Forest Soil Organic Carbon Research Articles

Factors affecting soil organic carbon in a Phyllostachys edulis forest

Phyllostachys edulis plays an important role in maintaining carbon cycling. We examined the effects of soil properties on organic carbon content in a P. edulis forest on Dagang Mountain, Jiangxi Province, China. Based on correlation and stepwise multiple regression analyses, the effects of seven soil factors on organic carbon and their sensitivities to change were studied using path and sensitivity analyses. The results revealed differences in the interconnections and intensities of soil factors on organic carbon. Soil porosity, field capacity, and ammonium nitrogen levels were the main factors affecting organic carbon in the ecosystem. Soil porosity had a strong direct effect on organic carbon content and a strong indirect effect through field capacity. Field capacity and ammonium nitrogen levels mainly affected organic carbon directly. Field capacity, soil porosity, and ammonium nitrogen content, as well as bulk density, β-glucosidase activity, and invertase activity, were sensitive factors. Polyphenol oxidase activity was insensitive. Our study provides a theoretical basis for understanding the effects of soil factors on organic carbon, which can be utilised to improve P. edulis forest management strategies and promote carbon sequestration capacities.

Varieties of active soil organic carbon of four forest types with varying incubation temperatures in Fengyang Mountain.

As an important part of the active soil organic carbon (SOC) pool, microbial biomass carbon (MBC) is the impetus and key to nutrient and energy cycling in the soil ecosystem. To provide important scientific references for forest soil carbon sequestration and greenhouse gas emission in this region, the warming effect on forest soil MBC and dissolved organic carbon (DOC) were determined using an incubation experiment. The content dynamics of MBC and DOC of an evergreen broadleaf forest, a Cunninghamia lanceolata forest, a mixed coniferous and broadleaf forest, and a Cryptomeria fortunei forest in the National Nature Reserve of Fengyang Mountain were studied using an incubation experiment at 10℃, 20℃, and 30℃. Results showed that SOC and DOC in the Cryptomeria fortunei forest were significantly higher (P 20℃ > 10℃; whereas, DOC was 10℃ > 20℃ > 30℃. For the same temperature, MBC and DOC in the evergreen broadleaf forest as well as the mixed coniferous and broadleaf forest declined larger on the whole than in the C. lanceolata forest and the C. fortunei forest after the 56 days of incubation experiment.

Prediction of spatial soil organic carbon distribution using Sentinel-2A and field inventory data in Sariska Tiger Reserve

Dynamic and vigorous top soil is the source for healthy flora, fauna, and humans, and soil organic matters are the underpinning for healthy and productive soils. Organic components in the soil play significant role in stimulating soil productivity processes and vegetation development. This article deals with the scientific demand for estimating soil organic carbon (SOC) in forest using geospatial techniques. We assessed distribution of SOC using field and satellite data in Sariska Tiger Reserve located in the Aravalli Hill Range, India. This study utilized the visible and near-infrared reflectance data of Sentinel-2A satellite. Three predictor variables namely Normalized Difference Vegetation Index, Soil Adjusted Vegetation Index, and Renormalized Difference Vegetation Index were derived to examine the relationship between soil and SOC and to identify the biophysical characteristic of soil. Relationship between SOC (ground and predicted) and leaf area index (LAI) measured through satellite data was examined through regression analysis. Coefficient of correlation (R 2) was found to be 0.95 (p value < 0.05) for predicted SOC and satellite measured LAI. Thus, LAI can effectively be used for extracting SOC using remote sensing data. Soil organic carbon stock map generated through Kriging model for Landsat 8 OLI data demonstrated variation in spatial SOC stocks distribution. The model with 89% accuracy has proved to be an effective tool for predicting spatial distribution of SOC stocks in the study area. Thus, optical remote sensing data have immense potential for predicting SOC at larger scale.

Impact of land use conversion on soil organic carbon stocks in an agro-pastoral ecotone of Inner Mongolia

Soil organic carbon (SOC) stocks in terrestrial ecosystems vary considerably with land use types. Grassland, forest, and cropland coexist in the agro-pastoral ecotone of Inner Mongolia, China. Using SOC data compiled from literature and field investigations, this study compared SOC stocks and their vertical distributions among three types of ecosystems. The results indicate that grassland had the largest SOC stock, which was 1.5- and 1.8-folds more than stocks in forest and cropland, respectively. Relative to the stock in 0–100 cm depth, grassland held more than 40% of its SOC stock in the upper 20 cm soil layer; forest and cropland both held over 30% of their respective SOC stocks in the upper 20 cm soil layer. SOC stocks in grazed grasslands were remarkably promoted after ≥20 years of grazing exclusion. Conservational cultivation substantially increased the SOC stocks in cropland, especially in the 0–40 cm depth. Stand ages, tree species, and forest types did not have obvious impacts on forest SOC stocks in the study area likely due to the younger stand ages. Our study implies that soil carbon loss should be taken into account during the implementation of ecological projects, such as reclamation and afforestation, in the arid and semi-arid regions of China.

Effect of altitude and aspect on soil organic carbon and nitrogen stocks in the Himalayan Mawer Forest Range

The soil organic carbon (SOC) and nitrogen (N) stocks in mountainous forests are influenced by the forest diversity, topographic features, and climate change impacts. Role of forest SOC and N stocks in global C cycle has been a subject of great research recently, but the effect of topographic features on their dynamics at the stand level has received less attention especially under temperate conditions. In order to find out how topographic aspect and altitude affect SOC and N budgets, a study was conducted in the Himalayan Mawer Forest Range. We examined SOC and N stocks at two altitude zones (Z1: 1800–2200masl & Z2: 2200–2500masl) under North (N) and South (S) aspects at three soil depths (D1: 0–20cm, D2: 20–40cm and D3: 40–60cm). The SOC stock was found to be decreasing with altitude from 105.9Mgha−1 to 78.3Mgha−1 under N aspect and from 81.6Mgha−1 to 74.0Mgha−1 under S aspect. SOC stock was higher by 16.5% under N aspect as compared to S aspect. The results lead to the conclusion that altitude has a negative effect on SOC stabilization and therefore altitude and aspect effect may be included in SOC stock estimation equations.

13C NMR spectroscopy characterization of particle-size fractionated soil organic carbon in subalpine forest and grassland ecosystems

BackgroundSoil organic carbon (SOC) and carbon (C) functional groups in different particle-size fractions are important indicators of microbial activity and soil decomposition stages under wildfire disturbances. This research investigated a natural Tsuga forest and a nearby fire-induced grassland along a sampling transect in Central Taiwan with the aim to better understand the effect of forest wildfires on the change of SOC in different soil particle scales. Soil samples were separated into six particle sizes and SOC was characterized by solid-state 13C nuclear magnetic resonance spectroscopy in each fraction.ResultsThe SOC content was higher in forest than grassland soil in the particle-size fraction samples. The O-alkyl-C content (carbohydrate-derived structures) was higher in the grassland than the forest soils, but the alkyl-C content (recalcitrant substances) was higher in forest than grassland soils, for a higher humification degree (alkyl-C/O-alkyl-C ratio) in forest soils for all the soil particle-size fractions.ConclusionsHigh humification degree was found in forest soils. The similar aromaticity between forest and grassland soils might be attributed to the fire-induced aromatic-C content in the grassland that offsets the original difference between the forest and grassland. High alkyl-C content and humification degree and low C/N ratios in the fine particle-size fractions implied that undecomposed recalcitrant substances tended to accumulate in the fine fractions of soils.

Seasonal variation of soil carbon and nitrogen under five typical Pinus massoniana forests

ABSTRACTFive different typical Pinus massoniana forests were sampled in the Guizhou Province to evaluate the effects of seasons on soil organic carbon (SOC), total nitrogen (STN), and inorganic nitrogen (SIN) in these forests. More seasonal variation occurred in the topsoil than in lower layers. The SOC and STN contents varied the least amongst the soil layers, but the SIN contents had the largest values and ranges during autumn. Carbon-to-nitrogen ratio (C/N) exhibited neither a vertical change nor a seasonal trend. C/N was either maximal or minimal depending upon the sites during autumn, indicating that ecological process during summer soils would strongly change this. More gravel content resulted in higher litter stock, SOC, and STN level in low-productivity forests. A low phosphorus level might result in low SOC and STN contents in clay-rich soils. Low litter stock and clay content will result in low SOC and STN levels in coniferous and broad-leaved mixed forests in contrast to pure forests. The SOC and soil N contents in P. massoniana forests are apparently affected by different sampling seasons, particularly in topsoil. This should be taken into account when evaluating C and N contents and their respective storages in other forest types.

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.

Toward inventory-based estimates of soil organic carbon in forests of the United States.

Soil organic carbon (SOC) is the largest terrestrial carbon (C) sink on Earth; this pool plays a critical role in ecosystem processes and climate change. Given the cost and time required to measure SOC, and particularly changes in SOC, many signatory nations to the United Nations Framework Convention on Climate Change report estimates of SOC stocks and stock changes using default values from the Intergovernmental Panel on Climate Change or country-specific models. In the United States, SOC in forests is monitored by the national forest inventory (NFI) conducted by the Forest Inventory and Analysis (FIA) program within the U.S. Department of Agriculture, Forest Service. The FIA program has been consistently measuring soil attributes as part of the NFI since 2001 and has amassed an extensive inventory of SOC in forest land in the conterminous United States and southeast and southcentral coastal Alaska. That said, the FIA program has been using country-specific predictions of SOC based, in part, upon a model using SOC estimates from the State Soil Geographic (STATSGO) database compiled by the Natural Resources Conservation Service. Estimates obtained from the STATSGO database are averages over large map units and are not expected to provide accurate estimates for specific locations, e.g., NFI plots. To improve the accuracy of SOC estimates in U.S. forests, NFI SOC observations were used for the first time to predict SOC density to a depth of 100cm for all forested NFI plots. Incorporating soil-forming factors along with observations of SOC into a new estimation framework resulted in a 75% (48±0.78Mg/ha) increase in SOC densities nationally. This substantially increases the contribution of the SOC pool, from approximately 44% (17Pg) of the total forest ecosystem C stocks to 56% (28Pg), in the forest C budget of the United States.

Soil carbon stock assessment in the tropical dry deciduous forest of the Sathanur reserve forest of Eastern Ghats, India

ABSTRACTClimate change and carbon mitigation through forest ecosystems are some of the important topics in global perspective. Tropical dry forests are one of the most widely distributed ecosystems in tropics, which remain neglected in research. The soil organic carbon (SOC) stock was quantified on a large scale (30 1-ha plots) in the dry deciduous forest of the Sathanur reserve forest of Eastern Ghats. The SOC stock ranged from 16.92 to 44.65 Mg/ha with a mean value of 28.26 ± 1.35 Mg/ha. SOC exhibited a negative trend with an increase in soil depth. A significant positive correlation was obtained between SOC stocks and vegetation characteristics viz. tree density, shrub basal area, and herb species richness, while a significant negative correlation was observed with bulk density. The variation in SOC stock among the plots obtained in the present study could be due to differences in tree abundance, herb species richness, shrub basal area, soil pH, soil bulk density, soil texture etc. The present study generates a large-scale baseline data of dry deciduous forest SOC stock, which would facilitate SOC stock assessment at the national level as well as to understand its contribution on a global scale.

Legacy of medieval ridge and furrow cultivation on soil organic carbon distribution and stocks in forests

Land management history can influence soil organic carbon (SOC) stocks over centuries. In this study, the impact of medieval ridge and furrow cropland management on SOC in forests was assessed. Continuous clockwise ploughing in rectangular fields moved topsoil from the outer part of strip-shaped fields towards the centre, thus forming a corrugated microtopography with peripheral furrows and central ridges. This tillage technique led to the burial of former topsoil under the ridges.The effect of this human-created microtopography and the centuries old topsoil burial on forest SOC spatial distribution and stocks was investigated. Five sites with ridge and furrow field strips under deciduous forests on soils of differing texture in Germany were sampled, with three orthogonal transects of the field strips and a defined reference position where neither net soil removal nor accumulation occurred. Reforestation took place between the 17th and 19th century.At 0 to 10cm depth, average SOC content was 28±3gkg−1 at ridges, 37±3gkg−1 at reference positions and 47±5gkg−1 at furrows. SOC stocks were 7±5% lower at ridges and 8±4% higher at furrows than at reference positions. Enhanced C input at furrows through leaf litter accumulation was indicated by higher SOC content in the free light fraction at furrows (10±5gkg−1) than at ridges (6±3gkg−1), higher specific SOC mineralisation (37±4μgCO2-Cg−1 SOC at furrows and 31±3μgCO2-Cg−1 SOC at ridges) and wider C/N ratio at furrows (18±1) compared with ridges (17±1).Buried topsoil under ridges (20 to 33–52cm depth) did not contain significantly less SOC than corresponding samples at furrows and reference positions. However, SOC content was 0.4 to 0.9gkg−1 higher at ridges than at reference positions, indicating long-term preservation of former topsoil SOC by burial under ridges, although enhanced SOC stocks at ridges due to carbon burial could not be significantly confirmed for all sites. It can be concluded that preserved microtopography over centuries and ancient topsoil burial, a legacy of medieval ridge and furrow cultivation, still influences forest SOC spatial distribution and stocks.

Spatial variability of organic carbon and total nitrogen in the soils of a subalpine forested catchment at Mt. Taiyue, China

Soil organic carbon (SOC) and total nitrogen (TN) are critical indicators of soil quality and play a pivotal role in key biogeochemical process (i.e. soil carbon and nutrient cycling). Many studies have assessed soil organic matter and nutrients individually in different ecosystems. However, appropriate sampling density for accurate estimation of the spatial distribution of SOC in subalpine forests and quantification of the relative importance of influencing factors remains uncertain. In this study, a combination of conventional analytical and geostatistical methods was used to analyze the spatial variability and patterns of SOC and TN along a soil profile. A total of 444 soil samples, taken from three layers down to 60cm, were collected from the Jieshigou catchment area (5.64km2) of Mount Taiyue in northern China. Results show that a large spatial variability of SOC and TN appears in upper 40cm along an elevation and vegetation gradient, while strong spatial autocorrelation is present below 40cm (40-60cm). Range and degree of spatial autocorrelation for SOC were slightly larger than those of TN; All the same, both showed clustered spatial distribution. A distribution map of Kriging revealed that both SOC and TN concentrations in the Jieshigou catchment area decreased from west to east along the direction of the valley, which coincides with the overlay of topographic features. More than 40% of the variance in SOC and TN contents could be explained by topographical indices and normalized difference vegetation index (NDVI). SOC and TN significantly increased (from 23.6 to 56.8gkg−1) with age of larch plantations in the surface layer. Our results suggest that a stratified random sampling was proved a sufficiently reliable way for estimating the spatial distribution of SOC and TN.

Vertical distribution and persistence of soil organic carbon in fire-adapted longleaf pine forests

Longleaf pine (Pinus palustris Miller) forests in the southern United States are being restored and actively managed for a variety of goals including: forest products, biodiversity, C sequestration and forest resilience in the face of repeated disturbances from hurricanes and climate change. Managed southern pine forests can be sinks for atmospheric CO2 in forest biomass; however, the persistence of biomass in the environment or in forest products is limited, thus making soil C the primary long-term pool. Little is known about the size of extant soil C pools, residence time of soil C or the role that frequent burning plays in C stabilization in longleaf pine ecosystems. We sampled soil from a chronosequence of longleaf pine stands ranging in age from 5 to 87years to quantify the vertical distribution of soil organic carbon (SOC) stocks; both oxidizable (SOCOX) and oxidation resistant (SOCR) fractions, pyrogenic carbon (PyC) and the mean residence time (MRT) of SOC and its associated fractions. SOC stocks (0–1m) ranged from 44.1 to 98.1 (x¯=77.0) Mg C ha−1, and no effect of stand age or biomass accumulation on SOC stocks was detected. Soil C accumulation was associated with elevated clay and extractable Fe contents. While SOC concentration declined with soil depth, the proportion of SOCR in SOC increased with depth. PyC was a minor component of soil C, representing 5–7% of SOC and the proportion was not depth dependent. The MRT of SOC was hundreds of years near the surface and many thousands of years at depth. Though SOCR was less abundant than SOCOX, SOCR MRT was an order of magnitude greater than SOCOX MRT and had a strong influence on bulk SOC MRT. The majority of the PyC was in the less persistent SOCox and not associated with long-term C storage in soil. Despite the flow of C from biomass in the form of decay products, litter fall, root turnover and pulses of PyC, these soils preserve little of recent inputs, which may be rapidly oxidized, lost to the atmosphere from periodic fires or, in the case of PyC, may be transported out of the system via erosion. Our results indicate that these soils were not strong sinks for atmospheric CO2, especially when compared to C accumulation in biomass.

Heterogeneity in decomposition rates and annual litter inputs within fine-root architecture of tree species: Implications for forest soil carbon accumulation

Fine roots (⩽2mm in diameter) of tree species, traditionally regarded as homogenous entities, are expected to be major contributors of forest soil organic carbon (C), but their contributions need to be reevaluated given the heterogeneity in many root characteristics within fine-root architecture. Here, literature data were synthesized to analyze whether there existed heterogeneity of decomposition rates within fine-root architecture of tree species and quantify the relative contributions of roots of each branching order to forest soil C storage by combining variations in annual litter inputs among fine-root branching hierarchies. Results showed the presence of heterogeneity in decomposition rates within fine-root architecture, and this pattern was influenced by classification methods (root order or diameter class). Specifically, the heterogeneity in decomposition rates was shown only when fine roots were classified by root order, but not when fine roots were classified by diameter class. The slower decomposition rates of lower-order roots (the first two orders) were related to their higher acid-unhydrolyzable residues. Furthermore, lower-order roots were found to account for 57–71% of annual root litter inputs of the first five orders. Given the slow decomposition rates and high annual litter inputs of lower-order roots of tree species, our findings suggest that lower-order roots should be important contributors to forest soil organic C.

庐山不同森林植被下土壤活性有机碳的季节变化

研究庐山森林土壤有机碳库动态变化，无论对预测和维护森林生态系统长期生产力，还是为全球气候变化研究提供基础数据方面，均具有重要意义。以庐山6种森林植被下土壤为研究对象，对不同森林植被下土壤各种活性有机碳的季节变化及相互关系进行分析。结果表明：1) 常绿阔叶林下ROC、WSOC、MBC含量均为最高，但其相应组分占TOC的比重较小，说明各组分有机碳的含量与植被类型有着密切关系。2) ROC的季节变化幅度较小，WSOC、MBC的季节变化较为复杂；总体上ROC与MBC含量表现为夏季高而冬季低，WSOC则完全相反。 Studying forest soil organic carbon dynamic changes in Mt.Lushan has a significant meaning for forecasting and maintaining long-term productivity of forest ecosystems. It also provides basic data for the global climate change. In this paper, we take six kinds of soil under different forest vegetations in Mt.Lushan as samples to study changes of active soil organic carbon under various forest vegetations in different seasons and relationships between these soil organic carbons. The results showed that: evergreen broad-leaved forest has the highest levels in ROC, WSOC, MBC, but a smaller proportion of TOC comparing to other forest vegetations, which indicates that organic carbon has a close relationship with vegetation types. ROC’s seasonal change is not very big in extent. The seasonal variation in WSOC and MBC is more complicated. In a word, the content of ROC and MBC is high in warm seasons and low in cold seasons while WSOC is exactly the opposite.

Microbial community composition regulates SOC decomposition response to forest conversion in a Chinese temperate forest

AbstractForest conversion influences soil organic carbon (SOC) decomposition through cascading effects on forest structure, soil properties, and soil microbial communities. However, interactive effects of these drivers and the key pathways that mediate forest SOC decomposition remain relatively unexplored. In this study, we compared relative importance of variables describing forest structure, soil properties, and soil microbial community on affecting SOC decomposition response to the conversion of a broadleaved Korean pine mixed forest into three other forests in the Changbai Mountains of China. We quantified SOC decomposition rate of these four forest types by measuring incubation soil respiration (SR). We then employed univariate regressions to quantify effect size of individual factor on SOC decomposition rate. A structural equation model (SEM) was developed to analyze pathways, relative importance, and interactive effects of these factors on SR. Our results showed strong marginal effects of dissolved organic carbon (DOC) content, fungal Phospholipid fatty acids (PLFAs) to bacterial PLFAs ratio (F/B), broadleaved to conifer ratio (B/C), and total PLFAs content (TPC) on SR. Measured SOC decomposition rate was most closely related to F/B, which was in turn influenced primarily by soil C/N ratio and fraction of non‐oxidized carbon (NOC%). Our study identified “Aboveground forest composition → SOC chemistry → Soil microbial composition → SOC decomposition” as the key pathway by which forest conversion affected SOC decomposition. This research work highlights the critical role of soil microbial community composition in altering SOC decomposition response to forest conversion.

Coupled dynamics of iron and iron-bound organic carbon in forest soils during anaerobic reduction

The behavior of iron (Fe)-bound organic carbon (OC) under anoxic conditions in natural soils and sediments represents a critical knowledge gap for understanding the biogeochemical cycles of OC and Fe. In this study, we investigated the dynamics of Fe and OC in four forest soils in the presence of the dissimilatory Fe-reducing bacterium, Shewanella oneidensis MR-1. Over an 8-day reduction period, 3.8–9.9% of total OC was released to solution in conjunction with the reduction of 12.5–37.7% of reactive Fe. The fraction of OC released was correlated with the fraction of Fe reduced, indicating that the reductive release was the controlling factor for the mobilization of OC upon the anaerobic microbial reaction. During the reduction, the fractions of poorly crystalline Fe oxides decreased, coupling with an increase in the relative abundance of crystalline Fe oxides. Lability of OC (as reflected by water-extractable OC content) increased after microbial reduction, indicating the decreased stability of OC because of changes in mineral-OC interactions and the conformation of mineral-OC complexes. The reduction of Fe was closely related to bulk soil electron accepting capacity (0.15–0.34mmole−/mol C). Our findings demonstrate that the redox reactions of Fe, modified by the redox reactivity of OC, play an important role in regulating the stability and transformation of OC.

Spatial variations in soil organic carbon, nitrogen and phosphorus concentrations related to stand characteristics in subtropical areas

Our study aimed to determine whether, and to what extent, stand characteristics and topography affected spatial variations in soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) concentrations in subtropical forests. Soil samples were taken from a Choerospondias axillaris deciduous broadleaved forest and a Lithocarpus glaber–Cyclobalanopsis glauca evergreen broadleaved forest. Spatial variations in SOC, TN and TP concentrations and the factors affecting them were investigated using geostatistical analysis and stepwise linear regression, respectively. The L. glaber–C. glauca forest exhibited higher coefficients of variation (CVs) of SOC (35 %) and TN (34 %) concentrations than the C. axillaris forest (27 % for SOC and 21 % for TN), but the CV of TP concentration in the L. glaber–C. glauca forest (17 %) was lower than that in the C. axillaris forest (24 %). Stand characteristics contributed the most to spatial variations in SOC and TP, while soil texture made the greatest contribution to variations in TN. Topography contributed the least to variations in SOC, TN and TP. Stand characteristics, together with topography and soil texture, contributed to spatial variations in SOC, TN and TP concentrations. The contributions of stand characteristics differed in SOC, TN and TP due to their different cycling characteristics.

Impact of hydrologically driven hillslope erosion and landslide occurrence on soil organic carbon dynamics in tropical watersheds

AbstractThe dynamics of soil organic carbon (SOC) in tropical forests play an important role in the global carbon (C) cycle. Past attempts to quantify the net C exchange with the atmosphere in regional and global budgets do not systematically account for dynamic feedbacks among linked hydrological, geomorphological, and biogeochemical processes, which control the fate of SOC. Here we quantify effects of geomorphic perturbations on SOC oxidation and accumulation in two adjacent wet tropical forest watersheds underlain by contrasting lithology (volcaniclastic rock and quartz diorite) in the Luquillo Critical Zone Observatory. This study uses the spatially explicit and physically based model of SOC dynamics tRIBS‐ECO (Triangulated Irregular Network‐based Real‐time Integrated Basin Simulator‐Erosion and Carbon Oxidation) and measurements of SOC profiles and oxidation rates. Our results suggest that hillslope erosion at the two watersheds may drive C sequestration or CO2 release to the atmosphere, depending on the forest type and land use. The net erosion‐induced C exchange with the atmosphere was controlled by the spatial distribution of forest types. The two watersheds were characterized by significant erosion and dynamic replacement of upland SOC stocks. Results suggest that the landscape underlain by volcaniclastic rock has reached a state close to geomorphic equilibrium, and the landscape underlain by quartz diorite is characterized by greater rates of denudation. These findings highlight the importance of the spatially explicit and physical representation of C erosion driven by local variation in lithological and geomorphological characteristics and in forest cover.

Sources of errors and uncertainties in the assessment of forest soil carbon stocks at different scales-review and recommendations.

Spatially explicit knowledge of recent and past soil organic carbon (SOC) stocks in forests will improve our understanding of the effect of human- and non-human-induced changes on forest C fluxes. For SOC accounting, a minimum detectable difference must be defined in order to adequately determine temporal changes and spatial differences in SOC. This requires sufficiently detailed data to predict SOC stocks at appropriate scales within the required accuracy so that only significant changes are accounted for. When designing sampling campaigns, taking into account factors influencing SOC spatial and temporal distribution (such as soil type, topography, climate and vegetation) are needed to optimise sampling depths and numbers of samples, thereby ensuring that samples accurately reflect the distribution of SOC at a site. Furthermore, the appropriate scales related to the research question need to be defined: profile, plot, forests, catchment, national or wider. Scaling up SOC stocks from point sample to landscape unit is challenging, and thus requires reliable baseline data. Knowledge of the associated uncertainties related to SOC measures at each particular scale and how to reduce them is crucial for assessing SOC stocks with the highest possible accuracy at each scale. This review identifies where potential sources of errors and uncertainties related to forest SOC stock estimation occur at five different scales-sample, profile, plot, landscape/regional and European. Recommendations are also provided on how to reduce forest SOC uncertainties and increase efficiency of SOC assessment at each scale.