Increase Soil Organic Carbon Stocks Research Articles

Effects of simulated sea level rise on stocks and sources of soil organic carbon in Kandelia obovata mangrove forests

Changes in stocks and sources of soil organic carbon (OC) with sea level rise (SLR) are crucial to carbon processes and sequestration benefits in mangrove forests. Soil OC contents and stocks were compared among three 11 years-old Kandelia obovata mangrove forests with SLR of 0, 40 and 80 cm, respectively, simulating the current, future ~100 years’ and future ~200 years’ SLR of mangrove forests along Jiulong River Estuary, an important K. obovata area in Fujian, China. Contribution rates of mangrove plants and allochthonous suspended particulate organic matter in tidal water (SPOM) to soil OC were also estimated. Significantly higher soil OC contents and stocks were measured at site SLR 80 cm. Mean OC stocks at sites SLR 80 cm, SLR 40 cm and SLR 0 cm were 23.82, 16.68 and 17.85 kg C m−2, respectively. High soil labile organic carbon (LOC) contents were also found at site SLR 80 cm, while low profiles of soil δ13C values were measured at site SLR 40 cm. Mean contribution rates of mangrove plants to soil OC were 36.65%, 35.42% and 21.79% at sites SLR 80 cm, SLR 40 cm and SLR 0 cm, respectively, and significantly higher OC contribution rates of mangrove plants were found at site SLR 80 cm. These indicated that SLR will increase soil OC stock and mangrove contribution to soil OC.

Evaluation of biochar applications combined with alternate wetting and drying (AWD) water management in rice field as a methane mitigation option for farmers’ adoption

ABSTRACT Biochar application and alternate wetting and drying (AWD) are emerging as promising technologies recommended for reducing CH4 emissions and water consumption in rice cultivation. In this study, we hypothesized that both technologies could be practiced in combination and this could further reduce CH4 emissions and water consumption when compared to practicing alone. The effects of biochar application and its co-application with chemical fertilizer or compost under conventional or AWD water management on CH4 emissions, productivity of rice, water use, and SOC stock, as well as cost and income were investigated. The experiment was carried out in an irrigated paddy field in the central plain of Thailand during both in the wet and dry seasons. Relative to control (CT), biochar application (BI), its co-application with compost (BC) or chemical fertilizer (BF) reduced seasonal CH4 emissions by 40.6%, 29.5%, and 12.3%, respectively. BI and BC significantly (p < 0.05) reduced grain yield by 19.9% and 10.8%, respectively, while BF significantly increased grain yield by 3.70%. In addition, BI, BC, and BF significantly enhanced soil organic carbon (SOC) stock by 21.2%, 21.4%, and 18.3%, respectively. Compared to the CT, higher production costs were found in BC and BF, but the farmer’s net incomes were also higher in BF because of its higher grain yield. On the other hand, water management in all amendment treatments under AWD was resulted in the reduction of CH4 emissions by the average of 18.8% as compared to the conventional system. AWD decreased rice yield by an average of 2.29%. It significantly reduced irrigation water use by an average of 11.9%, resulting in reducing production cost for water pumping. The results show that the practice that combined biochar application, AWD and chemical fertilizer are feasible for CH4 emission mitigation, SOC stock increase and irrigation water saving without significant effects on yield and farmer income.

Whole-profile soil organic matter content, composition, and stability under cropping systems that differ in belowground inputs

Subsoils have been identified as a potential carbon sink because they typically have low soil organic carbon (SOC) concentrations and high SOC stability. One proposed strategy to increase SOC stocks is to enhance C inputs to the subsoil by increasing crop rotation diversity with deep-rooted perennial crops. Using three long-term field trials in Iowa (study durations of 60, 35, and 12 years), we examined the effects of contrasting cropping systems [maize (Zea mays L.)-soybean (Glycine max (L.) Merr) (= two-year system) vs. maize-soybean-oat (Avena sativa L.)/alfalfa (Medicago sativa L.)-alfalfa or maize-maize-oat/alfalfa-alfalfa (= four-year system)] on above- and below-ground C inputs, as well as the content, biochemical composition, and distribution of SOC among physical fractions differing in stability to 90 cm depth. Average annual total C inputs were similar in the two-year and four-year systems, but the proportion of C delivered belowground was 20–35 % greater in the four-year system. Despite the long duration of these studies, the effect of cropping system on SOC content to 90 cm was inconsistent across trials, ranging from −7 % to +16 % in the four-year relative to the two-year system. At the one site where SOC was significantly greater in the four-year system, the effect of cropping system on SOC content was observed in surface and subsoil layers rather than limited to the subsoil (i.e., below 30 cm). Cropping system had minimal effects on biochemical indicators of plant-derived organic matter or on the proportions of SOC in labile particulate organic matter versus stable mineral-associated organic matter. We conclude that adoption of cropping systems with enhanced belowground C inputs may increase total profile SOC, but the effect is minimal and inconsistent; furthermore, it has minor impact on the vertical distribution, biochemical composition, and stability of SOC in Mollisols of the Midwest U.S.

Impact of vegetation restoration on soil organic carbon stocks and aggregates in a karst rocky desertification area in Southwest China

The objectives of the study were as follows: (a) to determine the response of soil organic carbon (SOC) fractions to vegetation restoration; (b) to examine the contributions of aggregate-associated OC to total soil OC accumulation along vegetation restoration, (c) to identify the factors that affect SOC accumulation along natural vegetation restoration in a karst region in Southwest China. Four vegetation restoration stages, namely, grassland, shrubland, shrub-arbor mixed forestland, and arbor forestland, were compared with cropland (CL). Soil samples were collected at depths of 0–10 cm and separated into five aggregate size fractions. SOC, light fraction OC (LFOC), easily oxidizable OC (EOC), and aggregate-associated OCs were determined for different aggregate sizes and total soil. Natural vegetation restoration increased macroaggregate amount but decreased the fractions of meso- and microaggregates. Vegetation restoration significantly increased total SOC, EOC, and LFOC concentrations and stocks and soil aggregate-associated OC concentrations. The responses of EOC and LFOC in total soil and soil aggregates were more sensitive than those of SOC along with vegetation restoration. Aggregate-associated OC concentrations generally increased with a decrease in aggregate size. Macro- and microaggregate-associated OC stocks increased, but mesoaggregate-associated OC stocks decreased following the conversion of CL to a natural vegetation ecosystem. The increase in SOC stocks was primarily attributed to the macroaggregate-associated OC stocks and their changes. OC concentrations and stocks in total soil and the soil aggregates were significantly positively related to exchangeable calcium content. Vegetation restoration considerably affects the amount of soil aggregates, OC concentrations, and stocks in total soil and soil aggregates. Changes in OC concentrations and stocks can be more pronounced in the liable carbon fraction than in total SOC. The increase in SOC was mostly attributed to OC accumulation in macroaggregates. Exchangeable calcium also affected soil OC accumulation in total soil and soil aggregates.

Soil Organic Carbon Fractions and Stocks as Affected by Organic Fertilizers in Rice Paddy Soil

This research evaluated the effect of different organic fertilization (OF) on soil organic carbon (SOC) stock and fractions in rice paddy soil. The OF system consisted with NPK, NPK+Rice straw (NPK+RS), NPK+Cow compost (NPK+CC), NPK+Swine compost (NPK+SC) and no-fertilization (Control). Labile organic C fractions and humification rates were characterized and SOC stock was determined with bulk density and SOC content. Application of OF significantly SOC content by 7-12% and 23-33% over the NPK and control, while it decreased bulk density by 2-6% and 11-14%. The lowest bulk density was found in RS, which indicated the improvement of soil physical property. The highest SOC stock (38-39 kg C ha-1) was observed in SC and CC treatments. The SOC stock showed significant negative correlation with bulk density (p < 0.001), and positively correlated with humification rate, grain yield (p < 0.01) and HWEC and straw yield (p < 0.05), indicating key roles of SOC on soil quality. These results showed that the OF application largely increased SOC stocks, rice yield, and concentration of labile C and stable C. Therefore, application of OF is recommendable for suitable soil management strategy to improve soil fertility and increase crop yield in rice paddy field. Correlation analysis between soil organic carbon fractions under different fertilization system. Parameter SOC† Bulk density HA FA HA/FA§ HR¶ HA/SOM∮ FA/SOM∮ WEC∫ HWEC∫ Grain Straw SOC stock 1 B. density - < 0.001***† 1 HA 0.103 0.070 1 FA 0.013* 0.027* 0.008** 1 HA/FA 0.754 0.512 < 0.001*** 0.801 1 HR 0.008 - 0.010** < 0.001*** < 0.001*** 0.021* 1 HA/SOM 0.032* - 0.026* < 0.001*** 0.004** 0.001*** 0.001*** 1 FA/SOM 0.001*** - 0.007** 0.009** < 0.001*** 0.787 0.001*** 0.003** 1 WEC < 0.001 - 0.001*** 0.214 0.026* 0.986 0.035* 0.099 0.006** 1 HWEC 0.047* - 0.039* 0.431 0.045* 0.680 0.153 0.310 0.030* 0.07 1 Grain 0.011* - 0.004** 0.157 0.450 0.206 0.105 0.093 0.254 0.10 0.060 1 Straw 0.043* - 0.004** 0.433 0.857 0.360 0.356 0.315 0.586 0.039* 0.207 < 0.001*** 1 †*, **, and *** denote significance at the 5, 1, and 0.1% levels, respectively.†SOC: Soil organic carbon§HA/FA: Ratio of humic acid content and fulvic acid content¶HR: Humification rate∮HA/SOM and FA/SOM: Proportion of humic acid to SOM content and Proportion of fulvic acid to SOM content∫WEC and HWEC: Water extractable carbon and hot water extractable carbon

Carbon storage in a rainfed Mediterranean vertisol: Effects of tillage and crop rotation in a long‐term experiment

AbstractThe storage of carbon (C) in cultivated soils can be increased with the adoption of different practices. The objective of this study was to determine soil organic carbon (SOC) storage in the 0‐ to 90‐cm depth profile, in four different soil layers (0–15, 15–30, 30–60 and 60–90 cm) in a long‐term (29 years) experiment established in 1986 on a rainfed Mediterranean Vertisol in southern Spain. The treatments studied were: conventional tillage (CT) versus no‐tillage (NT); five 2‐year crop rotations (wheat–chickpea, wheat–sunflower, wheat–bare fallow, wheat–faba bean, and continuous wheat; and nitrogen (N) fertilizer applied to wheat at four rates (0, 50, 100 and 150 kg N ha−1). The SOC accumulation was higher in the 30–60‐cm layer (9.2 Mg ha−1) due to the size of the characteristic cracks of Vertisols under semiarid conditions. Over the 29‐year study period, the SOC in the 0–90‐cm layer increased by 23.6 Mg ha−1 due to the change in residue management. The NT treatment exhibited a higher mean annual rate of organic C accumulation compared with the CT treatment (1.0 and 0.66 Mg ha−1 year−1, respectively) due to the retention of the mulched residue. Additionally, crop rotation influenced the rate of organic C accumulation, with wheat‐faba bean, wheat‐sunflower and continuous wheat exhibiting the highest levels of C storage in comparison to the other treatments. In rainfed Mediterranean agriculture, the selection of no‐tillage along with a rotation with legumes is key to improving soil fertility and increasing C reserves and the rate of C accumulation by soil.Highlights The SOC increase over the 29‐year study period was 23.6 Mg ha−1 in the 0–90‐cm soil profile. The characteristic cracks of Vertisol increase SOC stocks in deeper soil layers. No tillage accumulated 66.2% more SOC than conventional tillage in the 0–90‐cm profile. The wheat–sunflower rotation had the highest annual rate of C sequestration (1 Mg ha−1 year−1) and wheat–chickpea the lowest (0.6 Mg ha−1 year−1) over 29 years.

Effect of Water and Rice Straw Management Practices on Soil Organic Carbon Stocks in a Double-Cropped Paddy Field

ABSTRACTWater and rice straw (RS) management practices can potentially affect the accumulation of soil organic carbon (SOC) in agricultural soils. Field experiments were conducted in two consecutive rice-growing seasons (wet and dry) to evaluate SOC stocks under different water (continuous flooding [CF], alternate wetting and drying [AWD]) and RS management practices (RS incorporation [RS-I], RS burning [RS-B], without RS incorporation and burning [WRS]) in a double-cropped paddy field. RS-I under AWD had higher volumetric water content than the same RS management under CF at tillering in both growing seasons. Total SOC was significantly higher under AWD at tillering in both wet and dry seasons and after harvesting in the dry season compared with CF. The same trend was also observed for C:N ratio at tillering and after harvesting in the dry season. RS-B plots had lower SOC stocks than RS-I and WRS plots across most of the measuring periods regardless of the growing seasons. SOC stocks were 33.09 and 39.31 Mg/ha at RS-B and RS-I plots, respectively, in the wet season, whereas the respective values were 21.45 and 24.55 Mg/ha in the dry season. Incorporation of RS enhanced SOC stocks under AWD irrigation, especially in the dry season before planting. Soil incorporation of RS in combination with AWD could be a viable option to increase SOC stocks in the double-cropped rice production region as it is strongly linked with soil fertility and productivity. However, the environmental consequences of RS incorporation in irrigated lowland rice production system should be taken into consideration before its recommendation for paddy field on a large scale.

Deep soil inventories reveal that impacts of cover crops and compost on soil carbon sequestration differ in surface and subsurface soils.

Increasing soil organic carbon (SOC) via organic inputs is a key strategy for increasing long-term soil C storage and improving the climate change mitigation and adaptation potential of agricultural systems. A long-term trial in California's Mediterranean climate revealed impacts of management on SOC in maize-tomato and wheat-fallow cropping systems. SOC was measured at the initiation of the experiment and at year 19, at five depth increments down to 2m, taking into account changes in bulk density. Across the entire 2m profile, SOC in the wheat-fallow systems did not change with the addition of N fertilizer, winter cover crops (WCC), or irrigation alone and decreased by 5.6% with no inputs. There was some evidence of soil C gains at depth with both N fertilizer and irrigation, though high variation precluded detection of significant changes. In maize-tomato rotations, SOC increased by 12.6% (21.8MgC/ha) with both WCC and composted poultry manure inputs, across the 2m profile. The addition of WCC to a conventionally managed system increased SOC stocks by 3.5% (1.44MgC/ha) in the 0-30cm layer, but decreased by 10.8% (14.86MgC/ha) in the 30-200cm layer, resulting in overall losses of 13.4MgC/ha. If we only measured soil C in the top 30cm, we would have assumed an increase in total soil C increased with WCC alone, whereas in reality significant losses in SOC occurred when considering the 2m soil profile. Ignoring the subsoil carbon dynamics in deeper layers of soil fails to recognize potential opportunities for soil C sequestration, and may lead to false conclusions about the impact of management practices on C sequestration.

Mitigation potential and environmental impact of centralized versus distributed BECCS with domestic biomass production in Great Britain

AbstractNew contingency policy plans are expected to be published by the United Kingdom government to set out urgent actions, such as carbon capture and storage, greenhouse gas removal and the use of sustainable bioenergy to meet the greenhouse gas reduction targets of the 4th and 5th Carbon Budgets. In this study, we identify two plausible bioenergy production pathways for bioenergy with carbon capture and storage (BECCS) based on centralized and distributed energy systems to show what BECCS could look like if deployed by 2050 in Great Britain. The extent of agricultural land available to sustainably produce biomass feedstock in the centralized and distributed energy systems is about 0.39 and 0.5 Mha, providing approximately 5.7 and 7.3 MtDM/year of biomass respectively. If this land‐use change occurred, bioenergy crops would contribute to reduced agricultural soil GHG emission by 9 and 11 /year in the centralized and distributed energy systems respectively. In addition, bioenergy crops can contribute to reduce agricultural soil ammonia emissions and water pollution from soil nitrate leaching, and to increase soil organic carbon stocks. The technical mitigation potentials from BECCS lead to projected CO2 reductions of approximately 18 and 23 /year from the centralized and distributed energy systems respectively. This suggests that the domestic supply of sustainable biomass would not allow the emission reduction target of 50 /year from BECCS to be met. To meet that target, it would be necessary to produce solid biomass from forest systems on 0.59 or 0.49 Mha, or alternatively to import 8 or 6.6 MtDM/year of biomass for the centralized and distributed energy system respectively. The spatially explicit results of this study can serve to identify the regional differences in the potential capture of CO2 from BECCS, providing the basis for the development of onshore CO2 transport infrastructures.

Can reduced tillage sustain sugarcane yield and soil carbon if straw is removed?

Straw removal for bioelectricity, cellulosic ethanol, or other bio-products has become a common practice in Brazilian sugarcane areas, but long-term effects on soil organic carbon (SOC) and yield are unknown. Our objective was to quantify tillage intensity and removal levels on SOC stocks and sugarcane yield on contrasting edaphoclimatic conditions in Brazil. Conventional tillage-CT and reduced tillage-RT coupled with three straw removal levels (none—NR; moderate—MR; total—TR) were combined to create six treatment combinations: CTNR, CTMR, CTTR, RTNR, RTMR, and RTTR, respectively. Sugarcane yield was measured annually and soil samples to a depth of 40 cm were collected and analyzed before and after 5 years of treatments. Neither tillage practices nor straw removal significantly affected sugarcane yield on sandy loam soil, but CTMR, CTTR, and RTTR showed substantial yield reductions on clayey soil. SOC stocks for RTNR increased by as much as 1.64 Mg ha−1 year−1 on clayey soil, while CTMR and CTTR favored depletion of SOC stocks on both soils. Overall, each Mg of straw returned to the soil increased SOC stock by 95 kg ha−1 for clayey soil under both tillage practices and 55 kg ha−1 for sandy loam under the CT system. Furthermore, straw removal decreased microbial biomass C and b-glucosidase activity. These findings suggest that the adoption of RT attenuates the adverse impacts of straw removal on SOC stocks while ensuring sugarcane yields for a more sustainable bioenergy production in Brazil.

Biochar Application and Soil Carbon Stocks in Semi-Arid Vertisols

Biochar application as a soil amendment has improved crop productivity, soil properties, and long-term carbon (C) storage in soils. In order to ascertain these positive effects of biochar, a six-year study was conducted on deep black soil in a typical semi-arid setup of Karnataka. Biochar (prepared from Prosopis juliflora¬) was applied and mixed at rates ranging from 2.5 to 20 t ha-1 to rabi sorghum under rainfed conditions before the start of the study. The six treatment combinations were evaluated through their effects on run-off, soil loss, sorghum yields, soil properties, and increase in soil organic carbon (SOC) stock at the end of the study period. Sorghum yield increased with the levels of biochar application, with yields under 10 and 20 t ha-1 levels of addition being statistically at par. Soil loss was not significantly affected by biochar application, and so were most of the soil properties. However, there was an increase in SOC stock to the tune of 0.28 and 0.43 t ha-1y-1 through biochar application @ 10 and 20 t ha-1, respectively, pointing at the C sequestration potential of biochar. Increased crop yields by applying biochar can be attributed to enhanced soil aggregation and water holding capacity and increased nutrient cycling and uptake by plant roots.

Residue management induced changes in soil organic carbon and total nitrogen under different tillage practices in the North China Plain

Crop residue retention has been considered a practicable strategy to improve soil organic carbon (SOC) and total nitrogen (TN), but the effectiveness of residue retention might be different under varied tillage practices. To evaluate the effects of residue management on the distribution and stocks of SOC and TN under different tillage practices, a bifactorial experiment with three levels for tillage practices (no-tillage, rotary tillage, and conventional tillage) and two levels for residue managements (residue retention and residue removal) was conducted in the North China Plain (NCP). Results showed that after a short experimental duration (3−4 years), concentrations of SOC and TN in the 0−10 cm layer were higher under no-tillage than under conventional tillage, no matter whether crop residues were retained or not. Residue retention increased SOC and TN concentrations in the upper layers of soil to some degree for all tillage practices, as compared with residue removal, with the greatest increment of SOC concentration occurred in the 0−10 cm layer under rotary tillage, but in the 10−30 cm layer under conventional tillage. The stocks of SOC in the 0−50 cm depth increased from 49.89 Mg ha−1 with residue removal to 53.03 Mg ha−1 with residue retention. However, no-tillage did not increase SOC stock to a depth of 50 cm relative to conventional tillage, and increased only by 5.35% as compared with rotary tillage. Thus, residue retention may contribute more towards SOC sequestration than no-tillage. Furthermore, the combination between residue retention and no-tillage has the greatest advantage in enhancing SOC and TN in the NCP region.

Soil organic carbon stock on the Majorca Island: Temporal change in agricultural soil over the last 10 years

For quite a long time, soil organic carbon stock (SOCS) has reduced from reconverting forest areas into agricultural land or by inadequate agronomic practices. However in recent decades in Mediterranean areas, abandonment of agricultural areas due to lack of economic profits and to promoting tourism has fostered a change in land use that has impacted soil carbon. In line with this, the Majorca Island (Spain) could be a good study area to evaluate temporary changes in carbon stocks in soils in areas with a Mediterranean climate. The present study analysed the spatial distribution of SOCS and evaluated the temporal changes over 11 years in agricultural soils in relation to the influence of land use and land management’. The global carbon budget (estimated at 30 cm depth) on the Majorca Island was estimated at 31.23 Tg C, and showed wide spatial variability. As expected, SOCS was higher in the forest areas of the Tramuntana Mountain Range (>100 Mg ha−1), while the lowest contents were found in agricultural use (79.3 Mg ha−1), located mainly in the centre and south of this island.The total C stored from 2006 to 2017 increased by 2.62 Tg C (15%) in agricultural areas on the island. We noted a major increase in SOCS in the agricultural zones on mountain slopes (>2-fold higher), associated with abandoned crops in terrace cultivation, but grassland systems also increased. Nonetheless, the present study shows a sharp drop in SOCS in the centre and south of this island. This decrease was more pronounced in annual crops (−14.5%), which could be attributed to intensive soil management and increased irrigated land. Land abandonment has been indicated as the main potential to carbon sequestration in soil, but this potential for carbon sequestration is a finite process. Certain changes in agricultural practices on Majorca, and in rural Mediterranean areas in general, are necessary to avoid carbon loss in cropland soils. Adoption of conservation agriculture practices, e.g. cover crops, crop rotation, organic additions and reduced tillage techniques, can help increase SOC levels in rural Mediterranean areas.

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.

The effects of land abandonment and long‐term afforestation practices on the organic carbon stock and lignin content of Mediterranean humid mountain soils

Afforestation is an important strategy that can decrease atmospheric carbon by sequestering carbon in biomass and soil. In Spain, an active afforestation programme was adopted in the 1950s when the soil was severely eroded after widespread abandonment of arable land. The Araguás catchment (Central Spanish Pyrenees) is a good example of this programme because it was afforested with both Pinus sylvestris L. (PS) and Pinus nigra J.F.Arnold (PN). The soil organic carbon (SOC) stock and lignin content (based on the vanillyl, syringyl and cinnamyl contents) of these afforested soils were examined and compared to those of bare soil, secondary succession and meadow soils. Both the SOC stock and lignin content were used to evaluate the effects of land‐use changes on soil. Curie‐point pyrolysis with tetramethylammonium hydroxide was used to assess the lignin content. In the bare soil, there was none of the lignin compounds. The largest SOC stock and lignin content occurred under PN and secondary succession sites. A decreasing trend for the lignin content, related to the limited organic matter input and the longer degradation period, was observed at deeper horizons in all soils except meadows. These meadow soils also showed increased SOC stocks in deeper horizons. Land abandonment reduced the SOC stock although no significant differences were observed in the organic carbon incorporation assessed through lignin content (and if this was so it was restricted to the top centimetre or so). According to the results, PN was the best afforestation practice for increasing SOC stock and lignin content in soil. Pinus sylvestris afforestation was less successful than secondary succession at increasing SOC sequestration and lignin content.Highlights Effects of long‐term afforestation and land abandonment assessed in Mediterranean humid mountain soils. Soil organic carbon (SOC) stock and lignin content were used as indicators. Bare soil had the smallest SOC stock and lignin content. Afforestation with Pinus nigra was the best practice, increasing SOC stock and lignin content.

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.

Deep soil flipping increases carbon stocks of New Zealand grasslands.

Sequestration of soil organic carbon (SOC) has been recognized as an opportunity to off‐set global carbon dioxide (CO2) emissions. Flipping (full inversion to 1–3 m) is a practice used on New Zealand's South Island West Coast to eliminate water‐logging in highly podzolized sandy soils. Flipping results in burial of SOC formed in surface soil horizons into the subsoil and the transfer of subsoil material low in SOC to the “new” topsoil. The aims of this study were to quantify changes in the storage and stability of SOC over a 20‐year period following flipping of high‐productive pasture grassland. Topsoils (0–30 cm) from sites representing a chronosequence of flipping (3–20 years old) were sampled (2005/07) and re‐sampled (2017) to assess changes in topsoil carbon stocks. Deeper samples (30–150 cm) were also collected (2017) to evaluate the changes in stocks of SOC previously buried by flipping. Density fractionation was used to determine SOC stability in recent and buried topsoils. Total SOC stocks (0–150 cm) increased significantly by 69 ± 15% (179 ± 40 Mg SOC ha‐1) over 20 years following flipping. Topsoil burial caused a one‐time sequestration of 160 ± 14 Mg SOC ha‐1 (30–150 cm). The top 0–30 cm accumulated 3.6 Mg SOC ha‐1 year‐1. The chronosequence and re‐sampling revealed SOC accumulation rates of 1.2–1.8 Mg SOC ha‐1 year‐1 in the new surface soil (0–15 cm) and a SOC deficit of 36 ± 5% after 20 years. Flipped subsoils contained up to 32% labile SOC (compared to <1% in un‐flipped subsoils) thus buried SOC was preserved. This study confirms that burial of SOC and the exposure of SOC depleted subsoil results in an overall increase of SOC stocks of the whole soil profile and long‐term SOC preservation.

National estimation of soil organic carbon storage potential for arable soils: A data-driven approach coupled with carbon-landscape zones

Soil organic carbon (SOC) is important for its contributions to agricultural production, food security, and ecosystem services. Increasing SOC stocks can contribute to mitigate climate change by transferring atmospheric CO2 into long-lived soil carbon pools. The launch of the 4 per 1000 initiative has resulted in an increased interest in developing methods to quantity the additional SOC that can be stored in soil under different management options. In this work, we have made a first attempt to estimate SOC storage potential of arable soils using a data-driven approach based on the French National Soil Monitoring Network. The data-driven approach was used to determine the maximum SOC stocks of arable soils for France. We first defined different carbon-landscape zones (CLZs) using clustering analysis. We then computed estimates of the highest possible values using percentile of 0.8, 0.85, 0.9 and 0.95 of the measured SOC stocks within these CLZs. The SOC storage potential was calculated as the difference between the maximum SOC stocks and current SOC stocks for topsoil and subsoil. The percentile used to determine highest possible SOC had a large influence on the estimates of French national SOC storage potential. When the percentile increased from 0.8 to 0.95, the national SOC storage potential increased by two to three-fold, from 336 to 1020 Mt for topsoil and from 165 to 433 Mt for subsoil, suggesting a high sensitivity of this approach to the selected percentile. Nevertheless, we argue that this approach can offer advantages from an operational point of view, as it enables to set targets of SOC storage taking into account both policy makers' and farmers' considerations about their feasibility. Robustness of the estimates should be further assessed using complementary approaches such as mechanistic modelling.

Net Global Warming Potential of Spring Wheat Cropping Systems in a Semiarid Region

Investigations of global warming potential (GWP) of semiarid cropping systems are needed to ascertain agriculture’s contributions to climate regulation services. This study sought to determine net GWP for three semiarid cropping systems under no-tillage management in the northern Great Plains of North America: spring wheat (Triticum aestivum L.)—fallow (SW-F), continuous spring wheat (CSW) and spring wheat—safflower (Carthamus tinctorius L.)—rye (Secale cereale L.) (SW-S-R). Management records, coupled with published carbon dioxide (CO2) emission estimates, were used to determine emissions from production inputs and field operations. Static chamber methodology was used to measure soil-atmosphere methane (CH4) and nitrous oxide (N2O) fluxes over a 3-year period and changes in profile soil organic carbon (SOC) stocks were determined over 18 years. Carbon dioxide emissions associated with production inputs and field operations were greatest for CSW, intermediate for SW-S-R and lowest for SW-F. All cropping systems were minor CH4 sinks (≤0.5 kg CH4-C ha−1 yr−1) and moderate N2O sources (1.0 to 2.8 kg N2O-N ha−1 yr−1). No differences in SOC stocks were observed among cropping systems (P = 0.78), nor did SOC stocks change significantly from baseline conditions (P = 0.82). Summing across factors, net GWP was positive for SW-F and CSW, implying net greenhouse gas (GHG) emission to the atmosphere, while net GWP for SW-S-R was negative, implying net GHG uptake. Net GWP, however, did not differ among cropping systems (P = 0.17). Management practices that concurrently improve N use efficiency and increase SOC stocks are needed for semiarid cropping systems to be net GHG sinks.

Vertical dynamics of soil organic carbon and its influence on Chengdu Plain paddy soils from the 1980s to the 2010s

ABSTRACTThe vertical dynamics of paddy soil organic carbon (SOC) play an important role in soil quality and carbon cycling. In this study, we used an exponential decay function to estimate the vertical dynamics of SOC content and determined its influencing factors in a typical paddy soil area of the Chengdu Plain from the 1980s to the 2010s. Our results show an overall increase in SOC stocks at 0–100 cm from 11.8 Tg in the 1980s to 13.7 Tg in the 2010s. SOC content increased at depths of 0–40 cm and declined at depths of 40–100 cm over the past three decades. The exponential decay function parameters C0 and k significantly increased by 31.4% and 18.2% respectively, which suggests the vertical pattern of SOC distribution changed. The increase in nugget effects of C0 and k and the decrease in the relative contributions of the parent material, subgroup and distance-to-river indicate that extrinsic factors played increasing roles in the vertical variation of SOC content. Our study concludes that rice planting has led to vertical variations of SOC content and decreased the effects of intrinsic factors on the vertical variation of SOC content of Chengdu Plain paddy soils over the past three decades.