Soil Organic Carbon Turnover Time Research Articles

Minor Effects of Canopy and Understory Nitrogen Addition on Soil Organic Carbon Turnover Time in Moso Bamboo Forests

Increased atmospheric nitrogen (N) deposition has greatly influenced soil organic carbon (SOC) dynamics. Currently, the response of SOC to atmospheric N deposition is generally detected through understory N addition, while canopy processes have been largely ignored. In the present study, canopy N addition (CN) and understory N addition (UN, 50 and 100 kg N ha−1 year−1) were performed in a Moso bamboo forest to compare whether CN and UN addition have consistent effects on SOC and SOC turnover times (τsoil: defined as the ratio of SOC stock and soil heterotrophic respiration) with a local NHx:NOy ratio of 2.08:1. The experimental results showed that after five years, the SOC content of canopy water addition without N addition (CN0) was 82.9 g C kg−1, while it was 79.3, 70.7, 79.5 and 74.5 g C kg−1 for CN50, CN100, UN50 and UN100, respectively, and no significant difference was found for the SOC content between CN and UN. Five-year N addition did not significantly change τsoil, which was 34.5 ± 7.4 (mean ± standard error) for CN0, and it was 24.9 ± 4.8, 22.4 ± 4.9, 30.5 ± 4.0 and 22.1 ± 6.5 years for CN0, CN50, CN100, UN50 and UN100, respectively. Partial least squares structural equation modeling explained 93% of the variance in τsoil, and the results showed that soil enzyme activity was the most important positive factor controlling τsoil. These findings contradicted the previous assumption that UN may overestimate the impacts of N deposition on SOC. Our findings were mainly related to the high N deposition background in the study area, the special forest type of Moso bamboo and the short duration of the experiment. Therefore, our study had significant implications for modeling SOC dynamics to N deposition for high N deposition areas.

Radiocarbon evidence of organic carbon turnover response to grassland grazing: A soil aggregate fraction perspective

Grassland grazing, driven human activities, represents a prevalent form of land use conversion. While numerous studies have examined the impact of such conversions on soil carbon cycling, they primarily focus on the content and storage of soil organic carbon (SOC). However, research on the turnover dynamics of SOC and the underlying mechanisms triggered by land use conversions remains relatively scarce. In this study, radiocarbon (14C) tracing technology was applied to investigate the effects of grassland grazing on SOC turnover in the Saihanba area of Hebei province, China. The results revealed that the turnover time of SOC in pasture grassland was shortened by approximately 250 years compared to meadow grassland, suggesting that grazing diminishes the ability of topsoil to stabilize SOC. Furthermore, our findings indicate that grazing leads to a decrease in soil CO2 flux by 0.50 g C m−2 y−1 under aggregates larger than 250 μm and those between 63 and 250 μm. Conversely, the CO2 flux under aggregates, specifically those between 2 and 63 μm and less than 2 μm, increased by 0.96 g C m−2 y−1. This shift suggests a significant increase in the contribution of older SOC pools to the overall soil CO2 flux. Our study provides novel insights into SOC cycling in the context of grassland grazing, highlighting the importance of understanding SOC turnover dynamics for effective land management.

The real and apparent priming effects following litter incorporation as modeled under different moisture regimes

The addition of litter (maize straw) to soil accelerates activity of microorganisms and significantly changes the mineralization of soil organic carbon (SOC), which are referred to as “apparent” and “real” priming effects (PEs). In this study, the mechanisms of PEs under different moisture conditions were explored using a simple mechanistic model that was dependent on the size of the SOC pool (%C), microbial biomass, and microbial activity (the proportion of actively growing microbial biomass relative to the total soil biomass). In an incubation experiment, soil cores were incubated with 13C-labeled maize straw (δ13C = 600.9 ± 2.53‰) under two moisture regimes: constant moisture (60% of water holding capacity throughout incubation) and drying and rewetting treatments (7-day long drying and rewetting cycles in a 76-day laboratory incubation). Microcosms were sampled after 1, 7, 10, 15, 21, 60, and 76 days, and cumulative soil respiration, microbial biomass and cumulative PEs were measured during the incubation period. The model was parameterized with native SOC and 13C-labeled litter carbon (C) data and simulated microbial metabolic processes, microbial turnover, soil organic matter (SOM) dynamics, and apparent and real PEs. The model contained two special features. The first feature was the assumption of sequential utilization of native SOC and litter-derived C with distinctly different stability and properties, where the organic carbon (OC) must first be solubilized into dissolved organic carbon (DOC) before it can be consumed by microorganisms. The second feature was the first attempt to evaluate the continuous turnover of native SOC, native SOM-derived and litter-derived microbial biomass C under varying moisture conditions. The optimized model explained 93% of the observed cumulative priming effect (CPE) under constant moisture and 94% of the CPE under drying and rewetting treatments, respectively. The simulation results showed that the conversion of native SOC and litter C to DOC was rate-limited, and positive apparent PE dominated the priming process. Although more C was allocated to respiration rather than growth, the compensation by a longer turnover time of microbial C eventually led to a higher sequestration of soil C under the drying and rewetting treatment compared to those under constant moisture. The turnover time of native SOC was significantly affected by the turnover of native-SOM derived and litter-derived microbial biomass C, and the turnover of litter-derived microorganisms may play a key role in regulating real PE. This study demonstrates that the combination of mathematical modeling and 13C-labeled litter varying in solubility and recalcitrance can effectively distinguish C sources and elucidate the mechanisms of PEs in soils under different moisture regimes.

Soil carbon stocks in stable tropical landforms are dominated by geochemical controls and not by land use.

Soil organic carbon (SOC) dynamics depend on soil properties derived from the geoclimatic conditions under which soils develop and are in many cases modified by land conversion. However, SOC stabilization and the responses of SOC to land use change are not well constrained in deeply weathered tropical soils, which are dominated by less reactive minerals than those in temperate regions. Along a gradient of geochemically distinct soil parent materials, we investigated differences in SOC stocks and SOC (Δ14 C) turnover time across soil profile depth between montane tropical forest and cropland situated on flat, non-erosive plateau landforms. We show that SOC stocks and soil Δ14 C patterns do not differ significantly with land use, but that differences in SOC can be explained by the physicochemical properties of soils. More specifically, labile organo-mineral associations in combination with exchangeable base cations were identified as the dominating controls over soil C stocks and turnover. We argue that due to their long weathering history, the investigated tropical soils do not provide enough reactive minerals for the stabilization of C input in either high input (tropical forest) or low-input (cropland) systems. Since these soils exceeded their maximum potential for the mineral related stabilization of SOC, potential positive effects of reforestation on tropical SOC storage are most likely limited to minor differences in topsoil without major impacts on subsoil C stocks. Hence, in deeply weathered soils, increasing C inputs may lead to the accumulation of a larger readily available SOC pool, but does not contribute to long-term SOC stabilization.

The Climate Control of Soil Organic Carbon Dynamics Inferred From Speleothem Radiocarbon Ages

AbstractThe complexity of processes affecting soil organic carbon (SOC) turnover on spatio‐temporal scales often hinders the extrapolation of results from specific sites to larger scales. This study presents Holocene speleothem U‐Th ages paired with 14C ages of carbonate and dissolved organic carbon (DOC) through three caves located on a north‐south transect through China. The deviations of speleothem 14CDOC ages from the U‐Th ages show clearly spatial variability, and they are positively correlated with mean ages of modern SOC and soil turnover time, suggesting that deviations can be used to infer the SOC turnover. We further demonstrate that slow SOC turnover (large deviation) was associated with weak monsoon (low temperature/less precipitation) on temporal scales. Our findings reveal that climate dominates the speleothem 14CDOC ages and SOC turnover. As global warming likely will intensify, the accelerated turnover of SOC, particularly at higher latitude areas, may partially offset the existing soil carbon stock.

Soil Property, Rather than Climate, Controls Subsoil Carbon Turnover Time in Forest Ecosystems across China

Subsoil (0.2–1 m) organic carbon (C) accounts for the majority of soil organic carbon (SOC), and SOC turnover time (τ, year) is an important index of soil C stability and sequestration capacity. However, the estimation of subsoil τ and the identification of its dominant environmental factors at a regional scale is lacking in regards to forest ecosystems. Therefore, we compiled a dataset with 630 observations to investigate subsoil τ and its influencing factors in forest ecosystems across China using the structural equation model (SEM). The results showed a large variability of subsoil τ from 2.3 to 896.2 years, with a mean (± standard deviation) subsoil τ of 72.4 ± 68.6 years; however, the results of one-way analysis of variance (ANOVA) showed that subsoil τ differed significantly with forest types (p = 0.01), with the slowest subsoil τ obtained in deciduous-broadleaf forests (82.9 ± 68.7 years), followed by evergreen-needleleaf forests (77.6 ± 60.8 years), deciduous-needleleaf forests (75.3 ± 78.6 years), and needleleaf and broadleaf mixed forests (71.3 ± 80.9 years), while the fastest subsoil τ appeared in evergreen-broadleaf forests (59.9 ± 40.7 years). Subsoil τ negatively correlated with the mean annul temperature, occurring about three years faster with a one degree increase in temperature, indicating a faster subsoil SOC turnover under a warming climate. Subsoil τ significantly and positively correlated with microbial activities (indicated by microbial C and nitrogen), highlighting the importance of microbial communities in regulating subsoil C dynamics. Climate, forest types, forest origins, vegetation, and soil variables explained 37% of the variations in subsoil τ, as indicated by the SEM, and the soil property was the most important factor affecting subsoil τ. This finding challenged previous perception that climate was the most important factor driving subsoil C dynamics, and that dominant drivers varied according to climate zones. Therefore, recognizing different dominant factors in predicting subsoil C dynamics across climate zones would improve our understanding and reduce the uncertainties regarding subsoil C dynamics in biogeochemical models under ongoing climate change.

Effect of grazing exclusion on emission of greenhouse gases and soil organic carbon turnover in alpine shrub meadow

Grazing exclusion (GE) is a management option used widely to restore degraded grassland and improve grassland ecosystems. However, the impacts of GE on soil properties and greenhouse gas emissions of alpine shrub meadow are still unclear, especially long-term GE of more than ten years. To fill part of this gap, we examined the effects of long-term GE of alpine shrub meadow on soil nutrients, soil properties, greenhouse gas emissions (CO2 and CH4) and soil organic carbon (SOC) turnover. When compared to grazed grassland (GG), long-term GE resulted in: 1) greater SOC, nitrogen (N), and phosphorous (P) content, especially in the 20–30 cm soil layer; 2) greater soil C:N, C:P and N:P ratios in the 20–30 cm depth; 3) greater soil CO2, but lesser CH4 emission during the growing season; and 4) much faster SOC turnover time (0–30 cm). GE of more than ten years can increase grassland C reserves and improve the C sequestration capacity of the ecosystem. Results from this study can have important implications in developing future grassland management policies on soil nutrient balances, restoration of degraded grassland and controlling shrub expansion.

Subsoil organic carbon turnover is dominantly controlled by soil properties in grasslands across China

Soil organic carbon turnover time (τ, year) is an important indicator of soil carbon stability and a major factor determining soil carbon sequestration capacity. Many previous studies have investigated τ in the topsoil layers, but little is known about subsoil τ and its environmental drivers. We estimated subsoil τ (the ratio of subsoil organic carbon (OC) stock to net primary production) using field observations from the published literatures and employed regression analysis and the structural equation model (SEM) to identify the effects of climate, soil, and vegetation variables on subsoil τ in China’s grasslands. The results showed that the average subsoil τ over 0.2–1 m varied greatly from 5.5 to 702.2 years, with a mean (±standard deviation) of 118.5 ± 97.8 years. Subsoil τ varied significantly among different grassland types, with 164.0 ± 112.0 years for alpine meadow, 107.0 ± 47.9 years for alpine steppe, 177.0 ± 143.0 years for temperate desert steppe, 96.6 ± 88.7 years for temperate meadow steppe, and 101.0 ± 75.9 years for temperate typical steppe. Subsoil τ was significantly negatively correlated (p < 0.05) with normalized difference vegetation index, leaf area index, and gross primary production. Mean annual (air) temperature and precipitation had a negative impact on τ, indicating a faster turnover of soil carbon with a changing climate. Based on SEM, soil properties mainly drove subsoil τ, challenging our current understanding and providing evidence that different factors control topsoil and subsoil τ. In this sense, different environmental factors should be considered to reliably predict soil carbon dynamics at the top of 1 m and subsoils in biogeochemical models or Earth system models at regional or global scales.

Climate and geochemistry interactions at different altitudes influence soil organic carbon turnover times in alpine grasslands

Soil organic carbon turnover time (τ) is a key component of the carbon cycle. However, the impact of geochemical and climatic conditions on τ remains poorly understood in high alpine regions. We investigated climatic variables [e.g. mean annual precipitation (MAP) and mean annual temperature (MAT)], as well as geochemical variables [e.g. soil total element content (Mn, Ti, Fe, Si, Al, Mg, Ca, Na, K, and P), soil clay content, and soil pH] and estimated τ in the 0–30 cm soil layer at 169 alpine grassland sites along a 3000 km-long transect on the Tibetan Plateau. We found that τ ranged from 4 to 289 years on the Tibetan Plateau and showed a decreasing trend from the northwest to southeast and an increasing trend with altitude. The estimated τ was 717863 years (mean with 95% confidence interval) in alpine meadows, which did not significantly differ from alpine steppes (768765 yr). Overall, using boosted regression tree analysis, geochemistry was the most important controlling factor for τ (54% of the relative effect on τ), followed by climate (36%), and altitude (10%). When examining the relative contribution of individual variables, we found that MAP was the primary predictor of τ, followed by soil Si content, and altitude. Notably, variation in τ was explained by precipitation rather than temperature. Altitude indirectly affected τ by regulating climatic and soil geochemical conditions. The direct negative effect of climate on τ was opposite the positive indirect effect of climate on τ via soil geochemistry. These results highlight the importance of considering the interactions of climatic and geochemical factors as well as hydrological conditions when predicting how carbon turnover in the soils of semi-arid alpine grasslands will respond to future climate change.

Change in soil organic carbon and its climate drivers over the Tibetan Plateau in CMIP5 earth system models

Soil organic carbon (SOC) is the largest carbon pool in the terrestrial carbon cycle, which is closely linked to climate change and global warming feedbacks. Based on proxy observation data and outputs from the fifth phase of the Coupled Model Inter-comparison Project (CMIP5), we analyzed quantitatively the spatial distribution and temporal change for SOC using boosted regression trees (BRT) over the Tibetan Plateau (TP). The ensemble proxy observation SOC stock data indicated a decreasing spatial distribution from southeast to northwest over the TP. We used data from ten CMIP5 earth system models (ESMs) for SOC, which exhibited differences. However, the CMIP5 multi-model ensemble (MME) result presented a similar spatial distribution pattern to the ensemble proxy observation SOC, while SOC storage and turnover times from the CMIP5 MME model were less than the ensemble proxy observation. The BRT results indicated that air temperature (Ta) accounted for 44.81% of the relative contribution and was the most influential variable on MME SOC. This was followed by net primary production (NPP), with a 19.09% relative contribution. The relative influence of the top 10 cm soil temperature, total soil water, and precipitation on MME SOC was 13.55%, 12.68%, and 9.87%, respectively. Using the BRT method to determine the spatial distribution of relative contribution of SOC for these five variables demonstrated that Ta was mainly higher over the central and northwestern regions of the TP, and NPP was higher over the western central regions and along the plateau’s eastern edge. The statistical frequency of maximum relative contribution to SOC for the five variables indicated that the relative contribution of NPP covered the largest area, with 32% of the total grid numbers, followed by Ta, with 25%.

Estimation of Permafrost SOC Stock and Turnover Time Using a Land Surface Model With Vertical Heterogeneity of Permafrost Soils

AbstractWe developed vertically resolved soil biogeochemistry (carbon and nitrogen) module and implemented it into a land surface model, ISAM. The model captures the vertical heterogeneity of the northern high latitudes permafrost soil organic carbon (SOC). We also implemented Δ14C to estimate SOC turnover time, a critical determinant of SOC stocks, sequestration potential, and the carbon cycle feedback under changing atmospheric CO2 concentration [CO2] and climate. ISAM accounted for the vertical movement of SOC caused by cryoturbation and its linkage to frost heaving process, oxygen availability, organo‐mineral interaction, and depth‐dependent environmental modifiers. After evaluating the model processes using the site and regional level heterotrophic respiration, SOC stocks, and soil Δ14C profiles, the vertically resolved soil biogeochemistry version of the model (ISAM‐1D) estimated permafrost SOC turnover time of 1,443 years, which is about 3 times more than the estimation based on the without vertically resolved version of ISAM (ISAM‐0D). ISAM‐1D‐simulated SOC stocks for permafrost regions was 319 Pg C in the top 1 m soil depth by the 2000s, about 80% higher than the estimates based on ISAM‐0D. ISAM‐1D SOC stock and turnover time were compared well with the observations. However, the longer SOC turnover time preserves less SOC stocks due to the lower carbon use efficiency (CUE) for SOC than ISAM‐0D and thus respires more SOC than being transferred downward by cryoturbation. ISAM‐1D simulated reduced SOC sequestration (3.7 Pg C) compared to ISAM‐0D (4.8 Pg C) and published Earth system models (ESMs) over the 1860s–2000s, due to weaker [CO2]‐carbon cycle and stronger climate‐carbon cycle feedbacks, highlighting the importance of the vertically heterogeneous soil for understanding the permafrost SOC sinks.

Soil organic carbon turnover recovers faster than plant diversity in the grassland when high nitrogen addition is ceased: Derived from soil 14C evidences

Responses of terrestrial ecosystems to decreased nitrogen (N) deposition has brought considerable attention. Soil organic carbon (SOC) turnover is an important parameter in the terrestrial ecosystem model and ecosystem management. However, how SOC turnover responds to decreased N deposition has not yet been evaluated. Here we addressed this issue by the comparison of the SOC turnover in the treatment that receiving high N addition for 4 years and then none for 6 years (48 g N·m−2·yr−1, N48-cessation) with that derived from the control treatment (0 g N·m−2·yr−1, N0) in Inner Mongolia grassland, China. In this study, 14C isotope technology was used to directly reveal SOC turnover time. Our results showed that SOC turnover time in the N48-cessation treatment had no statistical difference compared with that in the N0 treatment after 6 years of N cessation, indicating that its SOC turnover had recovered from high N enrichment. The reduction of SOC storage (274 g/m2) in N48-cessation treatment from 2010 to 2014 also proved this recovery. Moreover, we also found that the species richness and Shannon diversity index in the N48-cessation treatment were significantly lower than those in the N0 treatment. It indicated that the recovery of SOC turnover was faster than plant diversity in the grassland when high N addition was ceased. This study suggested that the grasslands experienced high N deposition should reduce grazing and mowing in order to increase soil carbon input and offset the negative effect of the rapid recovery of SOC turnover on soil C storage when N deposition was decreased.

Global subsoil organic carbon turnover times dominantly controlled by soil properties rather than climate

Soil organic carbon (SOC) in the subsoil below 0.3 m accounts for the majority of total SOC and may be as sensitive to climate change as topsoil SOC. Here we map global SOC turnover times (τ) in the subsoil layer at 1 km resolution using observational databases. Global mean τ is estimated to be 1015_{729}^{1414} yr (mean with 95% confidence interval), and deserts and tundra show the shortest (146_{114}^{188} yr) and longest (3854_{2651}^{5622} yr) τ respectively. Across the globe, mean τ ranges from 9 (the 5% quantile) to 6332 years (the 95% quantile). Temperature is the most important factor negatively affecting τ, but the overall effect of climate (including temperature and precipitation) is secondary compared with the overall effect of assessed soil properties (e.g., soil texture and pH). The high-resolution mapping of τ and the quantification of its controls provide a benchmark for diagnosing subsoil SOC dynamics under climate change.

Changes in soil organic matter stability with depth in two alpine ecosystems on the Tibetan Plateau

Soil organic carbon (SOC) decomposition can potentially feedback to climate change. However, the biotic, abiotic and inherent factors controlling the stability of soil carbon, and changes in these factors with soil depth, remain poorly understood. In this study, we combined a number of complementary methods to quantify the biological, thermal, chemical, molecular and isotopic indices of soil organic matter (SOM) stability along the soil profile (0–70 cm) in two contrasting alpine ecosystems (meadow and shrubland) on the Tibetan Plateau. Firstly, we conducted an aerobic lab–incubation experiment on root–free, sieved soils. The number of days to respire 5% of initial SOC, a biological index of SOM stability, decreased with soil depth. Moreover, the temperature at which half of SOM mass loss (TG–T50), a thermal index of SOM stability, increased with soil depth. Additionally, hot–water extractable organic carbon (HWEOC) per gram SOC, a chemical index of SOM stability, showed weak (meadow) and little (shrubland) declining trend with depth. Further, we used Fourier–transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy to characterize the molecular composition of SOM. The index of recalcitrance of FTIR spectra and the combined index of aliphaticity and aromaticity of NMR spectra both increased with depth, suggesting that the molecular composition of SOM was more complex with increasing depth. Finally, the isotopic values of SOM (13C and 15N) and the 14C–based SOC turnover time both increased with depth, indicating that the isotopic indices of SOM stability also increased with depth. Overall, our results suggest that the thermal, chemical, molecular and isotopic indices of SOM stability were mutually correlated and all showed increasing trend with increasing soil depth in the two alpine ecosystems, although the biological index (as measured by aerobic incubation of root–free sieved soils) showed the opposite results.

Temperature and moisture are minor drivers of regional-scale soil organic carbon dynamics

Storing large amounts of organic carbon, soils are a key but uncertain component of the global carbon cycle, and accordingly, of Earth System Models (ESMs). Soil organic carbon (SOC) dynamics are regulated by a complex interplay of drivers. Climate, generally represented by temperature and moisture, is regarded as one of the fundamental controls. Here, we use 54 forest sites in Switzerland, systematically selected to span near-independent gradients in temperature and moisture, to disentangle the effects of climate, soil properties, and landform on SOC dynamics. We estimated two SOC turnover times, based on bulk soil 14C measurements (τ14C) and on a 6-month laboratory soil incubation (τi). In addition, upon incubation, we measured the 14C signature of the CO2 evolved and quantified the cumulated production of dissolved organic carbon (DOC). Our results demonstrate that τi and τ14C capture the dynamics of contrasting fractions of the SOC continuum. The 14C-based τ14C primarily reflects the dynamics of an older, stabilised pool, whereas the incubation-based τi mainly captures fresh readily available SOC. Mean site temperature did not raise as a critical driver of SOC dynamics, and site moisture was only significant for τi. However, soil pH emerged as a key control of both turnover times. The production of DOC was independent of τi and not driven by climate, but primarily by the content of clay and, secondarily by the slope of the site. At the regional scale, soil physicochemical properties and landform appear to override the effect of climate on SOC dynamics.

Soil organic carbon changes following degradation and conversion to cypress and tea plantations in a tropical mountain forest in Kenya

This study investigates, in a montane forest in Kenya, the changes in amount and stability of soil organic carbon (SOC) as a consequence of: a) forest degradation, by comparing primary and degraded forests; b) the replacement of degraded forests with cypress and tea plantations, by considering sites installed at different time in the past. The SOC concentrations and stocks were determined in different layers to 1 m depth, and the SOC turnover time (TT) derived by measuring the 14C concentration in the layers within the 0–30 cm depth. A significant SOC decline was evident in the 0–5 and 5–15 cm layers of degraded forest while, on the long term, both plantations induced a significant SOC increase in the 0–30 cm depth. The longer TT’s and lower SOC concentrations in the upper layers of degraded rather than primary forests imply an impact of forest degradation on the decomposition of the fast cycling SOC. Similarly, the shorter TT with increasing plantations age implies differences in SOC stabilization mechanisms between plantations and forests. Cypress and tea plantations established on degraded forests stimulate a long term SOC accrual but at the same time decrease the stability of the SOC pool.

Depth heterogeneity of soil organic carbon dynamics in a heavily grazed alpine meadow on the northeastern Tibetan Plateau: A radiocarbon-based approach

A deep understanding of soil organic carbon (SOC) dynamics on the Tibetan Plateau is crucial for predicting the response of the alpine carbon pool to future climate changes. Here we present information about SOC stocks and turnover time in a heavily grazed alpine meadow on the Tibetan Plateau based on radiocarbon (14C) measurements and inverse modeling. Our results reveal that the alpine meadow soil is composed of three distinct carbon pools. The topsoil represents the active carbon pool, which has a carbon inventory-weighted mean turnover time of 64 years. The CO2 efflux due to the mineral soil respiration from this pool is ca. 48.45 g C m-2 yr-1, accounting for 91 % of the total heterotrophic soil respiration. The intermediate carbon pool has an inventory-weighted mean turnover time of 780 years and the rate of mineral soil respiration in this pool is an order of magnitude lower than that in the active carbon pool. The parental materials feature the passive carbon pool, which has millennia-long turnover time and the mineral soil respiration is trivial. Comparing our results with other 14C-based studies suggests that grazing intensity may alter not only the SOC stocks but also the soil respiration rate in the alpine grassland ecosystem. The heavily grazed alpine meadow broadly fits the exponential relationship between turnover time and mean annual air temperature identified by meta-analysis of published data from the Tibetan Plateau, alpine grassland, and forest sites elsewhere.

Association with pedogenic iron and aluminum: effects on soil organic carbon storage and stability in four temperate forest soils

Soil organic carbon (SOC) can be stabilized via association with iron (Fe) and aluminum (Al) minerals. Fe and Al can be strong predictors of SOC storage and turnover in soils with relatively high extractable metals content and moderately acidic to circumneutral pH. Here we test whether pedogenic Fe and Al influence SOC content and turnover in soils with low Fe and Al content and acidic pH. In soils from four sites spanning three soil orders, we quantified the amount of Fe and Al in operationally-defined poorly crystalline and organically-complexed phases using selective chemical dissolution applied to the soil fraction containing mineral-associated carbon. We evaluated the correlations of Fe and Al concentrations, mean annual precipitation (MAP), mean annual temperature (MAT), and pH with SOC content and 14C-based turnover times. We found that poorly crystalline Fe and Al content predicted SOC turnover times (p < 0.0001) consistent with findings of previous studies, while organically-complexed Fe and Al content was a better predictor of SOC concentration (p < 0.0001). Greater site-level MAP (p < 0.0001) and colder site-level MAT (p < 0.0001) were correlated with longer SOC turnover times but were not correlated with SOC content. Our results suggest that poorly crystalline Fe and Al effectively slow the turnover of SOC in these acidic soils, even when their combined content in the soil is less than 2% by mass. However, in the strongly acidic Spodosol, organo-metal complexes tended to be less stable resulting in a more actively cycling mineral-associated SOC pool.

Urbanization Drives SOC Accumulation, Its Temperature Stability and Turnover in Forests, Northeastern China

Global urbanization is a vital process shaping terrestrial ecosystems but its effects on forest soil carbon (C) dynamics are still not well defined. To clarify the effects of urbanization on soil organic carbon (SOC) variation, 306 soil samples were collected and analyzed under two urban–rural gradients, defined according to human disturbance time and ring road development in Changchun, northeast China. Forest SOC showed a linear increase with increasing human disturbance time from year 1900 to 2014 (13.4 g C m−2 year−1), and a similar trend was found for the ring road gradient. Old-city regions had the longest SOC turnover time and it increased significantly with increasing urbanization (p = 0.011). Along both urban–rural gradients SOC stability toward temperature variation increased with increasing urbanization, meaning SOC stability in old-city regions was higher than in new regions. However, none of the urban–rural gradients showed marked changes in soil basal respiration rate. Both Pearson correlation and stepwise regression proved that these urbanization-induced SOC patterns were closely associated with landscape forest (LF) proportion and soil electrical conductivity (EC) changes in urban–rural gradients, but marginally related with tree size and compositional changes. Overall, Changchun urbanization-induced SOC accumulation was 60.6–98.08 thousand tons, accounting for 12.8–20.7% of the total forest C biomass sequestration. Thus, China’s rapid urbanization-induced SOC sequestration, stability and turnover time, should be fully estimated when evaluating terrestrial C balance.

Fallow associated with autumn-plough favors structure stability and storage of soil organic carbon compared to continuous maize cropping in Mollisols

Aggregate formation and stability of soil organic carbon (SOC) differ in different farming systems, probably due to differences in effects of tillage and residue management. This study used a 24-year field experiment to compare the effects of continuous maize cropping and natural fallow on aggregate formation and SOC storage in various aggregate-size classes and density fractions of a Chinese Mollisol. Soils collected from the upper 0.2-m layer were wet-sieved into four aggregate-size classes (>2, 0.25–2, 0.053–0.25 and 0.25 mm) and increased that of micro-aggregates (<0.25 mm) compared to the initial value while the opposite was observed in the natural fallow system. The fallow system generally had greater SOC concentration in the occluded fraction, higher proportion of newly-derived C as % total SOC in the light fraction and greater contribution of total residue C to new C in macro-aggregates and light fractions compared to the continuous maize system. Furthermore, the fallow system resulted in shorter turnover time of SOC than the continuous maize system. Natural fallow associated with autumn-plough improved soil structural stability and SOC storage while continuous maize cropping with residue removal decreased SOC sequestration and soil aggregate stability.