Globally rising soil heterotrophic respiration over recent decades.

To investigate the responses of different components of soil respiration to environmental factors at different timescales in a vineyard ecosystem. The trenching method was used to separate total soil respiration (TSR) into autotrophic respiration (AR) and heterotrophic respiration (HR). Soil respiration rates were measured by an LI-8100 automated flux system. On the hourly scale, there were contrasting responses in TSR, HR and AR to soil temperature at 5 cm (ST5), with clockwise hysteresis loop responses of TSR and HR to ST5 but a counterclockwise hysteresis loop between AR and ST5. The daily TSR didn’t exponentially response to ST5 during the growing season. On the monthly scale, the relationship between TSR and ST5 showed a counterclockwise hysteresis loop. Meanwhile, the diel respiration peak lagged the peak of gross primary productivity (GPP), but the monthly peak of TSR preceded the monthly peak of GPP. The daily TSR and the daily soil water content at 5 cm (SWC5) in different months showed a quadratic relationship, but there was an exponential correlation between the monthly TSR and the monthly SWC5. The relationship between soil respiration and environmental factors derived for a specific timescale cannot be directly applied to other timescales.

Heterotrophic respiration is a major component of the soil C balance however we critically lack understanding of its variation upon conversion of peat swamp forests in tropical areas. Our research focused on a primary peat swamp forest and two oil palm plantations aged 1 (OP2012) and 6 years (OP2007). Total and heterotrophic soil respiration were monitored over 13 months in paired control and trenched plots. Spatial variability was taken into account by differentiating hummocks from hollows in the forest; close to palm from far from palm positions in the plantations. Annual total soil respiration was the highest in the oldest plantation (13.8 ± 0.3 Mg C ha−1 year−1) followed by the forest and youngest plantation (12.9 ± 0.3 and 11.7 ± 0.4 Mg C ha−1 year−1, respectively). In contrast, the contribution of heterotrophic to total respiration and annual heterotrophic respiration were lower in the forest (55.1 ± 2.8%; 7.1 ± 0.4 Mg C ha−1 year−1) than in the plantations (82.5 ± 5.8 and 61.0 ± 2.3%; 9.6 ± 0.8 and 8.4 ± 0.3 Mg C ha−1 year−1 in the OP2012 and OP2007, respectively). The use of total soil respiration rates measured far from palms as an indicator of heterotrophic respiration, as proposed in the literature, overestimates peat and litter mineralization by around 21%. Preliminary budget estimates suggest that over the monitoring period, the peat was a net C source in all land uses; C loss in the plantations was more than twice the loss observed in the forest.

Summary Land-surface models are important tools for simulation of the past, present, and future capacity of terrestrial ecosystems to absorb anthropogenic CO2 emissions. However, fluvial carbon (C) transfers are presently neglected in these models. Using the Amazon basin as a case study, we show that this negligence leads to significant underestimation of the net uptake of atmospheric C while terrestrial C storage changes are overestimated. These biases arise from the fact that C—in reality, leached from soils and exported through the river network—is instead represented as partly being respired and partly being stored in soils. Moreover, these biases scale mainly to the fluvial C export to the coast, despite aquatic CO2 emission to the atmosphere being the major pathway of riverine C exports. We further show that fluvial C transfers may change significantly in response to changes in either hydrology or in atmospheric C uptake by vegetation.

The contribution of decomposing soil organic carbon (SOC) to total annual soil respiration (SR) was evaluated by radiocarbon measurements at a Scots pine stand growing on a plaggen soil in the Belgian Campine region. Two approaches were used to estimate the contribution of different C pools to SR. In the first approach, the variations in 14C content of soil CO2 efflux were monitored during one year (2003) and compared to the atmospheric and SOC 14C signatures to determine the contribution of “fast” (root respiration and fast decomposing SOC) and “slow” cycling C pools to total SR. In the second approach an estimate of the total heterotrophic soil respiration (Rh), comprising the slow cycling C and the heterotrophic part of the fast-cycling C pools, was derived applying a box model based on the amount of the bulk SOC pool and its 14C-derived mean residence time (MRT). The quantification of the Rh and the decomposition rate of the slow-cycling SOC allows to indirectly determining the contribution of the heterotrophic C that decompose within a year. Measurements of total SR performed in the field allowed assessing the contribution of the different C pools to total soil C efflux. On an annual basis, the fast-cycling C was the main contributor to SR, about 85%, while the contribution of the slow-cycling C (with MRT >1 yr) to total SR was 15%. Total annual Rh was 36% of total SR, which is in the lower range reported for temperate coniferous forests. The comparison of Rh with other estimates for the same site (47–50% of total SR) suggest a possible underestimation of the C flux from the mineral soil. In fact, the “very old” C contained in the plaggen horizon strongly affects the signature of the mostly young C leaving the soil. In conclusion, our results indicate that the contribution of SOC decomposition to total soil CO2 flux in this forest is less than 40%, and at least half of it comes from organic compounds less than 1 year old.

This study was conducted to evaluate the dynamics of soil respiration (total soil and heterotrophic respiration) following fertilizer application in red pine forests. Fertilizer (N:P:K = 113:150:37 kg/ha), which reflects current practices in Korean forest, was applied in April 2011, and total soil and heterotrophic respiration rates were monitored from April 2011 to March 2012. Monthly variation of total soil and heterotrophic respiration rates were similar between the fertilizer and control treatments, as soil temperature was the dominant factor controlling the both rates. Total soil respiration rates during the study period were not significantly different between the fertilizer (0.504 g CO2 m-2 h-1) and control (0.501 g CO2 m-2 h-1) treatments. However, the proportion of heterotrophic respiration was higher in the fertilizer (78% of total soil respiration rates) than in the control (62% of total soil respiration rates) treatments. These results suggest that current fertilizer practices in Korea forest soil do not substantially affect total soil respiration rates.

Understanding the drivers and mechanisms underlying variations in autotrophic (Ra), heterotrophic (Rh), and total soil respiration (Rs) is essential for improving carbon management in croplands. In this four-year field study (2018–2021) conducted in a maize system, we compared practices without straw (NS) and with straw mulch (SM). Random Forest analysis identified bulk density, soil temperature, soil inorganic carbon, and urease activity as the primary determinants of Ra, Rh, and Rs. Straw mulch improved soil biodiversity, species richness, α-diversity, and evenness. Piecewise structural equation modeling further showed that soil microenvironmental changes regulate substrate availability by altering soil biological properties, which subsequently shape bacterial communities and microbial metabolism, ultimately controlling Rs. Compared with NS, on average over the four-year study period, SM substantially enhanced Ra and Rh by 96.9% and 82.2%, respectively, and increased maize grain yield and aboveground biomass by 9.7% and 6.2%. Notably, SM shifted the net ecosystem carbon budget from negative to positive, indicating a pronounced carbon sink effect. Overall, our findings highlight the pivotal role of straw amendments in regulating Rs and its components and elucidating the pathways through which these effects occur. These results provide mechanistic evidence supporting SM as a climate-smart agricultural practice that can simultaneously enhance crop productivity and contribute to agricultural carbon sequestration strategies and carbon neutrality targets. Effects and driving mechanisms of straw additions on soil respiration and net ecosystem carbon budget. The outer brown dashed line represents the indirect effect of soil properties and bacterial structure affecting soil respiration. Rh: soil heterotrophic respiration; Rs: soil respiration; NPP: net primary productivity; NEP: net ecosystem production; NECB: net ecosystem carbon budget. NS: no straw; SM: straw mulch. * Represents the level of significance (* P < 0.05; ** P < 0.01) and ns represents non statistically significant. • Straw mulch had a bidirectional promotion effect on Ra (+96.9%) and Rh (+82.2%). • Straw mulch had a significant C sink effect. • Straw mulch increased the species richness, α-diversity, evenness, and modularity degree. • Main factors affecting Rs was soil substrate availability, followed by soil microenvironment.

Abstract. Satellite-based models have been widely used to simulate vegetation gross primary production (GPP) at the site, regional, or global scales in recent years. However, accurately reproducing the interannual variations in GPP remains a major challenge, and the long-term changes in GPP remain highly uncertain. In this study, we generated a long-term global GPP dataset at 0.05∘ latitude by 0.05∘ longitude and 8 d interval by revising a light use efficiency model (i.e., EC-LUE model). In the revised EC-LUE model, we integrated the regulations of several major environmental variables: atmospheric CO2 concentration, radiation components, and atmospheric vapor pressure deficit (VPD). These environmental variables showed substantial long-term changes, which could greatly impact the global vegetation productivity. Eddy covariance (EC) measurements at 95 towers from the FLUXNET2015 dataset, covering nine major ecosystem types around the globe, were used to calibrate and validate the model. In general, the revised EC-LUE model could effectively reproduce the spatial, seasonal, and annual variations in the tower-estimated GPP at most sites. The revised EC-LUE model could explain 71 % of the spatial variations in annual GPP over 95 sites. At more than 95 % of the sites, the correlation coefficients (R2) of seasonal changes between tower-estimated and model-simulated GPP are larger than 0.5. Particularly, the revised EC-LUE model improved the model performance in reproducing the interannual variations in GPP, and the averaged R2 between annual mean tower-estimated and model-simulated GPP is 0.44 over all 55 sites with observations longer than 5 years, which is significantly higher than those of the original EC-LUE model (R2=0.36) and other LUE models (R2 ranged from 0.06 to 0.30 with an average value of 0.16). At the global scale, GPP derived from light use efficiency models, machine learning models, and process-based biophysical models shows substantial differences in magnitude and interannual variations. The revised EC-LUE model quantified the mean global GPP from 1982 to 2017 as 106.2±2.9 Pg C yr−1 with the trend 0.15 Pg C yr−1. Sensitivity analysis indicated that GPP simulated by the revised EC-LUE model was sensitive to atmospheric CO2 concentration, VPD, and radiation. Over the period of 1982–2017, the CO2 fertilization effect on the global GPP (0.22±0.07 Pg C yr−1) could be partly offset by increased VPD (-0.17±0.06 Pg C yr−1). The long-term changes in the environmental variables could be well reflected in global GPP. Overall, the revised EC-LUE model is able to provide a reliable long-term estimate of global GPP. The GPP dataset is available at https://doi.org/10.6084/m9.figshare.8942336.v3 (Zheng et al., 2019).

A growing literature is reporting on how the terrestrial carbon cycle is experiencing year-to-year variability because of climate anomalies and trends caused by global change. As CO 2 concentration records in the atmosphere exceed 50 years and as satellite records reach over 30 years in length, we are becoming better able to address carbon cycle variability and trends. Here we review how variable the carbon cycle is, how large the trends in its gross and net fluxes are, and how well the signal can be separated from noise. We explore mechanisms that explain year-to-year variability and trends by deconstructing the global carbon budget. The CO 2 concentration record is detecting a significant increase in the seasonal amplitude between 1958 and now. Inferential methods provide a variety of explanations for this result, but a conclusive attribution remains elusive. Scientists have reported that this trend is a consequence of the greening of the biosphere, stronger northern latitude photosynthesis, more photosynthesis by semi-arid ecosystems, agriculture and the green revolution, tropical temperature anomalies, or increased winter respiration. At the global scale, variability in the terrestrial carbon cycle can be due to changes in constituent fluxes, gross primary productivity, plant respiration and heterotrophic (microbial) respiration, and losses due to fire, land use change, soil erosion, or harvesting. It remains controversial whether or not there is a significant trend in global primary productivity (due to rising CO 2, temperature, nitrogen deposition, changing land use, and preponderance of wet and dry regions). The degree to which year-to-year variability in temperature and precipitation anomalies affect global primary productivity also remains uncertain. For perspective, interannual variability in global gross primary productivity is relatively small (on the order of 2 Pg-C y -1) with respect to a large and uncertain background (123 +/- 4 Pg-C y -1), and detected trends in global primary productivity are even smaller (33 Tg-C y -2). Yet residual carbon balance methods infer that the terrestrial biosphere is experiencing a significant and growing carbon sink. Possible explanations for this large and growing net land sink include roles of land use change and greening of the land, regional enhancement of photosynthesis, and down regulation of plant and soil respiration with warming temperatures. Longer time series of variables needed to provide top-down and bottom-up assessments of the carbon cycle are needed to resolve these pressing and unresolved issues regarding how, why, and at what rates gross and net carbon fluxes are changing.

The effect of land use change from agricultural to short rotation woody crops on soil respiration and its temperature sensitivity is not fully understood. We studied the effect of land use change on soil respiration, in a factorial experiment, 4 years after converting an agricultural field to a Walker poplar (Populus deltoides × Populus × petrowskyana var. Walker) plantation in the boreal region of northeastern Alberta. Overall, total soil respiration was greater in the agricultural plots (planted to alfalfa, Medicago sativa L.) than in the poplar plots. Soil respiration and soil temperature at the 10-cm depth in both land uses had similar seasonal and diurnal variations. The season-long temperature sensitivity (Q10) of daily and nighttime soil respiration in the alfalfa plots was greater than that in the poplar plots, with Q10 values of 5.4 vs. 4.9, respectively. Our data also show that, 4 years after land use conversion, the heterotrophic respiration was smaller in the poplar plots than in the adjacent agricultural plots, indicating potential benefits of plantation establishment in reducing heterotrophic respiration. However, the temperature sensitivity of soil respiration based on monthly nighttime rates, which minimizes the plant phenological influences, was greater in the poplar plantation, suggesting that soil respiration could become greater in the poplar plantation under a future warmer climate. We conclude that establishment of poplar plantations, which are known to have a fast rate of biomass production for long-term carbon storage, may help mitigate climate change by reducing heterotrophic and total soil respiration in the Canadian boreal region, but the long-term implications (e.g., changes in the temperature sensitivity of soil respiration over time) need to be further studied.

Soil respiration has become the main way of farmland ecosystem carbon emissions. Soil respiration and its responses to soil moisture and soil temperature under straw and biochar returning were investigated. Combined soil CO2 fluxes system(ACE-002/OPZ/SC) with the method of root exclusion, this study conducted a long-term field experiment in the national monitor station of soil fertility and fertilizer efficiency of purple soils. The total soil respiration and heterotrophic respiration rate and the soil hydrothermal factors were measured during the growth period of rape and maize in rape-maize rotation systems, and the difference between total soil respiration and heterotrophic respiration was calculated as the contribution of root respiration to soil respiration. There were five treatments including CK(no organic material), CS(straw), CSD(straw+microorganism), BC(biochar), CSBC(50%straw+50%biochar), which were replicated three times. The results showed that straw and biochar returning significantly affected the seasonal variations and the peak of soil respiration. In addition to BC treatment, other treatments promoted soil respiration and cumulative emissions of soil CO2. Soil respiration rate was significantly different under different treatments, the changes in soil respiration rates showed a single peak curve under all treatments, the seasonal variations in soil respiration rates under rape was 0.12-2.29 μmol·(m2·s)-1, displaying an order of CS > CSD > CSBC > CK > BC. Soil respiration was pretty complex in maize season, the seasonal variation in soil respiration rates under rape was 1.02-15.32 μmol·(m2·s)-1, displaying an order of CSD > CS > CSBC > CK > BC, the changes in soil respiration rate presented a double peak curve under CS and CSD and CSBC treatments and a single peak curve under BC and CK treatments. Heterotrophic respiration could explain 86.50%-93.94% of seasonal variations in the soil total respiration, and the contribution of root respiration(26.49%-32.86%) was significantly lower than CK treatment(53.65%).Straw and biochar returning did not change soil temperature and soil moisture. Soil temperature at 5 cm depth had significant effects on the change dynamics of soil respiration rates, but soil moisture did not. Soil temperature at 5 cm depth could explain 82%-94% of the variations in soil respiration. The values of temperature sensitivity coefficient changed from 3.28 to 4.47. Compared with CK treatment, Q10 of CS, CSD and CSBC decreased by 26.62%, 18.12%, 20.58%, respectively, while BC increased by 12.53%. There was no synergistic effect between soil temperature and soil moisture on soil respiration, the dynamic changes of soil respiration rate could be simulated by single factor index function of soil temperature. Overall, soil respiration was significantly promoted by returning of straw, straw+microorganism, straw+biochar, while it was inhibited by returning of biochar.

In order to study the effects of acid rain on forest soil respiration, three plots were selected in the conifer/broad-leaved mixed forests in the Three Gorges Reservoir area of the Jinyun Mountain (Chongqing) from January 2016 to April 2017. Two groups of treatments were set up:a trenched treatment and an untrenched treatment. Each group was treated with four simulated acid rain gradients of pH 4.5 (control), 4.0, 3.25 and 2.5. The characteristics of total soil respiration and heterotrophic respiration under the four simulated acid rain treatments were measured. Soil temperature and moisture were measured during the respiration measurements, and soil samples were collected to study the effects of soil pH, the carbon-nitrogen ratio, and fine root biomass on soil respiration. The results indicated that the total soil respiration and heterotrophic respiration of trenched and untrenched plots showed a similar seasonal variation trend. The annual mean soil respiration rates of the CK, T4.0, T3.25, and T2.5 treatments were 1.89, 1.88, 1.75, and 1.74 μmol·(m2·s1)-1, respectively, and the annual mean soil respiration rates of the RCCK, RCT4.0, RCT3.25 and RCT2.5 treatments were 1.37, 1.32, 1.19, 1.08 μmol·(m2·s1)-1, respectively. There was no significant differences between the quarterly average of the total soil respiration and heterotrophic respiration before June 2016 (P>0.05). However, after October 2016, the difference was significant (P<0.01) with CK > pH 4.0 > pH 3.25 > pH 2.5. The cumulative soil CO2 emissions of the T4.0, T3.25, and T2.5 treatments in 2016 decreased by 3.89%, 9.64%, and 11.24% respectively, compared with the control, and the RCT4.0, RCT3.25, and RCT2.5 treatments decreased by 6.79%, 13.23%, and 25.56%, respectively. The simulated acid rain treatments reduced the ratio of heterotrophic respiration in the total soil respiration, and the degree of reduction increased with the pH of the simulated acid rain, indicating that the effect of acid rain on the inhibition of heterotrophic respiration exceeded that of autotrophic respiration. Although the simulated acid rain treatments increased the temperature sensitivity of soil respiration (Q10), it had no significant effect on soil temperature and humidity (P>0.05). The soil carbon-nitrogen ratio and fine root biomass were significantly greater in comparison to the control after October 2016. Soil respiration was significantly positively correlated with fine root biomass and significantly negatively correlated with the soil carbon-nitrogen ratio. Soil temperature and water contributed only slightly to soil respiration under the four simulated acid rain treatments. Fine root biomass and the soil carbon-nitrogen ratio were critical factors for variation of soil respiration under acid rain.

Impacts of fire on soil respiration and its components: A global meta-analysis

The C dynamics of a young (7 years old) smallholder oil palm plantation on peat soil in South Kalimantan (Indonesia) was investigated by directly assessing soil carbon dioxide (CO2) emissions and C sequestration as net primary production (NPP) over a period of 19 months, from September 2018 to March 2020. Soil CO2 efflux was measured monthly using a closed chamber system in ‘near (1m)’ and ‘far (3m)’ positions relative to tree bases, in order to measure total soil respiration (SR) and peat decomposition (PD), respectively. Simultaneously, litter (frond) decomposition (LD) was measured using a litter bag method. NPP was calculated as the sum of above-ground and below-ground biomass production. The C fluxes via SR, PD and LD were estimated to be 23.1 ± 6.13, 15.4 ± 4.37 (mean ± SD) and 0.38 Mg ha-1 yr-1, respectively. NPP (as C) was estimated to be 1090 g m-2 yr-1. NPP was low, mostly due to immaturity of the plantation. Heterotrophic respiration (HR) (= PD + LD) was 15.8 Mg ha-1 yr 1, resulting in the plantation acting as a net C source of 4.9 Mg ha−1 yr-1 (= NPP - HR).

BackgroundThe carbon pools of forest soils play a vital role in global carbon sequestration and emissions. Forest management can regulate the sequestration and output of forest soil carbon pools to a certain extent; however, the kinetics of the effects of forest density on soil carbon pools require further investigation.MethodsWe established sample plots with stand density gradients in three different aged Larix principis-rupprechtii plantations and quantified the soil respiration, soil organic carbon (SOC), soil dissolved organic carbon (DOC), microbial biomass carbon (MBC), light fraction organic carbon (LFOC), and readily oxidized carbon (ROC).Results and conclusionsDuring the growth and development of plantations, stand density is an essential factor that impacts soil respiration and its associated elements. Moderate density was observed to promote both the soil and heterotrophic respiration rates and the sequestration of MBC and LFOC, whereas it inhibited the sequestration of ROC. The soil, heterotrophic, and autotrophic respiration rates of older forest stands were relatively rapid, whereas the contents of SOC, MBC, LFOC, DOC, and ROC were higher and more sensitive to changes in stand density. The MBC, LFOC, and ROC in soil labile organic carbon were closely related to both the soil and heterotrophic respiration, but not the SOC. Among them, the LFOC and MBC played the roles of “warehouse” and “tool” and were significantly correlated with soil and heterotrophic respiration. The ROC, as a “raw material”, exhibited a significantly negative correlation with the soil and heterotrophic respiration. When the soil and heterotrophic respiration rates were rapid, the ROC content in the soil maintained the low level of a “dynamically stabilized” state. The stand density regulated heterotrophic respiration by affecting the soil labile organic carbon, which provided an essential path for the stand density to regulate soil respiration.

Comprehensive synthesis of spatial variability in carbon flux across monsoon Asian forests