Impact of tropical land‐use change on soil organic carbon stocks – a meta‐analysis

Land‐use changes are the second largest source of human‐induced greenhouse gas emission, mainly due to deforestation in the tropics and subtropics. CO₂ emissions result from biomass and soil organic carbon (SOC) losses and may be offset with afforestation programs. However, the effect of land‐use changes on SOC is poorly quantified due to insufficient data quality (only SOC concentrations and no SOC stocks, shallow sampling depth) and representativeness. In a global meta‐analysis, 385 studies on land‐use change in the tropics were explored to estimate the SOC stock changes for all major land‐use change types. The highest SOC losses were caused by conversion of primary forest into cropland (?25%) and perennial crops (?30%) but forest conversion into grassland also reduced SOC stocks by 12%. Secondary forests stored less SOC than primary forests (?9%) underlining the importance of primary forests for C stores. SOC losses are partly reversible if agricultural land is afforested (+29%) or under cropland fallow (+32%) and with cropland conversion into grassland (+26%). Data on soil bulk density are critical in order to estimate SOC stock changes because (i) the bulk density changes with land‐use and needs to be accounted for when calculating SOC stocks and (ii) soil sample mass has to be corrected for bulk density changes in order to compare land‐use types on the same basis of soil mass. Without soil mass correction, land‐use change effects would have been underestimated by 28%. Land‐use change impact on SOC was not restricted to the surface soil, but relative changes were equally high in the subsoil, stressing the importance of sufficiently deep sampling.     

Changes in soil organic carbon under perennial crops

This study evaluates the dynamics of soil organic carbon (SOC) under perennial crops across the globe. It quantifies the effect of change from annual to perennial crops and the subsequent temporal changes in SOC stocks during the perennial crop cycle. It also presents an empirical model to estimate changes in the SOC content under crops as a function of time, land use, and site characteristics. We used a harmonized global dataset containing paired‐comparison empirical values of SOC and different types of perennial crops (perennial grasses, palms, and woody plants) with different end uses: bioenergy, food, other bio‐products, and short rotation coppice. Salient outcomes include: a 20‐year period encompassing a change from annual to perennial crops led to an average 20% increase in SOC at 0–30 cm (6.0 ± 4.6 Mg/ha gain) and a total 10% increase over the 0–100 cm soil profile (5.7 ± 10.9 Mg/ha). A change from natural pasture to perennial crop decreased SOC stocks by 1% over 0–30 cm (?2.5 ± 4.2 Mg/ha) and 10% over 0–100 cm (?13.6 ± 8.9 Mg/ha). The effect of a land use change from forest to perennial crops did not show significant impacts, probably due to the limited number of plots; but the data indicated that while a 2% increase in SOC was observed at 0–30 cm (16.81 ± 55.1 Mg/ha), a decrease in 24% was observed at 30–100 cm (?40.1 ± 16.8 Mg/ha). Perennial crops generally accumulate SOC through time, especially woody crops; and temperature was the main driver explaining differences in SOC dynamics, followed by crop age, soil bulk density, clay content, and depth. We present empirical evidence showing that the FAO perennialization strategy is reasonable, underscoring the role of perennial crops as a useful component of climate change mitigation strategies.     

A global meta‐analysis of soil organic carbon response to corn stover removal

Corn (Zea mays L.) stover is a global resource used for livestock, fuel, and bioenergy feedstock, but excessive stover removal can decrease soil organic C (SOC) stocks and deteriorate soil health. Many site‐specific stover removal experiments report accrual rates and SOC stock effects, but a quantitative, global synthesis is needed to provide a scientific base for long‐term energy policy decisions. We used 409 data points from 74 stover harvest experiments conducted around the world for a meta‐analysis and meta‐regression to quantify removal rate, tillage, soil texture, and soil sampling depth effects on SOC. Changes were quantified by: (a) comparing final SOC stock differences after at least 3 years with and without stover removal and (b) calculating SOC accrual rates for both treatments. Stover removal generally reduced final SOC stocks by 8% in the upper 0–15 or 0–30 cm, compared to stover retained, irrespective of soil properties and tillage practices. A more sensitive meta‐regression analysis showed that retention increased SOC stocks within the 30–150 cm depth by another 5%. Compared to baseline values, stover retention increased average SOC stocks temporally at a rate of 0.41 Mg C ha^?1 year^?1 (statistically significant at p < 0.01 when averaged across all soil layers). Although SOC sequestration rates were lower with stover removal, with moderate (<50%) removal they can be positive, thus emphasizing the importance of site‐specific management. Our results also showed that tillage effects on SOC stocks were inconsistent due to the high variability in practices used among the experimental sites. Finally, we conclude that research and technological efforts should continue to be given high priority because of the importance in providing science‐based policy recommendations for long‐term global carbon management.     

Afforestation effects on SOC in former cropland: oak and spruce chronosequences resampled after 13 years

Chronosequences are commonly used to assess soil organic carbon (SOC) sequestration after land‐use change, but SOC dynamics predicted by this space‐for‐time substitution approach have rarely been validated by resampling. We conducted a combined chronosequence/resampling study in a former cropland area (Vestskoven) afforested with oak (Quercus robur) and Norway spruce (Picea abies) over the past 40 years. The aims of this study were (i) to compare present and previous chronosequence trends in forest floor and top mineral soil (0–25 cm) C stocks; (ii) to compare chronosequence estimates with current rates of C stock change based on resampling at the stand level; (iii) to estimate SOC changes in the subsoil (25–50 cm); and (iv) to assess the influence of two tree species on SOC dynamics. The two chronosequence trajectories for forest floor C stocks revealed consistently higher rates of C sequestration in spruce than oak. The chronosequence trajectory was validated by resampling and current rates of forest floor C sequestration decreased with stand age. Chronosequence trends in topsoil SOC in 2011 did not differ significantly from those reported in 1998, however, there was a shift from a negative rate (1998: ?0.3 Mg C ha^?1 yr^?1) to no change in 2011. In contrast SOC stocks in the subsoil increased with stand age, however, not significantly (P = 0.1), suggesting different C dynamics in and below the former plough layer. Current rates of C change estimated by repeated sampling decreased with stand age in forest floors but increased in the topsoil. The contrasting temporal change in forest floor and mineral soil C sequestration rates indicate a shift in C source‐sink strength after approximately 40 years. We conclude that afforestation of former cropland within the temperate region may induce soil C loss during the first decades followed by a recovery phase of yet unknown duration.     

Soil carbon sequestration and land use change associated with biofuel production: empirical evidence

Soil organic carbon (SOC) change can be a major impact of land use change (LUC) associated with biofuel feedstock production. By collecting and analyzing data from worldwide field observations of major LUCs from cropland, grassland, and forest to lands producing biofuel crops (i.e. corn, switchgrass, Miscanthus, poplar, and willow), we were able to estimate SOC response ratios and sequestration rates and evaluate the effects of soil depth and time scale on SOC change. Both the amount and rate of SOC change were highly dependent on the specific land transition. Irrespective of soil depth or time horizon, cropland conversions resulted in an overall SOC gain of 6–14% relative to initial SOC level, while conversion from grassland or forest to corn (without residue removal) or poplar caused significant carbon loss (9–35%). No significant SOC changes were observed in land converted from grasslands or forests to switchgrass, Miscanthus, or willow. The SOC response ratios were similar in both 0–30 and 0–100 cm soil depths in most cases, suggesting SOC changes in deep soil and that use of top soil only for SOC accounting in biofuel life cycle analysis (LCA) might underestimate total SOC changes. Soil carbon sequestration rates varied greatly among studies and land transition types. Generally, the rates of SOC change tended to be the greatest during the 10 years following land conversion and had declined to approach 0 within about 20 years for most LUCs. Observed trends in SOC change were generally consistent with previous reports. Soil depth and duration of study significantly influence SOC change rates and so should be considered in carbon emission accounting in biofuel LCA. High uncertainty remains for many perennial systems and forest transitions, additional field trials, and modeling efforts are needed to draw conclusions about the site‐ and system‐specific rates and direction of change.     

Projected changes in mineral soil carbon of European croplands and grasslands, 1990–2080

We present the most comprehensive pan‐European assessment of future changes in cropland and grassland soil organic carbon (SOC) stocks to date, using a dedicated process‐based SOC model and state‐of‐the‐art databases of soil, climate change, land‐use change and technology change. Soil carbon change was calculated using the Rothamsted carbon model on a European 10 × 10′ grid using climate data from four global climate models implementing four Intergovernmental Panel on Climate Change (IPCC) emissions scenarios (SRES). Changes in net primary production (NPP) were calculated by the Lund–Potsdam–Jena model. Land‐use change scenarios, interpreted from the narratives of the IPCC SRES story lines, were used to project changes in cropland and grassland areas. Projections for 1990–2080 are presented for mineral soil only. Climate effects (soil temperature and moisture) will tend to speed decomposition and cause soil carbon stocks to decrease, whereas increases in carbon input because of increasing NPP will slow the loss. Technological improvement may further increase carbon inputs to the soil. Changes in cropland and grassland areas will further affect the total soil carbon stock of European croplands and grasslands. While climate change will be a key driver of change in soil carbon over the 21st Century, changes in technology and land‐use change are estimated to have very significant effects. When incorporating all factors, cropland and grassland soils show a small increase in soil carbon on a per area basis under future climate (1–7 t C ha^?1 for cropland and 3–6 t C ha^?1 for grassland), but when the greatly decreasing area of cropland and grassland are accounted for, total European cropland stocks decline in all scenarios, and grassland stocks decline in all but one scenario. Different trends are seen in different regions. For Europe (the EU25 plus Norway and Switzerland), the cropland SOC stock decreases from 11 Pg in 1990 by 4–6 Pg (39–54%) by 2080, and the grassland SOC stock increases from 6 Pg in 1990 to 1.5 Pg (25%) under the B1 scenario, but decreases to 1–3 Pg (20–44%) under the other scenarios. Uncertainty associated with the land‐use and technology scenarios remains unquantified, but worst‐case quantified uncertainties are 22.5% for croplands and 16% for grasslands, equivalent to potential errors of 2.5 and 1 Pg SOC, respectively. This is equivalent to 42–63% of the predicted SOC stock change for croplands and 33–100% of the predicted SOC stock change for grasslands. Implications for accounting for SOC changes under the Kyoto Protocol are discussed.     

Depth‐dependent soil organic carbon dynamics of croplands across the Chengdu Plain of China from the 1980s to the 2010s

Agricultural soils have tremendous potential to sequester soil organic carbon (SOC) and mitigate global climate change. However, agricultural land use has a profound impact on SOC dynamics, and few studies have explored how agricultural land use combined with soil conditions affect SOC changes throughout the soil profile. Based on a paired soil resampling campaign in the 1980s and 2010s, this study investigated the SOC changes of the soil profile caused by agricultural land use and the correlations with parent material and topography across the Chengdu Plain of China. The results showed that the SOC content increased by 3.78 g C/kg in the topsoil (0–20 cm), but decreased in the 20–40 cm and 40–60 cm soil layers by 0.90 and 1.26 g C/kg respectively. SOC increases in topsoil were observed for all types of agricultural land. Afforestation on former agricultural land also caused SOC decreases in the 20–60 cm soil layers, while SOC decreases only occurred in the 40–60 cm soil layer for agricultural land using a traditional crop rotation (i.e. traditional rice–wheat/rapeseed rotation) and with rice–vegetable rotations converted from the traditional rotations. For each agricultural land use, SOC decreases in deep soils only occurred in high relief areas and in soils formed from Q4 (Quaternary Holocene) grey‐brown alluvium and Q4 grey alluvium that had a relatively low soil bulk density and clay content. The results indicated that SOC change caused by agricultural land use was depth dependent and that the effects of agricultural land use on soil profile SOC dynamics varied with soil characteristics and topography. Subsoil SOC decreases were more likely to occur in high relief areas and in soils with low soil bulk density and low clay content.     

六盘山林区几种土地利用方式对土壤有机碳矿化影响的比较

 应用土壤培养法,比较分析了六盘山林区天然次生林（杂灌林、山杨（Populus davidanda）和辽东栎（Quercus liaotungensis）林）、农田、草地和人工林（13 a、18 a和25 a华北落叶松（Larix principis-rupprechtii））土壤在30℃和60%田间饱和含水量条件下培养180 d有机碳矿化速率的差异（以180 d累计释放的CO2-C计）。结果显示：农田和草地土壤碳矿化释放的CO2-C含量（180 d释放的gCO2-C·kg-1干土）分别比天然次生林低65%和23%,差异主要在0～40土层;人工林比农田和草地分别高155%和 17%,差异主要在0～70 cm土层。农田土壤碳矿化释放的CO2-C分配比例（即180 d释放CO2-C/土壤 C）比天然次生林平均低12%,草地比天然次生林平均高18%,差异主要在0～40 cm土层;人工林比农田平均高29%,草地比人工林平均高9%,差异主要在0～50 cm土层。不同土地利用方式土壤碳矿化释放的CO2-C含量的差异比其分配比例的差异大。土壤碳矿化释放的CO2-C含量和分配比例总体上都随土层加深而递减。分配比例随土层加深而递减的幅度方面,不同土地利用方式间的差异不大;含量随土层加深而递减的幅度方面,天然次生林和人工林比农田和草地中大;随土层递减的幅度方面,土壤碳矿化释放的CO2-C含量比其分配比例大。这主要由不同土地利用方式土壤碳输入和稳定性等差异所致。结果说明土壤碳矿化速率随天然次生林变成农田或草地而下降,随在农田或草地上造林而增加, 矿化速率变化幅度比分配比例的变化幅度大。另外,土地利用变化也使不同土层土壤碳矿化速率和分配比例差异改变,其中速率改变的幅度比分配比例改变的幅度大。     

土地利用变化对土壤有机碳贮量的影响

通过对比分析六盘山林区典型天然次生林(杂灌林、山杨和辽东栎林)与农田、草地及农田、草地与人工林(13、18和25年生华北落叶松)邻近样地土壤有机碳含量和密度及其在土壤剖面上分布的差异,研究了天然次生林变成农田或草地及农田或草地造林后对土壤有机碳贮量的影响,结果表明,土壤有机碳含量方面,农田和草地比天然次生林分别低54％和27％,差异主要在0～50cm土层;农田和草地比人工林分别低42％和26％,差异主要在0～40cm土层,土壤有机碳密度方面,农田和草地比天然次生林分别低35％和14％,差异主要在0～50cm土层;农田比人工林低23％,草地比人工林高4％,差异主要在0～30cm土层．天然次生林和人工林土壤有机碳含量和密度随土层加深而递减的幅度比农田或草地大．这些差异主要由土地利用变化引起的土壤有机碳输入与输出及根系分布的变化所致．结果说明六盘山林区天然次生林破坏变成草地或农田后土壤有机碳含量和密度(主要是0～50cm土层)将下降,而农田中造林后土壤有机碳含量和密度(主要是0～30cm土层)又将增加,草地上造林后土壤有机碳含量增加而密度变化不大。另外,土壤有机碳含量和密度在土壤剖面上的分布也将随土地利用变化而发生改变。     

黄土丘陵区土壤有机碳固存对退耕还林草的时空响应

研究了黄土丘陵区土壤有机碳固存对退耕还林草的时空响应特征,分析了退耕还林草对土壤有机碳的近期影响和长期效应。结果表明,1)从黄土丘陵区退耕还林草的土壤固碳效应整体而言,相对于坡耕地,退耕还林和退耕撂荒具有显著的土壤碳增汇效应,而退耕还草、退耕还果没有明显土壤碳增汇效应。以天然草地土壤有机碳密度为目标,撂荒地表层土壤有机碳增汇潜力可达8.3 t/hm2。2)以10a为界,退耕还林草的近期土壤碳增汇效应不明显,而10a后土壤碳增汇效应逐渐明显,退耕还林、还灌、撂荒和坡耕地的固碳效应差异显著。3)在评估黄土丘陵区退耕还林草的土壤固碳效应时应当注重长期固碳效应。4)退耕还林草的土壤固碳效应主要受还林草方式及年限的影响,二者分别可解释55.6%和24.1%的有机碳变异性;地形因子可解释8.5%的有机碳变异性。在评估该区退耕还林的土壤固碳效应时应当充分考虑退耕年限和地形因子的影响。5)人工刺槐林地、人工柠条林地以及撂荒地深层土壤(100—200 cm)有机碳密度占2 m土体有机碳密度的35%—40%,而且随着植被恢复深层土壤有机碳密度显著增加。6)在估算黄土丘陵区退耕还林土壤固碳效应时应该考虑深层碳累积。如果按1 m土层的土壤有机碳密度计算,会严重低估退耕还林草的土壤固碳量。     

Land‐use conversion and changing soil carbon stocks in China's ‘Grain‐for‐Green’ Program: a synthesis

The establishment of either forest or grassland on degraded cropland has been proposed as an effective method for climate change mitigation because these land use types can increase soil carbon (C) stocks. This paper synthesized 135 recent publications (844 observations at 181 sites) focused on the conversion from cropland to grassland, shrubland or forest in China, better known as the ‘Grain‐for‐Green’ Program to determine which factors were driving changes to soil organic carbon (SOC). The results strongly indicate a positive impact of cropland conversion on soil C stocks. The temporal pattern for soil C stock changes in the 0–100 cm soil layer showed an initial decrease in soil C during the early stage (<5 years), and then an increase to net C gains (>5 years) coincident with vegetation restoration. The rates of soil C change were higher in the surface profile (0–20 cm) than in deeper soil (20–100 cm). Cropland converted to forest (arbor) had the additional benefit of a slower but more persistent C sequestration capacity than shrubland or grassland. Tree species played a significant role in determining the rate of change in soil C stocks (conifer < broadleaf, evergreen < deciduous forests). Restoration age was the main factor, not temperature and precipitation, affecting soil C stock change after cropland conversion with higher initial soil C stock sites having a negative effect on soil C accumulation. Soil C sequestration significantly increased with restoration age over the long‐term, and therefore, the large scale of land‐use change under the ‘Grain‐for‐Green’ Program will significantly increase China's C stocks.     

Animal manure application and soil organic carbon stocks: a meta‐analysis

The impact of animal manure application on soil organic carbon (SOC) stock changes is of interest for both agronomic and environmental purposes. There is a specific need to quantify SOC change for use in national greenhouse gas (GHG) emission inventories. We quantified the response of SOC stocks to manure application from a large worldwide pool of individual studies and determined the impact of explanatory factors such as climate, soil properties, land use and manure characteristics. Our study is based on a meta‐analysis of 42 research articles totaling 49 sites and 130 observations in the world. A dominant effect of cumulative manure‐C input on SOC response was observed as this factor explained at least 53% of the variability in SOC stock differences compared to mineral fertilized or unfertilized reference treatments. However, the effects of other determining factors were not evident from our data set. From the linear regression relating cumulative C inputs and SOC stock difference, a global manure‐C retention coefficient of 12% ± 4 (95% Confidence Interval, CI) could be estimated for an average study duration of 18 years. Following an approach comparable to the Intergovernmental Panel on Climate Change, we estimated a relative SOC change factor of 1.26 ± 0.14 (95% CI) which was also related to cumulative manure‐C input. Our results offer some scope for the refinement of manure retention coefficients used in crop management guidelines and for the improvement of SOC change factors for national GHG inventories by taking into account manure‐C input. Finally, this study emphasizes the need to further document the long‐term impact of manure characteristics such as animal species, especially pig and poultry, and manure management systems, in particular liquid vs. solid storage.     

Temporal dynamics of soil organic carbon after land‐use change in the temperate zone – carbon response functions as a model approach

Land‐use change (LUC) is a major driving factor for the balance of soil organic carbon (SOC) stocks and the global carbon cycle. The temporal dynamic of SOC after LUC is especially important in temperate systems with a long reaction time. On the basis of 95 compiled studies covering 322 sites in the temperate zone, carbon response functions (CRFs) were derived to model the temporal dynamic of SOC after five different LUC types (mean soil depth of 30±6 cm). Grassland establishment caused a long lasting carbon sink with a relative stock change of 128±23% and afforestation on former cropland a sink of 116±54%, 100 years after LUC (mean±95% confidence interval). No new equilibrium was reached within 120 years. In contrast, there was no SOC sink following afforestation of grasslands and 75% of all observations showed SOC losses, even after 100 years. Only in the forest floor, there was carbon accumulation of 0.38±0.04 Mg ha^?1 yr^?1 in afforestations adding up to 38±4 Mg ha^?1 labile carbon after 100 years. Carbon loss after deforestation (?32±20%) and grassland conversion to cropland (?36±5%), was rapid with a new SOC equilibrium being reached after 23 and 17 years, respectively. The change rate of SOC increased with temperature and precipitation but decreased with soil depth and clay content. Subsoil SOC changes followed the trend of the topsoil SOC changes but were smaller (25±5% of the total SOC changes) and with a high uncertainty due to a limited number of datasets. As a simple and robust model approach, the developed CRFs provide an easily applicable tool to estimate SOC stock changes after LUC to improve greenhouse gas reporting in the framework of UNFCCC.     

Changes in soil carbon sequestration and soil respiration following afforestation on paddy fields in north subtropical China

Aims Although many studies have reported net gains of soil organic carbon (SOC) after afforestation on croplands, this is uncertain for Chinese paddy rice croplands. Here, we aimed to evaluate the effects of afforestation of paddy rice croplands on SOC sequestration and soil respiration (R s). Such knowledge would improve our understanding of the effectiveness of various land use options on greenhouse gas mitigation in China.Methods The investigation was conducted on the Chongming Island, north subtropical China. Field sites were reclaimed from coastal salt marshes in the 1960s, and soils were homogeneous with simple land use histories. SOC stocks and R s levels were monitored over one year in a paddy rice cropland, an evergreen and a deciduous broad-leaved plantation established on previous paddy fields and a reference fallow land site never cultivated. Laboratory incubation of soil under fast-changing temperatures was used to compare the temperature sensitivity (Q 10) of SOC decomposition across land uses.Important findings After 15–20 years of afforestation on paddy fields, SOC concentration only slightly increased at the depth of 0–5cm but decreased in deeper layers, which resulted in a net loss of SOC stock in the top 40cm. Seasonal increase of SOC was observed during the rice-growing period in croplands but not in afforested soils, suggesting a stronger SOC sequestration by paddy rice cropping. However, SOC sequestered under cropping was more labile, as indicated by its higher contents of dissolved organic carbon and microbial biomass. Also, paddy soils had higher annual R s than afforested soils; R s abruptly increased after paddy fields were drained and plowed and remained distinctively high throughout the dry farming period. Laboratory incubation revealed that paddy soils had a much higher Q 10 of SOC decomposition than afforested soils. Given that temperature was the primary controller of R s in this region, it was concluded that despite the stronger SOC sequestration by paddy rice cropping, its SOC was less stable than in afforested systems and might be more easily released into the atmosphere under global warming.     

Surficial gains and subsoil losses of soil carbon and nitrogen during secondary forest development

Reforestation of formerly cultivated land is widely understood to accumulate above‐ and belowground detrital organic matter pools, including soil organic matter. However, during 40 years of study of reforestation in the subtropical southeastern USA, repeated observations of above‐ and belowground carbon documented that significant gains in soil organic matter (SOM) in surface soils (0–7.5 cm) were offset by significant SOM losses in subsoils (35–60 cm). Here, we extended the observation period in this long‐term experiment by an additional decade, and used soil fractionation and stable isotopes and radioisotopes to explore changes in soil organic carbon and soil nitrogen that accompanied nearly 50 years of loblolly pine secondary forest development. We observed that accumulations of mineral soil C and N from 0 to 7.5 cm were almost entirely due to accumulations of light‐fraction SOM. Meanwhile, losses of soil C and N from mineral soils at 35 to 60 cm were from SOM associated with silt and clay‐sized particles. Isotopic signatures showed relatively large accumulations of forest‐derived carbon in surface soils, and little to no accumulation of forest‐derived carbon in subsoils. We argue that the land use change from old field to secondary forest drove biogeochemical and hydrological changes throughout the soil profile that enhanced microbial activity and SOM decomposition in subsoils. However, when the pine stands aged and began to transition to mixed pines and hardwoods, demands on soil organic matter for nutrients to support aboveground growth eased due to pine mortality, and subsoil organic matter levels stabilized. This study emphasizes the importance of long‐term experiments and deep measurements when characterizing soil C and N responses to land use change and the remarkable paucity of such long‐term soil data deeper than 30 cm.     

六盘山林区几种土地利用方式下土壤活性有机碳的比较

 应用KMnO4氧化法测定分析了六盘山林区天然次生林（杂灌林、山杨（Populus davidanda）和辽东栎（Querces liaotungensis）林）、农田、草地和人工林（13、18和25年华北落叶松（Larix principis-rupprechtii））土壤活性有机碳含量及分配比例的差异。结果表明：农田和草地土壤活性有机碳含量比天然次生林分别低60%和36%,差异主要在0～70 cm土层;人工林比农田和草地分别高129%和29%,差异主要在0～50 cm土层。农田和草地土壤活性有机碳分配比例比天然次生林分别低11%和4%以上, 差异主要在0～20 cm与70～110 cm土层;人工林比农田和草地分别高13.3%和5.3%,差异主要在0～110 cm土层。土壤活性有机碳含量和分配比例随土层加深而递减,其中天然次生林和人工林土壤活性有机碳含量随土层加深而递减的幅度比农田和草地中大,农田土壤活性有机碳分配比例随土层加深而递减幅度较大。不同土地利用方式间土壤活性有机碳含量的差异比活性有机碳分配比例的差异大,土壤活性有机碳含量随土层加深而递减的幅度比分配比例随土层加深而递减的幅度大。这可能由土壤有机碳的输入、稳定性、质量和根系分布等差异所致。结果说明土壤活性有机碳含量和分配比例随天然次生林变成农田或草地而降低,随农田或草地中造林而增加,且土壤活性有机碳含量的变化幅度比分配比例大。另外,土壤活性有机碳含量和分配比例在土壤剖面的分布也随土地利用变化而改变,其中活性有机碳含量的变化幅度比分配比例大。     

Effect of land‐use and land‐cover change on mangrove blue carbon: A systematic review

Mangroves shift from carbon sinks to sources when affected by anthropogenic land‐use and land‐cover change (LULCC). Yet, the magnitude and temporal scale of these impacts are largely unknown. We undertook a systematic review to examine the influence of LULCC on mangrove carbon stocks and soil greenhouse gas (GHG) effluxes. A search of 478 data points from the peer‐reviewed literature revealed a substantial reduction of biomass (82% ± 35%) and soil (54% ± 13%) carbon stocks due to LULCC. The relative loss depended on LULCC type, time since LULCC and geographical and climatic conditions of sites. We also observed that the loss of soil carbon stocks was linked to the decreased soil carbon content and increased soil bulk density over the first 100 cm depth. We found no significant effect of LULCC on soil GHG effluxes. Regeneration efforts (i.e. restoration, rehabilitation and afforestation) led to biomass recovery after ~40 years. However, we found no clear patterns of mangrove soil carbon stock re‐establishment following biomass recovery. Our findings suggest that regeneration may help restore carbon stocks back to pre‐disturbed levels over decadal to century time scales only, with a faster rate for biomass recovery than for soil carbon stocks. Therefore, improved mangrove ecosystem management by preventing further LULCC and promoting rehabilitation is fundamental for effective climate change mitigation policy.     

From forest to cropland and pasture systems: a critical review of soil organic carbon stocks changes in Amazonia

The impact of deforestation on soil organic carbon (SOC) stocks is important in the context of climate change and agricultural soil use. Trends of SOC stock changes after agroecosystem establishment vary according to the spatial scale considered, and factors explaining these trends may differ sometimes according to meta‐analyses. We have reviewed the knowledge about changes in SOC stocks in Amazonia after the establishment of pasture or cropland, sought relationships between observed changes and soil, climatic variables and management practices, and synthesized the δ¹³C measured in pastures. Our dataset consisted of 21 studies mostly synchronic, across 52 sites (Brazil, Colombia, French Guiana, Suriname), totalling 70 forest–agroecosystem comparisons. We found that pastures (n = 52, mean age = 17.6 years) had slightly higher SOC stocks than forest (+6.8 ± 3.1 %), whereas croplands (n = 18, mean age = 8.7 years) had lower SOC stocks than forest (?8.5 ± 2.9 %). Annual precipitation and SOC stocks under forest had no effect on the SOC changes in the agroecosystems. For croplands, we found a lower SOC loss than other meta‐analyses, but the short time period after deforestation here could have reduced this loss. There was no clear effect of tillage on the SOC response. Management of pastures, whether they were degraded/nominal/improved, had no significant effect on SOC response. δ¹³C measurements on 16 pasture chronosequences showed that decay of forest‐derived SOC was variable, whereas pasture‐derived SOC was less so and was characterized by an accumulation plateau of 20 Mg SOC ha^?1 after 20 years. The large uncertainties in SOC response observed could be derived from the chronosequence approach, sensitive to natural soil variability and to human management practices. This study emphasizes the need for diachronic and long‐term studies, associated with better knowledge of agroecosystem management.     

Evaluation of the ECOSSE model for simulating soil organic carbon under Miscanthus and short rotation coppice‐willow crops in Britain

In this paper, we focus on the impact on soil organic carbon (SOC) of two dedicated energy crops: perennial grass Miscanthus x Giganteus (Miscanthus) and short rotation coppice (SRC)‐willow. The amount of SOC sequestered in the soil is a function of site‐specific factors including soil texture, management practices, initial SOC levels and climate; for these reasons, both losses and gains in SOC were observed in previous Miscanthus and SRC‐willow studies. The ECOSSE model was developed to simulate soil C dynamics and greenhouse gas emissions in mineral and organic soils. The performance of ECOSSE has already been tested at site level to simulate the impacts of land‐use change to short rotation forestry (SRF) on SOC. However, it has not been extensively evaluated under other bioenergy plantations, such as Miscanthus and SRC‐willow. Twenty‐nine locations in the United Kingdom, comprising 19 paired transitions to SRC‐willow and 20 paired transitions to Miscanthus, were selected to evaluate the performance of ECOSSE in predicting SOC and SOC change from conventional systems (arable and grassland) to these selected bioenergy crops. The results of the present work revealed a strong correlation between modelled and measured SOC and SOC change after transition to Miscanthus and SRC‐willow plantations, at two soil depths (0–30 and 0–100 cm), as well as the absence of significant bias in the model. Moreover, model error was within (i.e. not significantly larger than) the measurement error. The high degrees of association and coincidence with measured SOC under Miscanthus and SRC‐willow plantations in the United Kingdom, provide confidence in using this process‐based model for quantitatively predicting the impacts of future land use on SOC, at site level as well as at national level.     

Carbon emissions from agricultural expansion and intensification in the Chaco

Carbon emissions from land‐use changes in tropical dry forest systems are poorly understood, although they are likely globally significant. The South American Chaco has recently emerged as a hot spot of agricultural expansion and intensification, as cattle ranching and soybean cultivation expand into forests, and as soybean cultivation replaces grazing lands. Still, our knowledge of the rates and spatial patterns of these land‐use changes and how they affected carbon emissions remains partial. We used the Landsat satellite image archive to reconstruct land‐use change over the past 30 years and applied a carbon bookkeeping model to quantify how these changes affected carbon budgets. Between 1985 and 2013, more than 142 000 km² of the Chaco's forests, equaling 20% of all forest, was replaced by croplands (38.9%) or grazing lands (61.1%). Of those grazing lands that existed in 1985, about 40% were subsequently converted to cropland. These land‐use changes resulted in substantial carbon emissions, totaling 824 Tg C between 1985 and 2013, and 46.2 Tg C for 2013 alone. The majority of these emissions came from forest‐to‐grazing‐land conversions (68%), but post‐deforestation land‐use change triggered an additional 52.6 Tg C. Although tropical dry forests are less carbon‐dense than moist tropical forests, carbon emissions from land‐use change in the Chaco were similar in magnitude to those from other major tropical deforestation frontiers. Our study thus highlights the urgent need for an improved monitoring of the often overlooked tropical dry forests and savannas, and more broadly speaking the value of the Landsat image archive for quantifying carbon fluxes from land change.