Soil Organic C Stocks Research Articles

Soil Organic Carbon Changes to Increasing Cropping Intensity and No‐Till in a Semiarid Climate

Core Ideas After 10‐yr, NT annual cropping systems resulted in greater soil organic C mass relative to tilled fallow‐wheat. Cropping system intensity was more important to SOC mass than tillage. Changes in SOC mass were directly related to NPP and total C inputs. Annual cropping with NT management resulted in greater SOC mass (0–30 cm) than F‐W systems. SOC accretion was greatest in the CRP system. Soil organic C (SOC) in the semiarid Northern Great Plains (NGP) can benefit from increasing cropping intensity. We evaluated the effect of annual cropping on stocks of SOC adjusted for equivalent mass (SOCEM; 0–30 cm) and the change in SOC adjusted for equivalent mass (ΔSOCEM) over 10‐yr at a field site (45°40′N, 111°09′ W) near Bozeman, MT. The experiment consisted of two fallow‐wheat (Triticum aestivum L.; F‐W) rotations under till and no‐till (NT), and five annual cropping systems under NT factored with two N levels (moderate and high), and alfalfa–perennial grass system (Conservation Reserve Program, CRP). After 10‐yr, we found SOCEM of five NT annual cropping ( = 37.4 Mg ha–1) was significantly (P < 0.01) greater than NT F‐W (35.1 Mg ha–1), and till F‐W (33.7 Mg ha–1). The greatest SOCEM was found in CRP (39.9 Mg ha–1). Accretion of SOCEM was observed in three systems with the largest gains occurring in the CRP (0.24 Mg ha–1 yr–1), followed by NT continuous wheat (0.13 Mg ha–1 yr–1) and NT pea (Pisum sativum L.)/oil seed–wheat (0.09 Mg ha–1 yr–1). Soil organic C loss was observed in all other systems with the largest loss in the till F‐W (–0.29 Mg ha–1 yr–1). Among the cropping systems, ∆SOCEM was directly related to net primary productivity (NPP; r2 = 0.73) and total C (TC; shoot + root + rhizodeposit) inputs (r2 = 0.86). We found SOCEM was maintained at 7.0 Mg ha–1 yr–1 of net primary productivity (NPP) and 2.6 Mg ha–1 yr–1 of TC inputs with accretion and loss occurring above and below these thresholds.

Dynamics of biomass and carbon sequestration across a chronosequence of masson pine plantations

AbstractThe changes of forest biomass stock and carbon (C) sequestration with stand ages at fixed intervals in the different vegetation components remain unknown. Using the masson pine (Pinus massoniana) relative growth equation, biomass carbon stocks were obtained in four masson pine plantations at 12 year intervals (3 years, 15 years, 27 years, and 39 years). Meanwhile, the changes in soil organic C (SOC) stock with stand ages were also estimated. The biomass stock varied from 1.41 to 265.33 Mg ha−1, 6.87 to 7.49 Mg ha−1, and 2.66 to 4.86 Mg ha−1 in the tree, shrub, and herb layers. Carbon concentrations in plant tissues were 51.6%, 39.0%, and 42.2% in the tree, shrub, and herb layers. The aboveground biomass C contributed 81.7% and 60.5% in the tree and shrub layers, and the root to shoot (R/S) ratio of the tree and shrub layer biomass averaged 0.23 and 0.69. Biomass C stock increased significantly (p < 0.05) with forest age, whereas the changes in biomass accumulation rate decreased significantly (p < 0.05). The annual net C sequestration increased with age from 0.47 to 9.83 Mg ha−1 yr−1 in the tree layer but decreased in the shrub and herb layers. The SOC content decreased with soil depth but increased with age, whereas the SOC stock increased with depth and age. However, the total ecosystem C stock increased significantly (p < 0.05) with stand age suggesting that age is the controlling factor of photosynthetic and biological processes and thus changes in biomass accumulation and C sequestration in masson pine plantations. Therefore, in‐depth studies are needed for continuous monitoring of the changes in nutrients and elements cycling with stand ages in this forest ecosystem.

SOM and Biomass C Stocks in Degraded and Undisturbed Andean and Coastal Nothofagus Forests of Southwestern South America

Grazing and over-exploitation can severely degrade soil in native forests. Considering that productivity in ecosystems is related to soil organic matter (SOM) content and quality, the objectives of this study were to: (1) determine the influence of degraded (DEF), partly-degraded (PDF), and undisturbed (UNF) Nothofagus forests on the stocks of carbon (C) in tree biomass and SOM; (2) evaluate fractions of SOM as indicators of sustainable management; and (3) use the Century model to determine the potential gains of soil organic C (SOC). The forests are located in the Andes and Coastal mountains of southern Chile. The SOM was fractionated to separate the light fraction (LF), macroaggregates (>212 µm), mesoaggregates (212–53 µm), and microaggregates (<53 µm). In two measurement periods, the SOC stocks at 0–20 cm and 20–40 cm depths in macroaggregates were on average 100% higher in the Andean UNF, and SOC was over twice as much at 20–40 cm depth in Andean DEF. Century simulations showed that improved silvopastoral management would gradually increase total SOC in degraded soils of both sites, especially the Ultisol with a 15% increase between 2016 and 2216 (vs. 7% in the Andisol). Greater SOC in macroaggregates (p < 0.05) of UNF indicate a condition of higher sustainability and better management over the years.

Does organic farming accumulate carbon in deeper soil profiles in the long term?

Organic farming systems provide the opportunity to deliver more soil ecosystem services than conventional practices. One such service could be soil organic C (SOC) accumulation, but recent debates suggest that this is unclear. Furthermore, organic farming potential to accumulate SOC for the entire soil profile is not well known. We quantified the cumulative SOC stocks, aggregate-associated SOC, and particulate organic matter (POM) concentrations for the 0–100cm depth of the soil profile in a comparative crop rotation experiment in eastern Nebraska after >20yr of management. We studied: 1) conventional farming, 2) conventional farming with diversified rotation, 3) organic rotation with alfalfa (Medicago sativa L.) as green manure, and 4) organic rotation receiving 37Mgha−1yr−1 cattle manure. All the treatments had been managed for 40yr except the organic rotation with green manure, which had been in place for 20yr. Organic farming increased SOC stock, aggregate-associated SOC, and POM concentrations but only in the 0–15cm soil depth. The SOC stock under organic cattle manure system was 19% greater than under conventional farming (33.1Mgha−1) and 13% greater than under conventional farming with diversified rotation (34.8Mgha−1). The SOC stock under organic rotation with green manure (36.6Mgha−1) was 10% greater than in conventional farming. Results suggest that organic cropping systems accumulated SOC at 0.16MgCha−1yr−1 with cattle manure and 0.18MgCha−1yr−1 with green manure. Growing alfalfa for only two years in the 4-yr organic rotation probably limited its potential to increase SOC for the whole soil profile. Overall, in the long term, organic farming increases SOC stock but only in the upper 15cm of the soil profile.

Effect of Microbial Carbon, Nitrogen, and Phosphorus Stoichiometry on Soil Carbon Fractions under a Black Locust Forest within the Central Loess Plateau of China

Core Ideas Soil C fractions were significantly correlated with microbial C/P and N/P ratios. Soil C fractions were extremely affected by microbial C/P and N/P ratios. Microbial C, N, and P stoichiometry have the potential to affect C storage. Microbial C/N/P stoichiometry ratios are of interest because of important ecosystem fluxes of C, N, and P, such as mineralization and immobilization, and they can help to characterize the cycling of soil elements, especially regarding soil C sequestration. However, it is not clear how soil microbial C/N/P stoichiometry influences C cycling. In this study, the concentrations of microbial biomass C, microbial biomass N, microbial biomass P, and soil C fractions were measured in 45‐, 40‐, and 25‐yr‐old black locust (Robinia pseudoacacia L.) forest soils and a sloped cropland. The results showed that soil organic C (SOC) concentrations or stocks and the percentage increment of the SOC stocks were highest under 45‐yr‐old black locust (RP45a). The concentrations of each C fraction in the black locust forest soils (45a, 40a, and 25a) were higher than that of sloped cropland in the 0‐ to 30‐cm soil profile. The concentrations of microbial C, N, and P in RP45a were also higher by an average of 29.6 to 232.9, 9.2 to 17.9, and 0.6 to 1.1 mg kg−1, respectively, compared with the same concentrations in sloped cropland at the 0‐ to 30‐cm soil depth. Redundancy analysis indicated that soil C fractions, especially for particulate organic C (53–2000 and >2000 μm), were significantly correlated with the microbial C/P and N/P ratios, and the “best” model selection indicated that the microbial N/P and C/P ratios significantly changed with soil C fractions with regard to stand age and soil depth in the black locust forests. Therefore, the present study demonstrated that C fractions respond to microbial C, N, and P stoichiometry after farmland‐to‐forest conversion and hence have the potential to affect C storage in the loess hilly region.

Differences in soil organic carbon pools and biological activity between organic and conventionally managed rice-wheat fields

Regular input of organic matter under organic system of agriculture may bring about quantitative and qualitative changes in soil organic matter (SOM) and could impact microbial and biochemical properties of soil. While the effect of organic amendments on quantitative changes in soil organic C (SOC) is fairly well-documented, the relationship between physical fractions of SOM and microbial and biochemical activity of soils is relatively less quantified. Using an on-farm approach, we studied the effect of organic and conventional systems of agriculture on soil microbial and enzyme activities and microbial community composition in order to establish their relationship with physical fractions of SOC in rice-wheat sequence. Organic system of management improved SOC stocks by 2.2 Mg ha−1 in the top 15-cm soil, and the accrued C occurred as coarse particulate (cPOC) and mineral-associated organic C (MinOC). Organic production systems were characterised by higher microbial biomass C (MBC), mineralisable C, enzyme activities and population of bacteria, fungi and actinomycetes in soil than the conventional system of management. A regression model, which included dehydrogenase, cellulase, xylanase, MBC and clay, could predict differences in SOC between organic and conventionally managed fields, indicating the need for using several indicators together for characterising the two systems of management. Organic agriculture favourably impacted microbial and C-cycle enzyme activities, which regulated organic matter decomposition and stabilisation in soil.

Soil organic carbon contents, aggregate stability, and humic acid composition in different alpine grasslands in Qinghai-Tibet Plateau

Alpine grassland soils on Qinghai-Tibet Plateau store approximately 33.5 Pg of organic carbon (C) at 0–0.75 m depth and play an important role in the global carbon cycle. We investigated soil organic C (SOC), water-soluble organic C (WSOC), easily oxidizable organic C (EOC), humic C fractions, aggregate-associated C, aggregate stability, and humic acid (HA) composition along an east-west transect across Qinghai-Tibet Plateau, and explored their spatial patterns and controlling factors. The contents of SOC, WSOC, EOC, humic C fractions and aggregate-associated C, the proportions of macro-aggregates (2-0.25) and micro-aggregates (0.25-0.053 mm), and the aggregate stability indices all increased in the order alpine desert < alpine steppe < alpine meadow. The alkyl C, O-alkyl C, and aliphatic C/aromatic C ratio of HA increased as alpine desert < alpine meadow < alpine steppe, and the trends were reverse for the aromatic C and HB/HI ratio. Mean annual precipitation and aboveground biomass were significantly correlated with the contents of SOC and its fractions, the proportions of macro- and micro-aggregates, and the aggregate stability indices along this transect. Among all these C fractions, SOC content and aggregate stability were more closely associated with humic C and silt and clay sized C in comparison with WSOC, EOC, and macro- and micro-aggregate C. The results suggested that alpine meadow soils containing higher SOC exhibited high soil aggregation and aggregate stability. Mean annual precipitation should be the main climate factor controlling the spatial patterns of SOC, soil aggregation, and aggregate stability in this region. The resistant and stable C fractions rather than labile C fractions are the major determinant of SOC stocks and aggregate stability.

Climate change impacts on soil organic carbon stocks of Mediterranean agricultural areas: A case study in Northern Egypt

Mediterranean agricultural areas are characterised by low soil organic C (SOC) contents and as a consequence they are often degraded and highly vulnerable to environmental changes. Climate change is expected to have a large impact upon these areas but they may be key for mitigation of its effects given their potential for soil C sequestration. Several approaches have been proposed to evaluate climate change impacts on SOC stocks, being soil C models amongst the most effective tools to assess C stocks, dynamics and distribution and to predict trends under climate change scenarios. In this study, we applied the CarboSOIL model and global climate models to predict and analyse the effects of short- (2030), medium- (2050) and long-term (2100) climate changes on SOC contents at standard soil depths (0–25, 25–50 and 50–75cm) in a Mediterranean arid area (El Fayoum, Northern Egypt) for different land use types. The validation of CarboSOIL with field data proved the consistency of the model. Overall decreases of SOC contents in the topsoil soil layer and increases in the subsoil layers are expected in the short, medium and long term. However, intensity of these changes will depend on the land use type and our results suggest that agricultural land uses relying on irrigation will be particularly vulnerable to losses of SOC stocks. This study demonstrates the importance of assessing SOC contents and dynamics along the soil profile and the potential for soil C sequestration particularly in the subsoil. The methods used in this research can be applied in other Mediterranean areas with available information on soil, land use and climate.

Sampling and Data Analysis Optimization for Estimating Soil Organic Carbon Stocks in Agroecosystems

Core Ideas SOC stocks are highly variable at the agroecosystem scale. Optimized sampling and estimation approaches are crucial for low cost SOC assessment. Systematic sampling provided thorough geographic and attribute space coverage. Ordinary kriging outperformed regression kriging, simple mean, and SSURGO approaches. Low‐cost approaches for measuring soil organic C (SOC) stocks are essential for verifying farm management effects on C sequestration in agroecosystems. This study compared sampling and data analysis optimization approaches for estimating SOC stocks of a complex agroecosystem. Soil samples were collected from a 232‐ha area of a working dairy farm with multiple land uses, crop rotations, topographic features, and manure application rates. The SOC stocks were estimated by (i) simple mean, (ii) ordinary kriging, (iii) regression kriging, and (iv) using the USDA Soil Survey Geographic (SSURGO) database. Relationships among sampling schemes, estimation approaches, auxiliary information, and SOC stocks were explored. Slope, elevation, soil type, and land use types displayed a high degree of variation at the study site, yet relationships with SOC stocks were weak or nonsignificant. Systematic sampling provided the best coverage of both geographic and attribute space, allowing reliable variogram estimates and RMSEs that increased little with reduced sample number. Random and stratified random sampling approaches resulted in reduced accuracy. Ordinary kriging had lower RMSEs than either regression kriging or the simple mean, and total SOC stocks estimates fluctuated less when sample sizes were reduced. Using SSURGO overestimated total SOC stocks by up to 5.1% compared with the other approaches, suggesting that this approach may be worthy of evaluation for circumstances with a low budget and low confidence level requirements. Systematic sampling and ordinary kriging can provide an optimal strategy for estimating SOC stocks in agroecosystems with complex topography and land uses.

Consolidating soil carbon turnover models by improved estimates of belowground carbon input.

World soil carbon (C) stocks are third only to those in the ocean and earth crust, and represent twice the amount currently present in the atmosphere. Therefore, any small change in the amount of soil organic C (SOC) may affect carbon dioxide (CO2) concentrations in the atmosphere. Dynamic models of SOC help reveal the interaction among soil carbon systems, climate and land management, and they are also frequently used to help assess SOC dynamics. Those models often use allometric functions to calculate soil C inputs in which the amount of C in both above and below ground crop residues are assumed to be proportional to crop harvest yield. Here we argue that simulating changes in SOC stocks based on C input that are proportional to crop yield is not supported by data from long-term experiments with measured SOC changes. Rather, there is evidence that root C inputs are largely independent of crop yield, but crop specific. We discuss implications of applying fixed belowground C input regardless of crop yield on agricultural greenhouse gas mitigation and accounting.

Sampling for Soil Carbon Stock Assessment in Rocky Agricultural Soils

Core Ideas Soil mass is the greatest source of uncertainty for C stocks on rocky soils. Coring methods underestimate soil rock fragment content. Hammer coring methods underestimate soil mass. Mass‐based C stock reporting can overcome coring method bias. Rotary corers are a cost‐competitive and less biased alternative to standard corers. Coring methods commonly employed in soil organic C (SOC) stock assessment may not accurately capture soil rock fragment (RF) content or soil bulk density (ρb) in rocky agricultural soils, potentially biasing SOC stock estimates. Quantitative pits are considered less biased than coring methods but are invasive and often cost‐prohibitive. We compared fixed‐depth and mass‐based estimates of SOC stocks (0.3‐m depth) for hammer, hydraulic push, and rotary coring methods relative to quantitative pits at four agricultural sites ranging in RF content from &lt;0.01 to 0.24 m3 m−3. Sampling costs were also compared. Coring methods significantly underestimated RF content at all rocky sites, but significant differences (p &lt; 0.05) in SOC stocks between pits and corers were only found with the hammer method using the fixed‐depth approach at the &lt;0.01 m3 m−3 RF site (pit, 5.80 kg C m−2; hammer, 4.74 g C m−2) and at the 0.14 m3 m−3 RF site (pit, 8.81 kg C m−2; hammer, 6.71 kg C m−2). The hammer corer also underestimated ρb at all sites as did the hydraulic push corer at the 0.21 m3 m−3 RF site. No significant differences in mass‐based SOC stock estimates were observed between pits and corers. Our results indicate that (i) calculating SOC stocks on a mass basis can overcome biases in RF and ρb estimates introduced by sampling equipment and (ii) a quantitative pit is the optimal sampling method for establishing reference soil masses, followed by rotary and then hydraulic push corers.

Response of paddy soil organic carbon accumulation to changes in long-term yield-driven carbon inputs in subtropical China

A decrease in C inputs from the return of crop residues to soil has occurred in many regions worldwide in recent years. The effects of this decline in C inputs could provide valuable information for assessing the long-term impact of litter C inputs on soil organic C (SOC) in rice paddy soils. The present study aimed to evaluate the response of rice paddy SOC accumulation to changes in actual C inputs in subtropical China, with emphasis on the response of C accumulation to declining C inputs. For this, we used a long-term field experiment on paddy soil in a rice-rice (Oryza sativa L.) cropping system running from 1990 to 2014. The four treatments were CK (control, no fertilizer), OM (organic matter application), NPK (N, P, and K fertilizer application), and NPKOM (NPK and organic matter application). Organic matter application for the OM and NPKOM treatments included rice straw and green manure that were left in the field after harvest and chopped, along with rice residues with stubbles and roots. In all treatments, C sequestration showed an increasing trend (from 0.207 to 0.880gkg−1yr−1) in the early and middle stages of the experiment (1990–2006) followed by a decreasing trend (from −0.429 to −0.064gkg−1yr−1) in the late stage (2007–2014). The trends were more pronounced for the OM and NPKOM treatments than for their CK and NPK counterparts. The changes in SOC stocks were consistent with changes in C inputs (p<0.01). During the late stage, yield and litter inputs from crop residues and green manure decreased, quickly affecting SOC stock in paddy soils. This declining trend in annual rice yields was mainly caused by the decline in first rice yields, accounting for 42.3–91.5% of the decrease in annual C inputs. Insufficient P or N and K supply and unfavorable climatic factors (decreases in sunshine duration and both maximum and minimum temperatures) are possible reasons for the decline in first rice yields and green manure biomass in the late stage. Collectively, the results suggest that C stocks in high-productivity paddy soils respond very sensitively to a decline in C inputs. This raises the risk of loss of C stock in paddy soil if, in the long run, a large return of C to soil with crop residues or by other sources, e.g., green manure, cannot be achieved.

Increased cropping intensity improves crop residue inputs to the soil and aggregate-associated soil organic carbon stocks

Many South American agroecosystems are based mainly on soybean [Glycine max (L.) Merr.] as a sole crop in the year, which has increased concerns regarding soil conservation and ecosystems sustainability. The increase in cropping intensity (CI) has been suggested as a strategy to improve crop residue inputs, which in turn, may increase soil aggregation and soil organic C (SOC) storage, while maintaining or even increasing total sequence yields. Our objective was to evaluate the relationships between CI and crop residue input with SOC storage and soil aggregation in two contrasting northeastern Argentinean Pampas soils under no-till. Two parallel experiments were established in a Mollisol and a Vertisol evaluating six cropping sequences, starting from soybean monoculture and increasing the number of crops per year and crop diversity. Crop residue inputs to the soil (aboveground biomass, belowground biomass and total biomass), grain yield, the amount of macroaggregates (MA), SOC stored inside macroagregates (SOCMA) and total SOC stocks were measured in both soils two years after the beginning of cropping sequences, at three soil depths. Soil organic C stocks, MA and SOCMA were all positively related with CI in both soils at 0–5cm depth. All soil variables were lowest in simple rotations (soybean monoculture) and increased in more complex rotations (double cropping with cereals and legumes), although differences were significant (P<0.05) only in the top soil (0–5cm depth). Grain yields and crop residues followed a similar pattern being higher in rotations that included maize (with yields expressed as grain mass or as glucose equivalent mass) and lower in soybean monocultures. The highest protein yields were obtained in sequences with wheat and soybean double cropping. Increases in CI under no-till seem to be a useful strategy to improved residue inputs, soil aggregates and SOC stocks. Our results provide valuable evidence for stakeholders and policy-makers to improve SOC sequestration and soil health in agroecosystems of humid temperate croplands.

Crop yield and SOC responses to biochar application were dependent on soil texture and crop type in southern Quebec, Canada

AbstractChanges to soil nutrient availability and increases for crop yield and soil organic C (SOC) concentration on biochar‐amended soil under temperate climate conditions have only been reported in a few publications. The objective of this work was to determine if biochar application rates up to 20 Mg ha−1 affect nutrient availability in soil, SOC stocks and yield of corn (Zea mays L.), soybean (Glycine max L.), and switchgrass (Panicum virgatum L.) on two coarse‐textured soils (loamy sand, sandy clay loam) in S Quebec, Canada. Data were collected from field experiments for a 3‐y period following application of pine wood biochar at rates of 0, 10, and 20 Mg ha−1. For corn plots, at harvest 3 y after biochar application, 20 Mg biochar ha−1 resulted in 41.2% lower soil NH $ _4^+ $ on the loamy sand; the same effect was not present on the sandy clay loam soil. On the loamy sand, 20 Mg biochar ha−1 increased corn yields by 14.2% compared to the control 3 y after application; the same effect was not present on the sandy clay loam soil. Biochar did not alter yield or nutrient availability in soil on soybean or switchgrass plots on either soil type. After 3 y, SOC concentration was 83 and 258% greater after 10 and 20 Mg ha−1 biochar applications, respectively, than the control in sandy clay loam soil under switchgrass production. The same effect was not present on the sandy clay loam soil. A 67% higher SOC concentration was noted with biochar application at 20 Mg ha−1 to sandy clay loam soil under corn.

Determining Soil Bulk Density for Carbon Stock Calculations: A Systematic Method Comparison

Core Ideas Little is known about the methodological errors of bulk density quantification. Soil probes are easy to handle, but systematic methods comparisons are lacking. We compared methods among three soil ring types and three soil probe types at four sites. Accurate and effective determination of soil bulk density (BD) is needed to monitor soil organic C (SOC) stocks and SOC stock changes. However, BD measurements are often lacking in soil inventories and BD is estimated by pedotransfer functions with substantial uncertainty. In a systematic method comparison, we evaluated different methods for BD determination in the field by comparing the performance of MINI (5 cm3) and BIG (250 cm3) sample rings and of three driving hammer probes differing in diameter, material, and extraction method. Bulk density determined with 100‐cm3 sample rings was defined as the reference method (REF). All methods were tested at five depth increments in nine subplots at four sites with differing soil texture and SOC content. All methods determined BD in the depth increments with low systematic error (8% for probes and 2% for sample rings). The random error of the probe samples was, on average, 50% higher than that of the ring samples when the cores of the probes were adequately corrected for compaction or stretching. The BD was significantly overestimated (by 2%) when determined with MINI rings, and the variation in BD was not reduced with BIG sample rings rather than the smaller REF sample rings. The performance of the driving hammer technique varied widely among probe types and sites. The sheath probe had the smallest systematic error of all probes tested and is recommended for soil inventories. All methods for estimating BD had smaller errors than pedotransfer functions.

Ecosystem carbon stock across a chronosequence of spruce plantations established on cutovers of a high-elevation region

This study quantified the above- and belowground carbon (C) stocks across a chronosequence of spruce (Picea asperata) plantations established on cutovers and explored the turning point after which the increase in biomass C slowed or biomass C decreased for guiding forest management. We assessed above- and belowground plant biomass stocks at 11 sites in three regions, representing 12- to 46-year-old spruce plantations established on clear-cut areas in the eastern Tibetan Plateau, China. Biomass and C stocks of trees, understory vegetation, and forest floor litter were determined from plot-level inventories and destructive sampling. Fine root (<2 mm) biomass and mineral soil organic C (SOC) stock were estimated from soil cores. Tree biomass was quantified using allometric equations based on diameter at breast height (DBH) and height (H). Plant biomass C stocks in spruce plantations rapidly increased from 12 to 20 years at a rate of 7.8 Mg C ha−1 year−1, but decreased from 25 to 46 years at a rate of 0.79 Mg C ha−1 year−1. SOC stocks in spruce plantations gradually decreased from 12 to 46 years at a rate of 4.4 Mg C ha−1 year−1. Total C stock in the ecosystem remained unchanged for the first 20 years after the planting of spruce on cutovers, because the buildup of C stock in spruce biomass during the first 20 years was offset by the decrease in SOC. From 21 to 46 years after the reforestation, ecosystem C stock even decreased at a rate of 5.2 Mg C ha−1 year−1. The contribution of the understory vegetation, forest floor litter, and fine root to ecosystem C stock was low (<5.0 %) in the spruce plantations. Ecosystem C stock in the spruce forest established on the cutover in the eastern Tibetan Plateau was related to stand age. During the first 20 years, this ecosystem was C neutral. However, aged (20–46 years) spruce plantation ecosystem can be a C source if no management was implemented to revitalize tree growth, promote understory vegetation, and enhance SOC accumulation.

Repeated monitoring of organic carbon stocks after eight years reveals carbon losses from intensively managed agricultural soils in Western Germany

AbstractPast land‐use changes, intensive cropping with large proportions of root crops, and preferred use of mineral fertilizer have been made responsible for proceeding losses of soil organic C (SOC) in the plough layer. We hypothesized that in intensive agriculturally managed regions changes in SOC stocks would be detectable within a decade. To test this hypothesis, we tracked the temporal development of the concentrations and stocks of SOC in 268 arable sites, sampled by horizon down to 60 cm in the Cologne‐Bonn region, W Germany, in 2005 and in 2013. We then related these changes to soil management data and humus balances obtained from farmers' surveys. As we expected that changes in SOC concentrations might at least in part be minor, we fractionated soils from 38 representative sites according to particle size in order to obtain C pools of different stability.We found that SOC concentrations had increased significantly in the topsoil (from 9.4 g kg−1 in 2005 to 9.8 g kg−1 in 2013), but had decreased significantly in the subsoil (from 4.1 g kg−1 in 2005 to 3.5 g kg−­1 in 2013). Intriguingly, these changes were due to changes in mineral‐bound SOC rather than to changes in sand‐sized organic matter pools. As bulk density decreased, the overall SOC stocks in the upper 60 cm exhibited a SOC loss of nearly 0.6 t C (ha · y)−1 after correction by the equivalent soil mass method. This loss was most pronounced for sandy soils [−0.73 t SOC (ha · y)−1], and less pronounced for loamy soils [−0.64 t SOC (ha · y)−1]; silty soils revealed the smallest reduction in SOC [−0.3 t SOC (ha · y)−1]. Losses of SOC occurred even with the overall humus balances having increased positively from about 20 kg C (ha · y)−1 (2003–2005) to about 133 kg C (ha · y)−1 (2005–2013) due to an improved organic fertilization and intercropping. We conclude that current management may fail to raise overall SOC stocks. In our study area SOC stocks even continued to decline, despite humus conservation practice, likely because past land use conversions (before 2005) still affect SOC dynamics.

Conversion from forests to pastures in the Colombian Amazon leads to contrasting soil carbon dynamics depending on land management practices.

Strategies to mitigate climate change by reducing deforestation and forest degradation (e.g. REDD+) require country- or region-specific information on temporal changes in forest carbon (C) pools to develop accurate emission factors. The soil C pool is one of the most important C reservoirs, but is rarely included in national forest reference emission levels due to a lack of data. Here, we present the soil organic C (SOC) dynamics along 20years of forest-to-pasture conversion in two subregions with different management practices during pasture establishment in the Colombian Amazon: high-grazing intensity (HG) and low-grazing intensity (LG) subregions. We determined the pattern of SOC change resulting from the conversion from forest (C3 plants) to pasture (C4 plants) by analysing total SOC stocks and the natural abundance of the stable isotopes (13) C along two 20-year chronosequences identified in each subregion. We also analysed soil N stocks and the natural abundance of (15) N during pasture establishment. In general, total SOC stocks at 30cm depth in the forest were similar for both subregions, with an average of 47.1±1.8MgCha(-1) in HG and 48.7±3.1MgCha(-1) in LG. However, 20years after forest-to-pasture conversion SOC in HG decreased by 20%, whereas in LG SOC increased by 41%. This net SOC decrease in HG was due to a larger reduction in C3-derived input and to a comparatively smaller increase in C4-derived C input. In LG both C3- and C4-derived C input increased along the chronosequence. N stocks were generally similar in both subregions and soil N stock changes during pasture establishment were correlated with SOC changes. These results emphasize the importance of management practices involving low-grazing intensity in cattle activities to preserve SOC stocks and to reduce C emissions after land-cover change from forest to pasture in the Colombian Amazon.

Eucalyptus and Pinus stand density effects on soil carbon sequestration

Although afforestation of pasture and agricultural systems can increase C sequestration in soils, studies have shown wide variation in the magnitude, timing and direction of soil organic C (SOC) dynamics, depending on site conditions, management practices, and previous land use. Effects of stocking density on SOC have yet to be elucidated, however. The objectives proposed for this work were to quantify: (i) SOC content and distribution for the top 30cm of the A horizon under native pasture compared with that following 8years of afforestation (Eucalyptus grandis and Pinus taeda planted at different densities); (ii) SOC accumulation in the AB and the top of Bt horizons in afforested soils; and (iii) the contribution of the new vegetation (Eucalyptus and Pinus) to SOC. A field trial with three stand-densities of E. grandis and P. taeda and corresponding native pasture was established and soil samples were collected and assayed for five layers of the A horizon (0–5, 5–10, 10–15, 15–20, 20–30cm), and the AB and Bt1 horizons. Soil organic C concentration and δ13C were determined, and the total SOC stock and C derived from the Eucalyptus and Pinus vegetation (young C) were calculated. Our results suggest that there was likely no significant change in SOC stocks in response to 8years of afforestation with either Eucalyptus or Pinus. No significant differences in SOC stocks in the upper 30cm soil layer as a whole were found among pasture, Eucalyptus, and Pinus treatments. By contrast, SOC in the AB and Bt1 horizons was significantly higher under afforested sites than native pasture (P<0.001 and P<0.039, respectively). Soil δ13C in the afforested treatments (0–30cm) reflects the contribution of the new vegetation (Eucalyptus or Pinus) to SOC. Net accumulation of new SOC from the planted trees in the top 15cm soil layer was equivalent to 0.20MgCha−1yr−1 for Eucalyptus and 0.30MgCha−1yr−1 for Pinus. Assuming initial declines in SOC following afforestation have largely ended and young C sequestration rates continue at the same rate, SOC stocks in the top 15cm layer would reach those found under pasture after 15 and 11years for Eucalyptus and Pinus plantations, respectively. Measurements of additional sites afforested with Eucalyptus and Pinus of different ages are needed to draw firm conclusions about the net SOC balance in these afforested pasture soils.

Potential of grassland rehabilitation through high density-short duration grazing to sequester atmospheric carbon

According to the World Resources Institute (2000), a relative increase of carbon (C) stocks in world soils by 0.4% per year would be sufficient to compensate all anthropogenic greenhouse gas emissions. Several land management practices such as the suppression of tillage in agroecosystems and livestock exclusion in grasslands had initially been thought to store more carbon into the soil, but recent research puts this into question. In a context where finding effective C sequestration methods is urgent, the main objective of this study was to assess the ability of an innovative grassland management practice based on high density and short duration (HDSD) grazing to sequester atmospheric C into soils. The study was performed in a degraded communal rangeland in South Africa where soil organic C (SOC) depletion ranged from 5 to 95% depending on the degradation level, which varied from non-degraded (ND; with grass above ground coverage, Cov of 100%), degraded (D1; 50<Cov<75%), D2 (25<Cov<50%) and HD (highly degraded: Cov<5%). The ability of HDSD (1200cows ha−1 for 3days a year) to replenish SOC stocks was compared to four commonly used strategies: (1) livestock exclosure (E); (2) livestock exclosure with topsoil tillage (ET); (3) livestock exclosure with NPK fertilization (2:3:3, 22 at 0.2tha−1) (EF); (4) annual burning (AB); all treatments being compared to traditional free grazing control. A total of 540 soil samples were collected in the 0–0.05m soil layer for all treatments and degradation intensities. After two years, topsoil SOC stocks were significantly increased under EF and HDSD, by an average of 33.4±0.5 and 12.4±2.1g C m2 y−1, respectively. In contrast, AB reduced SOC stocks by 3.6±3.0gCm2y−1, while the impact of E and ET was not significant at P<0.05. HDSD replenished SOC stocks the most at D1 and D2 (6.7 and 7.4%y−1) and this was explained by grass recovery, i.e. a significant increase in soil surface coverage by grass and grass production. HDSD is cost-effective, and thus has great potential to be widely adopted by smallholder farmers.