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As one of the main terrain factors, the slope aspect generally shows remarkable effects on microclimate and vegetation distribution. The purpose of this study was to determine the potential effects of the slope aspect on soil respiration and the soil microbial community in a temperate mountain forest. A field investigation was carried out in a mountain oak forest located at different slope aspects (northeast, northwest, southeast, and southwest), and soil respiration was continuously measured for 12 months. The soil's bacterial and fungal taxa were analyzed during the growing season. Our results showed that average soil respiration on the southwest and southeast slope aspects was 72.9% higher than the average on the northeast and northwest slope aspects. The coefficient of variation of soil respiration had the highest value on the northwest aspect (20.9%) and the lowest value on the southeast aspect (6.9%). The southeast slope had significantly higher soil respiration and temperature sensitivity compared to the other three slope aspects. The slope aspect substantially affected the soil's bacterial and fungal r/K strategy, showing the lowest values on the northwest aspect and the highest values on the southeast aspect. Differences in bacterial r/K, the ratio of microbial biomass carbon (MBC) to soil organic carbon (SOC), and SOC among slope aspects contributed to a 43%, 38%, and 32% variation in soil respiration, respectively. The variation of soil temperature across slope aspects showed an indirect effect on soil respiration through changing bacterial r/K and MBC/SOC. Our findings highlight that terrain plays a critical role in regulating the spatial heterogeneity of soil respiration in mountain forests, which could be explained by the differences in SOC and microbial community composition across slope aspects. [ABSTRACT FROM AUTHOR]

Quantification of autotrophic (Ra) and heterotrophic (Rh) components of soil respiration (Rs) could greatly improve our understanding of terrestrial carbon cycling. Here, we investigated the effect of nitrogen (N) and phosphorus (P) supply on soil respiration rates and its components, and seasonal cumulative soil CO2 efflux in a semiarid grassland. A two‐factor experiment of N (0, 50 and 100 kg N ha−1 yr−1) and P (0, 18 and 36 kg P ha−1 yr−1) supply was conducted on the Loess Plateau in 2018–2021. Our results indicated that the promoting effect of combination of N and P supply on soil respiration and its components was greater than that of N/P supply alone. N and/or P supply increased the cumulative soil CO2 efflux in both non‐growing and growing seasons, and the non‐growing season CO2 efflux accounted for ∼25% of the growing season CO2 efflux. The increase of cumulative CO2 efflux from Ra in response to N and/or P supply was significantly greater than that of Rh in 2018 and 2019, while it shifted to the opposite, that is, a greater increase in Rh than in Ra in 2020 and 2021. Our results suggest that 100 kg N ha−1 yr−1 supply alone is an optimum fertilization scheme to trade‐off grassland productivity and soil CO2 emissions. Our results highlighted that the promoting effects of N and P fertilization on different soil CO2 efflux components could be reversed in a relatively short period (4 years), and this should be considered when nutrient addition is adapted to restore degraded grasslands. Plain Language Summary: Nitrogen (N) and/or phosphorus (P) addition is a widely‐used restoration measure in global grassland, on the other hand nutrient addition could subsequently alter soil CO2 emissions via directly/indirectly effects on plant root growth and soil microbial activity. Our study quantified the effect of N and/or P supply on soil CO2 emissions from soil respiration and its two components through a 4‐year field experiment. We found that N and/or P supply simultaneously promoted soil respiration initially within 1–2 years after fertilization, although this effect diminished after 3 years of soil nutrient addition. 100 kg N ha−1 yr−1 supply alone is the optimum fertilization amount to tradeoff productivity improvement and soil CO2 emissions in a semiarid grassland. Key Points: N and/or P supply significantly increased soil CO2 emissions from semiarid grasslandsThe cumulative soil CO2 efflux was four times higher in the growing than in the non‐growing seasonThe promoting effect of N and/or P supply on soil CO2 emissions weakened with fertilization duration [ABSTRACT FROM AUTHOR]

Increased nitrogen (N) application has profound effects on CO2 flux in croplands. The aim of this study is to investigate the effects of long-term N addition on soil respiration (SR) in a rainfed winter wheat (Triticum aestivum L.) field in the Loess Plateau of China. Two wheat cultivars were planted under three levels of N application (0, 180, and 360 kg N ha−1 year−1) in non-irrigated cropland from 2004 to 2013. The diurnal and seasonal SR variations and abiotic and biotic factors were measured during the growing seasons in 2012–2013. The results showed that N180 and N360 increased the cumulative CO2 flux by 30.3% and 32.4% on average after 5 and 10 years of N application, respectively. Multiple regressions revealed that the seasonal SR was mainly controlled by the soil temperature (ST), at a depth of 8 cm, and the leaf area index. Diurnal SR was mainly controlled by the ST and the net photosynthesis rate. Long-term N application stimulated SR by increasing the photosynthetic leaf area and temperature sensitivity. Overall, N application at a rate of 360 kg N ha−1 year−1 did not reach the threshold for limiting SR in the investigated semi-humid rainfed wheat cropland in the Loess Plateau of China. [ABSTRACT FROM AUTHOR]

On a global scale, soil respiration (Rs), representing the CO2 flux between the soil surface and the atmosphere, ranks as the second-largest terrestrial carbon (C) flux. Understanding the dynamics between Rs and its autotrophic (Ra) and heterotrophic (Rh) components is necessary for accurately evaluating and predicting global C balance and net ecosystem production under environmental change. In this study, we conducted a two-year root exclusion experiment in subtropical China's Camphor (Cinnamomum camphora (L.) Presl.) forests to assess seasonal changes in Ra and Rh and their relative contributions to Rs. Additionally, we examined the influence of environmental factors on the dynamics of Ra, Rh, and Rs. Our results showed that seasonal mean Rs values were 2.88 µmol m−2 s−1, with mean Ra and Rh of 1.21 and 1.67 µmol m−2 s−1, respectively, in the studied forests. On an annual basis, the annual values of mean Rs in the studied forests were 405 ± 219 g C m−2 year−1, with Rh and Ra accounting for 240 ± 120 and 164 ± 102 g C m−2 year−1, respectively. The seasonal mean ratio of Rh to Rs (Rh/Rs) was 58%, varying from 45 to 81%. Seasonal changes in Rs and Rh were strongly correlated with soil temperature but not soil water content. Both Rh and Rs increased exponentially with the average soil temperature measured in the topsoil layer (about 5 cm), with Q10 values of 2.02 and 1.73 for Rh and Rs, respectively. Our results suggest that the composition and activity of soil microbes and fauna play a primary role in releasing carbon flux from soil to the atmosphere in the studied forest ecosystems. [ABSTRACT FROM AUTHOR]

Among 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. [ABSTRACT FROM AUTHOR]

Abstract Among 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.

Soil CO2 efflux (Fsoil) is a significant contributor of labile CO2 to the atmosphere. The Himalayas, a global climate hotspot, condense several climate zones on account of their elevational gradients, thus, creating an opportunity to investigate the Fsoil trends in different climate zones. Presently, the studies in the Indian Himalayan region are localized to a particular forest type, climate zone, or area of interest, such as seasonal variation. We used a portable infrared gas analyzer to investigate the Fsoil rates in Himalayan tropical to alpine scrub forest along a 3100-m elevational gradient. Several study parameters such as seasons, forest types, tree species identity, age of trees, distance from tree base, elevation, climatic factors, and soil physico-chemical and enzymatic parameters were investigated to infer their impact on Fsoil regulation. Our results indicate the warm and wet rainy season Fsoil rates to be 3.8 times higher than the cold and relatively dry winter season. The tropical forest types showed up to 11 times higher Fsoil rates than the alpine scrub forest. The temperate Himalayan blue pine and tropical dipterocarp sal showed significant Fsoil rates, while the alpine Rhododendron shrubs the least. Temperature and moisture together regulate the rainy season Fsoil maxima. Spatially, Fsoil rates decreased with distance from the tree base (ρ = − 0.301; p < 0.0001). Nepalese alder showed a significant positive increase in Fsoil with stem girth (R2 = 0.7771; p = 0.048). Species richness (r, 0.81) and diversity (r, 0.77) were significantly associated with Fsoil, while elevation and major edaphic properties showed a negative association. Surface litter inclusion presented an elevation-modulated impact. Temperature sensitivity was exorbitantly higher in the sub-tropical pine (Q10, 11.80) and the alpine scrub (Q10, 9.08) forests. We conclude that the rise in atmospheric temperature and the reduction in stand density could enhance the Fsoil rates on account of increased temperature sensitivity. [ABSTRACT FROM AUTHOR]

Spatiotemporal heterogeneity in soil CO2 efflux (FS) underlies one of our greatest gaps in understanding global carbon (C) cycles. Though scientists recognize this heterogeneity, FS sampling schemes often average across spatial heterogeneity or fail to capture fine temporal heterogeneity, and many ecosystem models assume flat terrain. Here, we test the idea that simple, remotely sensible terrain variables improve regression models of spatiotemporal variation in FS. We used automatic chambers that, for the first time, capture FS in complex temperate forest terrain at fine temporal resolution with 177,477 hourly FS measurements at 8 locations from ridgetop to valley along planar and swale hillslopes, across three years ranging from dry to record wet precipitation. In two of these years, we measured FS weekly at 50 additional locations distributed across the 8-ha catchment. Growing season Fs estimates were 1.25 times greater when sampling hourly versus weekly. At ridgetops, growing season FS increased by an average of 463 gC m−2 180 day−1 (75.9%) from dry to wet years, while valleys decreased by 208 gC m−2 180 day−1 (− 20.1%). This bidirectional response to interannual moisture was identified in distinct Random Forest models of Fs for convergent (water accumulating) or non-convergent (water shedding) hillslope positions. We hypothesize that different FS constraints drive these opposing responses—water availably to biota limits FS from ridgetops while slow oxygen diffusion limits FS from wet valleys. Accounting for hillslope position and shape reduces variance of FS estimates in complex terrain, which could improve FS sampling, C budgets, and modeling. [ABSTRACT FROM AUTHOR]

Fire frequency is increasing with climate warming in the boreal regions of interior Alaska, with short fire return intervals (< 50 years) becoming more common. Recent studies suggest these "reburns" will reduce the insulating surface organic layer (SOL) and seedbanks, inhibiting black spruce regeneration and increasing deciduous cover. These changes are projected to amplify soil warming, increasing mineral soil organic carbon (SOC) decomposition rates, and impair re-establishment of understorey vegetation and the SOL. We examined how reburns changed soil temperature, heterotrophic soil respiration (RH), and understorey gross primary production (GPP), and related these to shifts in vegetation composition and SOL depths. Two distinct burn complexes previously covered by spruce were measured; both included areas burned 1x, 2x, and 3x over 60 years and mature (≈ 90 year old) spruce forests underlain by permafrost. A 2.7 °C increase in annual near-surface soil temperatures from 1x to 3x burns was correlated with a decrease in SOL depths and a 1.9 Mg C ha−1 increase in annual RH efflux. However, near-surface soil warming accounted for ≤ 23% of higher RH efflux; increases in deciduous overstorey vegetation and root biomass with reburning better correlated with RH than soil temperature. Reburning also warmed deeper soils and reduced the biomass and GPP of understory plants, lessening their potential to offset elevated RH and contribute to SOL development. This suggests that reburning led to losses of mineral SOC previously stored in permafrost due to warming soils and changes in vegetation composition, illustrating how burn frequency creates pathways for accelerated regional C loss. [ABSTRACT FROM AUTHOR]

The relationship between soil heterotrophic respiration (R h) and soil moisture has been often studied using disturbed soil samples and simple gravimetric and volumetric soil moisture indicators. The objective of this study was to investigate the relationship between R h and soil moisture in terms of water-filled porosity (WFP), matric potential (Ψ m), and relative soil gas diffusivity (D p / D o) using undisturbed soil cores obtained under different land covers. Soil CO 2 efflux, WFP, and Ψ m were measured in undisturbed soil samples (250 cm3) collected in the 0–5 cm soil layer (without any vegetation or living roots) under laboratory conditions by combining a CO 2 gas analyzer, a scale, and precision mini-tensiometers. For each site and land cover, we also measured soil chemical properties, soil physical properties, and soil microbial composition using phospholipid fatty acid analysis. Grassland soils had the largest total microbial biomass (6275 ng g−1), followed by soils from riparian (5327 ng g−1), and cropland (2745 ng g−1) sites. Bacteria were the dominant group representing 46% (SD = 5%) of the total microbial biomass across all sites and land covers. Maximum R h was 1.88 (SD = 0.40) μmol CO 2 m−2 s−1 in grassland, 1.64 (SD = 0.82) μmol CO 2 m−2 s−1 in riparian, and 0.94 (SD = 0.56) μmol CO 2 m−2 s−1 in cropland soils. Considering all land cover and soil types, our observations revealed that peak R h occurred at mean WFP = 0.81 , Ψ m = −6 kPa, and D p / D o = 0.003. Thus, we recommend avoiding the traditional field capacity definition of −33 kPa for representing peak microbial activity. Water-filled porosity was a more consistent predictor of R h than Ψ m or D p / D o across soils with contrasting organic matter content, total microbial biomass, soil texture, and soil structure. • Soil heterotrophic respiration as a function of matric potential, water-filled porosity, and soil gas diffusivity. • Soil moisture conditions ranged from air-dry to saturation using undisturbed soil cores obtained from different land covers. • Peak respiration occurred at WFP = 0.81, Ψ m = −6 kPa, and D p / D o = 0.003 across nine sites and three land covers. • Water-filled porosity was the most consistent predictor of soil heterotrophic respiration. • We recommend avoiding the traditional field capacity definition of −33 kPa for representing peak microbial activity. [ABSTRACT FROM AUTHOR]

Soil electrical conductivity (EC) is an important indicator of soil health. It affects crop yields and suitability, many soil properties like plant nutrient availability, and activity of soil microorganisms which influence key soil processes including the emission of greenhouse gases such as carbon dioxide i.e. soil respiration. While it is well known that major drivers of soil EC are soil temperature and moisture content, less is known on the relation between soil EC and respiration. Therefore, the objectives of this research were to determine relation of soil EC and soil microclimate (soil temperature and moisture), as well relation of soil EC and soil respiration under three different land covers. A study on soil EC, microclimate and respiration under bare soil, winter wheat and winter barley was carried out from November 2020 until July 2021 on experimental field near Osijek city, continental Croatia. The results showed that EC is more related to soil microclimate elements i.e. soil temperature and soil moisture content than on soil respiration. Between 17% and 47% of EC can be explained by soil microclimate elements and none i.e. only 4% to 12% by soil respiration. [ABSTRACT FROM AUTHOR]

Soil respiration (RS) is an important component of the C cycle because it contributes significant CO2 emissions to the atmosphere that result from metabolism and respiration of its autotrophic and heterotrophic components. However, the relative importance of different biophysical controls that drive the variability of this flux and their influence along forest succession pathways is still unknown. We incorporate multiyear RS, ecosystem flux and meteorological measurements in old-growth (OG), mid-secondary (MS) and early-secondary (ES) tropical dry forests (TDFs) with the goal of assessing the temporal variation of RS and identifying the biophysical controls at each site by applying structural equation models (SEM). Along forest succession, RS followed the pattern of precipitation events; we identified by the end of the wet season that RS was sustained by a longer period at OG, while in MS and ES, RS decreased according to the soil moisture availability. According to SEM, soil moisture and soil temperature exert an effect on the variability of RS in all sites. However, we found that RS was also controlled by the vapor pressure deficit at MS and gross primary production at OG and ES. Our results suggest that seasonality has a different impact on RS along forest succession in TDFs found in northwestern Mexico and highlights the relevance of considering additional biophysical controls of RS for a better understanding this critical process of the C cycle. [ABSTRACT FROM AUTHOR]

Soil respiration in forests contributes to significant carbon dioxide emissions from terrestrial ecosystems but it varies both spatially and seasonally. Both abiotic and biotic factors influence soil respiration but their relative contribution to spatial and seasonal variability remains poorly understood, which leads to uncertainty in models of global C cycling and predictions of future climate change. Here, we hypothesize that tree diversity, soil diversity, and soil properties contribute to local-scale variability of soil respiration but their relative importance changes in different seasons. To test our hypothesis, we conducted seasonal soil respiration measurements along a local-scale environmental gradient in a temperate forest in Northeast China, analyzed spatial variability of soil respiration and tested the relationships between soil respiration and a variety of abiotic and biotic factors including topography, soil chemical properties, and plant and soil diversity. We found that soil respiration varied substantially across the study site, with spatial coefficients of variation (CV) of 29.1%, 27.3% and 30.8% in spring, summer, and autumn, respectively. Soil respiration was consistently lower at high soil water content, but the influence of other factors was seasonal. In spring, soil respiration increased with tree diversity and biomass but decreased with soil fungal diversity. In summer, soil respiration increased with soil temperature, whereas in autumn, soil respiration increased with tree diversity but decreased with increasing soil nutrient content. However, soil nutrient content indirectly enhanced soil respiration via its effect on tree diversity across seasons, and forest stand structure indirectly enhanced soil respiration via tree diversity in spring. Our results highlight that substantial differences in soil respiration at local scales was jointly explained by soil properties (soil water content and soil nutrients), tree diversity, and soil fungal diversity but the relative importance of these drivers varied seasonally in our temperate forest. [ABSTRACT FROM AUTHOR]

The amount of CO2 gas emissions in drained peatland for oil palm cultivation has been widely reported. However, the research addressing the contribution of litter respiration to peat and total respiration and its relationship with several environmental factors is found rare. The aim of this study was to measure peat and heterogeneous litter respiration of drained tropical peat in one year at a distance of 2.25 m and 4.50 m from mature oil palm trees of 14 years using the chamber method (Licor Li-830). In addition to CO2 efflux, we measured other environmental parameters, including peat temperature (10 cm depth), air temperature, groundwater table (GWL), and rainfall. Results showed that the mean total peat respiration (Rt) was 12.06 g CO2 m-2day-1, which consisted of 68% (8.24 g CO2 m-2day-1) peat (Rp) and root (Rr) respiration and 32% (3.84 g CO2 m-2day-1) of litter respiration (Rl) at the distance of 2.25 m from the palm tree. Meanwhile, at a farther distance, the Rt was 12.49 g CO2m-2day-1, the contribution of Rp was 56% (6.78 g CO2 m-2day-1), and Rl was higher than the closest distance (46%; 5.71 g CO2 m-2day-1). Thus, one-year observation resulting the mean Rt and Rr was 0.07–0.08 Mg CO2 ha-1 day-1, while Rl was 0.04–0.06 Mg CO2 ha-1 day-1. The means of Rt, Rp, and Rl were significantly different in the dry season than those recorded in the rainy season. The climatic-related variable such as peat and air temperature were chiefly governing respiration in peat under mature oil palm plantation, whereas the importance of other variables present at particular conditions. This paper provides valuable information concerning respiration in peat, especially for litter contribution and its relationship with environmental factors in peatland, contributing to global CO2 emission.

Soil respiration is an important pathway of soil organic carbon losses in temperate grasslands; however, it is rarely studied across broad management intensity gradients in a landscape. Using the soda-lime method, we measured in-situ soil CO2 efflux with single measurements of long exposure time (i.e. 3 day long) in 150 grasslands in three German regions in early summer 2018 and 2019. The grasslands ranged from unfertilized and grazed grasslands to intensively fertilized and frequently harvested ones. To assess effects of grazing and fertilization intensities and plant diversity on soil CO2 efflux, we used Structural Equation Modeling to account for direct effects and indirect effects through soil and plant organic matter quantity and quality. Soil CO2 efflux was suppressed by limited water availability caused by naturally occurring droughts in both study years. Under the prevailing environmental conditions, grazing intensity, plant biomass and plant C:N ratio were not related to soil CO2 efflux. In contrast, fertilization intensity was positively associated with soil CO2 efflux (standardized coefficient of net effect: + 0.04 in 2018 and + 0.03 in 2019). This was because fertilization led to lower plant species richness and, thus, to lower C:N ratios in soils, which were associated with higher soil CO2 efflux (plant species richness net effect: −0.09 in 2018 and −0.18 in 2019; soil C:N ratio direct effect: −0.23 in 2018 and −0.33 in 2019). Intensively managed grasslands have higher soil respiration than extensively managed, plant species-rich grasslands even under the extreme conditions of natural droughts. [ABSTRACT FROM AUTHOR]

Bioenergy is one of the most considered alternatives to fossil fuels. Short-rotation woody crops (SRWCs) as bioenergy sources are capable of alleviating energy constraints and sequestering atmospheric CO2. However, studies investigating soil carbon (C) dynamics at SWRC plantations are scarce. We studied American sycamore (Platanus occidentalis) as a model tree species for SRWC at different planting densities ((1) 0.5 × 2.0 m (10,000 trees·ha−1 or tph), (2) 1.0 × 2.0 m (5000 tph), and (3) 2.0 × 2.0 m (2500 tph)) to examine seasonal variation in total soil respiration (Rtotal), partitioned into heterotrophic (Rh) and autotrophic (Ra) respiration, and we evaluated climatic and biological controls on soil respiration. Rtotal and Rh exhibited larger seasonal variation than Ra (p < 0.05). During the nongrowing seasons, the average Rtotal was 0.60 ± 0.21 g·C·m−2·day−1 in winter and 1.41 ± 0.73 g·C·m−2·day−1 in fall. During the growing season, Rtotal was 2–7 times higher in spring (3.49 ± 1.44 g·C·m−2·day−1) and summer (4.01 ± 1.17 g·C·m−2·day−1) than winter. Average Rtotal was 2.30 ± 0.63 g·C·m−2·day−1 in 2500 tph, 2.43 ± 0.64 g·C·m−2·day−1 in 5000 tph, and 2.41 ± 0.75 g·C·m−2·day−1 in 10,000 tph treatments. Average Rh was 1.72 ± 0.40 g·C·m−2·day−1 in 2500 tph, 1.57 ± 0.39 g·C·m−2·day−1 in 5000 tph, and 1.93 ± 0.64 g·C·m−2·day−1 in 10,000 tph, whereas Ra had the lowest rates, with 0.59 ± 0.53 g·C·m−2·day−1 in 2500 tph, 0.86 ± 0.51 g·C·m−2·d−1 in 5000 tph, and 0.48 ± 0.34 g·C·m−2·day−1 in 10,000 tph treatments. Rh had a greater contribution to Rtotal (63%–80%) compared to Ra (20%–37%). Soil temperature was highly correlated to Rtotal (R2 = 0.92) and Rh (R2 = 0.77), while the correlation to Ra was weak (R2 = 0.21). Rtotal, Rh, and Ra significantly declined with soil water content extremes (e.g., <20% or >50%). Total root biomass in winter (469 ± 127 g·C·m−2) was smaller than in summer (616 ± 161 g·C·m−2), and the relationship of total root biomass to Rtotal, Rh, and Ra was only significant during the growing seasons (R2 = 0.12 to 0.50). The litterfall in 5000 tph (121 ± 16 g DW·m−2) did not differ (p > 0.05) from the 2500 tph (108 ± 16 g DW·m−2) or 10,000 tph (132 ± 16 g DW·m−2) treatments. In no circumstances were Rtotal, Rh, and Ra significantly correlated with litterfall amount across planting densities and seasons (p > 0.05). Overall, our results show that Rtotal in American sycamore SRWC is dominated by the heterotrophic component (Rh), is strongly correlated to soil environmental conditions, and can be minimized by planting at a certain tree density (5000 tph). [ABSTRACT FROM AUTHOR]

The relevance of soil greenhouse gas (GHG) fluxes from temperate floodplain forests has yet remained elusive. We studied the soil methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) dynamics at three forest sites along a flooding gradient in the Danube National Park (Austria) to estimate annual GHG budgets and to assess if and how seasonal flooding affects individual GHG fluxes. Soil surface GHG fluxes were determined along with GHG concentrations in soil air and pore-water at a non-flooded (NF), an infrequently-flooded (IF), and a frequently-flooded (FF) site. Both study years were characterized by dry summers, and only the FF site was flooded during the study period. Soils at all sites were annual CH4 sinks (NF: − 4.50 ± 0.85, IF: − 2.54 ± 0.57, FF: − 0.67 ± 1.06 kg CH4-C ha−1 year−1) and the sink strength correlated positively with soil moisture. Pulse-like CH4 emissions were not observed during or after flooding. Soil N2O fluxes showed large temporal and spatial variations, without any significant differences between sites (average NF: 6.5 ± 7.1, IF: 10.4 ± 14.3, FF: 9.4 ± 10.5 µg N2O-N m−2 h−1). Pulse N2O emissions (up to ~ 80 µg N2O-N m−2 h−1) occurred during freeze/thaw events, but not during or after flooding. Mean annual soil CO2 effluxes at NF and IF were 9.4 ± 1.1 and 9.4 ± 2.1 t C ha−1 year−1, respectively. Soil CO2 efflux was significantly higher at the FF site (18.54 ± 6.21 t C ha−1 year−1). High soil air CO2 concentrations (> 10%) in aerated deeper soil layers indicated a substantial contribution of the usually waterlogged sub-soils to the summertime soil CO2 efflux at the FF site. Overall, our results suggest that the studied temperate floodplain forest soils do not absorb/emit substantially more CH4 and N2O than soils of comparable upland forests, whereas low groundwater level can lead to periodically enhanced CO2 emissions from normally waterlogged soil layers. [ABSTRACT FROM AUTHOR]

We analyzed the relationship between forest productivity (joint effect of forest maturity and soil quality) and total soil respiration (µmol CO2 m−2 s−1) in semi-arid juniper woodlands (young woodlands growing in abandoned farmlands with deeper soils, and mature woodlands in lithic soils), and investigated the seasonal variation in soil CO2 efflux as a function of soil temperature and the soil water content. We measured the soil CO2 efflux from twelve cylinders in the soil over a three-year period using LI-6400 equipment. The results show that, in the more productive site (young woodland), soil CO2 efflux was higher due to greater respiration, mainly in the driest periods. Soil respiration followed a seasonal trend, being higher in spring and decreasing in cold periods. In both juniper woodlands and especially in the older forest, the CO2 efflux rates were low (<2.5 for Q10), typical of slow-growing species. Soil respiration was controlled by soil temperature without drought and in the temperate-warm season, whereas respiration showed sensitivity to soil water content in periods when edaphic humidity was low (but only in the more productive, young forest, which seemed to show better adaptation to drought), and under high soil moisture (soil water > 25%) for both woodlands, coinciding with warm temperatures in the spring. This period also corresponded to the highest CO2 efflux recorded in both woodlands. The accumulation of organic C seems to also be important to maintain elevated soil respiration in summer, especially in young woodlands. Thus, apart from microclimatic conditions, factors related to productivity regulate respiratory activity. [ABSTRACT FROM AUTHOR]

Under ongoing global climate change, drought periods are predicted to increase in frequency and intensity in the future. Under these circumstances, it is crucial for tree's survival to recover their restricted functionalities quickly after drought release. To elucidate the recovery of carbon (C) transport rates in c. 70‐year‐old Norway spruce (Picea abies [L.] KARST.) after 5 years of recurrent summer droughts, we conducted a continuous whole‐tree 13C labeling experiment in parallel with watering. We determined the arrival time of current photoassimilates in major C sinks by tracing the 13C label in stem and soil CO2 efflux, and tips of living fine roots. In the first week after watering, aboveground C transport rates (CTR) from crown to trunk base were still 50% lower in previously drought‐stressed trees (0.16 ± 0.01 m h−1) compared to controls (0.30 ± 0.06 m h−1). Conversely, CTR below ground, that is, from the trunk base to soil CO2 efflux were already similar between treatments (c. 0.03 m h−1). Two weeks after watering, aboveground C transport of previously drought‐stressed trees recovered to the level of the controls. Furthermore, regrowth of water‐absorbing fine roots upon watering was supported by faster incorporation of 13C label in previously drought‐stressed (within 12 ± 10 h upon arrival at trunk base) compared to control trees (73 ± 10 h). Thus, the whole‐tree C transport system from the crown to soil CO2 efflux fully recovered within 2 weeks after drought release, and hence showed high resilience to recurrent summer droughts in mature Norway spruce forests. This high resilience of the C transport system is an important prerequisite for the recovery of other tree functionalities and productivity. [ABSTRACT FROM AUTHOR]

Tropical forests are the most productive terrestrial ecosystems, fixing over 40 Pg of carbon from the atmosphere each year. A substantial portion of this carbon is allocated below‐ground to roots and root‐associated micro‐organisms. However, there have been very few empirical studies on the dynamics of this below ground transfer, especially in tropical forests where carbon allocation processes are influenced by high plant species diversity.We used a whole‐stand girdling experiment to halt the below‐ground transfer of recent photosynthates in a lowland tropical forest in Borneo. By girdling 209 large trees in a 0.48 ha plot, we determined: (a) the contribution of recent photosynthate to root‐rhizosphere respiration and; (b) the relationships among the disruption of this below‐ground carbon supply, tree species composition and mortality.Mortality of the 209 trees was 62% after 370 days, with large variation among species and particularly high mortality within the Dipterocarpaceae (99%) and Fagaceae (100%) families. We also observed a higher risk of mortality following girdling for species with lower wood density.Soil CO2 emissions declined markedly (36 ± 5%) over ~50 days following girdling in three of six monitored subplots. In the other three subplots there was either a marginal decline or no response of soil CO2 emissions to girdling. The decrease in soil CO2 efflux was larger in subplots with dominance of Dipterocarpaceae.Synthesis. Our results indicate high spatial variation in the coupling of below‐ground carbon allocation and root‐rhizosphere respiration in this tropical forest, with a closer coupling in forest dominated by Dipterocarpaceae. Our findings highlight the implications of tree species composition of tropical forests in affecting the dynamics of below‐ground carbon transfer and its release to the atmosphere. [ABSTRACT FROM AUTHOR]

Silvicultural practices applied in managed forest plantations may help counteract the effects of climate change by influencing soil surface CO2 efflux (Fs). Understanding the effects of silvicultural practices on Fs will provide unbiased estimates of carbon fluxes and allow better silvicultural decisions for carbon sequestration. Therefore, we assessed how Fs differed seasonally across silvicultural practices (i.e., stocking levels, clone, fertilization and weed control treatments) and evaluated the effects of soil temperature (Ts) and soil volumetric water content (θv) on Fs across these practices for a mid-rotation (14 year-old) Pinus radiata plantation in the Canterbury region of New Zealand. There were significant differences in Fs (p < 0.05) over the four seasons, three levels of stocking, and five clones. The effects of fertilization and weed control applied 12 years previously on Fs were insignificant. Annual estimate of Fs (mean ± 1 standard deviation) from the study site was 22.7 ± 7.1 t ha−1 a−1 in the form of CO2 (6.2 ± 2.1 t ha−1 a−1 in the form of C). Fs values were consistently higher in plots with 1250 stems ha−1 compared to 2500 stems ha−1, which may be related to a strong soil resource limitation because of the close spacing in the latter plantation. Significant differences in Fs across clones suggest that variations in carbon partitioning might explain their growth performance. Silvicultural treatments influenced Fs response to soil temperature (p < 0.05), resulting in models explaining 28–49% of the total variance in Fs. These findings provide insights into how silvicultural management decisions may impact Fs in mid-rotation radiata pine plantations, contributing towards developing more precise and unbiased plantation carbon budgets. [ABSTRACT FROM AUTHOR]

Soil respiration in forests contributes to significant carbon dioxide emissions from terrestrial ecosystems but it varies both spatially and seasonally. Both abiotic and biotic factors influence soil respiration but their relative contribution to spatial and seasonal variability remains poorly understood, which leads to uncertainty in models of global C cycling and predictions of future climate change. Here, we hypothesize that tree diversity, soil diversity, and soil properties contribute to local-scale variability of soil respiration but their relative importance changes in different seasons. To test our hypothesis, we conducted seasonal soil respiration measurements along a local-scale environmental gradient in a temperate forest in Northeast China, analyzed spatial variability of soil respiration and tested the relationships between soil respiration and a variety of abiotic and biotic factors including topography, soil chemical properties, and plant and soil diversity. We found that soil respiration varied substantially across the study site, with spatial coefficients of variation (CV) of 29.1%, 27.3% and 30.8% in spring, summer, and autumn, respectively. Soil respiration was consistently lower at high soil water content, but the influence of other factors was seasonal. In spring, soil respiration increased with tree diversity and biomass but decreased with soil fungal diversity. In summer, soil respiration increased with soil temperature, whereas in autumn, soil respiration increased with tree diversity but decreased with increasing soil nutrient content. However, soil nutrient content indirectly enhanced soil respiration via its effect on tree diversity across seasons, and forest stand structure indirectly enhanced soil respiration via tree diversity in spring. Our results highlight that substantial differences in soil respiration at local scales was jointly explained by soil properties (soil water content and soil nutrients), tree diversity, and soil fungal diversity but the relative importance of these drivers varied seasonally in our temperate forest.

The total soil CO2 efflux and its constituents were analyzed along the gradients of pollution of forest ecosystems by the emissions from nonferrous metallurgy plants in the Kola Subarctic. The CO2 efflux from the soil surface was measured by the closed chamber method. The summer efflux was calculated using the regression dependences of CO2 emission on air temperature. The root and microbial respirations were separated using the field method of substrate-induced respiration. The soil CO2 efflux reached relatively high values in the background pine (200 g C/m2/summer), birch (460 g C/m2/summer), and spruce (420 g С/m2/summer) forests. The bulk of CO2 produced in the soils under these forests was due to root respiration (45–70%). Closer to pollution sources, a decrease in microbial and plant biomass, depletion of mineral nutrients, and accumulation of heavy metals were observed in soils. As a result, the CO2 efflux decreased by up to 1.5 times in the zone of defoliation of the trees, 10 times in the zone of technogenic sparse forests, and 20 times in the zone of technogenic barrens. Defoliation of forests was accompanied by some activation of microbial respiration because of the additional input of nutritional substrate for microorganisms. The degradation and death of vegetation resulted in an expectable decrease in root respiration and its complete suppression in technogenic barrens. The results of this study indicate that monitoring of the soil CO2 efflux helps to identify the specific features in the functioning of forest ecosystems during technogenic degradation and to develop efficient methods for their remediation under industrial pollution in the Arctic. [ABSTRACT FROM AUTHOR]

Atmospheric nitrogen (N) deposition rates affect soil N dynamics, influencing soil respiration (RS) rates. However, for the Mediterranean region, the effect of changes in atmospheric N deposition on RS are not well constrained yet. We investigated the interplay between increased N deposition and tree species composition on RS at a Scots pine—Pyrenean oak ecotone in Central Spain, and whether the observed responses were mediated by changes on selected soil properties. Throughout 3 years, we simulated two N deposition rates—10 (medium) and 40 kg N ha−1 a−1 (high)—over the background deposition (control) in neighbouring stands in which tree species composition (pine or oak) shapes soil stoichiometry and microbial communities. We monitored RS on a monthly basis during 3 years; in addition, we performed targeted measurements 24 h after the N fertilization events to assess short-term soil responses. During winter and summer, RS did not respond to enhanced N deposition rates. In spring and autumn, higher RS rates were observed in the medium-fertilization, but the size and duration of this effect was tree species dependent. We suggest that climate seasonality modulates the response of RS to N availability, with tree species effects becoming relevant only when environmental conditions are adequate. RS in fertilized plots was larger from February to May and in September under pine, while under oak a response was observed only in April, probably due to differences in native soil stoichiometry under each tree species. Overall, RS showed high stability during 3 years of N enrichment in this Mediterranean ecotone area. However, we observed short-term soil responses after N fertilization events—loss of base cations, soil acidification and reduced microbial biomass—which emphasize the need to investigate consequences for the belowground C and N cycles if chronic N enrichment persists in the long run. [ABSTRACT FROM AUTHOR]

The study focuses on the effect of chemical amelioration of dolomitic limestone (doses of 0, 2, 3, 4, 6, 9 and 26 t ha-1) on soil respiration in a Norway spruce monoculture in mid-altitudinal elevation during one-year period after application. Firstly, the soil respiration was measured in situ as monthly CO2 efflux from the soil surface horizon in the period May to October 2016. Secondly, basal respiration, microbial biomass carbon and metabolic quotient of the organic H and organo-mineral A horizons were assessed under laboratory conditions within one year after the treatment. Soil CO2 efflux increased by 3 to 31% and by 29 to 98% for the ameliorant of 2 and 26 t ha-1, respectively, compared to the unlimed control treatment. The CO2 efflux was significantly driven by external conditions such as soil moisture and temperature, especially in the last seasonal months. Basal respiration of the H horizon increased up to a dose of 9 t ha-1 but decreased at 26 t ha-1. In the A horizon, microbial activity increased in all the limed variants compared to the non-limed variant. A similar trend was observed in microbial carbon and the metabolic quotient of the soil. Our results prove that the ameliorant doses commonly used in the forestry sector (3-4 t ha-1) substantially increase the soil microbial activity during (soil CO2 efflux) and after (laboratory data) the first year after application. This results in the accelerated mineralization of soil organic material and subsequent loss from the forest ecosystem.

Soil respiration (Rs), the efflux of CO2 from soils to the atmosphere, is a major component of the terrestrial carbon cycle, but is poorly constrained from regional to global scales. The global soil respiration database (SRDB) is a compilation of in situ Rs observations from around the globe that has been consistently updated with new measurements over the past decade. It is unclear whether the addition of data to new versions has produced better‐constrained global Rs estimates. We compared two versions of the SRDB (v3.0 n = 5173 and v5.0 n = 10,366) to determine how additional data influenced global Rs annual sum, spatial patterns and associated uncertainty (1 km spatial resolution) using a machine learning approach. A quantile regression forest model parameterized using SRDBv3 yielded a global Rs sum of 88.6 Pg C year−1, and associated uncertainty of 29.9 (mean absolute error) and 57.9 (standard deviation) Pg C year−1, whereas parameterization using SRDBv5 yielded 96.5 Pg C year−1 and associated uncertainty of 30.2 (mean average error) and 73.4 (standard deviation) Pg C year−1. Empirically estimated global heterotrophic respiration (Rh) from v3 and v5 were 49.9–50.2 (mean 50.1) and 53.3–53.5 (mean 53.4) Pg C year−1, respectively. SRDBv5's inclusion of new data from underrepresented regions (e.g., Asia, Africa, South America) resulted in overall higher model uncertainty. The largest differences between models parameterized with different SRDVB versions were in arid/semi‐arid regions. The SRDBv5 is still biased toward northern latitudes and temperate zones, so we tested an optimized global distribution of Rs measurements, which resulted in a global sum of 96.4 ± 21.4 Pg C year−1 with an overall lower model uncertainty. These results support current global estimates of Rs but highlight spatial biases that influence model parameterization and interpretation and provide insights for design of environmental networks to improve global‐scale Rs estimates. [ABSTRACT FROM AUTHOR]

Purpose: The study aimed to determine the interactive effect of temperature with nutrients on maize residues decomposition in soil. Materials and methods: We conducted an incubation of 87 days by applying maize straw (δ13C value of −11.2‰) to soil (δ13C value of −26.3‰) with low (N0), medium (NM), and high (NH) level of nutrients addition, at two temperature levels of 5 °C (T-L) and 25 °C (T-H). We measured the cumulative CO2-C efflux, residues decomposition, temperature sensitivity (Q10), and extracellular enzyme activities. Results and discussion: Increased temperature significantly increased cumulative CO2 efflux and straw decomposition, with an enhanced rate of active (Ka) and slow (Ks) pools of soil and residues C. The mean values of Q10 ranged from 1.4 to 1.6 for the total CO2 efflux and 1.4 to 1.7 for maize straw decomposition. The outcome might be due to temperature-dependent microbial activation at 25 °C. The activities of β-glucosidase, α-glucosidase, cellobiohydrolase, and β-xylosidase enzymes were positively correlated with cumulative CO2 emissions at 25 °C suggesting microbial regulation on SOM decomposition. We found a U-shaped pattern of nutrients regulation on the temperature sensitivity of maize straw decomposition, with the lowest Q10 under NM. Conclusions: Our findings suggest that nutrients regulated the temperature effects on residue C decomposition by adjusting microbial activity (extracellular enzyme activities). Consequently, it may lead to soil C sequestration under the current climate change scenario. [ABSTRACT FROM AUTHOR]

Various processes that occur in forest soils have significant roles in the forest carbon cycle and contribute to soil CO2 efflux. Soil CO2 efflux occurs via a combination between the metabolism of soil microorganisms and the respiration of plant roots. It releases around 10 times more CO2 than all anthropogenic factors combined. For this reason, diurnal and seasonal carbon efflux rates were measured in Lithuanian Scots pine (P. sylvestris) forests to determine soil CO2 efflux rates and the relative importance of various climatic factors that can influence belowground conditions. The aim of this study was to determine the factors that have the strongest influence on variation in forest soil CO2 efflux and the mutual influence between the factors, in order to attain a clearer understanding of C sinks. We took measurements of soil CO2 efflux to examine diurnal and seasonal variation. For the continuous measurement of CO2 efflux, an ADC BioScientific LCpro+ soil respiration analysis system was used. The main parameters that were measured were CO2 efflux (µmol m-2 s-1), air temperature (°C) in the chamber, soil temperature (°C) at a depth of 10 cm and water evaporation (mmol m-2 s-1). The diurnal rate values fluctuated within a certain range (2.78-6.17 µmol m-2 s-1), while seasonal variations fluctuated more, but the average CO2 efflux rate was between 2 - 4 µmol m-2 s-1. The highest CO2 efflux rate recorded during daytime was from 16:00-20:00, and the lowest one was early in the morning, between 05:00-08:00. During the seasonal soil CO2 efflux study, it was found that peak forest growth was related to peak efflux rates. These rates were observed from early to mid-June. The collected data suggest that SM (soil moisture) has a weaker connection with the efflux rate but had an impact on Ts (soil temperature), dampening its effect on soil CO2 efflux. These results demonstrate that the soil CO2 efflux of P. sylvestris forests in the cool climatic zone of Lithuania depend most heavily on soil temperature and soil moisture. [ABSTRACT FROM AUTHOR]

Previous research has indicated that a potentially large portion of root‐respired CO2 can move internally through tree xylem, but these reports are relatively scarce and have generally been limited to short observations. Our main objective was to provide a continuous estimate of the quantity and variability of root‐respired CO2 that moves either internally through the xylem (FT) or externally through the soil to the atmosphere (FS) over most of a growing season. Nine trees were measured in a Populus deltoides stand for 129 days from early June to mid‐October. We calculated FT as the product of sap flow and dissolved [CO2] in the xylem (i.e., [CO2*]) and calculated FS using the [CO2] gradient method. During the study, stem and soil CO2 concentrations, temperature, and sap flow were measured continuously. We determined that FT accounted for 33% of daily total belowground CO2 flux (i.e., FS + FT; FB) during our observation period that spanned most of a growing season. Cumulative daily FT was lower than FS 74% of the time, equivalent to FS 26% of the time, and never exceeded FS. One‐third of the total CO2 released by belowground respiration over most of the growing season in this forest stand followed the FT pathway rather than diffusing into the soil. The magnitude of FT indicates that measurements of FS alone substantially underestimate total belowground respiration in some forest ecosystems by systematically underestimating belowground autotrophic respiration. The variability in FT observed during the growing season demonstrated the importance of making long‐term, high‐frequency measurements of different flux pathways to better understand physiological and ecological processes and their implications to global change. [ABSTRACT FROM AUTHOR]

Aboveground litter not only is an important source of nutrients to soil microbes but also regulates the microclimate in topsoil. How the changes in aboveground litter quantity would affect the microbial biogeochemical cycles is still unclear. Here we conducted a litter input manipulation experiment in a temperate mixed forest to investigate how different amounts of litter input affect soil organic carbon (SOC) and soil respiration via their regulation on soil microbes. We found that although neither SOC stock nor soil CO2 efflux was affected by litter manipulation, soil microbial characteristics had responded after four years of litter addition or removal treatments. Microbial biomass carbon (MBC) in the O horizon was higher in litter addition plots than in litter removal plots as a result of the changed availability of labile C under litter treatments. Both double litter and no litter treatments changed microbial compositions, which was probably due to the increased soil pH in no litter treatment and the increased labile C in double litter treatment. The null change in soil respiration could be attributed to the offset between the negative effect of decreased substrate and the positive effect of increased temperature on soil respiration in litter removal plots. Due to the important role of soil microbes in carbon cycling, the altered microbial properties under litter manipulation treatments suggested the inevitable changes in biogeochemical cycling in the long run and call for long-term studies on SOC dynamics in the future. [ABSTRACT FROM AUTHOR]

Aims: This study investigates how precipitation, temperature and seasonality (as a proxy of plant productivity) affect the temporal and spatial variability of soil CO2 efflux in two dry semiarid grasslands with different degrees of land degradation. Methods: We measured soil CO2 efflux over four years under plant, biological soil crust and bare soil patches and estimated annual soil carbon losses in both, a natural and a degraded grassland, by means of generalised additive mixed models considering temporal autocorrelation in the data. Results: Soil CO2 efflux ranged from 0.08 to 3.70 and from 0.10 to 3.01 μmol CΟ2 m−2 s−1 in the natural and degraded grasslands, respectively. Daily soil CO2 efflux was mostly affected by moisture in the degraded grassland (25.4%), while in the natural grassland was affected jointly by seasonality, temperature and moisture (27.5%). Overall, the highest soil carbon fluxes were measured in soils covered by biological soil crusts (1.24 ± 0.02 and 1.10 ± 0.02) and the lowest in bare soils (1.11 ± 0.02 and 0.82 ± 0.02 μmol CΟ2−2 s−1) in the natural and degraded sites, respectively. Cumulative soil carbon fluxes were mainly driven by temperature and previous precipitation (over three months). The highest soil carbon losses were estimated in the driest year (2009) and the lowest in the wettest (2010) with almost twice the amount of rainfall. The main difference between these years was the timing of the events that mostly occurred in the moments of maximum plant activity with optimum temperatures in spring in the dry year. Conclusions: Changes in precipitation patterns will affect soil carbon fluxes more than rainfall amount, particularly in degraded grasslands. Therefore, considering all climate drivers together with plant activity is essential to predict how climate change will affect soil biological processes in drylands. [ABSTRACT FROM AUTHOR]

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.

Carbonates in soils may act as a potential source of CO2 efflux from calcareous soils to the atmosphere. However, its seasonal dynamics and relationships with soil temperature or soil moisture are not well‐understood. A 5‐yr warming experiment was performed to evaluate the effects of symmetric (all season) and asymmetric (seasonal) warming on soil organic fraction (SOC)‐derived efflux and carbonate‐derived CO2 efflux (partitioned by δ13C signature) from a calcareous soil in the karst region of southwest China. In region where precipitation mostly falls during summer, an average of 2.0°C warming under both symmetric and asymmetric warming scenarios produced 6–34% and 11–14% increases in the SOC‐derived efflux and carbonate‐derived efflux, respectively. The SOC‐derived efflux in the symmetric warming scenario was 1.6 times that of the asymmetric warming scenarios, but no changes were found for the carbonate‐derived efflux. The SOC‐derived efflux was positively correlated with both soil temperature and soil moisture, suggesting an entailment with biotic processes. In contrast, the carbonate‐derived efflux was positively correlated with soil temperature, but was poorly correlated with soil moisture. Thus, the seasonal dynamics of the carbonate‐derived efflux in calcareous soils were likely driven by abiotic processes. [ABSTRACT FROM AUTHOR]

Summary: In arctic ecosystems, climate change has increased plant productivity. As arctic carbon (C) stocks predominantly are located belowground, the effects of greater plant productivity on soil C storage will significantly determine the net sink/source potential of these ecosystems, but vegetation controls on soil CO2 efflux remain poorly resolved.In order to identify the role of canopy‐forming species in belowground C dynamics, we conducted a girdling experiment with plots distributed across 1 km2 of treeline birch (Betula pubescens) forest and willow (Salix lapponum) patches in northern Sweden and quantified the contribution of canopy vegetation to soil CO2 fluxes and belowground productivity.Girdling birches reduced total soil CO2 efflux in the peak growing season by 53%, which is double the expected amount, given that trees contribute only half of the total leaf area in the forest. Root and mycorrhizal mycelial production also decreased substantially. At peak season, willow shrubs contributed 38% to soil CO2 efflux in their patches.Our findings indicate that C, recently fixed by trees and tall shrubs, makes a substantial contribution to soil respiration. It is critically important that these processes are taken into consideration in the context of a greening arctic because productivity and ecosystem C sequestration are not synonymous. See also the Commentary on this article by Baltzer & Sonnentag, 227: 1591–1593. [ABSTRACT FROM AUTHOR]

Ungulate herbivory can alter functional plant communities of early-successional forest ecosystems. The consequences of such vegetation changes on soil carbon cycling are still not fully understood. Here, we used an ungulate exclusion experiment to investigate how different levels of herbivory and associated changes in vegetation succession modulate soil CO2 efflux and its heterotrophic and autotrophic sources following windthrow in temperate mountain forests. Our results indicate that only high levels of ungulate herbivory and associated vegetation shifts from tree to rather grass dominated plant communities affect soil CO2 fluxes. We did not find evidence that a moderate herbivory level and accompanied smaller shifts in the functional plant community affect soil CO2 fluxes. A greater soil CO2 efflux under the influence of high herbivory pressure was primarily attributed to accelerated heterotrophic respiration, likely due to warmer soil conditions. Moreover, autotrophic respiration from grass roots and associated microbial communities is suggested to contribute to higher soil CO2 fluxes. We conclude that intense herbivory and accompanied successional changes in the functional plant community enhance soil carbon losses following forest windthrow. This might have negative consequences for the soil carbon stocks and for the climate system. [ABSTRACT FROM AUTHOR]

The response of dissolved inorganic carbon (DIC) leaching to rising temperature and precipitation is hardly known for forest soils on carbonate bedrock. We established field lysimeters, filled with soil from a humus-rich A horizon of a Rendzic Leptosol and detrital dolomite (C horizon) and conducted a two-factorial climate manipulation [control (C), soil warming (H, +4°C), irrigation (I, +50% precipitation), soil warming and irrigation (H+I)] to examine the effects of increased temperature and precipitation on DIC and DOC leaching, soil air CO2 concentrations, and soil CO2 efflux. We followed an isotopic approach to assess the biotic and abiotic DIC fractions. Soil warming (H) had no effect on DIC leaching and seepage DIC concentrations. Irrigation (I and H+I) increased annual DIC leaching by almost 100% as a matter of significantly increased seepage DIC concentrations and higher annual drainage. Isotopic partitioning of DIC fluxes suggested biotic contributions between 60 and 100% and no significant variation with warming or irrigation. Soil warming consistently increased the soil CO2 efflux by >50% independently of irrigation treatment. Soil air CO2 concentrations were not affected by soil warming alone (H). Irrigation (I and I+H) tended to increase subsoil (18 and 32 cm depth) soil air CO2 concentrations, while topsoil (6 cm) remained unaffected by irrigation. DIC leaching losses were about 4–5 times higher than DOC leaching losses, which showed a similar treatment response (I and H+I > C and H). Annual DIC leaching amounted to between 20 ± 2 (C) and 39 ± 2 (H+I) g m−2, representing ~ 2–5% of the total annual gaseous soil CO2 loss. Our results suggest that climate change, especially changing precipitation, could significantly affect the DIC export from carbonate forest soils, thereby affecting their carbon sequestration potential.

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. [ABSTRACT FROM AUTHOR]

Temporal variations in soil CO2 efflux and its direct links to soil microbial biomass and environmental factors in high‐elevation regions are poorly understood. In this study, I investigated the variations in soil CO2 efflux over time and their relationships to soil organic matter, soil microbial biomass and environmental factors in an alpine meadow site on the Qinghai–Tibetan Plateau. The results showed notable monthly to seasonal changes in soil CO2 efflux in alpine meadows. The soil CO2 efflux was lower overall from January to May and from October to December than from June to September (p < 0.05); on a seasonal basis, the soil CO2 efflux was lower during the spring, autumn and winter than during the summer (p < 0.05). In addition, linear regression analysis showed that the temporal variations in soil CO2 efflux over the whole year decreased with soil microbial biomass carbon content (p < 0.05). Soil CO2 efflux was also positively related to air temperature and snow cover depth (p < 0.05). Multiregression and principal component regression analyses indicated that temporal variations in soil CO2 efflux were mainly caused by monthly and seasonal changes in air temperature and soil microbial biomass. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Soil respiration (Rs), the soil‐to‐atmosphere CO2 flux produced by microbes and plant roots, is a critical but uncertain component of the global carbon cycle. Our current understanding of the variability and dynamics is limited by the coarse spatial resolution of existing estimates. We predicted annual Rs and associated uncertainty across the world at 1‐km resolution using a quantile regression forest algorithm trained with observations from the global Soil Respiration Database spanning from 1961 to 2011. This model yielded a global annual Rs estimate of 87.9 Pg C/year with an associated global uncertainty of 18.6 (mean absolute error) and 40.4 (root mean square error) Pg C/year. The estimated annual heterotrophic respiration (Rh), derived from empirical relationships with Rs, was 49.7 Pg C/year over the same period. Predicted Rs rates and associated uncertainty varied widely across vegetation types, with the greatest predicted rates of Rs in evergreen broadleaf forests (accounting for 20.9% of global Rs). The greatest prediction uncertainties were in northern latitudes and arid to semiarid ecosystems, suggesting that these areas should be targeted in future measurement campaigns. This study provides predictions of Rs (and associated prediction uncertainty) at unprecedentedly high spatial resolution across the globe that could help constrain local‐to‐global process‐based models. Furthermore, it provides insights into the large variability of Rs and Rh across vegetation classes and identifies regions and vegetation types with poor model performance that should be prioritized for future data collection. Plain Language Summary: Soils emit large amounts of carbon dioxide to the atmosphere every year via the process of soil respiration, which greatly exceeds emissions from human sources. However, rates of soil respiration are highly variable in space, which limits our ability to balance global carbon budgets and forecast climate change. We used a novel application of a machine learning approach to predict annual rates of soil respiration at high resolution (1 km) across the globe and examined spatial patterns of the associated uncertainty of these predictions. Predictions were made based on how observations of soil respiration were related to climate (annual temperature and annual and seasonal precipitation) and vegetation information. Predicted annual soil respiration and prediction uncertainty varied across ecosystem types and regions. Our predictions suggest that evergreen tropical forests dominate global annual soil respiration emissions. Dryland, wetland, and cold ecosystems had the highest associated prediction uncertainties, suggesting that future soil respiration measurements would be especially useful in these areas. The high spatial resolution of our predictions will help researchers studying the carbon cycle at local to global scales. Key Points: A high spatial resolution machine learning approach was used for estimating soil respiration across the worldPredictions of soil respiration varied widely across ecosystem classes, and allowed for suitable estimates of heterotrophic respirationAssociated prediction uncertainty was highest in high latitudes and data scarce regions, which should be targets for future measurements [ABSTRACT FROM AUTHOR]