Variation In Soil Respiration Research Articles

Soil respiration and CH4 consumption covary on the plot scale

Assessing the greenhouse gas (GHG) balance of a landscape or land use can be challenging since greenhouse gas fluxes change over time and can be very different locally. While the spatial variability of carbon dioxide (CO2) fluxes and nitrous oxide (N2O) on the plot scale can hardly be explained, there is first evidence that methane (CH4) consumption covaries with CO2 fluxes in upland forest soils. Yet, if this spatial correlation is persistent over time is unclear. We studied the spatial variability in soil respiration (CO2 fluxes) and soil CH4 consumption in a homogenous, planted forest on an 800 m2 plot. The study was repeated after two weeks at sampling points next to those of the first study. Both studies showed that CH4 consumption was higher at locations with higher soil respiration, and that the correlation between CH4 consumption and soil respiration was persistent between the studies. This result agrees with the interpretation of the rhizosphere as most important source of soil CO2 and potentially preferred habitat for methanotrophic bacteria. The correlation found in the first study allowed predicting CH4 consumption rates of the second study based on the observed CO2 fluxes at the new measurement locations with a standardized root-mean-square error (SRMSE) of 26%. Since soil respiration measurements are much easier to conduct, they would be a good means to complement the more laborious and cost-intensive measurements of CH4 consumption in order to get a more comprehensive estimate of the representative CH4 consumption of a site.

The intra- and inter-annual responses of soil respiration to climate extremes in a semiarid grassland

Increasing frequency and magnitude of climate extremes could fundamentally affect terrestrial carbon (C) cycling. However, as the second-largest terrestrial C flux, soil respiration (SR) responses to climate extremes are not well understood. Here, we investigated the effects of drought, heat wave and drought plus heat wave on SR in a semiarid grassland during the growing season over 3 years. The results indicated that drought consistently reduced the growing-season mean SR, especially during the period of drought treatment, while heat wave, alone or when combined with drought, had little effect on SR. The decreased SR under drought at the intra-annual timescale could be primarily attributable to restriction of low soil moisture to microbial biomass, as there were no consistent changes in plant community (aboveground biomass, richness and abundance). In contrast, the inter-annual variation in SR was positively related to plant community abundance, richness and aboveground biomass in additional to soil water availability, but was not significantly affected by microbial biomass. Our study highlighted that incorporating microbial biomass in C cycling models can improve simulation of seasonal dynamics of soil respiration while incorporating plant community characteristics can benefit prediction of variation in SR across multiple years in semiarid grasslands.

Seasonal variations in the response of soil respiration to rainfall events in a riparian poplar plantation

Rainfall events have profound influence on the soil carbon release in different forest ecosystems. However, seasonal variations in soil respiration (RS) response to rainfall events and associated regulatory processes are not well documented in riparian forest ecosystems to date. We continuously measured soil respiration in a riparian plantation ecosystem from 2015 to 2018 to explore the relationships between soil respiration and rainfall events. Across the 4 years, 83 individual rainfall events were identified for spring, summer and autumn. We found that mean RS rate after rain (post-RS) was significantly higher than that before rain (pre-RS) (p < 0.05) in spring, and the relative change in soil respiration (RSrc) increased against rainfall size due to the stimulation by the significant increases in soil moisture content (ΔSM). In contrast, mean post-RS was lower than pre-RS and RSrc was significantly decreased with the increasing rainfall size (p < 0.01) in summer and autumn. Reduced changes in soil temperature (ΔTS) and increased soil moisture content after rain (post-SM) contributed to the decreased RS due to frequently occurring heavy rain events in summer. Increased ΔSM following rainfall events coupled with groundwater level increase suppressed RSrc in autumn, even though increased ΔTS could offset the negative effects of SM on RS to some extent. In addition, we found that higher post-SM after large rainfall events (>10 mm day−1) changed the response of RS to soil temperature (TS) by reducing the temperature sensitivity (Q10) even in this riparian plantation ecosystem. Our study highlights the importance of integrating seasonal difference in soil respiration response to rainfall events and the impact of large rainfall events on soil C release for estimating forest soil carbon cycling at multiple scales.

Topography and plant community structure contribute to spatial heterogeneity of soil respiration in a subtropical forest

Soil respiration is the largest carbon (C) flux from terrestrial ecosystems into the atmosphere. Accurate estimates of the magnitude and distribution of soil respiration are critically important to models of global C cycling and predictions of future climate change. One of the greatest challenges to accurate large-scale estimation of soil respiration is its great spatial heterogeneity at the site level. Our study explored how soil respiration varies in space and the drivers that lead to this variance in a natural subtropical evergreen broadleaf forest in Southern China. We conducted a two-year soil respiration measurement for 168 randomly selected sampling points in a 4 ha plot. We measured the spatial variance of soil respiration and tested its correlation to a variety of abiotic and biotic factors including topography, aboveground plant community structure, soil environmental factors, soil organic matter, and microbial community structure. We found that soil respiration was highly varied across the study plot, with a spatial variation coefficient (CV) of 32.75%. The structural equation modeling (SEM) analysis showed that elevation influenced tree species diversity, productivity, and soil water content, which in turn affected soil respiration via soil C content, clay content, fungal:bacterial ratio, annual litterfall, and fine root biomass. 31% of the total spatial variation of soil respiration was accounted for in the SEM, mostly by elevation, soil C content, annual litterfall biomass, tree species diversity as estimated by the Simpson's index, and soil water content, with standardized total effects of 0.31, −0.31, 0.29, 0.19, and −0.18, respectively. Our data demonstrated that soil respiration was highly spatially varied at the fine scale, and was primarily regulated by factors of topography and plant community structure. More studies investigating the spatial variation of soil respiration are therefore needed to better understand and assess terrestrial ecosystem C cycling.

Seasonal evaluation of biotic and abiotic factors suggests phosphorus retention in constructed floodplains in three agricultural streams

Floodplain restoration constructed via the two-stage ditch in agricultural streams has the potential to enhance nutrient retention and prevent the eutrophication of downstream ecosystems. Identifying the role of biotic and abiotic factors influencing soluble reactive phosphorus (SRP) retention in floodplains is of interest given that changing redox conditions associated with floodplain inundation can result in a release of geochemically sorbed SRP to the water column. In three agricultural waterways (Indiana, USA), we conducted seasonal measurements of a suite of biogeochemical pools (total P, bioavailable P and Fe) and processes (SRP flux and microbial respiration) from multiple floodplain transects, along with their adjacent stream sediments, to determine the role of biotic and abiotic processes on floodplain SRP retention or release. Across floodplain soils, organic matter explained a significant amount of variation in soil respiration, and SRP flux from the water column to the floodplain soils was driven by the molar ratio of Fe: P, with values >6 indicating potential SRP sorption due to increased available sorption sites. We developed a mass balance model at a single site to relate seasonal floodplain processes with water column SRP export, above and below the study reach, using measurements in this study combined with data from the literature. Grab sample data suggest that the reach retained 26% of incoming SRP, which the mass balance model attributed to seasonal synergy between plant assimilation in spring and summer (removing P from floodplain soils) and abiotic P sorption during winter and spring inundation (adding SRP to the floodplain). Retention of SRP was higher in floodplain soils compared to stream sediments based on the modeled SRP budget. Thus, we suggest that these constructed floodplains will maximize SRP retention from the water column if they inundate regularly, have floodplain soils with Fe:P > 3–6, and that promote sustained plant life.

Soil Properties Control Microbial Carbon Assimilation and Its Mean Residence Time

Microbial assimilation and stabilization of soil organic carbon (SOC) is an important process in global carbon cycling. For an improved understanding of climate-induced changes in ecosystem C dynamics, it is important to know the group-specific turnover of microbial C. Consequently, we wanted to answer the questions if fungi store newly assimilated C longer than bacteria and if climatic and edaphic properties of different regions affect microbial C assimilation and its subsequent release. This study presents results from a 112-day long field experiment where endogenous soil microorganisms were labelled with 13C labelled glucose to follow the dynamics of newly assimilated C in two study regions (Kraichgau and Swabian Alb). Whereas microbial assimilation of newly added C was higher in Kraichgau than in Swabian Alb, the opposite result was obtained for the mean residence time (MRT) of microbial biomass C (76 days in Kraichgau and 93 days in Swabian Alb). The accelerated turnover rates of microbial C in the warmer soils of Kraichgau with lower clay content might be an important mechanism explaining the differences in SOC content between both regions. Gram-positive bacteria assimilated more 13C-glucose into their biomass than fungi and the MRT of C was higher in bacteria as compared to fungi in both regions. Beside these dynamic substrate utilization strategies, we could proof cross feeding by gram-negative bacteria. Carbon MRT in fungi was region specific and was best represented by a two-pool model; the initial MRT ranged between 5 to 39 days and was, in the end, higher than 4 years. This provides evidence of a possible shift in fungal community composition; fast growing fungi dominant in the early phase and internal redistribution of C in the second phase of decomposition. Our study identified microbial group and region specific MRT of freshly assimilated C as an important parameter, which might help to explain the commonly found variation in soil respiration and SOC stabilization.

The interplay between abiotic factors and below-ground biological interactions regulates carbon exports from peatlands

Climate change projections indicate increases in CO2 emissions and DOC release from peatlands, thereby potentially transforming them from sinks to carbon sources. The individual effects of abiotic and biotic factors on peatland carbon exports and the interactions among all these factors are still poorly understood. Therefore, we conducted a field study over two years in a peatland complex by selecting four habitats differing in their dominant vegetation (shrub, grass, sedge and moss) in order to assess the links between abiotic and biotic factors and carbon release at each habitat. Accounting for the wide variability across habitats allowed us to examine the direct and indirect effects of abiotic and biotic factors on carbon cycling at the ecosystem level (i.e. the whole peatland complex). Results showed that the habitats dominated by vascular plants had the highest microbial and above-ground plant biomass, as well as the highest values of Acari and Collembola biomass, whereas Enchytraeidae biomass was dominant in the moss habitat. Furthermore, at ecosystem level, the SEM explained 75% of the total soil respiration variation and 33% of the DOC variation. Accordingly, soil respiration was directly linked to enchytraeid biomass, with these invertebrates being positively related to soil moisture. In contrast, both soil temperature and soil moisture had direct effects on microbial and Acari biomass, and the interaction between Acari biomass and below-ground plant biomass was responsible for DOC release. These findings provide new insights into the role of soil biota in carbon dynamics in peatlands, by showing that the direct effects of abiotic factors on biotic communities (both plants and soil organisms) determine how much carbon is lost from these systems (either as CO2 e or DOC).

Temporal changes in the spatial variability of soil respiration in a meadow steppe: The role of abiotic and biotic factors

How and why spatial patterns of soil respiration (SR) emerge and change or persist over time, and what makes spatial patterning of SR and environment variables interdependent or non-interdependent, is poorly understood. Our objectives were to quantify temporal changes in spatial patterns of SR for a grassland dominated by Leymus chinensis and to explore the underlying ecological mechanisms. We conducted simultaneous measurements of SR, along with a suite of other aboveground and belowground properties, for 75 sampling points in May, July (when SR was measured twice due to a distinct temporal fluctuation in soil water content and higher temperatures) and September. No spatial structure was detected for SR in May. The degree of spatial dependence of SR in September was lower than on 17 July and 23 July, while the autocorrelation range for SR in September (7.2 m) was higher than on 17 July (1.7 m) and 23 July (3.8 m). SR was explained by soil physicochemical properties (soil organic carbon content, organic carbon:total nitrogen ratio and soil pH) in May, and soil pH was the most influential factor at this time. On 17 July, SR was explained by the combination of soil water content, bulk density, aboveground biomass, soil fungal:bacterial biomass ratio and C acquiring enzyme activity. Soil temperature, bulk density, soil electrical conductivity and aboveground biomass explained 53% of the spatial variability in SR on 23 July. SR was mostly controlled by soil temperature (positively) in September. These results indicated that the spatial patterns of SR varied remarkably across the four observations and evolved temporally as a result of seasonal shifts in environmental conditions and vegetation. Biological factors (e.g. plant and microbial biomass) were important determinants of the spatial pattern of SR in July, when the strongest spatial heterogeneity was observed.

Dynamics of soil respiration and its influencing factors in urban forests under nitrogen addition

Urban forest is an important carbon pool, soil respiration of which is an important part of terrestrial carbon cycle. To understand the dynamics and influencing factors of soil respiration in urban forest under the background of increasing nitrogen deposition, we conducted dynamic observation on soil respiration rate, temperature, moisture and chemical properties by adding 0 (CK), 50 (LN), 100 (HN) kg N·m-2·a-1 ammonium nitrate to a typical urban forest. The results showed that soil respiration had significant seasonal variation, which was not affected by nitrogen addition. Soil respiration was significantly correlated with soil temperature. The interaction between soil temperature and soil moisture could better explain the variation of soil respiration. Nitrogen addition changed temperature sensitivity of soil respiration, with the order of Q10 values as LN (2.12) > CK (2.10) > HN (2.05). Soil nitrate concentration, soil soluble organic carbon, pH, soil carbon to nitrogen ratio had significant correlation with soil respiration. The positive effect of nitrogen deposition on soil respiration was mainly in the growing season, with slightly inhibitive effect in the non-growing season.

Significant Diel Variation of Soil Respiration Suggests Aboveground and Belowground Controls in a Tropical Moist Forest in Puerto Rico

AbstractSoil respiration in tropical forests represents a major source of carbon dioxide (CO2) to the atmosphere. The magnitude of this large flux is projected to change in response to climate change, with global implications due to the disproportionate role of tropical forests in the carbon cycle. Evaluating diel patterns of soil respiration concomitantly with biophysical drivers is a valuable approach for elucidating the mechanisms controlling soil respiration. We measured hourly soil respiration rates in a tropical moist forest in Puerto Rico over a 3‐year period using automated chambers, as well as soil temperature/moisture, air temperature, relative humidity, and photosynthetically active radiation. Hourly soil respiration exhibited as much as threefold variation on diel time scales (monthly diel amplitude ranged from 1 to 7 μmol CO2 m−2 s−1), and both the magnitude and shape of diel patterns changed significantly from month to month. Soil respiration peaked in the morning and late afternoon with a midday decline that was evident during the warmest summer months. The relationship between soil respiration and soil temperature differed during daytime versus the night, with a nonlinear relationship in the daytime but a significant positive linear relationship at night. These findings suggest factors other than/in addition to temperature could be controlling soil respiration during the day; however, soil respiration did not correlate with any of the other measured biophysical variables. Overall, our results highlight the potential role of aboveground processes as drivers of soil respiration at diel time scales, especially in closed canopy tropical forests with low diel variation in soil temperature/moisture.

Accounting for soil respiration variability \u2013 Case study in a Mediterranean pine-dominated forest

The number of spots to monitor to evaluate soil respiration (Rs) is often chosen on an empirical or conventional basis. To obtain an insight into the necessary number of spots to account for Rs variability in a Mediterranean pine-dominated mixed forest, we measured Rs all year long on sixteen dates with a portable gas-analyser in 50 spots per date within an area 1/3 ha wide. Linear mixed-effects models with soil temperature and litter moisture as descriptors, were fitted to the collected data and then evaluated in a Monte Carlo simulation on a progressively decreasing number of spots to identify the minimum number required to estimate Rs with a given confidence interval. We found that monitoring less than 14 spots would have resulted in a 10% probability of not fitting the model, while monitoring 20 spots would have reduced the same probability to about 5% and was the best compromise between field efforts and quality of the results. A simple rainfall index functional to select sampling dates during the summer drought is proposed.

Responses of soil respiration to temperature under different mulching modes in a dryland corn field

Based on a 9-year field experiment, soil samples under straw mulching (SM), plastic film mulching (FM) and no mulching (CK) were incubated at 15, 25 and 35 ℃ for 60 d to investigate the responses of soil respiration to warming and its temperature sensitivity. The results showed that during the whole incubation period, soil respiration rate exhibited a unimodal distribution, while the cumulative soil respiration increased with an "S" curve. The cumulative soil respiration during the first 30 d accounted for about 75%-85% of total during the whole incubation period. The cumulative SM increased by 19.4% compared with CK, whereas no difference was detected between CK and FM. At 25 ℃ and 35 ℃, the mean soil respiration rate increased by 17.0% and 36.8%, and the cumulative CO2 release of soil respiration increased by 13.1% and 33.6%, respectively, compared with 15 ℃. No interaction was detected between mulching method and temperature. 97.7%-99.9% of variation in soil respiration could be explained by temperature change, with soil respiration being positively correlated with organic carbon and total nitrogen content. Compared with no mulching and plastic film mulching, straw mulching could significantly promote soil respiration by increasing the input of organic matter in the soil, but reduced the temperature sensitivity of soil respiration. Straw mulching rather than plastic film mulching would be more efficient at reducing CO2 emission in the Loess Plateau dryland farming area under the context of global warming.

Effects of Soil Temperature, Water Content, Species, and Fertilization on Soil Respiration in Bamboo Forest in Subtropical China

Understanding the change pattern of soil respiration (SR) and its drivers under different bamboo species and land management practices is critical for predicting soil CO2 emission and evaluating the carbon budget of bamboo forest ecosystems. A 24-month field study was performed in subtropical China to monitor SR in experimental plots of local bamboo (Phyllostachys glauca) without fertilization (PG) and commercial bamboo (Phyllostachys praecox) with and without fertilization (PPF and PP, respectively). The SR rate and soil properties were measured on a monthly timescale. Results showed that the SR rate ranged from 0.38 to 8.53 µmol CO2 m−2s−1, peaking in June. The PPF treatment had higher SR rates than the PP and PG treatments for most months; however, there were no significant differences among the treatments. The soil temperature (ST) in the surface layer (0–10 cm) was found to be the predominant factor controlling the temporal change pattern of the monthly SR rate in the PG and PP treatments (i.e., those without fertilization). A bivariate model is used to show that a natural factor—comprised of ST and soil water content (SWC)—explained 44.2% of the variation in the monthly SR rate, whereas biological (i.e., bamboo type) and management (i.e., fertilization) factors had a much smaller impact (less than 0.1% of the variation). The annual mean SR showed a significant positive correlation with soil organic matter (SOM; r = 0.51, P &lt; 0.05), total nitrogen (TN; r = 0.47, P &lt; 0.05), total phosphorus (TP; r = 0.60, P &lt; 0.01), clay content (0.72, P &lt; 0.05) and below-ground biomass (r = 0.60*), which altogether explain 69.0% of the variation in the annual SR. Our results indicate that the fertilization effect was not significant in SR rate for most months among the treatments, but was significant in the annual rate. These results may help to improve policy decisions concerning carbon sequestration and the management of bamboo forests in China.

Variations of soil respiration and its temperature sensitivity in the reversion process of desertification in the eastern Hobq Desert, China

To clarify the effects of desertification reversal on soil respiration rate (Rs) and its temperature sensitivity (Q10), five different reversal stages were selected: mobile dune, semi-fixed sandland, algae crust fixed sandland, lichen crust fixed sandland, and moss crust fixed sandland in the eastern Hobq Desert. Rs at different stages were measured by static chamber-gas chromatography and the Q10 was calculated. We analyzed the effects of environmental factors on Rs. The results showed that Rs gradually increased with sand fixation and vegetation succession: moss crust fixed sandland (0.78 μmol·m-2·s-1)> lichen crust fixed sandland (0.67 μmol·m-2·s-1)> algae crust fixed sandland (0.46 μmol·m-2·s-1)> semi-fixed sandland (0.42 μmol·m-2·s-1)> mobile dune (0.29 μmol·m-2·s-1). The Rs of growing season was higher than that of non-growing season. Q10 of Rs at different reversal stages followed the order: mobile dune (3.28)> semi-fixed sandland (2.93)> algae crust fixed sandland (2.54)> lichen crust fixed sandland (1.91)> moss crust fixed sandland (1.84). The Q10 of non-growing season was higher than that of growing season. There was positive correlation between Rs and soil temperature. Rs of mobile and semi-fixed sand was positively correlated with soil water content, but not in other three fixed sandlands. Rs was correlated with soil total nitrogen, organic carbon, bulk density, porosity, quantity of bacteria, quantity of actinomycetes and quantity of fungi. Our results indicated that in the process of desertification reversal, the increases of soil organic carbon and nitrogen content and the abundance of microbes, the improvement of soil texture and the accumulation of plant biomass could dramatically enhance soil respiration and reduce its temperature sensitivity, which were the main driving forces to change carbon cycle of desert soil, and mediate the effects of soil water on soil respiration.

Inter-annual variation of soil respiration and its spatial heterogeneity in a cool-temperate young larch plantation in northern Japan

Inter-annual variation of soil respiration and its spatial heterogeneity in a cool-temperate young larch plantation in northern Japan

Controls of Temporal Variations on Soil Respiration in a Tropical Lowland Rainforest in Hainan Island, China

Soil respiration represents the largest carbon (C) flux from terrestrial ecosystems to the atmosphere. We created a study site in tropical lowland rainforest and used static chamber method to measure the temporal variations of soil respiration and their relationship with environmental factors at monthly time scale. The temporal variations of soil respiration showed a seasonal pattern related to soil temperature (p < .01) and soil moisture (p < .05). We tested different regression models to explore the relationship between soil respiration and environmental factors. Soil respiration had a better fit with soil temperature than with soil moisture in single-factor models. At different temperatures, the Q10 values from different models changed in rather different ways. We found that the mixed quadratic model composite of soil temperature and moisture had the best-fitting effect (R2 = .74) on soil respiration and could better explain the seasonal variation. In a certain soil moisture range close to 15%, soil respiration increased with soil temperature. However, soil respiration became restricted when the moisture was greatly higher or lower than this value. Furthermore, at low soil temperatures (lower than 16°C), higher soil moisture could decrease soil respiration rapidly. Thus, soil respiration in a tropical lowland rainforest is co-controlled by soil temperature and moisture. This study expands our observations of soil respiration in tropical forests and how it responds to environmental factors, which is important for reducing errors in evaluation and scaling up of soil carbon flux in climate change studies.

Response of Soil Respiration and Microbial Biomass to Soil Salinity under Different Water Content in the Coastal Areas of Eastern China

Knowledge concerning the response of soil carbon transformation to soil moisture and salinity is essential for saline soil improvements and utilization. An incubation experiment with three moisture levels (50, 75 and 100% of field capacity) and six salinity levels (soil electrical conductivity of 1 : 5 soil suspension as 0.25, 1.01, 2.02, 3.21, 4.92 and 6.17 dS m–1 for S1, S2, S3, S4, S5 and S6 treatment) was conducted with the purpose to investigate the effect of moisture and salinity on soil respiration (SR) and soil microbial biomass carbon (SMBC). Both soil salinity and soil moisture significantly impact SR and SMBC, yet the interaction effect between them was not significant either for SR, or SMBC. SR and SMBC were found to positively correlate with soil moisture but increased first and then decreased along with increasing soil salinity with the maximum values observed at S3 and S2 salinity levels, respectively. It was found significantly higher at S1 or S2 than at S5 or S6 salinity levels. Furthermore, soil moisture was found to be the predominant factor with significant higher effect than soil salinity levels in determining the variation of SR and SMBC. The results suggested that both the active and total soil microbial communities slightly increased when soil salinity was low (0.25 to 1.01 or 2.02 dS m–1), while they were adversely affected by higher salinity (4.92 and 6.17 dS m–1). In addition, the reduction in SR at high salinity and low moisture levels is mostly caused by reduction in SMBC as it was found significantly correlated to SR.

Photosynthetic and environmental regulations of the dynamics of soil respiration in a forest ecosystem revealed by analyses of decadal time series

Although photosynthetic and environmental regulations of the dynamics of soil respiration have been frequently reported, few studies have so far tested their generality and interactive effects. Using decade-long continuous measurements of soil respiration and eddy covariance records of net ecosystem exchange of CO2 at a forest site in the central USA, we examined the linkage of photosynthesis and environmental factors with soil respiration. Results showed that gross primary production (GPP) regulated soil respiration with monthly mean time lags that varied between four to twelve hours. The variations in this time lag were affected by past trajectories of moisture and temperature. GPP played a more important role in regulating soil respiration during dry than wet seasons, probably due to stronger water limitation on soil respiration under dry conditions. Finally, we found that models incorporating GPP as an input explained more variation in soil respiration than using soil temperature and moisture alone. Our findings suggest that photosynthesis and environmental conditions interactively drive the dynamics of soil respiration.

Temporal variation in soil respiration and its sensitivity to temperature along a hydrological gradient in an alpine wetland of the Tibetan Plateau

Wetlands are predicted to experience lowered water tables due to permafrost degradation in the Tibetan Plateau. These changes may affect carbon cycle processes such as soil respiration (Rs). However, the magnitude, patterns and controls of Rs remain poorly understood in alpine wetlands with their distinct hydrological regimes. Here, we conducted a field study on Rs from 2012 to 2014 in three alpine ecosystems on the Tibetan Plateau—fen, wet meadow and meadow—with soil water decreases along hydrological gradients. From 2012 to 2014, the annual Rs was 128.9–193.3 g C m−2yr−1, 281.5–342.9 g C m−2yr−1, and 663.4–709.1 g C m−2yr−1 for the fen, wet meadow, and meadow, respectively. An abrupt increase in CO2 emissions was caused by the spring thawing of the frozen soil in the fen and wet meadow, contributing 20.4–37.6% and 13.2–17.4%, respectively, to the annual Rs. The diurnal variation in the Rs was site specific among the three ecosystems, with one peak at 1300 h in the fen and meadow and two peaks at 1300 h and 1900 h in the wet meadow. The temperature-independent components of the diurnal variation in Rs were generally explained by photosynthetically active radiation in the fen and wet meadow, but not in the meadow. The temperature sensitivity of the Rs (unconfounded Q10) varied significantly among the three ecosystems, with the highest values occurring in the wet meadow, implying that permafrost thaw-induced wetland drying from the fen to the wet meadow could enhance the response of CO2 emissions to climate warming but that further drying from the wet meadow to the meadow probably weakens the effect of warming on the Rs. Our study emphasized the important role of the hydrological regime in regulating the temporal variation in Rs and its response to climate warming.

Variations in Soil Respiration at Different Soil Depths and Its Influencing Factors in Forest Ecosystems in the Mountainous Area of North China

An in-depth understanding of the dominant factors controlling soil respiration is important to accurately estimate carbon cycling in forest ecosystems. However, information on variations in soil respiration at different soil depths and the influencing factors in forest is limited. This study examined the variations in soil respiration at two soil depths (0–10 and 10–20 cm) as well as the effects of soil temperature, soil water content, litter removal, and root cutting on soil respiration in three typical forest types (i.e., Pinus tabulaeformis Carrière, Platycladus orientalis (L.) Franco, and Quercus variabilis Bl.) in the mountainous area of north China from March 2013 to October 2014. The obtained results show that soil respiration exhibited strong seasonal variation and decreased with soil depth. Soil respiration was exponentially correlated to soil temperature, and soil respiration increased with soil water content until reaching threshold values (19.97% for P. tabulaeformis, 16.65% for P. orientalis, and 16.90% for Q. variabilis), followed by a decrease. Furthermore, interactions of soil temperature and water content significantly affected soil respiration at different soil depths of forest types, accounting for 68.9% to 82.6% of the seasonal variation in soil respiration. In addition to soil temperature and water content, aboveground litter and plant roots affected soil respiration differently. In the three forest types, soil respiration at two soil depths decreased by 22.97% to 29.76% after litter removal, and by 44.84% to 53.76% after root cutting. The differences in soil respiration reduction between the two soil depths are largely attributed to variations in substrate availability (e.g., soil organic content) and soil carbon input (e.g., litter and fine root biomass). The obtained findings indicate that soil respiration varies at different soil depths, and suggest that in addition to soil temperature and water content, soil carbon input and dissolved organic substances may exert a strong effect on forest soil respiration.