Variations of Soil Organic Carbon Following Land Use Change on Deep‐Loess Hillsopes in China

Land use change is a key factor driving changes in soil organic carbon (SOC) around the world. However, the changes in SOC following land use changes have not been fully elucidated, especially for deep soils (>100 cm). Thus, we investigated the variations of SOC under different land uses (cropland, jujube orchard, 7‐year‐old grassland and 30‐year‐old grassland) on hillslopes in the Yuanzegou watershed of the Loess Plateau in China based on soil datasets related to soils within the 0–100 cm. Furthermore, we quantified the contribution of deep‐layer SOC (200–1,800 cm) to that of whole soil profiles based on soil datasets within the 0–1,800 cm. The results showed that in shallow profiles (0–100 cm), land uses significantly (p  < 0·05) influenced the distribution of SOC contents and stocks in surface layer (0–20 cm) but not subsurface layers (20–100 cm). Pearson correlation analysis indicated that soil texture fractions and total N were significantly (p  < 0·05 or 0·01) correlated with SOC content, which may have masked effects of land use change on SOC. In deep profiles (0–1,800 cm), SOC stock generally decreased with soil depth. But deep soils showed high SOC sequestration capacity. The SOC accumulated in the 100–1,800 m equalled 90·6%, 91·6%, 87·5% and 88·6% of amounts in the top 100 cm under cropland, 7‐year‐old grassland, 30‐year‐old grassland and jujube orchard, respectively. The results provide insights into SOC dynamics following land use changes and stressed the importance of deep‐layer SOC in estimating SOC inventory in deep loess soils. Copyright © 2017 John Wiley & Sons, Ltd.     

Concentration and stock of carbon in the soils affected by land uses and climates in the western Himalaya,India

Soils are the third biggest sink of carbon on the earth. Hence, suitable land uses for a climatic condition are expected to sequester optimum atmospheric carbon in soils. But, information on how climatic conditions and land uses influence carbon accumulation in the soils on the Himalayan Mountains is not known. This study reports the impact of four climatic conditions (sub-tropical, altitude: 500–1200 m; temperate 1200–2000 m; lower alpine 2000–3000 m; upper alpine, 3000–3500 m) and four land uses (forest, grassland, horticulture, agriculture) on the concentrations and stocks of soil organic carbon (SOC) in upper (0–30 cm) and deeper (30–100 cm) soil depths on the western Himalayan Mountains of India. The study also explored the drivers which influenced the SOC stock build up on the mountains. Rainfall and soil moisture showed quadratic relations, whereas temperature declined linearly with the altitude. SOC stock as well as concentration was the highest (101.8 Mg ha^− 1 in 0–30 cm, 227.97 Mg ha^− 1 in 0–100 cm) in temperate and the lowest in sub-tropical climate (37 Mg ha^− 1 in 0–30 cm, 107.04 Mg ha^− 1 in 0–100 cm). Pattern of SOC stock build up across the altitude was: temperate > lower alpine > upper alpine > sub-tropical. SOC stocks in all land uses across the climatic conditions, except agriculture in lower alpine, was higher (0.7 to 41.6%) in the deeper than upper soil depth. SOC stocks in both the depths showed quadratic relations with soil temperature and soil moisture. Other factors like fine soil particles, land-use factor and altitude influenced positively whereas slope and pH, negatively to the SOC stocks. In all climatic conditions, other than temperate, SOC stocks were greater in natural ecosystems like forests and pastures (112.5 to 247.5 Mg ha^− 1) than agriculture (63 to 120.4 Mg ha^− 1). In temperate climate, SOC stock in agriculture (253.6 Mg ha^− 1) on well formed terraces was a little higher than forest (231.3 Mg ha^− 1) on natural slope. These observations, suggest that land uses on temperate climate may be treated as potential sinks for sequestration of the atmospheric carbon. However, agriculture in subtropical climate need to be pursued with due SOC protection measures like the temperate climate for greater sequestration of the atmospheric carbon.     

Soil Organic Carbon Accumulation in Abandoned Croplands on the Loess Plateau

The Grain to Green Program in China which began in 1999 led to the conversion of 0.64 million ha of cropland to grassland on steep sloping landscapes. However, the pattern of natural vegetation succession following cropland has not been well represented in previous regional syntheses of land use change effects on soil organic carbon (SOC). A chronosequence study focusing on the vegetation succession and soil carbon stocks was conducted in the center of the Loess Plateau. The chronosequence included fields of 0, 2, 5, 8, 9, 10, 12, 15 and 25 years of self‐restoration after cropland abandonment, as well as a natural grassland reference. Plant coverage, species richness and plant biomass increased significantly with time of cropland abandonment. Over time, the species composition more nearly resembled a natural grasslands community. Cropland abandonment replenished SOC stocks by 3.6 kg C m⁻² during the 25‐year self‐restoration, but the SOC accumulation was restricted to the upper soil profiles (0–60 cm). SOC accumulation rate was 88 g C m⁻² y⁻¹ in 0–30 cm and 55 g C m⁻² y⁻¹ in 30–60 cm soil depth, respectively. These carbon stocks were still significantly lower than those found in the natural grassland soil. Our results suggest that the recovery of plant communities and SOC stocks appears to be slow in this semiarid environment without revegetation effort along with appropriate field management, although the post‐agricultural soils have a high potential for carbon sequestration. Copyright © 2016 John Wiley & Sons, Ltd.     

Redistribution of Different Organic Carbon Fractions in the Soil Profile of a Typical Chinese Mollisol with Land-Use Change

Soil organic carbon (SOC) content depends significantly upon changes in land use and vegetation cover. This study aimed to examine the redistribution of whole soil OC, water-soluble OC (WSOC), and different density-separated OC fractions in soil profiles of 0–100 cm under different land uses and to elaborate the mechanism of C sequestration in response to the land use change. The land use types include maize plots with or without chemical fertilizer application (i.e., Maize-nitrogen, phosphorus, and potassium (NPK) and Maize-NF plots), plots with vegetation removed (No Vegetation), plots with grass (Grass), and alfalfa plant (Alfalfa). These plots used to be maize cropping system with NPK fertilizer for many years before 2003. Significant difference in SOC content generally occurred in soil layers of 0–40 cm among the different plots after 11 years of land-use change. Long-term continuous maize planting decreased SOC content; the significant SOC decrease occurred in Maize plot in the range of 9.3–23.4% for different soil layers compared with the initial soil sampled in 2003. In addition, SOC in Maize plot decreased by 3.6% and 8.5% at top two soil layers, respectively, in comparison with No Vegetation plot. The similar reduction of OC was observed in heavy OC fractions. The calculated sensitivity index for OC decreased in the order of light fraction > water-soluble fraction > the whole soil > heavy fraction. Therefore, the young and labile carbon fractions are much sensitive to land use change relative to the old and recalcitrant carbon fractions. This study indicated that land use changes led to a redistribution of SOC in soil profile, particularly at top soil layers, and conversion from arable land to natural grass cover or nitrogen-fixation plant cultivation such as alfalfa led to the enrichment of SOC at different depths of soil profile.     

子午岭林区不同植被恢复阶段土壤有机碳变化研究

研究了子午岭林区植被恢复过程中土壤有机碳含量、团聚体有机碳分布以及不同粒级团聚体有机碳对土壤有机碳的贡献率。研究结果表明,0—100 cm剖面上有机碳含量加权平均值随植被恢复年限逐渐升高,坡耕地0—100 cm土层土壤有机碳加权平均值为3.54 g/kg,弃耕地、草地、灌木和乔木阶段分别比坡耕地提高6.8%,36.6%,41.5%和73.6%;0—20 cm土层土壤有机碳含量随植被恢复的提高幅度明显高于20 cm以下土层;0—5和5—10 cm土层土壤各粒级团聚体有机碳含量随植被恢复年限逐渐增加,并有向大粒级（〉2 mm）团聚体中富集的趋势,10—20 cm土层土壤团聚体有机碳含量随植被恢复变化不明显;弃耕地、草地、灌木和乔木阶段0—20 cm土层〉5,5～2和2～1 mm粒级团聚体有机碳贡献率高于坡耕地,说明植被恢复0—20 cm土层土壤增加的有机碳更多地固定在〉1 mm粒级团聚体中。     

喀斯特石漠化区不同土地利用方式下土壤有机碳分布特征

为探究喀斯特土壤有机碳分布特征及其对人为干扰的响应,挖掘了2 854个土壤剖面,采集了22 786个土壤样品,分析了贵州省不同土地利用方式下土壤有机碳分布规律;并结合贵州省石漠化防治规划,初步估算了石漠化防治工程的土壤碳增汇贡献。结果表明:贵州省土壤有机碳呈现含量高、密度小的特征。表层土壤(0-20cm)有机碳平均含量25.07g/kg,平均密度仅为4.27kg/m~2。不同用地类型土壤表层有机碳含量大小为灌木林地乔灌木林地灌草地乔木林地弃耕地与荒地草地水田园地旱地与坡耕地;表层碳密度大小为水田灌木林地乔木林地乔灌木林地弃耕地与荒地灌草地旱地与坡耕地草地园地。0-60cm土层土壤有机碳含量对人为干扰较为敏感,60-100cm土层土壤有机碳含量差异较小。实施退耕还林,人工种草及人工造林等石漠化防治工程会明显促进土壤有机碳的积累,到2050年,贵州省0-10,0-20,0-30,0-100cm土层土壤有机碳将增加1.99×10~(13),3.37×10~(13),4.45×10~(13),6.29×10~(13) g。可见,喀斯特地区土壤有机碳具有含量高、密度低的显著特征,石漠化治理能有效增加喀斯特地区土壤碳汇。     

Response of soil properties and C dynamics to land-use change in the west of Loess Plateau

Land-use change (LUC) is widely considered a major factor that affects soil organic carbon (SOC) sequestration. The impacts of four LUC types on soil properties, SOC, particulate organic carbon (POC) and labile organic carbon (LOC) at the 0–100 cm depth were examined in the west of Loess Plateau, northwest China. Bulk density at the 20–40 cm depth increased significantly after native grassland conversion to cropland, while artificial grassland establishment and abandonment on former cropland caused reverse change. Soil water content in the profile increased 60–230% after cultivation and decreased 32–49% after abandonment (p < 0.01). The particle size distribution also showed a response to LUC. Only artificial grassland establishment caused an SOC sink of 32% at the 0–10 cm depth as well as two labile fractions. SOC tended to increase after cultivation and after abandonment, with 6% and 20% at soil surface, respectively. There were increasing trends in POC and LOC. After afforestation on former native grassland, SOC tended to decrease (23%) at the 0–10 cm depth while POC and LOC tended to increase (33% and 6%, respectively). Principal component analysis was successful in separating LUC through soil property parameters. Carbon sequestration is largely ascribed to increased below-ground production and tillage elimination after perennial alfalfa (Medicago sativa L.) plantation. Irrigation and fertilization activities contribute to SOC accumulation after cultivation to some extent. The self-restoration dynamic depending on time since abandonment is important to SOC change. A lower proportion of stabilized carbon results in a slow rate of SOC accumulation after afforestation. It is necessary to investigate the long-term dynamic after LUC.     

晋西黄土丘陵区不同土地利用类型对土壤碳氮储量的影响

通过对比晋西黄土区蔡家川流域人工刺槐林地、人工油松林地、退耕荒草地和农地土壤有机碳和总氮含量的差异性,探讨了土地利用变化对该流域土壤碳氮储量的影响。结果表明:（1）4种土地利用类型下,土壤有机碳和总氮均表现出表聚效应,且荒草地垂向降幅最大,各样地土壤有机碳和总氮均呈极显著线性正相关;（2）油松林地0—48 cm土层有机碳含量高于其他样地,刺槐林地0—48 cm土层总氮含量显著高于其他样地,而4种土地利用类型在48—96 cm土层和96—160 cm土层中有机碳和总氮含量差异不显著;（3）从蔡家川流域退耕土壤固碳和固氮效应整体而言,退耕林地具有显著的土壤碳氮固存效应,随着人工林地面积占比的增加,流域土壤总有机碳和总氮储量相应增加。     

基于最大似然法与矩法的黄土高原小流域土壤碳氮空间变异分析

以黄土高原寺底沟小流域为研究对象,根据不同土地利用方式采集46个样点的土壤样品,通过地统计方法对土壤有机碳和全氮的空间变异特征进行了分析。采用受限最大似然法(REML)和矩法(MOM)两种方法分别对变异函数进行了估计,通过交叉检验选择克里金预测效果较好的变异函数进行地统计插值。(1)与矩法(MOM)相比,在多数情况下受限最大似然法(REML)估计的变异函数进行克里金插值更加准确。(2)土层深度对土壤全氮空间变异影响较小,对土壤有机碳影响较大,表层土壤有机碳含量及变异程度明显高于下层土壤。(3)土地利用方式对土壤有机碳和全氮的空间分布有重要影响,灌木林和天然草地土壤有机碳和全氮水平最高,弃耕地其次,梯田、果园、人工草地最低,表明退耕还林对提高土壤碳氮水平有重要贡献。     

Long‐term changes of aggregate‐associated and labile soil organic carbon and nitrogen after conversion from forest to grassland and cropland in northern Turkey

Soil management systems can have great effect on soil chemical, physical and biological properties. Conversion of forest to grassland and cropland can alter C and N dynamics. The objective of this study was to evaluate the changes in aggregate‐associated and labile soil organic C and N fractions after conversion of a natural forest to grassland and cropland in northern Turkey. This experiment was conducted on plots subject to three different adjacent land uses (forest, grassland and cropland). Soil samples were taken from 0–5, 5–15 and 15–30 cm depths from each land use. Some soil physical (soil texture, bulk density), chemical (soil pH, soil organic matter, lime content, total organic C and N, inorganic N, free and protected organic C) and biological (microbial biomass C and N, mineralizable C and N) properties were measured. The highest and lowest bulk densities were observed in grassland (1.41 g cm⁻³) and cropland (1.14 g cm⁻³), respectively. Microbial biomass C and total organic C in forest were almost twice greater than grassland and four‐times greater than cropland. Cultivation of forest reduced total organic N, mineralizable N and microbial biomass N by half. The great portion of organic C was stored in macroaggregates (>250 µm) in all the three land uses. Free organic C comprised smaller portion of soil organic C in all the three land uses. Thus, this study indicated that long‐term conversion of forest to grassland and cropland significantly decreased microbial biomass C, mineralizable C and physically protected organic C and the decreases were the greatest in cropland. Copyright © 2011 John Wiley & Sons, Ltd.     

区域绿洲农田土壤有机碳分布及其影响因子研究

以干旱区典型绿洲农田区——玛纳斯县中部农田为研究区,以土壤有机碳为研究对象,结合野外土壤调查及实验室分析数据研究了土壤有机碳的垂直分布特征,并分析土壤质地、地形、土地利用、作物类型等不同因子对农田土壤有机碳的影响。结果表明:玛纳斯县中部农田土壤有机碳是自然环境综合因素的结果,土壤有机碳含量随着土壤深度的增加不断减小;不同土壤质地土壤有机碳含量的特征为:粘壤土粉壤土沙壤土;不同地形因子中坡向与农田0~30、30~60 cm层的土壤有机碳含量呈显著正相关,海拔与农田60~100 cm层的土壤有机碳含量呈显著正相关;不同土地利用方式下土壤有机碳含量有较大差异,果园的土壤有机碳含量最高,荒地的土壤有机碳含量最低;不同作物类型土壤有机碳含量特征为:玉米地酒葡萄地棉花地,且差异显著。     

塔里木盆地北缘绿洲不同土地利用方式下土壤有机碳含量及其碳矿化特征

[目的]探讨土地利用方式对土壤有机碳含量及碳矿化的影响,为塔里木盆地北缘绿洲土壤生态系统的保护和恢复建设提供理论依据。[方法]基于野外采样和室内培养试验,分析土壤有机碳含量的基本特征,利用回归分析法拟合出土壤有机碳矿化动态变化过程。[结果]矿化累积释放的CO2含量大小依次为:果园棉田人工林弃耕地荒草地盐碱地沙地。不同土地利用方式土壤有机碳矿化反应趋势相同,1~6d为快速分解阶段,日均矿化量高但反应时间短,6~28d为缓慢分解阶段,动态变化与前者相反。有机碳矿化率大小依次为:沙地荒草地盐碱地弃耕地人工林棉田果园,沙地最高,达(10.36±0.24)%,表明沙地土壤有机碳稳定性最差,而果园具有较强的固定有机碳能力。[结论]土地利用方式对土壤有机碳矿化及其固碳能力均有显著影响。     

岩溶山地不同土地利用方式土壤颗粒有机碳和矿物结合态有机碳的分布特征

对重庆中梁山岩溶山地不同土地利用方式下0-40cm土壤颗粒有机碳和矿物结合态有机碳的含量和分布特征进行了研究.结果表明:不同土地利用方式土壤有机碳含量平均值表现为:林地>菜地>草地>橘园地>弃耕地.除橘园地外,其它各土地利用类型土壤细颗粒有机碳(FPOC)含量大于粗颗粒有机碳(CPOC).不同利用方式土壤颗粒有机碳含量在剖面层次中表现不同.0-20cm表层土壤CPOC含量表现为:橘园地>草地>菜地>林地>弃耕地,差异较大.土壤FPOC含量表现为:林地>草地>菜地>橘园地>弃耕地;20-40cm土壤CPOC和FPOC最高值出现在菜地,最低值出现在弃耕地.不同土地利用方式土壤矿物结合态有机碳(MOC)含量和土壤有机碳含量分布特征一致.除橘园地外土壤各组分有机碳分配比例大致表现为:MOC/SOC>CPOC/SOC>FPOC/SOC.相关分析表明,不同土地利用方式土壤SOC和POC呈正相关,相关性不一致.林地和草地呈极显著相关(P<0.01),弃耕地呈显著相关(P<0.05),菜地和橘园地相关性不显著.表明人为干扰和耕作措施会影响POC对SOC的贡献.     

Effects of vegetation restoration on soil organic carbon sequestration at multiple scales in semi-arid Loess Plateau,China

Soil organic carbon (SOC) sequestration by vegetation restoration is the theme of much current research. Since 1999, the program of “Grain for Green”has been implemented in the semi-arid Loess Plateau, China. Its scope represents the largest vegetation restoration activity in China. However, it is still unclear for the SOC sequestration effects of vegetation cover change or natural succession promoted by the revegetation efforts at different scales under the semi-arid conditions. In this study, the changes in SOC stocks due to the vegetation restoration in the middle of Loess Plateau were estimated at patch, hill slope transect and small watershed scale from 1998 to 2006. Soil samples were taken from field for the determination of cesium-137 (¹³⁷Cs) and SOC contents. Vegetation cover change from 1998 to 2006 at the small watershed scale was assessed using Geographic Information System. The results showed that cropland transforming to grassland or shrubland significantly increased SOC at patch scale. Immature woodland, however, has no significant effect. When vegetation cover has no transformation for mature woodland (25 years old), SOC has no significant increase implying that SOC has come to a stable level. At hill slope scale, three typical vegetation cover patterns showed different SOC sequestration effects of 8.6%, 24.6%, and 21.4% from 1998 to 2006, and these SOC increases mainly resulted from revegetation. At the small watershed scale, SOC stocks increased by 19% in the surface soil layer at 0–20 cm soil depth from 1998 to 2006, which was equivalent to an average SOC sequestration rate of 19.92 t C y^− 1 km^− 2. Meanwhile, SOC contents showed a significant positive correlation (P < 0.001) with the ¹³⁷Cs inventory at every soil depth interval. This implied significant negative impacts of soil erosion on SOC sequestration. The results have demonstrated general positive effects of vegetation restoration on SOC sequestration at multiple scales. However, soil erosion under rugged topography modified the spatial distribution of the SOC sequestration effects. Therefore, vegetation restoration was proved to be a significant carbon sink, whereas, erosion could be a carbon source in high erosion sensitive regions. This research can contribute to the performance assessment of ecological rehabilitation projects such as “Grain to Green” and the scientific understanding of the impacts of vegetation restoration and soil erosion on soil carbon dynamics in semi-arid environments.     

Soil organic carbon as affected by land use in young and old reclaimed regions of a coastal estuary wetland,China

Soil organic carbon (SOC) and selected soil properties were measured in fringe and ditch marshes and cropland of old and young reclaimed areas in a subtropical estuary in China in order to investigate the effects of land use and reclamation history on SOC. The results show that after the conversion of wetlands to cropland, a longer reclamation history (>20 yr) resulted in greater soil bulk density, salinity, clay and silt, and lower soil moisture, SOC and sand content, whereas a shorter reclamation history (<20 yr) induced smaller values for soil pH, moisture and sand. Ditch marshes had greater average SOC in the top 50 cm than fringe marshes and cropland. SOC decreased generally down soil profiles from 0 to 50 cm in depth, except for the obvious accumulation of SOC in deeper soils from old fringe and young ditch marshes. Ditch marshes had the greatest SOC densities in the top 50 cm in both regions compared to the other land uses. SOC densities in the top 50 cm were less in croplands than in fringe marshes in the young region, while there were no significant differences between them in the older one. Except for cropland, SOC densities in the top 50 cm of the fringe or ditch marshes in the old region were not significantly different from those in the young region. SOC in both regions was reduced by 13.53 × 10⁴ t (12.98%) in the top 50 cm of the marshes after conversion to cropland, whereas the regional SOC storage increased by 29.25 t when ditch marshes were included. The results from regression analysis show that bulk density and soil moisture significantly influenced SOC. The total SOC stored in both ditch marshes and croplands was higher compared to fringe marshes. The regional SOC storage in the top 50 cm was not reduced after reclamation due to C accumulation in the ditch marshes. The regional effects of cultural practices should be taken into account in devising strategies for managing soils in coastal wetlands, particularly in the developing world.     

土地利用方式和地形对半干旱区土壤有机碳含量的影响

以内蒙古赤峰市敖汉旗为研究对象,以实地调查数据为基础,结合土地利用方式与地形的变化,对敖汉旗0～100 cm深度土壤有机碳含量的空间分布特征进行了研究,旨在对地区碳储量的估算和科学利用土地资源起到积极的借鉴作用。结果表明,敖汉旗土壤有机碳含量在0～100 cm深度的土壤剖面内的变化范围为0.23～20.71 g/kg,主要集中在40 cm以上土层,且随着土层深度的增加土壤有机碳平均含量逐渐降低;各土地利用方式下土壤有机碳含量均表现为:林地农地草地。土壤有机碳含量主要富集在高海拔区的平缓地段;受土壤侵蚀的影响,当坡度10°后,不同土地利用类型的有机碳含量均显著降低。     

土地利用变化对闽江河口湿地表层土壤有机碳含量及其活性的影响

对闽江河口原生植被芦苇沼泽,以及由其转化的不同其它土地利用类型(滩涂养殖地、水田、草地、撂荒地和池塘养殖地)的表层(0-50 cm)沉积物(或土壤)有机碳和活性有机碳含量的研究,结果表明,滩涂养殖地、水田、池塘养殖地、草地和撂荒地的土壤有机碳含量分别比芦苇沼泽地低27%,75%,67%,1%,60%;在有机碳储量方面,滩涂养殖地、水田、池塘养殖地和撂荒地比天然芦苇沼泽地分别低11%,50%,37%,24%,草地有机碳储量比芦苇高44%;草地土壤有机碳含量和储量随土层加深而递减的幅度比芦苇地大;水田有机碳含量和储量垂直变化不明显,弃耕后,表层有机碳含量提高,垂直变化明显。不同土地利用方式间土壤活性有机碳含量的差异比有机碳的差异大,与芦苇地相比,滩涂养殖地、水田、池塘、草地活性有机碳含量分别低24%,83%,84%,42%;撂荒10年的弃耕地与水稻田土壤相比,活性有机碳含量提高了47%。     

基于GIS的土壤有机碳储量核算及其对土地利用变化的响应

土地利用变化是影响土壤有机碳储量变化的重要驱动因素,为了进一步探讨土地利用变化对土壤碳储量的影响,该文根据土壤样点数据、土壤类型图、土地利用类型图,分析了江苏省1985年和2005年表层土壤有机碳密度的变化以及土地利用变化对表层土壤有机碳密度的影响,主要结论如下：1）江苏省表层土壤有机密度的空间变化趋势为：黄淮平原生态区南北差异明显,北部的沂沭泗平原丘岗以增加为主,南部的淮河下游平原以减少为主;沿海滩涂与海洋生态区持平为主;而长江三角洲平原生态区表现不一：沿江平原丘岗生态亚区以增加为主,而茅山宜溧低山丘陵生态亚区和太湖水网生态亚区均表现为有机碳密度的减少;2）各地类表层土壤有机碳密度均有所增加;耕地-林地、草地;草地-林地、建设用地;建设用地-耕地、草地、林地;水域的转出以及未利用地的转出等转换类型有利于土壤碳储量的增加、其他地类间的转换会造成一定的碳排放。     

Conversion of cropland into grassland: Implications for soil organic‐carbon stocks in two soils with different texture

Soil organic‐carbon (SOC) stocks are expected to increase after conversion of cropland into grassland. Two adjacent cropland and grassland sites—one with a Vertisol with 23 y after conversion and one with an Arenosol 29 y after conversion—were sampled down to 60 cm depth. Concentrations of SOC and total nitrogen (N_tot) were measured before and after density fractionation in two light fractions and a mineral‐associated fraction with C adsorbed on mineral surfaces. For the soil profiles, SOC stocks and radiocarbon (¹⁴C) concentrations of mineral associated C were determined. Carbon stocks and mineral‐associated SOC concentrations were increased in the upper 10 cm of the grassland soil compared to the cropland. This corresponded to the root‐biomass distribution, with 59% and 86% of the total root biomass at 0–5 cm soil depth of the grasslands. However, at the Arenosol site, at 10–20 cm depth, C in the mineral‐associated fraction was lost 29 y after the conversion into grassland. Over all, SOC stocks were not significantly different between grassland and cropland at both sites when the whole profile was taken into account. At the Arenosol site, the impact of land‐use conversion on SOC accumulation was limited by low total clay surface area available for C stabilization. Subsoil C (30–50 cm) at cropland of the Vertisol site comprised 32% of the total SOC stocks with high ¹⁴C concentrations below the plowing horizon. We concluded that fresh C was effectively translocated into the subsoil. Thus, subsoil C has to be taken into account when land‐use change effects on SOC are assessed.     

滇中尖山河小流域不同土地利用类型土壤活性有机碳分布特征

土壤活性有机碳对土地利用方式最为敏感,定量分析不同土地利用方式对土壤活性有机碳分布特征的影响对流域的土壤碳循环研究具有重要意义。从滇中尖山河小流域坡耕地、荒草地、林地、园地4种不同土地利用类型角度,系统地分析了0—10,10—20,20—30 cm土层土壤有机碳（SOC）、微生物有机碳（MBC）、易氧化有机碳（EOC）及可溶性有机碳（DOC）的分布特征及其相关性。结果表明:不同土地利用类型下土壤SOC,MBC,EOC,DOC整体均表现为园地 > 林地 > 坡耕地 > 荒草地;4种土地利用类型MBC,EOC,DOC整体上随着土层深度的增加而逐渐降低,且主要分布在0—20 cm土层,在20—30 cm土层含量较低（低于30%）;4种土地利用类型下SOC和MBC,EOC,DOC呈极显著正相关关系,MBC,EOC,DOC两两之间也表现出极显著正相关。综上,退耕还林以及在荒草地种植人工林可作为提高土壤有机碳及活性有机碳含量的有效措施,并将在减少流域水土流失和面源污染、改善土壤质量、恢复土壤肥力等方面起到重要作用。