wet forests to cold and dry forests, while Q10‐MAT (standardized to the MAT of each site) values displayed the opposite pattern. In contrast, biological factors (i.e. plant productivity and soil bacterial diversity) and soil factors (e.g. clay, pH, and C availability of microbial biomass C and dissolved organic C) played relatively small roles in the biogeographic patterns. Moreover, no significant relationship was found between Q10‐MAT and soil C quality, challenging the current C quality‐temperature hypothesis. Using a single, fixed Q10‐MAT value (the mean across all forests), as is usually done in model predictions, would bias the estimated soil CO2 emissions at a temperature increase of 3.0 °C. This would lead to overestimation of emissions in warm biomes, underestimation in cold biomes, and likely significant overestimation of overall C release from soil to the atmosphere. Our results highlight that climate‐related biogeographic variation in soil C responses to temperature needs to be included in next‐generation C cycle models to improve predictions of C‐climate feedback

Temperature responses of carbon dioxide fluxes from coarse dead wood in a black ash wetland

The invasive emerald ash borer (EAB, Agrilus planipennis Fairmaire) causes widespread ash tree mortality in North America, and the CO2 efflux (respiration, F) from coarse dead wood (CDW) following the EAB infestation is unknown. We examined seasonal variations in CO2 fluxes from various types of CDW (cut ash stumps, downed logs, and standing girdled dead stems) and the surfaces of soil and live stem in a black ash wetland in which EAB infestation was simulated. Responses of FCDW to seasonal changes in temperature were less sensitive than that of live stems. However, FCDW from the stump and log cross-section were significantly greater than the other component fluxes. The mean CO2 flux from girdled stems was similar to those from soil and live stems. The log and stump cross-sections may function as an unaccounted pathway of CO2 flux following pre-emptive or salvage harvests associated with EAB mitigation. The increases in the amount of CDW and temperature caused by canopy openness and subsequent increased insolation, and potential long-term increase in water level and CDW moisture might accelerate the respirational carbon loss from soil and CDW after black ash wetlands are infested by EAB. These results identify and quantify CO2 pathways in EAB affected wetlands, which can be used to improve respiration modeling and carbon accounting in black ash wetlands

Response of black ash wetland gaseous soil carbon fluxes to a simulated emerald ash borer infestation

The rapid and extensive expansion of emerald ash borer (EAB) in North America since 2002 may eliminate most existing ash stands, likely affecting critical ecosystem services associated with water and carbon cycling. To our knowledge, no studies have evaluated the coupled response of black ash (Fraxinus nigra Marsh.) wetland water tables, soil temperatures, and soil gas fluxes to an EAB infestation. Water table position, soil temperature, and soil CO2 and CH4 fluxes were monitored in nine depressional headwater black ash wetlands in northern Michigan. An EAB disturbance was simulated by girdling (girdle) or felling (ash-cut) all black ash trees with diameters greater than 2.5 cm within treated wetlands (n = 3 per treatment). Soil gas fluxes were sensitive to water table position, temperature, and disturbance. Soil CO2 fluxes were significantly higher, and high soil CH4 fluxes occurred more frequently in disturbed sites. Soil CH4 fluxes in ash-cut were marginally significantly higher than girdle during post-treatment, yet both were similar to control sites. The strong connection between depressional black ash wetland study sites and groundwater likely buffered the magnitude of disturbance-related impact on water tables and carbon cycling