Spatial variation in forest soil respiration: A systematic review of field observations at the global scale

As it is responsible for the second largest CO₂ flux in the terrestrial ecosystem, the accurate estimation and prediction of soil respiration (SR) are necessary, especially for forest ecosystems, which are a major contributor to the total terrestrial SR. Spatial variation is one of the challenges affecting the accurate estimation and prediction of forest SR in ecosystems. Although a number of studies have examined spatial variation in SR within individual forests, the magnitude and patterns of spatial variation in SR within forest ecosystems (CV of SR [%]) remain unexplored at the global scale. In this study, we collected 94 field observation studies to demonstrate the range and pattern of the CV of SR, and to clarify the controlling factors. Through our analysis, the CV of SR was found to range from 1.8 % to 89.3 % on the global scale; it was highest in the equatorial zone (39.0 % ± 13.8 %) and followed by the warm temperate zone (32.6 ± 14.5 %) and the snow zone (30.0 % ± 16.3 %). There was a significant negative correlation between the CV of SR and soil water content, bulk density, fine root biomass, and elevation at both the global scale and in each climatic zone (P < 0.01). Other factors such as total nitrogen content, mean of diameter at breast height, slope, etc., were also significantly correlated with the CV of SR, but the correlation was different among climatic zones. This study provides an overall perspective of the CV of SR by clarifying the range, patterns, and controlling factors at both the global scale and in each climatic zone. However, further research is needed, especially regarding the mechanisms between the CV of SR and its controlling factors.

Characteristics of soil CO2 emission and ecosystem carbon balance in wheat-maize rotation field with 4-year consecutive application of two lignite-derived humic acids

Humic acid originating from lignite is a popular resource of organic fertilizer. The effects of humic acid application on crop biomass and soil CO₂ emission charged the regional agro-ecosystem carbon balance. Two kinds of humic acid, obtained from lignite via H₂O₂-oxidation (OHA) and KOH-activation (AHA), were applied in a wheat-maize rotation located field at three levels of 500 (OHA1; AHA1), 1000 (OHA2; AHA2), and 1500 kg hm^-2 (OHA3; AHA3), only chemical fertilizer treatment (CF) as control to investigate the change of soil CO₂ emission, crop yield and ecosystem carbon balance in 2016-2019. During the four experimental years, the trend of cumulative efflux of soil CO₂ was increasing in medium and high dosage humic acid treatments. The grain yield of wheat and maize had the same trend as the cumulative efflux of soil CO₂ due to the increase of soil NO₃^--N and soil available P directly affected by humic acid application. The main factor of cumulative soil CO₂ efflux improvement was soil NO₃^--N and soil available P in 2016, while soil available potassium became key factor in 2019 with the step regression. Net ecosystem productivity (NEP) was used to assess ecosystem carbon balance, which was positive values showed atmospheric CO₂ sink under all the fertilization treatments and increased with the increase of humic acid use level. AHA2 and AHA3 treatments charged the higher NEP in 2019 than 2016. Meanwhile, AHA treatment presented a higher NEP average than OHA treatment with the same applied level. Crop yield and soil available P was the directly positive factor to NEP over four years under the fertilization by SEM analysis. It is recommended that AHA be applied at 1000 kg hm^-2 together with chemical fertilizers to achieve the higher crop yield and a sink of the atmospheric CO₂ in agricultural fields in North China.

Quantitative assessment of deep-seated CO2 leakage around CO2-rich springs with low soil CO2 efflux using end-member mixing analysis and carbon isotopes

This study examined a mountainous area with two hydrochemically distinct CO₂-rich springs to understand the origin, flow, and leakage of CO₂, which may provide implications for precise monitoring of CO₂ leakage in geological carbon storage (GCS) sites. The carbon isotopic compositions of dissolved inorganic carbon (DIC) in CO₂-rich water (δ¹³C_DIC) and those of soil CO₂ (δ¹³C_CO2) indicated a deep-seated CO₂ supply to the near-surface environment in the study area. The hydrochemical difference (e.g. pH, total dissolved solids) for the two CO₂-rich springs separated by 7 m, despite similar δ¹³C_DIC and partial pressure of CO₂, was considered as the result of different evolution of shallow groundwater affected by deep-seated CO₂ preferentially rising along fracture zones. Electrical resistivity tomography also suggested flow through fracture zones beneath the CO₂-rich springs, showing low resistivity compared to other surveyed zones. However, soil CO₂ efflux was low compared to that in other natural CO₂ emission sites, and in particular it was noticeably low near the CO₂-rich springs, whereas δ¹³C_CO2 was high close the CO₂-rich springs. The dissolution of CO₂ in the near-surface water body seemed to decrease the deep-seated CO₂ leakage through the soil layer, while δ¹³C_CO2 imprinted the source. End-member mixing analysis was performed to assess the contribution of deep-seated CO₂ to the low soil CO₂ efflux by assuming that atmospheric CO₂ and soil CO₂ (by respiration) as well as deep-seated CO₂ contribute to the soil CO₂ efflux. For each end-member, characteristic δ¹³C_CO2 and CO₂ concentrations were defined, and then their apportionment to soil CO₂ efflux was estimated. The resultant proportion of deep-seated CO₂ was up to 8.8%. Unlike the spatial distribution of high soil CO₂ efflux, high proportions exceeding 3% were found around the CO₂-rich springs along the east-west valley. The study results indicate that soil CO₂ efflux measurement should be combined with carbon isotopic analysis in GCS sites for CO₂ leakage detection because CO₂ dissolution in the underground water body may blur leakage detection on the surface. The implication of this study is the need to quantitatively assess the contribution of deep-seated CO₂ using the soil CO₂ concentration, soil CO₂ efflux, and δ¹³C_CO2 at each measurement site.

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.

Temperature sensitivity of soil respiration across multiple time scales in a temperate plantation forest

Soil respiration (Rs) is the largest carbon (C) flux from terrestrial ecosystems to the atmosphere. Predictions of Rs and associated feedback to climate change remain largely uncertain, in part due to the high temporal heterogeneity of temperature sensitivity (apparent Q₁₀) of Rs under a changing climate. Therefore, it is of critical importance to provide better insight into how Q₁₀ varies across multiple temporal scales. We investigated the diurnal, seasonal, and annual variabilities in the Q₁₀ of Rs using continuous Rs measurements (at hourly intervals) over six growing seasons in a mature temperate larch plantation in North China. We found that night-time values of Q₁₀ were slightly lower than daytime values. Large seasonal and annual fluctuations of Q₁₀ were observed, as illustrated by high coefficients of variation of 15.0% and 21.8%, respectively. The higher Q₁₀ in spring and autumn were primarily regulated by fine root growth and higher soil moisture after snow melt in spring, and leaf senescence in autumn. Lower Q₁₀ in summer may have been caused by limitations in substrate availability and microbial activity resulting from drought, which also caused a decoupling of Rs from soil temperature in summer. Furthermore, a bivariate nonlinear model incorporating both soil temperature and soil moisture best explained Q₁₀ variability. Generally, lower soil temperature and higher soil moisture lead to higher values of Q₁₀, indicating that climate warming could exert a negative effect on Q₁₀, partially offsetting the warming-induced increase in soil C loss. We provide long-term field experimental evidence that it would be inappropriate to estimate Rs on a multiyear scale using a fixed Q₁₀ value or a value obtained from one season and/or one year. Thus, we emphasize the importance of incorporating the seasonal and annual heterogeneities of Q₁₀ into C cycle model simulations under future climate change scenarios.

Effects of soil erosion and reforestation on soil respiration, organic carbon and nitrogen stocks in an eroded area of Southern China

Soil erosion and reforestation greatly affects the functionality of many terrestrial ecosystems. However, the effects of soil erosion and reforestation on soil respiration (SR), and soil organic carbon (SOC) and total nitrogen (TN) stocks remain unclear. Therefore, we investigated the changes in SR, and SOC and TN stocks at four different soil erosion levels (severely, moderately, lightly, and non-eroded) and two different aged Pinus massoniana plantations (8- and 36-year-old) in the hilly red soil regions of Southern China. Our results showed that soil erosion level and reforestation significantly influenced SR, and SOC and TN stocks. Meanwhile, the mean SR, and SOC and TN stocks all significantly decreased with erosion level but increased significantly with times since reforestation. Soil temperature (ST) could explain 70-92% of SR seasonal variation based on exponential models, whereas no significant relationship between SR and soil water content were found. Furthermore, the structural equation modeling indicated that SOC stocks at 0-20 cm had a much stronger effect on SR than ST. Meanwhile, the SOC stocks for 0-20 cm increased by 177% and 558% in the 8- and 36-year-old Pinus massoniana plantations in comparison with the severely eroded forestland, respectively. This study highlights that reforestation could be an effective strategy for restoring the carbon and nitrogen storage in eroded regions of Southern China and emphasizes the need to consider the effects of soil erosion and reforestation when assessing regional carbon budgets under different climate scenarios.

Responses of soil respiration to rainfall addition in a desert ecosystem: Linking physiological activities and rainfall pattern

The tight linkage between photosynthesis (A_n) and soil respiration (R_s) has been verified in many terrestrial ecosystems. However, it remains unclear whether this linkage occurs in desert ecosystems, where water is considered an important trigger of carbon cycling. A field experiment was performed under seven simulated rainfall amounts (0, 3, 5, 10, 15, 25, and 40 mm) with two co-existing desert plants (Reaumuria soongorica and Nitraria sphaerocarpa) in June (early growing season, EGS) and August (middle growing season, MGS) in 2016. A_n, R_s, predawn water potential (Ψ_pd), soil temperature (T_s) and soil moisture (S_wc) were measured for each treatment or control plot for 3 weeks. Our objective was to examine the effects of rainfall pattern on R_s and physiological responses of the two plants and the relationships between R_s and biotic and abiotic factors. No obvious variations in Ψ_pd or A_n were found under small rainfall events. However, when the rainfall amount exceeded 10 mm, both plants responded strongly, and the response patterns of R_s showed trends similar to those of A_n, which varied between species and seasons. Moreover, rain additions of 3-40 mm significantly increased R_s, and the relative changes in R_s (ΔR_s) of both species were much larger in the EGS than in the MGS. Importantly, abiotic factors may have controlled the variations in R_s under small rain events while A_n played a more important role in regulating the variations in R_s when the rainfall amount exceeded 10 mm for both species, suggest that the rainfall pattern-driven changes in R_s composition interact with physiological activity and abiotic factors to regulate the response of R_s to rainfall variability in desert ecosystems. Thus, climate change in the coming decades may lead to carbon sequestration by desert plants, which may cause desert ecosystems to act as carbon sinks.

Contribution of root respiration to total soil respiration in a semi-arid grassland on the Loess Plateau, China

Using the trenching method, a study was conducted in a grassland on the Loess Plateau of northern China in 2008 and 2009 to partition total soil respiration (Rt) into microbial respiration (Rm) and root respiration (Rr). Using the measurements of soil CO₂ diffusivity and soil CO₂ production, an analytical model was applied to correct the data, aiming to quantify the method-induced error. The results showed that Rm and Rr responded differently to biotic and abiotic factors and exhibited different diurnal and seasonal variations. The diurnal variation of Rm was strongly controlled by soil temperature, while Rr might be mainly controlled by photosynthesis. The combination of soil temperature and moisture could better explain the seasonal variation in Rm (r²=0.76, P<0.001). The seasonal variation of Rr was influenced mainly by the plant activity. The contribution of root respiration to total soil respiration (Rr/Rt ratio) also exhibited substantial diurnal and seasonal variations, being higher at nighttime and lower at daytime. In the different growing stages, the Rr/Rt ratios ranged from 15.0% to 62.0% in 2008 and 14.5% to 63.6% in 2009. The mean values of the Rr/Rt ratio in the growing season and the annual mean Rr/Rt ratio were 41.7% and 41.9%, respectively, during the experiment period. Different precipitation distributions in the two years did not change the yearly Rr/Rt ratio. Corrected with the analytical model, the trenching method in small root-free plots led to an underestimation of Rr and Rr/Rt ratio by 4.2% and 1.8%.

Temperature sensitivity of total soil respiration and its heterotrophic and autotrophic components in six vegetation types of subtropical China

The temperature sensitivity of soil respiration (Q₁₀) is a key parameter for estimating the feedback of soil respiration to global warming. The Q₁₀ of total soil respiration (R_t) has been reported to have high variability at both local and global scales, and vegetation type is one of the most important drivers. However, little is known about how vegetation types affect the Q₁₀ of soil heterotrophic (R_h) and autotrophic (R_a) respirations, despite their contrasting roles in soil carbon sequestration and ecosystem carbon cycles. In the present study, five typical plantation forests and a naturally developed shrub and herb land in subtropical China were selected for investigation of soil respiration. Trenching was conducted to separate R_h and R_a in each vegetation type. The results showed that both R_t and R_h were significantly correlated with soil temperature in all vegetation types, whereas R_a was significantly correlated with soil temperature in only four vegetation types. Moreover, on average, soil temperature explained only 15.0% of the variation in R_a in the six vegetation types. These results indicate that soil temperature may be not a primary factor affecting R_a. Therefore, modeling of R_a based on its temperature sensitivity may not always be valid. The Q₁₀ of R_h was significantly affected by vegetation types, which indicates that the response of the soil carbon pool to climate warming may vary with vegetation type. In contrast, differences in neither the Q₁₀ of R_t nor that of R_a among these vegetation types were significant. Additionally, variation in the Q₁₀ of R_t among vegetation types was negatively related to fine root biomass, whereas the Q₁₀ of R_h was mostly related to total soil nitrogen. However, the Q₁₀ of R_a was not correlated with any of the environmental variables monitored in this study. These results emphasize the importance of independently studying the temperature sensitivity of R_t and its heterotrophic and autotrophic components.

Influences of recovery from wildfire and thinning on soil respiration of a Mediterranean mixed forest

The ecosystem recovery after wildfire and thinning practices are both key processes that have great potential to influence fluxes and storage of carbon within Mediterranean semiarid ecosystems. In this study, started 7years after a wildfire, soil respiration (SR) patterns measured from 2008 to 2010 were compared between an unmanaged-undisturbed mature forest stand (UB site) and a naturally regenerated post-wildfire stand (B site) in a Mediterranean mixed forest in Spain. The disturbed stand included a control zone (unthinned forest, BUT site) and a thinned zone (BT site). Our results indicated that SR was lower at naturally regenerated after fire sites (BUT and BT) than at unburnt one. Soil under the canopy layer of pine and oak trees exhibited higher SR rates than bare or herbaceous layer soils, regardless of the site. The effect of thinning was only manifest, with a significant increase of SR, during the 1st year after thinning practices. SR showed a clear soil temperature-dependent seasonal pattern, which was strongly modulated by soil water content (SWC), especially in summer. Site-specific polynomial regression models were defined to describe SR responses, being mainly controlled by both soil temperature (Ts) and SWC at UB site, or Ts at burnt sites. The sensitivity of SR rate to Ts variations (Q₁₀) ranged between 0.20 and 6.89, with mean annual values varying between 0.92 and 1.35. Q₁₀ values were higher at BT than at UB-BUT sites. The results revealed a significant, non-linear dependence, of Q₁₀ on both Ts and SWC at UB site, and on Ts at both burnt sites. This study contributes to (i) improve the understanding of how natural recovery and management practices affect soil respiration in a Mediterranean forest during their early stages after fire disturbance and (ii) highlight the importance of Q₁₀ values <1 which emphasizes drought stress effect on SR temperature sensitivity.

Soil respiration dynamics in fire affected semi-arid ecosystems: Effects of vegetation type and environmental factors

Soil respiration (Rs) is the second largest carbon flux in terrestrial ecosystems and therefore plays a crucial role in global carbon (C) cycling. This biogeochemical process is closely related to ecosystem productivity and soil fertility and is considered as a key indicator of soil health and quality reflecting the level of microbial activity. Wildfires can have a significant effect on Rs rates and the magnitude of the impacts will depend on environmental factors such as climate and vegetation, fire severity and meteorological conditions post-fire. In this research, we aimed to assess the impacts of a wildfire on the soil CO₂ fluxes and soil respiration in a semi-arid ecosystem of Western Australia, and to understand the main edaphic and environmental drivers controlling these fluxes for different vegetation types. Our results demonstrated increased rates of Rs in the burnt areas compared to the unburnt control sites, although these differences were highly dependent on the type of vegetation cover and time since fire. The sensitivity of Rs to temperature (Q10) was also larger in the burnt site compared to the control. Both Rs and soil organic C were consistently higher under Eucalyptus trees, followed by Acacia shrubs. Triodia grasses had the lowest Rs rates and C contents, which were similar to those found under bare soil patches. Regardless of the site condition (unburnt or burnt), Rs was triggered during periods of higher temperatures and water availability and environmental factors (temperature and moisture) could explain a large fraction of Rs variability, improving the relationship of moisture or temperature as single factors with Rs. This study demonstrates the importance of assessing CO₂ fluxes considering both abiotic factors and vegetation types after disturbances such as fire which is particularly important in heterogeneous semi-arid areas with patchy vegetation distribution where CO₂ fluxes can be largely underestimated.

Continuous measurement of soil carbon efflux with Forced Diffusion (FD) chambers in a tundra ecosystem of Alaska

Soil is a significant source of CO2 emission to the atmosphere, and this process is accelerating at high latitudes due to rapidly changing climates. To investigate the sensitivity of soil CO2 emissions to high temporal frequency variations in climate, we performed continuous monitoring of soil CO2 efflux using Forced Diffusion (FD) chambers at half-hour intervals, across three representative Alaskan soil cover types with underlying permafrost. These sites were established during the growing season of 2015, on the Seward Peninsula of western Alaska. Our chamber system is conceptually similar to a dynamic chamber, though FD is more durable and water-resistant and consumes less power, lending itself to remote deployments. We first conducted methodological tests, testing different frequencies of measurement, and did not observe a significant difference between collecting data at 30-min and 10-min measurement intervals (averaged half-hourly) (p<0.001). Temperature and thaw depth, meanwhile, are important parameters in influencing soil carbon emission. At the study sites, we observed cumulative soil CO2 emissions of 62.0, 126.3, and 133.5gCm(-2) for the growing period, in sphagnum, lichen, and tussock, respectively, corresponding to 83.8, 63.7, and 79.6% of annual carbon emissions. Growing season soil carbon emissions extrapolated over the region equated to 0.17±0.06 MgC over the measurement period. This was 47% higher than previous estimates from coarse-resolution manual chamber sampling, presumably because it better captured high efflux events. This finding demonstrates how differences in measurement method and frequency can impact interpretations of seasonal and annual soil carbon budgets. We conclude that annual CO2 efflux-measurements using FD chamber networks would be an effective means for quantifying growing and non-growing season soil carbon budgets, with optimal pairing with time-lapse imagery for tracking local and regional changes in environment and climate in a warming Arctic.

Response of soil CO2 efflux to precipitation manipulation in a semiarid grassland

Soil CO2 efflux (SCE) is an important component of ecosystem CO2 exchange and is largely temperature and moisture dependent, providing feedback between C cycling and the climate system. We used a precipitation manipulation experiment to examine the effects of precipitation treatment on SCE and its dependences on soil temperature and moisture in a semiarid grassland. Precipitation manipulation included ambient precipitation, decreased precipitation (-43%), or increased precipitation (+17%). The SCE was measured from July 2013 to December 2014, and CO2 emission during the experimental period was assessed. The response curves of SCE to soil temperature and moisture were analyzed to determine whether the dependence of SCE on soil temperature or moisture varied with precipitation manipulation. The SCE significantly varied seasonally but was not affected by precipitation treatments regardless of season. Increasing precipitation resulted in an upward shift of SCE-temperature response curves and rightward shift of SCE-moisture response curves, while decreasing precipitation resulted in opposite shifts of such response curves. These shifts in the SCE response curves suggested that increasing precipitation strengthened the dependence of SCE on temperature or moisture, and decreasing precipitation weakened such dependences. Such shifts affected the predictions in soil CO2 emissions for different precipitation treatments. When considering such shifts, decreasing or increasing precipitation resulted in 43 or 75% less change, respectively, in CO2 emission compared with changes in emissions predicted without considering such shifts. Furthermore, the effects of shifts in SCE response curves on CO2 emission prediction were greater during the growing than the non-growing season.