Distinct controls over the temporal dynamics of soil carbon fractions after land use change

Soil organic carbon (SOC), the largest terrestrial carbon pool, plays a significant role in soil‐related ecosystem services such as climate regulation, soil fertility and agricultural production. However, its fate under land use change is difficult to predict. A major issue is that SOC comprised of numerous organic compounds with potentially distinct and poorly understood turnover properties. Here we use spatiotemporal measurements of the particulate (POC), mineral‐associated (MOC) and charred SOC (COC) fractions from 176 trials involving changes in land use to assess their underlying controls. We find that the initial pool sizes of each of the three fractions consistently and dominantly control their temporal dynamics after changes in land use (i.e. the baseline effects). The effects of climate, soil physicochemical properties and plant residues, however, are fraction‐ and time‐dependent. Climate and soil properties show similar importance for controlling the dynamics of MOC and COC, while plant residue inputs (in term of their quantity and quality) are much less important. For POC, plant residues and management practices (e.g. the frequency of pasture in crop‐pasture rotation systems) are substantially more important, overriding the influence of climate. These results demonstrate the pivotal role of measuring SOC composition and considering fraction‐specific stabilization and destabilization processes for effective SOC management and reliable SOC predictions.     

Zones of influence for soil organic matter dynamics: A conceptual framework for data and models

Soil organic matter (SOM) is an indicator of sustainable land management as stated in the global indicator framework of the United Nations Sustainable Development Goals (SDG Indicator 15.3.1). Improved forecasting of future changes in SOM is needed to support the development of more sustainable land management under a changing climate. Current models fail to reproduce historical trends in SOM both within and during transition between ecosystems. More realistic spatio‐temporal SOM dynamics require inclusion of the recent paradigm shift from SOM recalcitrance as an ‘intrinsic property’ to SOM persistence as an ‘ecosystem interaction’. We present a soil profile, or pedon‐explicit, ecosystem‐scale framework for data and models of SOM distribution and dynamics which can better represent land use transitions. Ecosystem‐scale drivers are integrated with pedon‐scale processes in two zones of influence. In the upper vegetation zone, SOM is affected primarily by plant inputs (above‐ and belowground), climate, microbial activity and physical aggregation and is prone to destabilization. In the lower mineral matrix zone, SOM inputs from the vegetation zone are controlled primarily by mineral phase and chemical interactions, resulting in more favourable conditions for SOM persistence. Vegetation zone boundary conditions vary spatially at landscape scales (vegetation cover) and temporally at decadal scales (climate). Mineral matrix zone boundary conditions vary spatially at landscape scales (geology, topography) but change only slowly. The thicknesses of the two zones and their transport connectivity are dynamic and affected by plant cover, land use practices, climate and feedbacks from current SOM stock in each layer. Using this framework, we identify several areas where greater knowledge is needed to advance the emerging paradigm of SOM dynamics—improved representation of plant‐derived carbon inputs, contributions of soil biota to SOM storage and effect of dynamic soil structure on SOM storage—and how this can be combined with robust and efficient soil monitoring.     

土壤有机质对农田管理措施的动态响应

土壤有机质在农田肥力、环境保护、农业可持续发展等方面具有重要意义。它不仅决定农作物产量,而且在全球碳素循环中起着重要作用。由于大气CO2浓度升高与全球气候变暖等一系列环境问题的加剧,全球碳素循环受到越来越多的关注。农田具有大气CO2源和库的双重潜力。历史上由于人类对农田的过度开垦和耕种,造成土壤有机质含量大幅度下降,降低了农田的作物产量潜力;同时导致大量的碳以CO2形式由陆地生态系统排放到大气圈,加剧了全球温室效应。大量研究结果表明,诸如耕作、种植制度、施肥等农田管理措施能够显著地影响土壤有机质动态,而免耕、提高复种指数、合理的轮作换茬、有机肥料和化肥的施用以及弃耕农田还林还草等保护性管理措施则能够提高农田土壤有机质含量,使农田起到大气CO2汇的作用。综述了近年来农田管理对土壤有机质动态影响研究方面的进展。     

Soil organic carbon changes as influenced by agricultural land use and management: a case study in Yanhuai Basin, Beijing, China

The effects of agricultural land use and management practices on soil organic carbon (SOC) are of great concern. In this study, SOC changes were investigated in sandy loam soils (Ustochrepts, USDA Soil Taxonomy) under orchard, vegetable, corn (Zea maize L.), and soybean (Glycine max L.) cultivation in northern China. The corn fields were further classified into three categories based on their inputs, i.e. high-input, mid-input, and low-input corn fields. In April 2005, a total of 197 soil samples were collected from 42 soil sites within 100 cm soil depth in Yanhuai Basin, Beijing, China. SOC contents were determined using rapid dichromate oxidation, and ANOVA statistical analysis was used to test the significances between land use and management practices at p<0.05. The results showed that: (1) the effects of land use and management practices on SOC primarily occurred within the topsoil (0–25 cm), and the SOC contents sharply decreased with the increase in soil depth. (2) SOC content and density values of orchard, vegetable, and high-input corn fields were higher than those of soybean, mid- and low-input corn fields.     

土地利用变化对土壤有机碳的影响研究进展

土壤有机碳是陆地碳库的重要组成部分,也是当前全球碳循环和全球变化研究的热点。土地利用/覆被变化及土地管理变化通过影响土壤有机碳的储量和分布,进而影响温室气体排放和陆地生态系统的碳通量。研究土地利用变化影响下的土壤有机碳储量及其动态变化规律,有助于加深理解全球气候变化与土地利用变化之间的关系。在阅读国内外有关文献的基础上,分别从土地利用及其管理方式变化的角度,概括了土地利用变化对土壤有机碳的影响过程与机理;针对当前研究的两大类方法,即实验方法和模型方法,分类详细介绍了它们各自的特点以及存在的一些问题。在此基础上,提出今后土地利用变化对土壤有机碳影响研究的发展趋势。     

黄土塬区苹果园土壤有机碳分布特征

以黄土塬区塬面和梁坡梯田5、10、15a和20a苹果园为对象,在行间距果树1.0、1.5m和2.0m处用土钻法分层采集0-100cm土样,LiquiTOCⅡ测定样品土壤有机碳(Soil Organic Carbon,SOC)含量,分析两种地形条件下各龄果园SOC的分布特征。结果表明:塬面5、10、15a和20a果园SOC分别为6.39、6.46、6.66g/kg和6.47g/kg,梁坡分别为5.83、6.05、6.54g/kg和6.09g/kg,两种地形条件下同龄果园SOC差异显著(P0.05),但龄间SOC消长趋势相似,均有15a的20a的10a的5a的;水平方向上,5a果园SOC沿树干向外增大,10a果园减小,15a和20a果园变化较小,塬面和梁坡同龄果园间SOC水平分布格局较一致;垂直方向上,同梁坡相比,塬面果园4个层次(0-10cm、10-20cm、20-50cm和50-100cm)的平均SOC较高;梁坡果园50-100cm土层的SOC龄间差异较大,20-50cm土层龄间差异较小,塬面果园50-100cm土层的SOC龄间变化较小;在"纯果园"利用阶段,果园利用方式并未引起SOC下降,深层SOC有明显的积累效应。     

Crop Management for Soil Carbon Sequestration

Reducing emissions of greenhouse gases (GHG) from agriculture is related to increasing and protecting soil organic matter (SOM) concentration. Agricultural soils can be a significant sink for atmospheric carbon (C) through increase of the SOM concentration. The natural ecosystems such as forests or prairies, where C gains are in equilibrium with losses, lose a large fraction of the antecedent C pool upon conversion to agricultural ecosystems. Adoption of recommended management practices (RMPs) can enhance the soil organic carbon (SOC) pool to fill the large C sink capacity on the world's agricultural soils. This article collates, reviews, and synthesizes the available information on SOC sequestration by RMPs, with specific references to crop rotations and tillage practices, cover crops, ley farming and agroforestry, use of manure and biosolids, N fertilization, and precision farming and irrigation. There is a strong interaction among RMPs with regards to their effect on SOC concentration and soil quality. The new equilibrium SOC level may be achieved over 25 to 50 years. While RMPs are being adapted in developed economies, there is an urgent need to encourage their adoption in developing countries. In addition to enhancing SOC concentration, adoption of RMPs also increases agronomic yield. Thus, key to enhancing soil quality and achieving food security lies in managing agricultural ecosystems using ecological principles which lead to enhancement of SOC pool and sustainable management of soil and water resources.     

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}.     

Microbiological indicators of soil quality and degradation following conversion of native forests to continuous croplands

Deforestation resulting from forest conversion to agricultural land use is an important issue worldwide. This phenomenon is known to influence the activity and size of soil microbial community due to changes in environmental conditions with subsequent losses of soil organic matter (SOM) and soil quality degradation. The objective of this study was to investigate the relationship between soil organic carbon (SOC) losses and enzyme activities following land use conversion from native forests to continuous croplands. The amount of soil microbial biomass carbon (SMBC) and the activity of five soil enzymes (i.e., urease, invertase, alkaline phosphatase, acid phosphatase and arylsulfatase) were measured in croplands derived from forests and adjacent natural forests all on similar soil type at Gorgan site located in Northeast Iran. The content of SMBC decreased (47–83%) with deforestation at both soil sampling depths (0–20 and 20–40 cm). With the exception of phosphatases, the absolute activities of soil enzymes (activity on a soil mass basis) tended to decrease significantly (15–35%) with continuous cultivation. However, the specific enzyme activities expressed either per unit of SOC or SMBC tended to increase (about 1.5–5.5 times) with conversion of forestlands to croplands. The significant positive correlation between enzyme activity per SMBC and C turnover rate may imply that a faster C cycle and loss due to deforestation is related to a greater enzymatic activity by a smaller size of microbial biomass in cropland soils. In brief, the specific activities of soil enzymes could be used to reveal SOM losses and soil degradation in natural forest ecosystems, and to identify changes in soil quality and fertility following deforestation. Changes or improvements in soil management such as cessation of cultivation or implementing agricultural practices that stop or minimize soil disturbance are most likely needed to stop further soil degradation, restore soil quality and rebuild SOC stocks to offset CO₂ emissions in these ecosystems.     

全球和生物群系尺度土壤有机碳分布控制因子及其分异规律

土壤有机碳(SOC)是陆地生态系统碳库的核心组成部分,其动态平衡受气候、土壤、植被、地形及人类活动等的影响,但在不同的空间尺度上,这些影响因素的相对重要性和差异还不明确。为阐明不同尺度和不同土层深度土壤有机碳密度(SOCd,kg/m³土壤)的环境影响因子差异,选用全球113571个土壤剖面SOCd测量数据以及38个环境协变量数据,利用数据挖掘方法,分析了全球尺度和生物群系尺度不同土层深度SOCd的控制因子,并量化了空间自相关对相关结果的影响。研究结果表明:仅空间自相关就能解释全球尺度不同土壤深度13%-20%的SOCd空间变异,但是随土壤深度的增加,空间自相关的解释率降低。在剔除空间自相关的影响后,分析结果表明:全球尺度上,气候因素对SOCd空间变异的解释率最高,但只能解释17%-20%,这种解释率在不同土层之间没有显著差异。在生物群系尺度上,除北方森林地区,气候因素能够解释SOCd空间变异的24%-37%;而在北方森林地区,地形是影响SOCd空间变异的重要因素,对SOCd的解释率为21%-43%。这些结果表明,SOCd的控制因子在不同的尺度上明显不同。无论是在全球尺度上,还是生物群系尺度上,如果不考虑空间自相关,地形的影响会被低估,其他环境因素的影响被严重高估。为了准确计算全球与生物群系尺度上各土层SOCd分布的控制因子及其分异情况,空间自相关必须被考虑。     

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.     

Smallholder farms have and can store more carbon than previously estimated

Increasing soil organic carbon (SOC) stocks is increasingly targeted as a key strategy in climate change mitigation and improved ecosystem resiliency. Agricultural land, a dominant global land use, provides substantial challenges and opportunities for global carbon sequestration. Despite this, global estimates of soil carbon sequestration potential often exclude agricultural land and estimates are coarse for regions in the Global South. To address these discrepancies and improve estimates, we develop a hybrid, data-augmented database approach to better estimate the magnitude of SOC sequestration potential of agricultural soils. With high-resolution (30 m) soil maps of Africa developed by the International Soils Database (iSDA) and Malawi as a case study, we create a national adjustment using site-specific soil data retrieved from 1160 agricultural fields. We use a benchmark approach to estimate the amount of SOC Malawian agricultural soils can sequester, accounting for edaphic and climatic conditions, and calculate the resulting carbon gap. Field measurements of SOC stocks and sequestration potentials were consistently larger than iSDA predictions, with an average carbon gap of 4.42 ± 0.23 Mg C ha⁻¹ to a depth of 20 cm, with some areas exceeding 10 Mg C ha⁻¹. Augmenting iSDA predictions with field data also improved sensitivity to identify areas with high SOC sequestration potential by 6%—areas that may benefit from improved management practices. Overall, we estimate that 6.8 million ha of surface soil suitable for agriculture in Malawi has the potential to store 274 ± 14 Tg SOC. Our approach illustrates how ground truthing efforts remain essential to reduce errors in continent-wide soil carbon predictions for local and regional use. This work begins efforts needed across regions to develop soil carbon benchmarks that inform policies and identify high-impact areas in the effort to increase SOC globally.     

汉中盆地秸秆还田撂荒和林地对土壤碳的影响

农业活动是温室气体重要的排放源,土壤碳库[土壤有机碳(SOC)和无机碳(SIC)]稍微变化会对大气CO_2产生很大影响。汉中盆地是南水北调的重要水源涵养地,在该区域秸秆还田、农田撂荒和林地是目前常见土地利用方式,但缺乏不同利用方式对SIC和SOC影响的研究。该研究采集该区域典型样地土壤,用滴定法和有机碳分析仪分别测定其SIC和SOC含量,研究3种土地利用方式对土壤碳库的影响。结果表明:SOC随土层深度最为敏感的是农田,其次是撂荒地,林地最不敏感。0~140 cm土层SOC碳密度,林地最大,是撂荒田的2.26倍,农田是撂荒田的1.37倍。深土层SOC碳密度,林地是撂荒田的2.44倍,农田是撂荒田的1.07倍。撂荒田的SIC密度最大,其次是农田,林地的SIC碳密度最低。在0~140 cm土层中,SIC密度依次为12.37、11.68和9.77 kg·m~2,撂荒田的SIC碳密度是林地的1.27倍。随着我国农村发展,土地利用管理出现新的方式,今后在估算土地利用管理方式对土壤碳影响时还需要综合考虑SOC和SIC。     

土壤有机碳动态:风蚀效应

土壤风蚀是引起土壤退化最广泛的形式和原因之一。土壤风蚀对土壤碳动态的影响机制一方面是土壤风蚀引起土壤退化使土壤生产力下降,输入土壤的碳数量减少;另一方面是富含有机碳的细粒物质直接移出系统。风蚀土壤碳的去向包括:(1)就近沉积,(2)沉积于水渠和河流,输入水体;(3)以粉尘形式运移,在远离风蚀区的地域沉积;(4)氧化释放至大气。风蚀引起土壤碳的迁移和沉积不仅导致土壤有机碳在地域间的再分布,使土壤性状的空间异质性增加,也显著改变了土壤系统中碳矿化的生物学过程。土壤有机碳的保持可以促进团聚体的形成,使土壤物理稳定性增加,减缓风蚀。对易风蚀土地进行退耕还林还草、实行保护性耕作等措施可以有效增加土壤碳的固存。     

长白山原始阔叶红松林土壤有机质组分小尺度空间异质性

土壤有机质(SOM)对于维持生态系统生产力具有非常重要的意义,有机质的组成、空间分布和空间关联性是影响和控制诸多生态系统过程的重要因素。应用地统计学方法,对长白山原始阔叶红松林局部尺度内0—20 cm土壤有机质与活性有机质的空间异质性进行了研究,并通过交叉半方差分析探讨了二者之间的相关性。研究结果表明:(1)总体上来说,土壤有机碳(SOC)、全氮(TN)、颗粒态有机碳(POC)和颗粒态有机氮(PON)空间异质性较小;而土壤微生物量碳(MBC)、微生物量氮(MBN)和表层(0—10 cm)溶解性有机碳(DOC)的空间异质性较大;(2)SOC、TN、MBC、DOC、POC和PON随着深度的增加空间自相关性增加;而溶解性有机氮(DON)的空间自相关性随深度的增加变化不大;(3)SOC与TN在表层和下层(10—20 cm)均存在空间上的正相关关系;(4)SOC、TN在表层和下层分别与MBC、MBN、DOC、DON和POC呈空间上的正相关性,但是与PON之间的空间相关关系较差;(5)不同土层深度的土壤活性有机质之间的相关关系存在差异。在表层,除POC,PON外,其余土壤活性有机质组分在空间上两两相关;但是随着土壤深度的增加,活性有机质变量之间在空间上两两相关。研究结果表明土壤有机质组分在长白山原始阔叶红松林小尺度内存在不同空间异质性和空间关联性,这为人们更好的理解森林生态系统功能(如土壤养分循环)提供了重要的理论依据。     

Carbon storage capacity of semi‐arid grassland soils and sequestration potentials in northern China

Organic carbon (OC) sequestration in degraded semi‐arid environments by improved soil management is assumed to contribute substantially to climate change mitigation. However, information about the soil organic carbon (SOC) sequestration potential in steppe soils and their current saturation status remains unknown. In this study, we estimated the OC storage capacity of semi‐arid grassland soils on the basis of remote, natural steppe fragments in northern China. Based on the maximum OC saturation of silt and clay particles <20 μm, OC sequestration potentials of degraded steppe soils (grazing land, arable land, eroded areas) were estimated. The analysis of natural grassland soils revealed a strong linear regression between the proportion of the fine fraction and its OC content, confirming the importance of silt and clay particles for OC stabilization in steppe soils. This relationship was similar to derived regressions in temperate and tropical soils but on a lower level, probably due to a lower C input and different clay mineralogy. In relation to the estimated OC storage capacity, degraded steppe soils showed a high OC saturation of 78–85% despite massive SOC losses due to unsustainable land use. As a result, the potential of degraded grassland soils to sequester additional OC was generally low. This can be related to a relatively high contribution of labile SOC, which is preferentially lost in the course of soil degradation. Moreover, wind erosion leads to substantial loss of silt and clay particles and consequently results in a direct loss of the ability to stabilize additional OC. Our findings indicate that the SOC loss in semi‐arid environments induced by intensive land use is largely irreversible. Observed SOC increases after improved land management mainly result in an accumulation of labile SOC prone to land use/climate changes and therefore cannot be regarded as contribution to long‐term OC sequestration.     

森林土壤有机碳分解对模拟增温的响应

由化石燃料燃烧和土地利用变化引起的全球气候变暖是地球上最严重的人为干扰之一,对陆地生态系统结构和功能产生重要的影响。土壤有机碳（SOC）是陆地生态系统最大的碳库,其微小变化都会影响全球碳平衡和气候变化。近30年来,国内外学者在不同森林生态系统相继开展了野外模拟增温对SOC分解的影响及其调控机制研究。基于在全球建立的26个野外模拟气候变暖实验平台,系统分析增温对森林生态系统SOC分解的影响格局和潜在机制,发现增温通常促进森林SOC的分解,对气候变暖产生正反馈作用。然而,因增温方式和持续时间、土壤微生物群落结构和功能的多样性、SOC结构和组成的复杂性、植物-土壤-微生物之间相互作用以及森林类型等不同而存在差异,导致人们对森林SOC分解响应气候变暖的程度及时空格局变化缺乏统一的认识,且各类生物和非生物因子的相对贡献尚不清楚。基于已有研究,从土壤微生物群落结构和功能、有机碳组分以及植物-土壤-微生物互作3个方面构建了气候变暖影响SOC分解的概念框架,并进一步阐述了今后的重点研究方向,以期深入理解森林生态系统碳-气候反馈效应,为制定森林生态系统管理措施和实现"碳中和"提供科学依据。1）加强模拟增温对不同森林生态系统（特别是热带亚热带森林生态系统） SOC分解的长期观测研究,查明SOC分解的时空动态特征;2）加强土壤微生物功能群与SOC分解之间关系的研究,揭示SOC分解对增温响应的微生物学机制;3）形成统一的SOC组分研究方法,揭示不同碳组分对增温的响应特征和机制;4）加强森林生态系统植物-土壤-微生物间相互作用对模拟增温的响应及其对SOC分解调控的研究;5）加强模拟增温与其他全球变化因子（例如降水格局变化、土地利用变化、大气氮沉降）对SOC分解的交互作用,为更好评估未来全球变化背景下森林土壤碳动态及碳汇功能的维持提供理论基础。     

Soil carbon dynamics following land‐use change varied with temperature and precipitation gradients: evidence from stable isotopes

Knowledge of soil organic matter (SOM) dynamics following deforestation or reforestation is essential for evaluating carbon (C) budgets and cycle at regional or global scales. Worldwide land‐use changes involving conversion of vegetation with different photosynthetic pathways (e.g. C₃ and C₄) offer a unique opportunity to quantify SOM decomposition rate and its response to climatic conditions using stable isotope techniques. We synthesized the results from 131 sites (including 87 deforestation observations and 44 reforestation observations) which were compiled from 36 published papers in the literatures as well as our observations in China's Qinling Mountains. Based on the ¹³C natural abundance analysis, we evaluated the dynamics of new and old C in top soil (0–20 cm) following land‐use change and analyzed the relationships between soil organic C (SOC) decomposition rates and climatic factors. We found that SOC decomposition rates increased significantly with mean annual temperature and precipitation in the reforestation sites, and they were not related to any climatic factor in deforestation sites. The mean annual temperature explained 56% of variation in SOC decomposition rates by exponential model (y = 0.0014e^0.1395x) in the reforestation sites. The proportion of new soil C increased following deforestation and reforestation, whereas the old soil C showed an opposite trend. The proportion of new soil C exceeded the proportion of old soil C after 45.4 years' reforestation and 43.4 years' deforestation, respectively. The rates of new soil C accumulation increased significantly with mean annual precipitation and temperature in the reforestation sites, yet only significantly increased with mean annual precipitation in the deforestation sites. Overall, our study provides evidence that SOC decomposition rates vary with temperature and precipitation, and thereby implies that global warming may accelerate SOM decomposition.     

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.     

土地利用和环境因子对表层土壤有机碳影响的尺度效应——以陕北黄土丘陵沟壑区为例

土壤表层有机碳对土地利用和环境因子的变化非常敏感,并具有尺度变异特征。研究不同尺度上表层土壤有机碳的空间分布及其与土地利用与环境因子的关系对于评价黄土丘陵沟壑区表层土壤有机碳状况具有重要意义。选择黄土丘陵沟壑区安塞集水区和集水区内典型小流域——沐浴小流域作为研究区,探讨两个尺度上,表层土壤有机碳的分布特征及其与土地利用、环境因子的关系。结果表明:(1)土地利用方式对有机碳的影响在不同尺度上差异明显,对于不同利用方式下的有机碳含量,沐浴小流域从高到低依次是荒草地林地灌木林地耕地,安塞集水区则依次为林地灌木林地耕地荒草地;(2)对于不同利用方式下的土壤有机碳密度,沐浴小流域从高到低依次是荒草地林地耕地灌木林地,安塞集水区则是林地耕地荒草地灌木林地;(3)在沐浴小流域和安塞集水区两个尺度上,坡向、坡度和植被盖度均与有机碳含量和有机碳密度正相关,而相对海拔、土地利用与有机碳密度负相关;(4)在小流域尺度上,海拔高度、坡位、土地利用与有机碳含量负相关,坡位与有机碳密度负相关,但是在集水区尺度上,相关性则与此相反。