Changes In Soil Organic Carbon Stocks Research Articles

Effects of past land use on soil organic carbon changes after dam construction

A large-scale and high-dropout water-level-fluctuation zone (WLFZ) was formed by the construction of cascaded large dam in the southwest hydropower base of China. The original land use patterns changed significantly by the formation of the WLFZ, which possibly affected the soil organic carbon (SOC) stocks and local carbon cycle. However, little is known about the effects of land uses before dam construction on the SOC stocks in the WLFZ. To address this, we chose the Nuozhadu mega reservoir built in 2014 in the upper Mekong River basin as our study area. We established five sampling transects around the reservoir and sampled them in 2015. Each transect contained three vertical sampling lines representing three land-use types: natural forest, farmland, and rubber plantations. SOC stocks were determined in the WLFZ and in the infralittoral reference zone (IRZ), and the interactions among SOC, land uses, soil erosion, inundation, and other soil physicochemical properties were analyzed to establish the factors controlling SOC in the WLFZ. The average soil organic carbon density (SOCD) was not significantly different between the WLFZ and IRZ, but it differed significantly among the three types of land uses. The SOC stocks in natural forests of the WLFZ and IRZ were significantly greater than those in the other land use types. The SOCD decreased with increasing elevation in the natural forest and rubber plantation in the WLFZ, while no significant trend was found in farmland. These results indicated that the construction of a large dam in southwestern China may not lead to significant changes in regional SOC stocks, but it can result in the redistribution of SOC in the large-scale and high-dropout WLFZ. Land use before dam construction can affect the background concentrations of SOC, soil physicochemical properties, and soil erosion rate in the WLFZ, which controlled the redistribution of SOC.

Mapping soil organic carbon stock change by soil monitoring and digital soil mapping at the landscape scale

Mapping soil organic carbon stock change by soil monitoring and digital soil mapping at the landscape scale

Effects of soil erosion and reforestation on soil respiration, organic carbon and nitrogen stocks in an eroded area of Southern China

Soil erosion and reforestation greatly affects the functionality of many terrestrial ecosystems. However, the effects of soil erosion and reforestation on soil respiration (SR), and soil organic carbon (SOC) and total nitrogen (TN) stocks remain unclear. Therefore, we investigated the changes in SR, and SOC and TN stocks at four different soil erosion levels (severely, moderately, lightly, and non-eroded) and two different aged Pinus massoniana plantations (8- and 36-year-old) in the hilly red soil regions of Southern China. Our results showed that soil erosion level and reforestation significantly influenced SR, and SOC and TN stocks. Meanwhile, the mean SR, and SOC and TN stocks all significantly decreased with erosion level but increased significantly with times since reforestation. Soil temperature (ST) could explain 70–92% of SR seasonal variation based on exponential models, whereas no significant relationship between SR and soil water content were found. Furthermore, the structural equation modeling indicated that SOC stocks at 0–20 cm had a much stronger effect on SR than ST. Meanwhile, the SOC stocks for 0–20 cm increased by 177% and 558% in the 8- and 36-year-old Pinus massoniana plantations in comparison with the severely eroded forestland, respectively. This study highlights that reforestation could be an effective strategy for restoring the carbon and nitrogen storage in eroded regions of Southern China and emphasizes the need to consider the effects of soil erosion and reforestation when assessing regional carbon budgets under different climate scenarios.

Controlling factors of carbon dynamics in grassland soils of Bavaria between 1989 and 2016

Grassland management and climate change may have profound effects on soil organic carbon (SOC) storage in temperate grasslands, but to date only limited information on the long-term SOC development derived from grassland monitoring sites is available. In 1986, the Bavarian State Research Center for Agriculture established 20 long-term grassland soil monitoring sites in the state of Bavaria, Germany. We analyzed SOC changes and their main controlling factors in these monitoring sites between 1989 and 2016 in four intervals using the machine learning algorithm conditional random forest. The results showed that changes of SOC stocks in grassland monitoring sites were affected by the multifactorial and interactive nature of changing climate conditions, pedogenic-topographic factors and management practices. Seasonal climate variables explained the highest variability of SOC stock changes in long-term grassland sites, followed by organic fertilizer application. Increasing autumn precipitation led to decreased SOC stocks, whereas increasing spring and summer precipitation led to increased SOC stocks on sites at high elevations with low slopes. Furthermore, we observed that the organic fertilizer application resulted in an increase and/or preservation of SOC stocks, whereas conventional farming with a low degree of organic fertilization resulted in SOC stock decreases. Regarding the acceleration of climate change and the complex interactions between site conditions, changing climate and management, there is an urgent need to collect improved management data in future long-term studies.

Modeling soil organic carbon evolution in long-term arable experiments with AMG model

Reliable models predicting soil organic carbon (SOC) evolution are required to better manage cropping systems with the objectives of mitigating climate change and improving soil quality. In this study, data from 60 selected long-term field trials conducted in arable systems in France were used to evaluate a revised version of AMG model integrating a new mineralization submodel. The drivers of SOC evolution identified using Random Forest analysis were consistent with those considered in AMG. The model with its default parameterization simulated accurately the changes in SOC stocks over time, the relative model error (RRMSE = 5.3%) being comparable to the measurement error (CV = 4.3%). Model performance was little affected by the choice of plant C input estimation method, but was improved by a site specific optimization of SOC pool partitioning. AMG shows a good potential for predicting SOC evolution in scenarios varying in climate, soil properties and crop management.

The ecosystem carbon sink implications of mountain forest expansion into abandoned grazing land: The role of subsoil and climatic factors

Woody encroachment is a widespread phenomenon resulting from the abandonment of mountain agricultural and pastoral practices during the last century. As a result, forests have expanded, increasing biomass and necromass carbon (C) pools. However, the impact on soil organic carbon (SOC) is less clear. The main aim of this study was to investigate the effect of woody encroachment on SOC stocks and ecosystem C pools in six chronosequences located along the Italian peninsula, three in the Alps and three in the Apennines. Five stages along the chronosequences were identified in each site. Considering the topsoil (0–30 cm), subsoil (30 cm-bedrock) and whole soil profile, the temporal trend in SOC stocks was similar in all sites, with an initial increment and subsequent decrement in the intermediate phase. However, the final phase of the woody encroachment differed significantly between the Alps (mainly conifers) and the Apennines (broadleaf forests) sites, with a much more pronounced increment in the latter case. Compared to the previous pastures, after mature forest (>62 years old) establishment, SOC stocks increased by: 2.1(mean) ± 18.1(sd) and 50.1 ± 25.2 Mg C·ha−1 in the topsoil, 7.3 ± 17.4 and 93.2 ± 29.7 Mg C·ha−1 in the subsoil, and 9.4 ± 24.4 and 143.3 ± 51.0 Mg C·ha−1 in the whole soil profile in Alps and Apennines, respectively. Changes in SOC stocks increased with mean annual air temperature and average minimum winter temperature, and were negatively correlated with the sum of summer precipitation. At the same time, all other C pools (biomass and necromass) increased by 179.1 ± 51.3 and 304.2 ± 67.6 Mg C·ha−1 in the Alps and the Apennines sites, respectively. This study highlights the importance of considering both the subsoil, since deep soil layers contributed 38% to the observed variations in carbon stocks after land use change, and the possible repercussions for the carbon balance of large areas where forests are expanding, especially under pressing global warming scenarios.

Admixing Fir to European Beech Forests Improves the Soil Greenhouse Gas Balance

Research highlights: The admixture of fir to pure European beech hardly affected soil-atmosphere CH4 and N2O fluxes but increased soil organic carbon (SOC) stocks at a site in the Black Forest, Southern Germany. Background and objectives: Admixing deep-rooting silver fir has been proposed as a measure to increase the resilience of beech forests towards intensified drying-wetting cycles. Hence, the goal of this study was to quantify the effect of fir admixture to beech forests on the soil-atmosphere-exchange of greenhouse gases (GHGs: CO2, CH4 and N2O) and the SOC stocks by comparing pure beech (BB) and mixed beech-fir (BF) stands in the Black Forest, Germany. Materials and methods: To account for the impact of drying-wetting events, we simulated prolonged summer drought periods by rainout shelters, followed by irrigation. Results: The admixture of fir to pure beech stands reduced soil respiration, especially during autumn and winter. This resulted in increased SOC stocks down to a 0.9 m depth by 9 t C ha−1 at BF. The mixed stand showed an insignificantly decreased sink strength for CH4 (−4.0 under BB and −3.6 kg C ha−1 year−1 under BF). With maximal emissions of 25 µg N m−2 h−1, N2O fluxes were very low and remained unchanged by the fir admixture. The total soil GHG balance of forest conversion from BB to BF was strongly dominated by changes in SOC stocks. Extended summer droughts significantly decreased the soil respiration in both BB and BF stands and increased the net CH4 uptake. Conclusions: Overall, this study highlights the positive effects of fir admixture to beech stands on SOC stocks and the total soil GHG balance. In view of the positive impact of increased SOC stocks on key soil functions such as water and nutrient retention, admixing fir to beech stands appears to be a suitable measure to mitigate climate change stresses on European beech stands.

Agricultural soil organic carbon stocks in the north-eastern Iberian Peninsula: Drivers and spatial variability

Estimating soil organic carbon (SOC) stocks under agriculture, assessing the importance of their drivers and understanding the spatial distribution of SOC stocks are crucial to predicting possible future SOC stocks scenarios under climate change conditions and to designing appropriate mitigation and adaptation strategies. This study characterized and modelled SOC stocks at two soil depth intervals, topsoil (0–30 cm) and subsoil (30–100 cm), based on both legacy and recent data from 7245 agricultural soil profiles and using environmental drivers (climate, agricultural practices and soil properties) for agricultural soils in Catalonia (NE Spain). Generalized Least Square (GLS) and Geographical Weighted Regression (GWR) were used as modelling approaches to: (i) assess the main SOC stock drivers and their effects on SOC stocks; (ii) analyse spatial variability of SOC stocks and their relationships with the main drivers; and (iii) predict and map SOC stocks at the regional scale. While topsoil variation of SOC stocks depended mainly on climate, soil texture and agricultural variables, subsoil SOC stocks changes depended mainly on soil attributes such us soil texture, clay content, soil type or depth to bedrock. The GWR model revealed that the relationship between SOC stocks and drivers varied spatially. Finally, the study was only able to predict and map topsoil SOC stocks at the regional scale, because controlling factors of SOC stocks at the subsoil level were largely unavailable for digital mapping. According to the resulting map, the mean SOC stock value for Catalan agriculture at the topsoil level was 4.88 ± 0.89 kg/m2 and the total magnitude of the carbon pool in agricultural soils of Catalonia up to 30 cm reached 47.9 Tg. The present study findings are useful for defining carbon sequestration strategies at the regional scale related with agricultural land use changes and agricultural management practices in a context of climate change.

Modeling soil organic carbon dynamics under shifting cultivation and forests using Rothc model

Shifting cultivation (jhum) and forest land are the main land uses in North Eastern region of Indian Himalaya, but in the long term this form of agriculture is not acceptable due to the soil degradation following the cutting of forest vegetation, and the consequent biodiversity loss, high erosion rates and nutrient loss through runoff. No information on soil organic carbon (SOC) dynamics and simulation studies are available, so an attempt was done using RothC model. The model was parameterized on measured SOC contents of forest and jhum sites, and average SOC stocks and changes were simulated for a period of 5 years in forest and jhum sites under the baseline and the projected climate change conditions available for Nagaland state (2021–2050). Forest sites under the baseline climate showed a steady-state condition, and simulated SOC decreased by 0.04 t C ha−1 yr−1during 5 years. In addition simulations indicated that forest land use could not benefit from climate change in a 10 years period (SOC changes showed a decreasing trend from 0.36 to 0.06 t C ha−1 yr−1). Jhum sites showed negative changes in SOC stocks both under the baseline (0.40 t C ha−1 yr−1) and the projected climate change conditions (0.50 t C ha−1 yr−1). Indeed, SOC decreases under climate change were inversely related to the duration of the jhum cropping cycle, almost linear in the first five years (from 0.65 to 0.41 t C ha−1 yr−1), and thereafter only slightly decreasing (from 0.39 to 0.32 t C ha−1 yr−1). We can conclude that under climate change conditions the jhum cropping cycle can be extended for a longer period without substantial effects on SOC decreases.

Stocks and Stoichiometry of Soil Organic Carbon, Total Nitrogen, and Total Phosphorus after Vegetation Restoration in the Loess Hilly Region, China

The Loess Plateau is an important region for vegetation restoration in China; however, changes in soil organic carbon (SOC), soil nutrients, and stoichiometry after restoration in this vulnerable ecoregion are not well understood. Typical restoration types, including orchardland, grassland, shrubland, and forestland, were chosen to examine changes in the stocks and stoichiometry of SOC, soil total nitrogen (TN), and soil total phosphorus (TP) at different soil depths and recovery times. Results showed that SOC stocks first increased and then stabilized in orchardland, grassland, and shrubland at 0–30 cm depths, while in forestland, SOC stocks gradually increased. Soil TN stocks first increased and then decreased in orchardland, shrubland, and forestland with restoration age at 0–30 cm depths, while soil TP stocks showed little variation between restoration types; at the same time, the overall C:N, C:P, and N:P ratios increased with restoration age. In the later stages of restoration, the stocks of SOC and soil TN at 0–30 cm soil depths were still lower than those in natural grassland and natural forest. Additionally, the SOC, soil TN, and soil TP stocks and the C:N, C:P, and N:P ratios decreased with soil depth. The forestland had the highest rate of change in SOC and soil TN stocks, at 0–10 cm soil depth. These results indicate a complex response of SOC, soil TN, and soil TP stocks and stoichiometry to vegetation restoration, which could have important implications for understanding C, N, and P changes and nutrient limitations after vegetation restoration.

Soil Organic Carbon, Aggregation, and Microbial Community Structure in Annual and Perennial Biofuel Crops

Core Ideas Removal of corn and sorghum residues did not affect soil carbon under no‐tillage. Soil carbon (0–15 cm) increased in 10‐yr stands of switchgrass and miscanthus. Soil carbon increased with root biomass, fungi abundance, and soil aggregate size. Perennial crops improved soil health while providing feedstock for biofuels. The substitution of cellulosic biofuel in place of conventional fuels could reduce greenhouse gas (GHG) emissions from transportation. However, changes in soil organic carbon (SOC) and soil health during biofuel crop production could have a major impact on the GHG balance of biofuels. We assessed temporal changes (10 yr) in SOC stocks to a 90 cm depth in Cumulic Hapludolls from central Kansas under perennial and annual cropping systems. The perennial crops were miscanthus (Miscanthus sacchariflorus) and switchgrass (Panicum virgatum L.). The annual cropping systems were continuous corn (Zea mays L.), and corn, dual purpose–grain sorghum (Sorghum bicolor (L.) Moench), sweet sorghum, and photoperiod‐sensitive sorghum (PS) in rotation with soybean [Glycine max (L.) Merr.]. All standing aboveground biomass was removed at harvest of corn, sorghum, and perennial crops. Stocks of SOC increased in the 0–15 cm depth under switchgrass and miscanthus by 0.8 and 1.3 Mg C ha−1 yr−1, respectively. The SOC stocks did not change at the other depths or at any depth in the annual cropping systems nor throughout the soil profile under any crops. Root biomass measured in the seventh year of the study was 3.7 to 7.8 times greater in perennials than in annual crops. Increases on SOC were correlated with greater root biomass, abundance of arbuscular mycorrhizae and saprophytic fungi, and soil aggregate diameter. These results demonstrate the potential for perennial biofuel crops to enhance C sequestration and improve soil quality while providing feedstock for production of cellulosic biofuel.

Drivers of long-term carbon dynamics in cropland: A bio-political history (France, 1852–2014)

Following the COP 21, it has been widely recognized that agricultural soils may contribute toward mitigating or enhancing climate change depending on their behavior as a sink or a source of carbon (C) to the atmosphere. Here, we studied the case of soil organic carbon (SOC) stocks in French cropland at the regional scale from 1852 to 2014 by coupling the GRAFS approach (Generalized Representation of Agro-Food System) with the AMG model (after its authors names: Andriulo, Mary, Guérif). Our results emphasize the role of crop net primary production (NPP) as the main driver of changes in SOC stocks through the input of crop residues. Three distinct phases of NPP were identified: (1) from 1852 to 1950, a slow rise in NPP occurred, which induced a moderate increase of cropland SOC pools (1–2‰ yr−1); (2) from 1950 to 1980, NPP increased considerably, resulting in a much higher rate of SOC accumulation (2–4‰ yr−1); (3) from 1980 to 2014, divergent trajectories were followed according to regions with either a slowdown in NPP growth with moderate increase in SOC stocks (0–2‰ yr−1), or a reversed trend toward declining NPP induced by a reduction of fodder production, replaced by imported feed which led to negative change of cropland SOC stocks. We highlight that these evolutions of carbon dynamics in cropland were closely linked to changes in the French social political context, from the second Empire to the Cold War and the advent of a neo-liberal era.As a whole, for France, we found a rate of C sequestration in the top 30 cm of cropland soils of about 1.7 Mt C yr−1 for the last two decades, less than one fifth of the current fossil-fuel-based CO2 emission rate of the French agricultural sector which exceeds 8 Mt C yr−1. Our study showed that in developed countries like France, where a sharp increase in cropland SOC stocks occurred owing to the modernization of agriculture, this should be considered as a “fossil-fuel-powered carbon sink”, which does only partly compensate for the direct emissions through mechanization and fertilizer production that induced it.

Composting and compost application: Trade-off between greenhouse gas emission and soil carbon sequestration in whole rice cropping system

Manure application showed contrasting effects on soil quality and greenhouse gas (GHG) emissions, in particular methane (CH4) emissions in a rice cropping system. Therefore, to mitigate CH4 emissions, stabilized manure like compost is recommended without considering the additional GHG emissions during the industrial processes and soil organic carbon (SOC) stock changes. To determine the integrated effect of compost utilization on the net global warming potential (GWP) of a rice cropping system, the fluxes of GHGs during the whole process were computed using a life cycle assessment (LCA) method. The model framework was composed of GHG fluxes from two compartments: the industrial activities, and the composting and rice cropping processes. Since manure application can increase SOC stock, the annual SOC stock changes were analyzed by the net ecosystem C budget (NECB). Manure applications significantly increased rice productivity and the net primary production (NPP) as a C input source without difference between fresh and composted manures. NPK+fresh manure application significantly increased CH4 and N2O emissions by 81% and 37% over the NPK treatment in rice cropping system, respectively, and depleted SOC stock with 1.3 Mg C ha−1 year−1, due to priming effect. As a result, NPK+fresh manure application increased the net GWP by 80% over the NPK treatment. In comparison, NPK+compost utilization decreased the net GWP by 30% over that of the NPK+fresh manure during the whole process. Manure composting increased the GWP of the industrial processes by 7%, but the 20% reduction of CH4 flux and 0.5 Mg C ha−1 year−1 of SOC stock increase significantly decreased the net GWP during the whole rice cropping process. As a result, the GHG intensity which means the net GWP per gain yield was not different between the NPK+composted manure and the NPK treatments. In conclusion compost application can be a reasonable soil management strategy to reduce GHG emission impact and to increase crop productivity in rice cropping systems.

Revisiting IPCC Tier 1 coefficients for soil organic and biomass carbon storage in agroforestry systems

Agroforestry systems comprise trees and crops, or trees and pastures within the same field. Globally, they cover approximately 1 billion hectares of land and contribute to the livelihoods of over 900 million people. Agroforestry systems have the capacity to sequester large quantities of carbon (C) in both soil and biomass. However, these systems have not yet been fully considered in the approach to C accounting developed by the Intergovernmental Panel on Climate Change, largely due to the high diversity of agroforestry systems and scarcity of relevant data. Our literature review identified a total of 72 scientific, peer-reviewed articles associated with biomass C storage (50) and with soil organic carbon (SOC) (122), containing a total of 542 observations (324 and 218, respectively). Based on a synthesis of the reported observations, we are presenting a set of Tier 1 coefficients for biomass C storage for each of the eight main agroforestry systems identified, including alley cropping, fallows, hedgerows, multistrata, parklands, shaded perennial-crop, silvoarable and silvopastoral systems, disaggregated by climate and region. Using the same agroforestry classification, we are presenting a set of stock change factors (FLU) and SOC accumulation/loss rates for three main land use changes (LUCs): cropland to agroforestry; forest to agroforestry; and grassland to agroforestry. Globally, the mean SOC stock change factors (± confidence intervals) were estimated to be 1.25 ± 0.04, 0.89 ± 0.07, and 1.19 ± 0.10, for the three main LUCs, respectively. However, these average coefficients hide huge disparities across and within different climates, regions, and types of agroforestry systems, highlighting the necessity to adopt the more disaggregated coefficients provided herein. We encourage national governments to synthesize data from local field experiments to generate country-specific factors for more robust estimation of biomass and SOC storage.

Unexpected higher decomposition of soil organic matter during cold fallow season in temperate rice paddy

In temperate paddy fields, rice is generally cultivated under flooding for 100–140 days during summer season, and thereafter, the soil is left without management under dried soil condition during cold fallow season. In this area rice straw is mostly removed for cattle feeding. Therefore, soil organic carbon (SOC) stock could be decreased with conventional practices, but the seasonal changes of SOC stock have not been evaluated well. To investigate SOC stock changes in temperate rice paddy, two seasonal C balances during rice cropping and fallow season were compared by the net ecosystem C budget (NECB) in the chemical fertilizer (control) and two different green manure treatments for three years. In the control treatment only chemical fertilizer was conventionally fertilized for rice cropping, but maintained without human control during the fallow season. In two green manure treatments, barley and hairy vetch (hereafter, vetch) were cultivated during the fallow season, and their whole biomasses were incorporated before rice transplanting. The conventional soil management with only chemical fertilization decreased small level of SOC stock (minus 0.70–0.84 Mg C ha−1 y−1 of NECB), which was attributed by high harvest removal (approximately 70% of total C input) and mineralized C loss (30%). Cover cropping and its biomass recycling as green manure was effective to increase SOC stock. High C/N ratio of non-leguminous barley produced high biomass yield, and its biomass incorporation significantly increased SOC stock with 1.21–3.43 Mg C ha−1 y−1 of annual NECB. In comparison low C/N ratio of leguminous vetch application increased slightly SOC stock (0.17–1.34 Mg C ha−1 y−1 of NECB), due to low biomass productivity and high mineralized C loss. However, irrespective with fertilization background, approximately 70% of mineralized C loss were occurred during the cold fallow season. Rice grain yields were similar between the control and hairy vetch treatment, but around 10% lower in barley treatment. Conclusively, high C/N ratio of cover crop cultivation during the fallow season could be better to increase SOC stock in mono-rice paddy, but additional nutrient management to increase rice productivity should be developed for barley utilization as green manure.

Changes in soil organic carbon stocks after conversion from forest to oil palm plantations in Malaysian Borneo

The continuous rise in the global demand for palm oil has resulted in large-scale expansion of industrial oil palm plantations—largely at the expense of primary and secondary forests. The potentially negative environmental impacts of these conversions have given rise to closer scrutiny. However, empirical data on the effects of conversion of forests to industrial oil palm plantations on soil organic carbon (SOC) stocks is scarce and patchy. We evaluated the changes in SOC stocks after conversion of tropical forest into oil palm plantations over the first and second rotation period in Sarawak, Malaysian Borneo. Soil samples were collected from three age classes of oil palm plantations converted from forest (49, 39 and 29 years ago respectively) with three replicate sites and four adjacent primary forest sites as reference. In each site under oil palm, the three management zones, namely weeded circle (WC), frond stacks (FS), and between palm (BP), were sampled separately. All soil samples were collected from five soil layers (0–5, 5–15, 15–30, 30–50 and 50–70 cm). Samples were analysed for SOC concentration, soil bulk density, pH and soil texture. Results showed SOC stocks declined by 42%, 24% and 18% after 29, 39 and 49 years of conversion respectively. Significant differences in SOC stocks were found among different management zones in the oil palm plantations, and the trend was similar for all age classes: FS > WC > BP, demonstrating the necessity of considering within-plantation variability when assessing soil C stocks. The largest differences between SOC stocks of the reference forest and converted plantations were found in the topsoil (0–15 cm depth) but differences were also found in the subsoil (>30 cm). Our results will contribute towards future modelling and life cycle accounting to calculate the carbon debt from the conversion of forest to oil palm plantations.

Relationship between Soil Organic Carbon Stocks and Clay Content under Different Climatic Conditions in Central China

Understanding the association between soil organic carbon (SOC) and texture under different climatic conditions is important for assessing the effects of future climate changes on SOC stocks. In this study, we conducted a climatic gradient experiment covering three climate types (humid, sub-humid, and semi-arid) with a steep rainfall ranging from 345 to 910 mm, and specifically determined SOC dynamics, clay content, and vegetation and soil characteristics. The results showed that, from semi-arid to humid regions, SOC stocks, SOC, and clay content increased synchronously and were closely related in layers of depths of both 0–10 and 10–20 cm. In contrast, under similar climatic conditions, SOC dynamics were mainly affected by vegetation and soil characteristics, especially total nitrogen and total phosphorus dynamics, but not the soil clay content. Therefore, these results suggest that the relationship between SOC stocks and clay content depended on scale sizes. Specifically, on a larger scale with different climatic gradients, the climate may partly determine the changes in SOC and clay dynamics, whereas, at a smaller scale where climate type does not vary considerably, the changes in SOC stocks and clay content may be related to vegetation diversity and soil nutrient dynamics. These results may contribute to future model development and the projection of changes in soil carbon storage.

Biochar Effects on Carbon Stocks in the Coffee Agroforestry Systems of the Himalayas

Coffee agroforestry is an emerging agricultural practice in the mid hills of Nepal. Smallholder farmers of low-income strata have progressively adopted coffee as a perennial crop over seasonal crops. A multi-year study was conducted to test effects of locally produced biochar derived from coffee wastes, e.g., pulp and husks, on carbon stocks of: i) coffee trees, and, ii) soil organic carbon (SOC) in selected coffee growing pockets. We conducted on-farm experimental trials in three different physiographical locations of the Nepal mid-hills, namely, Chandanpur (Site I at 1475masl), Panchkhal (Site II at 1075masl), and Talamarang (Site III at 821masl) where smallholders grow coffee together with other cereal crops and vegetables. We applied biochar to the soil at a rate of 5 Mgha-1, then, monitored the SOC and biomass growth of the coffee trees in the three treatment plots at sites I, II and III over two years beginning in 2013. The average stocks of aboveground carbon in coffee trees increased from 6.2±4.3 Mgha-1 to 9.1±5.2 Mgha-1 over the trial period of two years in biochar treated plots. The same in control plots increased from 5.6±2.8 Mgha-1 to 6.7±4.7 Mgha-1. In the biochar plots, the average increments of ABG carbon was 0.73 Mgh-1 while in the control it was 0.29 Mgh-1. Analysis of soil organic carbon of the plots indicated overall incremental change in carbon stocks in the coffee farms. During the base year, the average SOC stocks in the top 0-15cm layer of the soil at sites I, II, and III were estimated 74.88 ± 15.93; 63.96 ±16.71 and 33.05 ±4.42 Mgha-1 respectively. Although both the biochar treated and control plot registered incremental change in SOC stocks, the volumes were remarkably higher in the former than the latter. Compared to the baseline data, the changes in SOC stocks in the three biochar treated plots were 19.8, 49.8 and 45.3 Mgha-1, respectively, whereas in the control plots these were 8.3, 29.3 and 11.3 Mgha-1, respectively. The higher incremental rates of C-stocks in all the biochar treated plots in comparison to the corresponding control plots of the coffee agroforestry implies that application of biochar can enhance accumulation of carbon in the form of aboveground biomass and soil organic carbon.

SOC Stock Changes and Greenhouse Gas Emissions Following Tropical Land Use Conversions to Plantation Crops on Mineral Soils, with a Special Focus on Oil Palm and Rubber Plantations

The increasing global demand for vegetable oils has resulted in a significant increase in the area under oil palm in the tropics during the last couple of decades, and this is projected to increase further. The Roundtable on Sustainable Palm Oil discourages the conversion of peatlands to oil palm and rubber plantations. However, our understanding of the effects on soil organic carbon (SOC) stocks and associated greenhouse gas (GHG) emissions of land use conversion is incomplete, especially for mineral soils under primary forests, secondary forests, rubber and other perennial plantations in the tropics. In this review we synthesised information on SOC stocks and GHG emissions from tropical mineral soils under forest, oil palm and rubber plantations and other agroecosystems across the tropical regions. We found that the largest SOC losses occurred after land use conversion from primary forest to oil palm and rubber plantations. Secondary forest and pasture lands showed lower SOC losses as well as total GHG (CO2, N2O and CH4) emissions when converted to oil palm and rubber plantations. However, due to the limited data available on all three GHG emissions, there remains high uncertainty in GHG emissions estimates, and regional GHG accounting is more reliable. We recommend long-term monitoring of oil palm and other perennial plantations established on tropical mineral soils on different soil types and regions on SOC stock changes and total GHG emissions and evaluate appropriate management practices to optimise production and sustainable economic returns, and minimise environmental impact.

Increasing crop yields and root input make Canadian farmland a large carbon sink

Soil organic carbon (SOC) in agricultural lands is vital for global food production and greenhouse gas (GHG) mitigation. Accurate quantification of the change in SOC stocks at regional or national scales, which depends heavily on reliable spatiotemporal carbon (C) input data, remains a big challenge. Here we use the process-based RothC model to estimate change in SOC stocks across Canada for 1971 to 2015, based on calculated annual C flows between cropland and livestock sectors. Total C input to 0–20 cm soils from crops, manure, and biosolids in Canada increased by 81% from 1971 to 2015, which shifted Canadian agricultural lands from a CO2 source before 1990 (−1.1 Tg C yr−1) to a small sink during 1990–2005 (4.6 Tg C yr−1), and a larger sink thereafter (10.6 Tg C yr−1). The increasing trend of SOC stocks is mainly driven by the increases in crop yield; the enhanced C sink since 2005 reflects increasing C input largely driven by the increasing area and yield of canola. SOC sequestration showed a potential to offset ~34% or more of agricultural GHG emission since 1990. Increasing crop yields and adopting crop mixes that input proportionately more below-ground C, such as canola and oat, showed potential additional opportunity to sequester SOC, estimated at 1.7 Tg C yr−1 for 2016–2030 in Canada. This study illustrates that SOC sequestration is driven largely by plant C inputs, and shows that agronomic measures which augment C input through crop choices and yield-enhancing practices can profoundly benefit climate mitigation strategies.