Temperature Sensitivity Of Soil Organic Carbon Decomposition Research Articles

Warming Reduces Priming Effect of Soil Organic Carbon Decomposition Along a Subtropical Elevation Gradient

AbstractThe priming effects (PEs) of soil organic carbon (SOC) decomposition is a crucial process affecting the C balance of terrestrial ecosystems. However, there is uncertainty about how PEs will respond to climate warming. In this study, we sampled soils along a subtropical elevation gradient in China and conducted a 126‐day lab‐incubation experiment with and without the addition of 13C‐labeled high‐bioavailability glucose or low‐bioavailability lignin. Based on the mean annual temperature (MAT) of each elevation (9.3–16.4°C), a temperature increase of 4°C was used to explore how PEs mediate the decomposition of SOC in response to warming. Our results showed that the magnitude of glucose‐induced PEs (PEglucose) was higher than lignin‐induced PEs (PElignin), with both PEs linearly increasing with MAT. Across the MAT (i.e., elevation) gradient, short‐term warming had a constant magnitude of negative effects on PEglucose, whereas rising MAT exacerbated the negative effects of short‐term warming on PElignin. Moreover, the temperature sensitivity of SOC decomposition decreased after adding glucose and lignin across the MAT gradient, suggesting that fresh C inputs may prime the microbial breakdown of labile SOC under warming. Taken together, warming alleviated SOC loss due to PEs through varying mechanisms depending on substrate bioavailability. Warming mediated the PEglucose by increasing available nitrogen and weakening microbial nitrogen‐mining but inhibited the PElignin by shifting from microbial nitrogen‐mining to microbial co‐metabolization. Our findings highlight the role of warming in regulating the PEs and suggest that incorporating the suppression effect of warming on PEs can contribute to the accurate prediction of soil C dynamics in a warming world.

Climate-induced shifts in composition and protection regulate temperature sensitivity of carbon decomposition through soil profile

Through soil profile, both chemical composition of soil organic carbon (SOC) and edaphic physiochemical properties present a vertical gradient, likely resulting in depth-specific SOC dynamics in response to climate change (e.g., global warming). We assessed temperature sensitivity of SOC decomposition (Q10) by incubating (128 days) soils sampled across five sequential layer depths (i.e., 0–10, 10–20, 20–30, 30–50, and 50–100 cm) at ten sites along a ∼2500 m elevational transect (from ∼2100 m to ∼4600 m) covering various vegetation types (from evergreen broadleaved forest to alpine meadow) in southeast Tibet, China. The Q10 of SOC decomposition was significantly affected by both soil depth and elevation. However, depth-induced variation of Q10 was much smaller than that induced by the elevation gradient. Across the ten sites and five soil depths, chemical composition of SOC and its physiochemical protection against decomposition contributed >80% to the explained variance of Q10 values. Path analysis suggested that climate indirectly affected Q10 via its regulation on chemical composition of SOC and their physiochemical stabilization. The results from a carbon model constrained by the collected data further revealed that fast, slow and passive SOC pools exhibited significant difference in their Q10, resulted from different involvement of chemical composition and physicochemical protection in their decomposition. Our findings demonstrate similar temperature sensitivity of SOC decomposition across soil depths, but spatially heterogeneous temperature sensitivity due to climate-induced variability of both chemical recalcitrance of SOC and its physiochemical protection against decomposition.

Shrubification decreases soil organic carbon mineralization and its temperature sensitivity in alpine meadow soils

Shrubification is widespread in global grasslands. However, whether shrubification promotes the mineralization of soil organic carbon (SOC) remains uncertain. We compared sizes and compositions of soil non-cellulosic and amino sugar pools, soil stoichiometry, SOC decomposition and its temperature sensitivity between herbaceous and shrubby patches in alpine meadows on the Qinghai-Tibetan Plateau. The SOC content and soil stoichiometric C/P and N/P ratios in shrubby patches markedly increased with increasing plant size compared with herbaceous areas, indicating the increased P limitation to microorganisms relative to C and N with SOC accumulation under shrubs. The mass ratio of galactose plus mannose to arabinose plus xylose in the non-cellulosic carbohydrate pool of plant input was higher under shrubs than under grasses but was generally similar in soils between herbaceous and shrubby patches. This suggests the retarded microbial transformation of plant-derived carbohydrates under shrubs. Shrubs decreased accumulation of microbial necromass in SOC but increased the proportion of fungal origin in the microbial necromass. Lower total CO2 efflux per unit mass of SOC at either 15 °C or 25 °C from shrubby than from herbaceous patches over the 76-day incubation was mainly associated with the increased soil C/P or N/P ratio under shrubs. Shrubs decreased temperature sensitivity of SOC decomposition compared with grasses only at 20–40 cm soil depth, where microbial-synthesized substances had greater dominance over organic matter than at 0–20 cm depth and were less abundant under shrubs relative to grasses. Our findings show that increased P limitation to microorganisms in shrubification resulted in the decreased SOC decomposability and indicate that microbial-synthesized substances determined the temperature sensitivity of SOC mineralization. Therefore, shrubification mitigates CO2 emissions in grasslands by decreasing SOC mineralization and its temperature sensitivity in the context of global warming.

Alpine meadow degradation enhances the temperature sensitivity of soil carbon decomposition on the Qinghai–Tibetan plateau

Grassland degradation is widespread globally, yet limited information is available on the effects and mechanisms of grassland degradation regarding the response of soil organic carbon (SOC) to temperature change. This is especially true for alpine regions, which can have high SOC storage and are extreme vulnerability to global warming. Here, we studied the temperature sensitivity of SOC decomposition (Q10, proportional change in decomposition rate for a 10 °C difference in temperature) in both the topsoil (0–10 cm depth) and subsoil (20–30 cm) along an alpine meadow degradation gradient on the Qinghai–Tibetan plateau (QTP). Q10 values were increased in response to alpine meadow degradation (severely degraded (2.42) > moderately degraded (2.20) > non-degraded (2.11)) and were higher in subsoil (2.34) than in topsoil (2.14) as a whole. Soil carbon quantity and quality and extracellular enzyme activities all decreased significantly with increasing degradation levels and soil depths. Among all the factors considered (soil texture; soil pH; carbon quantity, availability, and quality; and enzyme activities), Q10 values were found to be primarily mediated by carbon quality and enzyme activities. This result supported the “carbon-quality temperature” hypothesis in degraded alpine grassland, and that considering soil carbon quality and enzyme activity could improve predictions of the feedbacks between soil carbon and global warming under grassland degradation. Our findings suggest that alpine meadow degradation will further increase the losses of SOC in a warming climate, making the ecosystem more vulnerable to climate change.

High microbial diversity stabilizes the responses of soil organic carbon decomposition to warming in the subsoil on the Tibetan Plateau

Soil microbes are directly involved in soil organic carbon (SOC) decomposition, yet the importance of microbial biodiversity in regulating the temperature sensitivity of SOC decomposition remains elusive, particularly in alpine regions where climate change is predicted to strongly affect SOC dynamics and ecosystem stability. Here we collected topsoil and subsoil samples along an elevational gradient on the southeastern Tibetan Plateau to explore the temperature sensitivity (Q10 ) of SOC decomposition in relation to changes in microbial communities. Specifically, we tested whether the decomposition of SOC would be more sensitive to warming when microbial diversity is low. The estimated Q10 value ranged from 1.28 to 1.68, and 1.80 to 2.10 in the topsoil and subsoil, respectively. The highest Q10 value was observed at the lowest altitude of forests in the topsoil, and at the highest altitude of alpine meadow in the subsoil. Variations in Q10 were closely related to changes in microbial properties. In the topsoil the ratio of gram-positive to gram-negative bacteria (G+:G-) was the predominant factor associated with the altitudinal variations in Q10 . In the subsoil, SOC decomposition showed more resilience to warming when the diversity of soil bacteria (both whole community and major groups) and fungi was higher. Our results partly support the positive biodiversity-ecosystem stability hypothesis. Structural equation modeling further indicates that variations in Q10 in the subsoil were directly related to changes in microbial diversity and community composition, which were affected by soil pH. Collectively our results provide compelling evidence that microbial biodiversity plays an important role in stabilizing SOC decomposition in the subsoil of alpine montane ecosystems. Conservation of belowground biodiversity is therefore of great importance in maintaining the stability of ecosystem processes under climate change in high-elevation regions of the Tibetan Plateau.

Does soil organic carbon quality or quantity govern relative temperature sensitivity in soil aggregates?

Soil aggregates govern soil organic carbon (SOC) sequestration. But, sparse understanding about the process leads to inaccuracy in predicting potential of soil to stabilize C in warming world. We appraised effects of 43 years of fertilization on relative temperature sensitivity of SOC decomposition (Q10) in soil aggregates to know whether SOC quality or quantity governs Q10. Treatments were: fallow, control, 100% recommended dose of nitrogen (N), N and phosphorus (NP), N, P and potassium (NPK), and NPK + farmyard manure (FYM) (NPK + FYM). Macroaggregates, microaggregates and silt + clay (s + c) fractions were incubated for 16 weeks at 25, 35 and 45 °C, SOC quality (R0) and Q10 were computed. SOC mineralization from macro- and micro- aggregates were 34 and 28% higher than s + c across the treatments. The s + c fraction of NPK + FYM had ~ 41, 40 and 24% higher C decay rate than NPK plots at 25, 35 and 45 °C, respectively. For s + c fraction Q10 increased over other aggregates. Mean Q10 of s + c fraction was ~ 18.3 and 17.5% higher than macro and micro-aggregate-C, respectively. R0 was the lowest for NPK + FYM, suggesting long-term manuring with balanced NPK significantly enhance recalcitrance of C. We observed Q10 of macroaggregates and s + c fraction is controlled by C quality but C quantity governs Q10 of microaggregates in Vertisol. Specifically, microaggregates of NPK + FYM were more temperature sensitive, and could be vulnerable to C loss. Hence, practices facilitating microaggregate formation should be avoided. Thus, we recommend manure application for facilitating C sequestration.

Land use affects temperature sensitivity of soil organic carbon decomposition in macroaggregates but not in bulk soils in subtropical Oxisols of Queensland, Australia

The turnover of soil organic carbon (SOC) under different land uses plays an important role in terrestrial carbon (C) cycling. However, the effects of warming on C decomposition dynamics have not been resolved due to seemingly inconsistent results among different experiments. Hence, the objective of this work was to assess soil aggregate-associated C and temperature sensitivity of SOC decomposition within bulk soils and aggregates under two contrasting land management systems in subtropical southern Queensland.We examined the temperature sensitivity of SOC decomposition (Q10) and activation energy required for soil C mineralization (Ea) from bulk soils (<8 mm), large macroaggregates (2−8 mm) and small macroaggregates (0.25−2 mm), as well as key C and N cycling enzyme activities in the surface (0−10 cm) soils of an Oxisol. The two land uses studied were: undisturbed rainforest and 115-year-old kikuyu grass (Pennisetum clandestinum) pasture. Bulk soils as well as their large and small macroaggregates were separately incubated at two temperatures (17 °C and 27 °C) for 124 d. We observed that the pasture soils had more small macroaggregates but less microaggregates than the forest soils. However, all fractions in the forest contained significantly higher SOC than those in the pasture. Potential soil enzyme activities per unit SOC within bulk soils and macroaggregates for N-acetyl β-glucosaminidase, α-glucosidase, and phenol oxidase were higher in the pasture than in the forest. The cumulative C mineralization values of bulk soils under the forest vegetation were 16 % higher than for pasture at 17 °C and 97 % higher than pasture at 27 °C. Cumulative C mineralization from large macroaggregates of both land uses were more than that from small macroaggregates. However, the Ea and Q10 values of bulk soils were similar for both the forest and pasture soils. For the large macroaggregates, the Ea value was ∼78 % higher for the pasture soil than for the forest soil, with the Q10 value also being 73 % higher in the pasture soil. However, the opposite was observed for the small macroaggregates, with Ea and Q10 values being higher in the forest soil. The contrasting nature of Q10 from large and small macroaggregates in Oxisols suggests that the feedback from SOC decomposition in response to changing temperature, that is, global warming is closely associated with soil aggregation.

Latitudinal pattern of soil lignin/cellulose content and the activity of their degrading enzymes across a temperate forest ecosystem

Temperate mixed forests, along with other high latitudinal ecosystems, are more vulnerable to global warming in comparison with warm sites, because of the slower carbon (C) turnover and higher soil organic carbon (SOC) accumulation. Lignin and cellulose are two major components of plant litter, and usually make contributions to the recalcitrant and labile SOC pool, respectively. Because the chemical composition of SOC plays key role in regulating the bioavailability of soil C pool, understanding the relationship between soil lignin or cellulose content and temperature are of great significance in evaluating the feedbacks between SOC pool and the future scenarios of global warming. The biological degradation of soil lignin or cellulose is mainly dependent on soil enzymatic activities, and thus, the response of ligninolytic and cellulolytic enzymes to increased temperature would determine C release under future warming scenarios. However, the responses of the soil lignin/cellulose content and the activity of cellulolytic and ligninolytic enzymes to increased mean annual temperature (MAT) have rarely been studied, and the factors driving these changes are not fully understood. Latitudinal gradients are often used for monitoring global-warming-related problems, because of its natural gradients of temperature. In this study, we demonstrate the latitudinal pattern of lignin/cellulose content and the activities of cellulose- and lignin- degrading enzymes in a temperate Broad-leaved Korean pine mixed forests distributed along a latitudinal gradient (with MAT ranging from −1.9 to 5.1 °C) in northeastern China. The linear mixed model revealed that soil lignin content was negatively correlated with MAT (Slope = −7.604, t = −2.608, P = 0.011), whereas soil cellulose content showed no response to increased MAT. The activity of soil polyphenol oxidase (PPO), one of the enzymes catalyze lignin decomposition, was higher in high-latitude sites, in contrast, the activity of the cellulase (CEL) complex was higher in low-latitude plots. Structural equation model (SEM) analysis indicates that MAT can directly influence soil lignin or cellulose content, and indirectly through changing NRCB, plant litter C/N, microbial biomass, and degrading enzymatic activities. The value of soil lignin/(lignin + cellulose) ratio and soil lignocellulose index (LCI, lignin/(lignin + holocellulose) ratio), varied between 0.8–0.9, and 0.6–0.8, respectively, indicating that the SOC pool in this temperate ecosystem is dominated by recalcitrant components. The negative correlations between MAT and LCI, soil lignin/(lignin + cellulose), and log (PPO + PER)/log(CEL) (Slope = −0.008, t = −2.363, P = 0.021; Slope = −0.004, t = −3.134, P = 0.003, and Slope = −0.057, t = −4.477, P < 0.001, respectively) suggested that the recalcitrance of SOC would decrease with elevated MAT. Thus, we propose the content and the proportion of recalcitrant carbon in soil organic matter will decrease under the projected global warming, and thus, the temperature sensitivity of SOC decomposition will accordingly to be predicted to decline. This may have consequences on SOC stability in this temperate forest ecosystem.

Long-term organic amendment reduces the temperature sensitivity of organic carbon decomposition in an upland soil of subtropical China

The temperature sensitivity of soil organic carbon (SOC) decomposition determines the feedback of soil carbon (C) pool to climate warming. In the present study, soil samples were collected from a long-term fertilization experiment (since 1986) in a double-corn cropping system. Laboratory soil incubations were conducted to investigate the effect of long-term fertilization on the temperature sensitivity of SOC. Results showed that compared to the initial level, long-term corn cropping without fertilization (control) led to a decline in SOC, while inorganic N, P, and K fertilizer application (NPK) maintained it. Organic amendment combined with inorganic NPK (NPKM) significantly increased SOC relative to the NPK treatment. Warming and organic amendment significantly promoted CO2 release. The temperature sensitivity (Q10) of soil CO2 emissions was significantly lower in the NPKM than in the NPK, while no significant differences in Q10 values were found between the control and either the NPKM or the NPK. Therefore, organic amendment could promote SOC sequestration, and significantly reduce the temperature sensitivity of SOC decomposition in the present subtropical upland soil.

Depth dependence of soil carbon temperature sensitivity across Tibetan permafrost regions

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

Temperature sensitivity of soil organic carbon decomposition increased with mean carbon residence time: Field incubation and data assimilation.

Temperature sensitivity of soil organic carbon (SOC) decomposition is one of the major uncertainties in predicting climate-carbon (C) cycle feedback. Results from previous studies are highly contradictory with old soil C decomposition being more, similarly, or less sensitive to temperature than decomposition of young fractions. The contradictory results are partly from difficulties in distinguishing old from young SOC and their changes over time in the experiments with or without isotopic techniques. In this study, we have conducted a long-term field incubation experiment with deep soil collars (0-70cm in depth, 10cm in diameter of PVC tubes) for excluding root C input to examine apparent temperature sensitivity of SOC decomposition under ambient and warming treatments from 2002 to 2008. The data from the experiment were infused into a multi-pool soil C model to estimate intrinsic temperature sensitivity of SOC decomposition and C residence times of three SOC fractions (i.e., active, slow, and passive) using a data assimilation (DA) technique. As active SOC with the short C residence time was progressively depleted in the deep soil collars under both ambient and warming treatments, the residences times of the whole SOC became longer over time. Concomitantly, the estimated apparent and intrinsic temperature sensitivity of SOC decomposition also became gradually higher over time as more than 50% of active SOC was depleted. Thus, the temperature sensitivity of soil C decomposition in deep soil collars was positively correlated with the mean C residence times. However, the regression slope of the temperature sensitivity against the residence time was lower under the warming treatment than under ambient temperature, indicating that other processes also regulated temperature sensitivity of SOC decomposition. These results indicate that old SOC decomposition is more sensitive to temperature than young components, making the old C more vulnerable to future warmer climate.

Dynamics of soil organic carbon mineralization in tea plantations converted from farmland at Western Sichuan, China.

Climate warming and land use change are some of the drivers affecting soil organic carbon (SOC) dynamics. The Grain for Green Project, local natural resources, and geographical conditions have resulted in farmland conversion into tea plantations in the hilly region of Western Sichuan. However, the effect of such land conversion on SOC mineralization remains unknown. In order to understand the temperature sensitivity of SOC decomposition in tea plantations converted from farmland, this study considered the different years (i.e., 2–3, 9–10, and 16–17 years) of tea plantations converted from farmland as the study site, and soil was incubated for 28 days at 15°C, 25°C, and 35°C to measure the soil respiration rate, amount, and temperature coefficient (Q10). Temperature and land use type interactively affected the SOC mineralization rate, and the cumulative amount of SOC mineralization in all the plots was the largest at 35°C. SOC mineralization was greater and more sensitive to temperature changes in the farmland than in the tea plantations. Compared with the control, tea plantation soils showed lower SOC mineralization rate and cumulative mineralization amount. The 16–17-year-old tea plantation with a low SOC mineralization amount and high SOC content revealed the benefits of carbon sequestration enhancement obtained by converting farmland into tea plantations. The first-order kinetic equation described SOC mineralization dynamics well. Farmland conversion into tea plantations appeared to reduce the potentially mineralizable carbon pool, and the age of tea plantations also had an effect on the SOC mineralization and sequestration. The relatively weak SOC mineralization temperature sensitivity of the tea plantation soils suggested that the SOC pool of the tea plantation soils was less vulnerable to warming than that of the control soils.

The temperature sensitivity of soil organic carbon decomposition is greater in subsoil than in topsoil during laboratory incubation

The turnover of soil organic carbon (SOC) in cropland plays an important role in terrestrial carbon cycling, but little is known about the temperature sensitivity (Q10) of SOC decomposition below the topsoil layer of arable soil. Here, samples of topsoil (0–20 cm) and subsoil (20–40 cm) layers were obtained from paddy fields and upland croplands in two regions of China. Using a sequential temperature changing method, soil respiration rates were calculated at different temperatures (8 °C to 28 °C) and fitted to an exponential equation to estimate Q10 values. The average SOC decomposition rate was 59% to 282% higher in the topsoil than in the subsoil layer because of higher labile carbon levels in the topsoil. However, Q10 values in the topsoil layer (5.29 ± 1.47) were significantly lower than those in the subsoil layer (7.52 ± 1.84). The pattern of Q10 values between the topsoil and subsoil was significantly negative to labile carbon content, which is consistent with the carbon quality-temperature hypothesis. These results suggest that the high temperature sensitivity of SOC decomposition in the subsoil layer needs to be considered in soil C models to better predict the responses of agricultural SOC pools to global warming.

Decomposition of soil organic carbon influenced by soil temperature and moisture in Andisol and Inceptisol paddy soils in a cold temperate region of Japan

Understanding the effects of temperature and moisture on soil organic carbon (SOC) dynamics is crucial to predict the cycling of C in terrestrial ecosystems under a changing climate. For single rice cropping system, there are two contrasting phases of SOC decomposition in rice paddy soils: mineralization under aerobic conditions during the off-rice season and fermentation under anaerobic conditions during the growth season. This study aimed to investigate the effects of soil temperature and moisture on SOC decomposition under the aerobic and subsequently anaerobic conditions. Two Japanese paddy soils (Andisol and Inceptisol) were firstly incubated under four temperatures (±5, 5, 15, and 25°C) and two moisture levels (60 and 100% water-filled pore space (WFPS)) under aerobic conditions for 24 weeks. Then, these samples were incubated for 4 weeks at 30°C and under anaerobic conditions. Carbon dioxide (CO2) and methane (CH4) productions were measured during the two incubation stages to monitor the SOC decomposition dynamics. The temperature sensitivity of SOC was estimated by calculation of the Q10 parameter. The total CO2 production after the 24-week aerobic incubation was significantly higher in both soils for increasing soil temperature and moisture (P < 0.01). During the subsequent anaerobic incubation, total decomposed C (sum of CO2 and CH4 productions) was significantly lower in samples that had been aerobically incubated at higher temperatures (15 and 25°C). Moreover, CH4 production was extremely low in all soil samples. Total decomposed C after the two incubation stages ranged from 256.8 to 1146.1 mg C kg−1 in the Andisol and from 301.3 to 668.8 mg C kg−1 in the Inceptisol. However, the ratios of total decomposed C to SOC ranged from 0.29 to 1.29% in the Andisol and from 2.21 to 4.91% in the Inceptisol. Both aerobic and anaerobic decompositions of SOC in two paddy soils were significantly affected by soil temperature and moisture. Maintaining optimal soil temperature and medium moisture during the off-rice season might be an appropriate agricultural management to mitigate CH4 emission in the following rice growth season. Although it is high in SOC content, Andisol has less biodegradable components compared to Inceptisol and this could be a probable reason for the distinct difference in temperature sensitivity of SOC decomposition between two paddy soils.

Comparing microbial carbon sequestration and priming in the subsoil versus topsoil of a Qinghai-Tibetan alpine grassland

Subsoils of alpine grasslands on the Qinghai-Tibetan Plateau represent a tremendous yet poorly investigated reservoir of soil organic carbon (SOC) on a global “hotspot” of warming. Compared with the temperature sensitivity of SOC decomposition, microbial anabolism of new carbon and priming of native SOC remain poorly constrained under warming-enhanced labile carbon input in these subsoils. Here we employed an innovative approach to investigate the sequestration of freshly added carbon in microbial necromass versus SOC priming in the top- (0–10 cm) and subsoils (30–40 cm) from a field experiment that simulated varied warming scenarios in an alpine grassland on the Qinghai-Tibetan Plateau. The 13C composition of microbial necromass-derived amino sugars was analyzed in tandem with respired CO2 and dissolved SOC components (including dissolved lignin) in an 86-day laboratory incubation with 13C-labeled glucose. A higher fraction of freshly added carbon was respired while a smaller proportion was stabilized as amino sugars in the subsoil relative to the topsoil, leading to a much lower microbial carbon accumulation efficiency at depth. Meanwhile, a higher relative priming effect was observed in the subsoil (47± 14%) compared to the topsoil (14± 4%), suggesting a higher vulnerability to substrate-induced SOC loss at depth, although such changes may be associated with higher glucose addition rate (relative to SOC) in the subsoil. Furthermore, enhanced winter warming significantly reduced degradable SOC (assessed by SOC mineralization and dissolved lignin content) in the subsoil and potentially intensified nitrogen limitation under labile carbon additions, which further decreased microbial carbon accumulation (in the form of amino sugars) in the subsoil without affecting the topsoil. These results collectively indicate a limited microbial carbon sequestration potential and a higher vulnerability to warming-induced substrate changes in the subsoil of this alpine grassland, which warrants better understanding to predict soil carbon responses to climate warming on the Qinghai-Tibetan Plateau.

Temperature sensitivity of soil organic carbon decomposition as affected by long-term fertilization under a soybean based cropping system in a sub-tropical Alfisol

Understanding temperature sensitivity of soil organic carbon (SOC) decomposition from bulk soils and aggregates of long-term fertilized plots is imperative to forecast soil C dynamics. We evaluated the impacts of 43 years of fertilization under a soybean (Glycine max) based cropping system on temperature sensitivity of SOC decomposition (Q10) in an Alfisol. Treatments were: no mineral fertilizer or manure (control), 100% recommended dose of nitrogen (N), N and phosphorus (NP), N, P and potassium (NPK), NPK+lime at 0.4Mgha−1 (NPK+L), 150% recommended NPK (150% NPK), and NPK+farmyard manure (FYM) at 10Mgha−1 (NPK+FYM). Bulk soils as well as macro- and micro-aggregates were incubated for 24days at 25°C and 35°C. Cumulative SOC mineralization (Ct) in the 0–15cm soil layer of bulk soils with NPK+FYM and NPK treated plots were similar but significantly higher than unfertilized control plots. However, both Ct and Q10 values in the NPK+FYM plots were higher than NPK in the 15–30cm soil layer. In the 0–15cm soil layer, NPK+FYM plots had ∼10 and 26% greater Q10 values of macro- and microaggregates than NPK. Activation energies required for bulk soils C mineralization was ∼2 and 3 times higher in NPK+FYM and NPK+L plots, respectively, compared with unfertilized control plots in that layer. Lime along with NPK application increased the activation energy of SOC decomposition. Thus, long-term NPK+FYM and NPK+L applications have great potential for less proportional SOC decomposition than NPK or unfertilized control plots under a temperature rise in these acid soils. However, NPK+FYM management practice is recommended as it has highest SOC accumulation and can have less SOC losses under a temperature rise.

Labile substrate availability controls temperature sensitivity of organic carbon decomposition at different soil depths

The decomposition of soil organic carbon (SOC) is intrinsically sensitive to temperature. However, the degree to which the temperature sensitivity of SOC decomposition (as often measured in Q10 value) varies with soil depth and labile substrate availability remain unclear. This study explores (1) how the Q10 of SOC decomposition changes with increasing soil depth, and (2) how increasing labile substrate availability affects the Q10 at different soil depths. We measured soil CO2 production at four temperatures (6, 14, 22 and 30 °C) using an infrared CO2 analyzer. Treatments included four soil depths (0–20, 20–40, 40–60 and 60–80 cm), four sites (farm, redwood forest, ungrazed and grazed grassland), and two levels of labile substrate availability (ambient and saturated by adding glucose solution). We found that Q10 values at ambient substrate availability decreased with increasing soil depth, from 2.0–2.4 in 0–20 cm to 1.5–1.8 in 60–80 cm. Moreover, saturated labile substrate availability led to higher Q10 in most soil layers, and the increase in Q10 due to labile substrate addition was larger in subsurface soils (20–80 cm) than in surface soils (0–20 cm). Further analysis showed that microbial biomass carbon (MBC) and SOC best explained the variation in Q10 at ambient substrate availability across ecosystems and depths (R2 = 0.37, P < 0.001), and MBC best explained the variation in the change of Q10 between control and glucose addition treatment (R2 = 0.14, P = 0.003). Overall, these results indicate that labile substrate limitation of the temperature sensitivity of SOC decomposition, as previously shown in surface soils, is even stronger for subsoils. Understanding processes controlling the labile substrate availability (e.g., with rising atmospheric CO2 concentration and land use change) should advance our prediction of the fate of subsoil SOC in a warmer world.

Temperature sensitivity of carbon dioxide production in aggregates and their responses to nitrogen addition in a freshwater marsh, Sanjiang Plain

Nitrogen (N) availability may affect the decomposition of soil organic carbon (SOC) in a wetland experiencing global warming. The distribution of SOC within the soil matrix may be an important factor determining its turnover. However, the temperature sensitivity of SOC decomposition in aggregate and its response to N addition are crucial in the context of global warming but to date have been less obvious. Thus, an incubation experiment was conducted to investigate these problems in a freshwater marsh of Northeast China. The results showed that the rates of SOC decomposition in macro-aggregate were lower than that in micro-aggregate at high temperature (15°C), whereas macro-aggregate was the main source of carbon fluxes due to the largest mass contribution to bulk soil. Increasing temperature had a positive effect, while N addition produced neutral and negative effects on SOC decomposition. The micro-aggregate had the largest temperature sensitivity () within a temperature range from 5 to 15°C. A low level of N input had no effect on value in macro- and micro-aggregate as well as bulk soil, whereas a high level of N input significantly decreased the temperature sensitivity in 2–1 mm, < 0.25 mm aggregates and bulk soil. Our results suggest that SOC decomposition in micro-aggregate may be more sensitive to increasing temperatures, whereas this positive feedback to global warming may be weakened by enhanced N deposition in the near future.

Herbivores regulate the sensitivity of soil organic carbon decomposition to warming

There is considerable interest in how the sensitivity of soil organic carbon (SOC) decomposition to warming is determined by the degree of recalcitrance (i.e., quality) of SOC. Although herbivores have widespread effects on the quality of plant litter, from which SOC is derived, herbivores are seldom considered in studies that examine the response of SOC degradation and greenhouse gas emissions to warming. In this study, we addressed the question: do herds of wild ungulates affect the temperature sensitivity of SOC decomposition in grasslands of Yellowstone National Park (YNP)? We examined the effects of ungulates on temperature sensitivity by comparing microbial respiration at different temperatures in incubated soils collected inside and outside long-term ungulate exclosures. Exclosure sites included grasslands on hilltops, slopes, and at the base of slopes that varied considerably in soil properties. Herbivores reduced the temperature sensitivity of SOC decomposition uniformly among sites by 20% (P = 0.002). The herbivore—induced decline in temperature sensitivity was probably governed by YNP grazers increasing the quality of grassland SOC. Results from this study suggest that herbivores regulate how global warming modifies grassland SOC decomposition, stability, and CO2 emissions.

Reduced substrate supply limits the temperature response of soil organic carbon decomposition

Controls on the decomposition rate of soil organic carbon (SOC), especially the more stable fraction of SOC, remain poorly understood, with implications for confidence in efforts to model terrestrial C balance under future climate. We investigated the role of substrate supply in the temperature sensitivity of SOC decomposition in laboratory incubations of coarse-textured North American soils sampled from paired native pine and hardwood forests located across a 20 °C gradient in mean annual temperature (MAT). In this study we show that for this wide range of forest soils, the supply of labile substrate, controlled through extended incubation and glucose additions, exerts a strong influence on the magnitude of SOC decomposition response to warming. When substrate supply was high, either in non-depleted soils or in soils first depleted of labile C through extended incubation but then amended with glucose, SOC decomposition rates responded to increased temperature with a mean Q10 of 2.5. In contrast, for the depleted soils with no substrate added, SOC responded to varying temperature with a mean Q10 of 1.4. Our laboratory study shows for upland forest soils that substrate supply can play a strong role in determining the temperature response of decomposing SOC. Previous studies have described the effect of substrate availability of temperature responses on soil respiration, but few have described the effect on decomposition of more stable SOC. Because substrate supply is likely to vary strongly – both spatially and temporally, these findings have important implications for SOC processing in natural systems.