Carbon Sequestration in Reclaimed Minesoils

Minesoils are drastically influenced by anthropogenic activities. They are characterized by low soil organic matter (SOM) content, low fertility, and poor physicochemical and biological properties, limiting their quality, capability, and functions. Reclamation of these soils has potential for resequestering some of the C lost and mitigating CO₂ emissions. Soil organic carbon (SOC) sequestration rates in minesoils are high in the first 20 to 30 years after reclamation in the top 15 cm soil depth. In general, higher rates of SOC sequestration are observed for minesoils under pasture and grassland management than under forest land use. Observed rates of SOC sequestration are 0.3 to 1.85 Mg C ha^{? 1} yr^{? 1} for pastures and rangelands, and 0.2 to 1.64 Mg C ha^{? 1} yr^{? 1} for forest land use. Proper reclamation and postreclamation management may enhance SOC sequestration and add to the economic value of the mined sites. Management practices that may enhance SOC sequestration include increasing vegetative cover by deep-rooted perennial vegetation and afforestation, improving soil fertility, and alleviation of physical, chemical and biological limitations by fertilizers and soil amendments such as biosolids, manure, coal combustion by-products, and mulches. Soil and water conservation are important to SOC sequestration. The potential of SOC sequestration in minesoils of the US is estimated to be 1.28 Tg C yr^?1, compared to the emissions from coal combustion of 506 Tg C yr^{? 1}.     

Soil organic carbon stock change due to land use activity along the agricultural frontier of the southwestern Amazon,Brazil, between 1970 and 2002

The southwestern portion of the Brazilian Amazon arguably represents the largest agricultural frontier in the world, and within this region the states of Rondônia and Mato Grosso have about 24% and 32% of their respective areas under agricultural management, which is almost half of the total area deforested in the Brazilian Amazon biome. Consequently, it is assumed that deforestation in this region has caused substantial loss of soil organic carbon (SOC). In this study, the changes in SOC stocks due to the land use change and management in the southwestern Amazon were estimated for two time periods from 1970–1985 and 1985–2002. An uncertainty analysis was also conducted using a Monte Carlo approach. The results showed that mineral soils converted to agricultural management lost a total of 5.37 and 3.74 Tg C yr^?1 between 1970–1985 and 1985–2002, respectively, along the Brazilian Agricultural Frontier in the states of Mato Grosso and Rondônia. Uncertainties in these estimates were ±37.3% and ±38.6% during the first and second time periods, respectively. The largest sources of uncertainty were associated with reference carbon (C) stocks, expert knowledge surveys about grassland condition, and the management factors for nominal and degraded grasslands. These results showed that land use change and management created a net loss of C from soils, however, the change in SOC stocks decreased substantially from the first to the second time period due to the increase in land under no‐tillage.     

Uncertainty analysis of soil organic carbon stock change in Canadian cropland from 1991 to 2001

National estimates of changes in the amount of soil organic carbon (SOC) in cropland requires an assessment of uncertainty for accounting and reporting under the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol. Canada has data sets on SOC stocks in croplands, historical changes in SOC levels due to management practices, and historical changes in the area of land devoted to certain soil management practices. We conducted an analysis of uncertainty of the change in SOC levels due to management practices in Canada from 1991 to 2001 using Monte Carlo analysis and a simple model. Probability distribution functions were determined for each of the inputs of the model, enabling us to assess the uncertainty for the output. The storage rate of SOC in cropland soils of Canada for the 10‐year period ranged from 3.2 to 8.3 Mt C yr^?1 at 95% confidence, with a mean of 5.7 Mt C yr^?1. Approximately 67% (about 3.8 Mt C yr^?1) of the increase in SOC storage in Canada occurred in Saskatchewan where the cropland area under no‐till increased from 10% to 35%, and the area of summer‐fallow declined from 43% to 20% during the study period. The large uncertainty in the effect of no‐till on SOC stock changes in the Gray‐Brown Luvisols of Ontario contributed most to the variance in the model output. If trends in agricultural management continue for the next 10‐year census period, the estimated SOC storage would comprise between 7% and 19% of the gap required to achieve the 6% reduction in 1990 greenhouse gas emission levels for Canada under the Kyoto Protocol.     

Climate- and crop-responsive emission factors significantly alter estimates of current and future nitrous oxide emissions from fertilizer use

The current Intergovernmental Panel on Climate Change (IPCC) default methodology (tier 1) for calculating nitrous oxide (N₂O) emissions from nitrogen applied to agricultural soils takes no account of either crop type or climatic conditions. As a result, the methodology omits factors that are crucial in determining current emissions, and has no mechanism to assess the potential impact of future climate and land‐use change. Scotland is used as a case study to illustrate the development of a new methodology, which retains the simple structure of the IPCC tier 1 methodology, but incorporates crop‐ and climate‐dependent emission factors (EFs). It also includes a factor to account for the effect of soil compaction because of trampling by grazing animals. These factors are based on recent field studies in Scotland and elsewhere in the UK. Under current conditions, the new methodology produces significantly higher estimates of annual N₂O emissions than the IPCC default methodology, almost entirely because of the increased contribution of grazed pasture. Total emissions from applied fertilizer and N deposited by grazing animals are estimated at 10 662 t N₂O‐N yr^?1 using the newly derived EFs, as opposed to 6 796 t N₂O‐N yr^?1 using the IPCC default EFs. On a spatial basis, emission levels are closer to those calculated using field observations and detailed soil modelling than to estimates made using the IPCC default methodology. This can be illustrated by parts of the western Ayrshire basin, which have previously been calculated to emit 8–9 kg N₂O‐N ha^?1 yr^?1 and are estimated here as 6.25–8.75 kg N₂O‐N ha^?1 yr^?1, while the IPCC default methodology gives a maximum emission level of only 3.75 kg N₂O‐N ha^?1 yr^?1 for the whole area. The new methodology is also applied in conjunction with scenarios for future climate‐ and land‐use patterns, to assess how these emissions may change in the future. The results suggest that by 2080, Scottish N₂O emissions may increase by up to 14%, depending on the climate scenario, if fertilizer and land management practices remain unchanged. Reductions in agricultural land use, however, have the potential to mitigate these increases and, depending on the replacement land use, may even reduce emissions to below current levels.     

Quantifying global greenhouse gas emissions from land‐use change for crop production

Many assessments of product carbon footprint (PCF) for agricultural products omit emissions arising from land‐use change (LUC). In this study, we developed a framework based on IPCC national greenhouse gas inventory methodologies to assess the impacts of LUC from crop production using oil palm, soybean and oilseed rape as examples. Using ecological zone, climate and soil types from the top 20 producing countries, calculated emissions for transitions from natural vegetation to cropland on mineral soils under typical management ranged from ?4.5 to 29.4 t CO₂‐eq ha^?1 yr^?1 over 20 years for oil palm and 1.2–47.5 t CO₂‐eq ha^?1 yr^?1 over 20 years for soybeans. Oilseed rape showed similar results to soybeans, but with lower maximum values because it is mainly grown in areas with lower C stocks. GHG emissions from other land‐use transitions were between 62% and 95% lower than those from natural vegetation for the arable crops, while conversions to oil palm were a sink for C. LUC emissions were considered on a national basis and also expressed per‐tonne‐of‐oil‐produced. Weighted global averages indicate that, depending on the land‐use transition, oil crop production on newly converted land contributes between ?3.1 and 7.0 t CO₂‐eq t oil production^?1 yr^?1 for palm oil, 11.9–50.6 t CO₂‐eq t oil production^?1 yr^?1 for soybean oil, and 7.7–31.4 t CO₂‐eq t oil production^?1 yr^?1 for rapeseed oil. Assumptions made about crop and LUC distribution within countries contributed up to 66% error around the global averages for natural vegetation conversions. Uncertainty around biomass and soil C stocks were also examined. Finer resolution data and information (particularly on land management and yield) could improve reliability of the estimates but the framework can be used in all global regions and represents an important step forward for including LUC emissions in PCFs.     

Estimation of forest carbon budget from land cover change in South and North Korea between 1981 and 2010

This paper quantified carbon budget in the past 30 years (1981–2010) and identified the impact of land cover change on carbon dynamics using vegetation integrated simulator for trace gases (VISIT) model. North Korea was converted from carbon sink to source with 10.72 ± 5.18 Tg C yr^?1 of net ecosystem production (NEP) in the 1980s, 3.00 ± 7.96 Tg C yr^?1 in the 1990s, and ?0.46 ± 5.13 Tg C yr^?1 in the 2000s. NEP in South Korea was 10.55 ± 1.09 Tg C yr^?1 in the 1980s, 10.47 ± 7.28 Tg C yr^?1 in the 1990s, and 6.32 ± 5.02 Tg C yr^?1 in the 2000s, showing a gradual decline. In North Korea, NEP was decreased by 0.52 Tg yr^?1 in the 1990s due to reduction of forest, and increased by 0.36 Tg yr^?1 in the 2000s due to expansion of cropland. In South Korea, it was decreased by 0.24 Tg yr^?1 in the 1990s as urban and built-up area expanded, and increased by 0.04 Tg yr^?1 in the 2000s with the expansion of forest. These results suggest the importance of forest and land cover management against deforestation for ensuring national carbon balance.     

Soil organic carbon changes in landscape units of Belgium between 1960 and 2000 with reference to 1990

The present study quantifies changes in soil organic carbon (SOC) stocks in Belgium between 1960, 1990 and 2000 for 289 spatially explicit land units with unique soil association and land‐use type, termed landscape units (LSU). The SOC stocks are derived from multiple nonstandardized sets of field measurements up to a depth of 30 cm. Approximately half of the LSU show an increase in SOC between 1960 and 2000. The significant increases occur mainly in soils of grassland LSU in northern Belgium. Significant decreases are observed on loamy cropland soils. Although the largest SOC gains are observed for LSU under forest (22 t C ha^?1 for coniferous and 29 t C ha^?1 for broadleaf and mixed forest in the upper 30 cm of soil), significant changes are rare because of large variability. Because the number of available measurements is very high for agricultural land, most significant changes occur under cropland and grassland, but the corresponding average SOC change is smaller than for forests (9 t C ha^?1 increase for grassland and 1 t C ha^?1 decrease for cropland). The 1990 data for agricultural LSU show that the SOC changes between 1960 and 2000 are not linear. Most agricultural LSU show a higher SOC stock in 1990 than in 2000, especially in northern Belgium. The observed temporal and spatial patterns can be explained by a change in manure application intensity. SOC stock changes caused by land‐use change are estimated. The SOC change over time is derived from observed differences between SOC stocks in space. Because SOC stocks are continuously influenced by a number of external factors, mainly land‐use history and current land management and climate, this approach gives only an approximate estimate whose validity is limited to these conditions.     

Driving forces of soil organic carbon evolution at the landscape and regional scale using data from a stratified soil monitoring

Soil monitoring programmes face significant challenges as there is an important trade‐off between detecting significant changes in soil properties on the one hand (which can be achieved by minimizing variability by higher sampling density or stratification approaches), and identifying the driving forces responsible for these changes on the other hand (which requires enough variability). This study aims to reconcile these two objectives by identifying the driving forces of soil organic carbon (SOC) evolution over a long period, based on an extensive but stratified soil monitoring programme. Data at both the finest level (questionnaires to the farmers) and the large scale (agricultural census, climate and soil databases for southern Belgium) were used in a cluster analysis, multiple linear regressions and mixed odels in order to discriminate between the driving forces involved. Results indicated that the negative ‘baseline effect’ (i.e. the inversely proportional effect of the initial SOC content on the SOC evolution) was responsible for an important part of the SOC variability. Consequently, the systems are not at steady state when starting the observations, although this assumption is used by most SOC dynamic models. Moreover, the baseline effect resulted in a trend of the soils to converge towards a regional SOC stock which significantly differed according to land use (36.4 t C ha^?1 for the plough depth of cropland and 92.2 t C ha^?1 for the 0–30 cm layer of grassland). Despite this strong effect, the main driving forces of the SOC decrease of cropland (?0.2 t C ha^?1 yr^?1) and SOC increase of grassland (+0.2 t C ha^?1 yr^?1) over a period of 50 years were discriminated. The agricultural management (cropland) and the clay content (grassland), together with the change in precipitation (to a lesser degree for cropland) were highlighted as the predominant factors involved in SOC evolution, when land use change is excluded. The use of questionnaires allowed to better understanding the impact of an intensive agricultural management on the SOC content, as the lowest SOC stocks were associated to the most intensively managed fields. The mixed models partly succeeded in predicting SOC evolution as they presented still large uncertainties after validation (mean error from 3% to 25%, root mean square error of prediction from 21% to 242%). While SOC monitoring schemes are increasingly being implemented, our results will likely apply to those using a similar design. It was shown that this strategy succeeded to reconcile both the SOC change detection and the distinction of the driving forces involved at the regional scale.     

Scale and uncertainty in modeled soil organic carbon stock changes for US croplands using a process‐based model

Process‐based model analyses are often used to estimate changes in soil organic carbon (SOC), particularly at regional to continental scales. However, uncertainties are rarely evaluated, and so it is difficult to determine how much confidence can be placed in the results. Our objective was to quantify uncertainties across multiple scales in a process‐based model analysis, and provide 95% confidence intervals for the estimates. Specifically, we used the Century ecosystem model to estimate changes in SOC stocks for US croplands during the 1990s, addressing uncertainties in model inputs, structure and scaling of results from point locations to regions and the entire country. Overall, SOC stocks increased in US croplands by 14.6 Tg C yr^?1 from 1990 to 1995 and 17.5 Tg C yr^?1 during 1995 to 2000, and uncertainties were ±22% and ±16% for the two time periods, respectively. Uncertainties were inversely related to spatial scale, with median uncertainties at the regional scale estimated at ±118% and ±114% during the early and latter part of 1990s, and even higher at the site scale with estimates at ±739% and ±674% for the time periods, respectively. This relationship appeared to be driven by the amount of the SOC stock change; changes in stocks that exceeded 200 Gg C yr^?1 represented a threshold where uncertainties were always lower than ±100%. Consequently, the amount of uncertainty in estimates derived from process‐based models will partly depend on the level of SOC accumulation or loss. In general, the majority of uncertainty was associated with model structure in this application, and so attaining higher levels of precision in the estimates will largely depend on improving the model algorithms and parameterization, as well as increasing the number of measurement sites used to evaluate the structural uncertainty.     

Bias and variance in model results associated with spatial scaling of measurements for parameterization in regional assessments

Models are central to global change analyses, but they are often parameterized using data that represent only a portion of heterogeneity in a region. This creates uncertainty in the results and constrains the reliability of model inferences. Our objective was to evaluate the uncertainty associated with differential scaling of parameterization data to model soil organic carbon stock changes as a function of US agricultural land use and management. Specifically, we compared analyses in which model parameters were derived from field experimental data that were scaled to the entire US vs. the same data scaled to climate regions within the country. We evaluated the effect of differential scaling on both bias and variance in model results. Model results had less variance by scaling data to the entire country because of a larger sample size for deriving individual parameter values, although there was a relatively large bias associated with this parameterization, estimated at 2.7 Tg C yr^?1. Even with the large bias, resulting confidence intervals from the two parameterizations had considerable overlap for the estimated national rate of SOC change (i.e. 77% overlap in those intervals). Consequently, the results were relatively similar when focusing on the uncertainty rather than solely on the mean estimate. In contrast, large biases created less overlap in confidence intervals for the change rates within individual climate regions, compared with the national estimates. For example, the overlap in resulting intervals from the two parameterizations was only 32% for the warm temperate moist region, with a corresponding bias of 3.1 Tg C yr^?1. These findings demonstrate that there is a greater risk of making erroneous inferences because of large biases if models are parameterized with broader scale information, such as an entire country, and then used to address impacts at a finer spatial scale, such as sub‐regions within a country. In addition, the study demonstrates a trade‐off between variance and bias in model results that depends on the scaling of data for model parameterization.     

Predicting soil C changes over sugarcane expansion in Brazil using the DayCent model

In recent years, the increase in Brazilian ethanol production has been based on expansion of sugarcane‐cropped area, mainly by the land use change (LUC) pasture–sugarcane. However, second‐generation (2G) cellulosic‐derived ethanol supplies are likely to increase dramatically in the next years in Brazil. Both these management changes potentially affect soil C (SOC) changes and may have a significant impact on the greenhouse gases balance of Brazilian ethanol. To evaluate these impacts, we used the DayCent model to predict the influence of the LUC native vegetation (NV)–pasture (PA)–sugarcane (SG), as well as to evaluate the effect of different management practices (straw removal, no‐tillage, and application of organic amendments) on long‐term SOC changes in sugarcane areas in Brazil. The DayCent model estimated that the conversion of NV‐PA caused SOC losses of 0.34 ± 0.03 Mg ha^?1 yr^?1, while the conversion PA‐SG resulted in SOC gains of 0.16 ± 0.04 Mg ha^?1 yr^?1. Moreover, simulations showed SOC losses of 0.19 ± 0.04 Mg ha^?1 yr^?1 in SG areas in Brazil with straw removal. However, our analysis suggested that adoption of some best management practices can mitigate these losses, highlighting the application of organic amendments (+0.14 ± 0.03 Mg C ha^?1 yr^?1). Based on the commitments made by Brazilian government in the UNFCCC, we estimated the ethanol production needed to meet the domestic demand by 2030. If the increase in ethanol production was based on the expansion of sugarcane area on degraded pasture land, the model predicted a SOC accretion of 144 Tg from 2020 to 2050, while increased ethanol production based on straw removal as a cellulosic feedstock was predicted to decrease SOC by 50 Tg over the same 30‐year period.     

Organic carbon in soils of Central Asia—status quo and potentials for sequestration

Expectations have been raised that carbon sequestration in soils could provide a short-term bridge to reduce the impacts of increasing carbon emissions until low-carbon technologies are available. To assess the role of Central Asia in this regard, the organic carbon in soils of Central Asia and losses in response to land use were quantified in a spatially explicit way. Based on literature information on soil organic carbon contents and in combination with the FAO-UNESCO Soil Map of the Word, the organic carbon stocks in the upper 30 cm of native soils of Central Asia were estimated to amount to 20,17?±?4,03 Pg. The extent of conversion of native land into agricultural land and the degradation of rangelands was assessed by a land use land cover change map of the region. This type of land use (change) was responsible for a reduction of the soil organic carbon by about 828?±?166 Tg C, or on average 4.1% of the total stocks. To this reduction, degradation of rangeland (observed on 4.9 Mha) with 50 Tg contributed only 6%. Most of the losses resulted from past conversion of rangelands into rainfed or irrigated agricultural land in the north of Kazakhstan. Hotspots of high soil organic matter depletion were former wetlands, drained for cultivation during the last decades. Assuming that improved agronomic and grazing management could be put in place and that therewith SOC levels in all of Central Asia’s cropland and degraded rangeland could be brought back to native levels in the next 50 years, each year 16.6 Tg C could be sequestered. This is equal to the sizeable amount of 15.5% of the 2004 annual anthropogenic C-emissions of the five Central Asian countries (107 Tg C yr^?1). However, Central Asia contributed only 1.4% of CO₂ that is set free worldwide by fossil fuel burning. Therefore, the mitigation effect on rising atmospheric CO₂ levels and climate change of such ambitious sequestration plans, if put into practice, would be hardly notable. The central Asian example shows that, unfortunately, the strategy of soil C sequestration as a stand-alone measure is not a viable bridge to a future in which alternative energy source can substitute fossil fuel burning, but can only be part of a set of mitigating measures.     

Storage and sequestration potential of topsoil organic carbon in China's paddy soils

Carbon (C) storage and sequestration in agricultural soils is considered to be an important issue in the study of terrestrial C cycling and global climatic change. The baseline C stock and the C sequestration potential are among the criteria for a region or a state to adopt strategies or policies in response to commitment to the Kyoto Protocol. Paddy soils represent a large portion of global cropland. However, little information on the potential of C sequestration and storage is available for such soils. In this paper, an estimation of the topsoil soil organic carbon (SOC) pool and the sequestration potential of paddy soils in China was made by using the data from the 2nd State Soil Survey carried out during 1979–1982 and from the nationwide arable soil monitoring system established since then. Results showed that the SOC density ranged from 12 to 226 t C ha^?1 with an area‐weighted mean density of 44 t C ha^?1, which is comparable to that of the US grasslands and is higher than that of the cultivated dryland soils in China and the US. The estimated total topsoil SOC pool is 1.3 Pg, with 0.85 Pg from the upper plow layer and 0.45 Pg from the plowpan layer. This pool size is ～2% of China's total storage in the top 1 m of the soil profiles and ～4% of the total topsoil pool, while the area percentage of paddy soil is 3.4% of the total land. The C pool in paddy soils was found predominantly in southeast China geographically and in the subgroups of Fe‐accumulating and Fe‐leaching paddy soils pedogenetically. In comparison with dryland cultivation, irrigation‐based rice cultivation in China has induced significant enrichment of SOC storage (0.3 Pg) in paddy soils. The induced total C sequestration equals half of China's total annual CO₂ emission in the 1990s. Estimates using different SOC sequestration scenarios show that the paddy soils of China have an easily attainable SOC sequestration potential of 0.7 Pg under present conditions and may ultimately sequester 3.0 Pg. Soil monitoring data showed that the current C sequestration rate is 12 Tg yr^?1. The total C sequestration potential and the current sequestration rate of the paddy soils are over 30%, while the area of the paddy soils is 26% that of China's total croplands. Therefore, practicing sustainable agriculture is urgently needed for enhancing SOC storage to realize the ultimate SOC sequestration of rice‐based agriculture of China, as the current C sequestration rate is significantly lower than the potential rate.     

Declining trend of carbon in Finnish cropland soils in 1974–2009

Soil organic matter not only affects soil properties and productivity but also has an essential role in global carbon (C) cycle. We studied changes in the topsoil C content of Finnish croplands using a dataset produced in nationwide soil monitoring. The monitoring network consisting of fields on both mineral and organic soils was established in 1974 and resampled in 1987, 1998, and 2009. Over the monitoring period from 1974 to 2009, cultivated soils showed a continuous decline in C concentration (g kg^?1). In organic soils, C concentration decreased at a mean rate of 0.2–0.3% yr^?1 relative to the existing C concentration. In mineral soils, the relative decrease was 0.4% yr^?1 corresponding to a C stock (kg m^?2) loss of 220 kg ha^?1 yr^?1. The change in management practices in last decades toward increasing cultivation of annual crops has contributed to soil C losses noted in this study. The results, however, suggest that the C losses result partly from other processes affecting cultivated soils such as climatic change or the continuing long‐term effect of forest clearance. We estimated that Finnish cropland soils store 161 Tg carbon nationwide in the topmost 15 cm of which 117 Tg is in mineral soils. C losses from mineral soils can therefore total up to 0.5 Tg yearly.     

The European carbon balance. Part 4: integration of carbon and other trace‐gas fluxes

Overviewing the European carbon (C), greenhouse gas (GHG), and non‐GHG fluxes, gross primary productivity (GPP) is about 9.3 Pg yr^?1, and fossil fuel imports are 1.6 Pg yr^?1. GPP is about 1.25% of solar radiation, containing about 360 × 10¹⁸ J energy – five times the energy content of annual fossil fuel use. Net primary production (NPP) is 50%, terrestrial net biome productivity, NBP, 3%, and the net GHG balance, NGB, 0.3% of GPP. Human harvest uses 20% of NPP or 10% of GPP, or alternatively 1‰ of solar radiation after accounting for the inherent cost of agriculture and forestry, for production of pesticides and fertilizer, the return of organic fertilizer, and for the C equivalent cost of GHG emissions. C equivalents are defined on a global warming potential with a 100‐year time horizon. The equivalent of about 2.4% of the mineral fertilizer input is emitted as N₂O. Agricultural emissions to the atmosphere are about 40% of total methane, 60% of total NO‐N, 70% of total N₂O‐N, and 95% of total NH₃‐N emissions of Europe. European soils are a net C sink (114 Tg yr^?1), but considering the emissions of GHGs, soils are a source of about 26 Tg CO₂ C‐equivalent yr^?1. Forest, grassland and sediment C sinks are offset by GHG emissions from croplands, peatlands and inland waters. Non‐GHGs (NH₃, NOx) interact significantly with the GHG and the C cycle through ammonium nitrate aerosols and dry deposition. Wet deposition of nitrogen (N) supports about 50% of forest timber growth. Land use change is regionally important. The absolute flux values total about 50 Tg C yr^?1. Nevertheless, for the European trace‐gas balance, land‐use intensity is more important than land‐use change. This study shows that emissions of GHGs and non‐GHGs significantly distort the C cycle and eliminate apparent C sinks.     

Potential Soil Carbon Sequestration and CO2 Offset by Dedicated Energy Crops in the USA

Energy crops are fast-growing species whose biomass yields are dedicated to the production of more immediately usable energy forms, such as liquid fuels or electricity. Biomass-based energy sources can offset, or displace, some amount of fossil-fuel use. Energy derived from biomass provides 2 to 3% of the energy used in the U.S.A.; but, with the exception of corn-(Zea mays L.)-to-ethanol, very little energy is currently derived from dedicated energy crops. In addition to the fossil-fuel offset, energy cropping might also mitigate an accentuated greenhouse gas effect by causing a net sequestration of atmospheric C into soil organic C (SOC). Energy plantations of short-rotation woody crops (SRWC) or herbaceous crops (HC) can potentially be managed to favor SOC sequestration. This review is focused primarily on the potential to mitigate atmospheric CO₂ emissions by fostering SOC sequestration in energy cropping systems deployed across the landscape in the United States. We know that land use affects the dynamics of the SOC pool, but data about spatial and temporal variability in the SOC pool under SRWC and HC are scanty due to lack of well-designed, long-term studies. The conventional methods of studying SOC fluxes involve paired-plot designs and chronosequences, but isotopic techniques may also be feasible in understanding temporal changes in SOC. The rate of accumulation of SOC depends on land-use history, soil type, vegetation type, harvesting cycle, and other management practices. The SOC pool tends to be enhanced more under deep-rooted grasses, N-fixers, and deciduous species. Carbon sequestration into recalcitrant forms in the SOC pool can be enhanced with some management practices (e.g., conservation tillage, fertilization, irrigation); but those practices can carry a fossil-C cost. Reported rates of SOC sequestration range from 0 to 1.6 Mg C ha^?1 yr^?1 under SRWC and 0 to 3 Mg C ha^?1 yr^?1 under HC. Production of 5 EJ of electricity from energy crops—a perhaps reasonable scenario for the U.S.A.—would require about 60 Mha. That amount of land is potentially available for conversion to energy plantations in the U.S.A. The land so managed could mitigate C emissions (through fossil C not emitted and SOC sequestered) by about 5.4 Mg C ha^?1 yr^?1. On 60 Mha, that would represent 324 Tg C yr^?1—a 20% reduction from current fossil-fuel CO₂ emissions. Advances in productivity of fast-growing SRWC and HC species suggest that deployment of energy cropping systems could be an effective strategy to reduce climate-altering effects of anthropogenic CO₂ emissions and to meet global policy commitments.     

Changes in soil organic carbon,nitrogen, pH and bulk density with the development of larch (Larix gmelinii) plantations in China

Under the government of China's environmental program known as Returning Farmland To Forests (RFTF), about 28 million hectares of farmland have been converted to tree plantation. This has led to a large accumulation of biomass carbon, but less is known about underground carbon‐related processes. One permanent plot (25 years of observation) and four chronosequence plot series comprising 159 plots of larch (Larix gmelinii) plantations in northeastern China were studied. Both methods found significant soil organic carbon (SOC) accumulation (96.4 g C m^?2 yr^?1) and bulk density decrease (5.7 mg cm^?3 yr^?1) in the surface soil layer (0–20 cm), but no consistent changes in deeper layers, indicating that larch planting under the RFTF program can increase SOC storage and improve the physical properties of surface soil. Nitrogen depletion (4.1–4.3 g m^?2 yr^?1), soil acidification (0.007–0.022 pH units yr^?1) and carbon/nitrogen (C/N) ratio increase (0.16–0.46 per year) were observed in lessive soil, whereas no significant changes were found in typical dark‐brown forest soil. This SOC accumulation rate (96.4 g m^?2 yr^?1) can take 39% of the total carbon sink capacity [net ecosystem exchange (NEE)] of larch forests in this region and the total soil carbon sequestration could be 87 Tg carbon within 20 years of plantation by approximating all larch plantations in northeastern China (4.5 Mha), showing the importance of soil carbon accumulation in the ecosystem carbon balance. By comparison with the rates of these processes in agricultural use, the RFTF program of reversing land use for agriculture will rehabilitate SOC, soil fertility and bulk density slowly (< 69% of the depletion rate in agricultural use), so that a much longer duration is needed to rehabilitate the underground function of soil via the RFTF program. Global forest plantations on abandoned farmland or function to protecting farmland are of steady growth and our findings may be important for understanding their underground carbon processes.     

Bioenergy Crops and Carbon Sequestration

Greenhouse gas (GHG) emissions constitute a global problem. The need for agricultural involvement in GHG mitigation has been widely recognized since the 1990s. The concept of C sinks, C credits, and emission trading has attracted special interests in herbaceous and woody species as energy crops and source of biofuel feedstock. Bioenergy crops are defined as any plant material used to produce bioenergy. These crops have the capacity to produce large volume of biomass, high energy potential, and can be grown in marginal soils. Planting bioenergy crops in degraded soils is one of the promising agricultural options with C sequestration rates ranging from 0.6 to 3.0 Mg C ha^?1 yr^?1. About 60 million hectares (Mha) of land is available in the United States and 757 Mha in the world to grow bioenergy crops. With an energy offset of 1 kg of C in biomass per 0.6 kg of C in fossil fuel, there exists a vast potential of offsetting fossil fuel emission. Bioenergy crops have the potential to sequester approximately 318 Tg C yr^?1 in the United States and 1631 Tg C yr^?1 worldwide. Bioenergy crops consist of herbaceous bunch-type grasses and short-rotation woody perennials. Important grasses include switchgrass (Panicum virgatum L.), elephant grass (Pennissetum purpureum Schum.), tall fescue (Fetusca arundinacea L.), etc. Important among short-rotation woody perennials are poplar (Populus spp.), willow (Salix spp.), mesquite (Prosopis spp.), etc. The emissions of CO₂ from using switchgrass as energy crop is 1.9 kg C Gj^?1 compared with 13.8, 22.3, and 24.6 kg C Gj^?1 from using gas, petroleum, and coal, respectively. Mitigation of GHG emissions cannot be achieved by C sinks alone, a substantial reduction in fossil fuel combustion will be necessary. Carbon sequestration and fossil fuel offset by bioenergy crops is an important component of a possible total societal response to a GHG emission reduction initiative.     

Soil C erosion and burial in cropland

Erosion influences the lateral and vertical distribution of soil in agricultural landscapes. A better understanding of the effects of erosion and redistribution on soil organic carbon (C) within croplands would improve our knowledge of how management practices may affect global C dynamics. In this study, the vertical and lateral distribution of soil organic C was characterized to evaluate the amounts and timescales of soil organic C movement, deposition and burial over the last 50 years in different agroecosystems across Canada. There was strong evidence that a substantial portion of eroded sediment and soil organic C was deposited as colluvium close to its source area, thereby burying the original topsoil. The deepest aggraded profile was in a potato field and contained over 70 cm of deposited soil indicating an accumulation rate of 152 Mg ha yr^?1; aggraded profiles in other sites had soil deposition rates of 40–90 Mg ha^?1 yr^?1. The largest stock of soil organic C was 463 Mg ha^?1 (to 60 cm depth) and soil C deposition ranged from about 2 to 4 Mg ha^?1 yr^?1 across all sites. A distinct feature observed in the aggraded profiles at every site was the presence of a large increase in soil organic C concentration near the bottom of the A horizon; the concentration of this C was greater than that at the soil surface. Compared to aggraded profiles, the SOC concentration in eroded profiles did not differ with depth, suggesting that dynamic replacement of soil organic C had occurred in eroded soils. A large amount of soil organic C is buried in depositional areas of Canadian croplands; mineralization of this stock of C appears to have been constrained since burial, but it may be vulnerable to future loss by management practices, land use change and a warming climate.     

Nitrogen Management Affects Carbon Sequestration in North American Cropland Soils

Agricultural soils in North America can be a sink for rising atmospheric CO₂ concentrations through the formation of soil organic matter (SOM) or humus. Humification is limited by the availability of nutrients such as nitrogen (N). Recommended management practices (RMPs) that optimize N availability foster humus formation. This review examines the management practices that contribute to maximizing N availability for optimizing sequestration of atmospheric CO₂ into soil humus. Farming practices that enhance nutrient use, reduce or eliminate tillage, and increase crop intensity, together, affect N availability and, therefore, C sequestration. N additions, from especially, livestock manure and leguminous cover crops are necessary for increasing grain and biomass yields and returning crop residues to the soil thereby increasing soil organic carbon (SOC) concentration. Conservation tillage practices enhance also the availability of N and increase SOC concentration. Increase in cropping intensity and/or crop rotations produce higher quantity and quality of residues, increase availability of N, and therefore foster increase in C sequestration. The benefit of C sequestration from N additions may be negated by CO₂ and N₂O emissions associated with production and application of N fertilizers. More studies need to be conducted to ascertain the benefits of adding N via manuring versus N fertilizer additions. Furthermore, site specific adaptive research is needed to identify RMPs that optimize soil N use efficiency while improving crop yield and C sequestration thereby curbing greenhouse gas (GHG) emissions. Due to the wide range of climate in North America, there is a large range of C sequestration potential in agricultural soils through N management. Humid croplands may have the potential to sequester 8–298 Tg C yr^?1 while dry croplands may sequester 1–35 Tg C yr^?1. These estimates, however, are highly uncertain and wide-ranging. Clearly, more research is needed to quantify, more precisely, the C sequestration potential across different N management scenarios especially in Mexico and Canada.