C 1s K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy for characterizing functional group chemistry of black carbon

Black carbon (BC) is considered ubiquitous in soil organic matter (OM) and therefore plays an important role in soil biogeochemistry. Its complexity, particularly within environmental matrices, presents a challenge for research, primarily as a result of techniques which may favor detection of certain functional group types rather than capturing total sample C. The objective of this study was to utilize carbon (C) 1s near edge X-ray absorption fine edge structure (NEXAFS) spectroscopy to characterize the C chemistry of a broad range of BC materials. Characteristic resonances in the NEXAFS spectra allowed direct molecular speciation of the total C chemistry of the reference materials, environmental matrices and potentially interfering materials, obtained from an earlier BC ring trial. Spectral deconvolution was used to further identify the functional group distribution of the materials. BC reference materials and soils were characterized by a large aromatic C region comprising around 40% of total absorption intensity. We were able to distinguish shale and melanoidin from BC reference materials on the basis of their unique spectral characteristics. However, bituminous coal shared chemical characteristics with BC reference materials, namely high aromaticity of more than 40% identified by way of a broad peak. Lignite also shared similar spectra and functional group distributions to BC reference materials and bituminous coal. We compared the results of spectral deconvolution with the functional group distributions obtained by way of direct polarization magic angle spinning (DPMAS) ¹³C nuclear magnetic resonance (NMR) spectroscopy. Correlations between aromatic type C values for DPMAS ¹³C NMR and NEXAFS gave r² = 0.633 (p < 0.05) and the values for NEXAFS were around 30–40% lower than for ¹³C NMR. Correlations were also drawn between the aromatic C/O-alkyl C ratio values for the two methods (r² = 0.49, p < 0.05). Overall, NEXAFS was applicable for a wide range of environmental materials, such as those measured, although some limitations for the technique were addressed.     

Molecular signature and sources of biochemical recalcitrance of organic C in Amazonian Dark Earths

Amazonian Dark Earths (ADE) are a unique type of soils developed through intense anthropogenic activities that transformed the original soils into Anthrosols throughout the Brazilian Amazon Basin. We conducted a comparative molecular-level investigation of soil organic C (SOC) speciation in ADE (ages between 600 and 8700 years B.P.) and adjacent soils using ultraviolet photo-oxidation coupled with ¹³C cross polarization-magic angle spinning nuclear magnetic resonance (CP-MAS NMR), synchrotron-based Fourier transform infrared-attenuated total reflectance (Sr-FTIR-ATR) and C (1s) near edge X-ray absorption fine structure (NEXAFS) spectroscopy to obtain deeper insights into the structural chemistry and sources of refractory organic C compounds in ADE. Our results show that the functional group chemistry of SOC in ADE was considerably different from adjacent soils. The SOC in ADE was enriched with: (i) aromatic-C structures mostly from H- and C-substituted aryl-C, (ii) O-rich organic C forms from carboxylic-C, aldehyde-C, ketonic-C and quinine-C, and (iii) diverse group of refractory aliphatic-C moieties. The SOC in adjacent soils was predominantly composed of O-alkyl-C and methoxyl-C/N-alkyl-C structures and elements of labile aliphatic-C functionalities. Our study suggests that the inherent molecular structures of organic C due to selective accumulation of highly refractory aryl-C structures seems to be the key factor for the biochemical recalcitrance and stability of SOC in ADE. Anthropogenic enrichment with charred carbonaceous residues from biomass-derived black C (BC) is presumed to be the precursor of these recalcitrant polyaromatic structures. Our results also highlight the complementary role that might be played by organic C compounds composed of O-containing organic C moieties and aliphatic-C structures that persisted for millennia in these anthropic soils as additional or secondary sources of chemical recalcitrance of SOC in ADE. These organic C compounds could be the products of: (i) primary recalcitrant biomolecules from non-BC sources or (ii) secondary processes involving microbial mediated oxidative or extracellular neoformation reactions of SOC from BC and non-BC sources; and stabilized through physical inaccessibility to decomposers due to sorption onto the surface or into porous structures of BC particles, selective preservation or through intermolecular interactions involving clay and BC particles.     

Protection of organic carbon in soil microaggregates via restructuring of aggregate porosity and filling of pores with accumulating organic matter

We examined relationships between the pore structure of microaggregates and the protection of organic matter (OM) within that structure. By using ultra-small angle X-ray scattering (USAXS) before and after combustion of microaggregates at 350 °C, we took advantage of differences in X-ray scattering contrast among soil minerals, OM, and air to evaluate the distribution of the total- and OM-filled porosity within microaggregates (53-250 μm in diameter). Systematic changes in microaggregate structure were observed for long-term field manipulations of land use (a chronosequence of tallgrass prairie restorations) and agricultural management (conventional tillage versus no-till at two levels of nitrogen fertilization). Our results imply that OM preservation arose from the evolution of the architectural system of microaggregates during their formation and stabilization. Soils and treatments with increasing OM in microaggregates were associated with encapsulation of colloidal OM by minerals, thereby creating protected OM-filled pores at the submicron scale within the microaggregate structure. For example, in the prairie chronosequence, microaggregates from the cultivated soil had the lowest concentration of OM, but 75% of the OM that had survived cultivation was in OM-filled pores. Following restoration, the concentration of OM in microaggregates increased rapidly, but the proportion of OM in OM-filled pores declined initially and then increased over time until 90% of the OM was in OM-filled pores. OM totally encapsulated within the pore structure can create spatial and kinetic constraints on microbial access to and degradation of OM. Encapsulation of OM increases the capacity for its protection relative to sorption on mineral surfaces, and comparison of its extent among treatments suggests important feedback loops. The use of USAXS, which has not previously been applied to the study of soil aggregate structures and the distribution of OM within those structures, provided new information on the mechanisms of OM protection in soil microaggregates, and insights relevant to strategies for enhancing carbon-sequestration in soil through changes in agricultural management practices and land use.     

Organic carbon and sulphur compounds in wetland soils: insights on structure and transformation processes using K-edge XANES and NMR spectroscopy

X-ray absorption near-edge structure (XANES) and nuclear magnetic resonance (NMR) spectroscopy were used in combination to characterize organic carbon structures in a series of wetland soils in Saskatchewan, and XANES spectroscopy was also used to examine sulphur speciation in the soils. The organic C contents of most of the wetland soils are consistently higher by a factor of two to five times compared to adjacent well-drained soils. NMR analyses indicate that the organic matter in the wetland soils consists of predominantly aliphatic structures such as carbohydrates and long chain poly(methylene) units which are refractory structures found in plant waxes. The poly(methylene) structures have a significant capacity to sorb nonpolar organic molecules. The phenolic OH and carboxyl group content of the wetland soils studied is an additional significant factor in their sequestering ability for heavy metals or pesticides. Carbon XANES spectroscopy shows that the surface (∼10 nm) layer of particulate organic matter has a structure dominated by aromatic, carbohydrate and carboxylic acid-like material apparently derived from partially degraded lignin and cellulose polymers which are adsorbed onto clay minerals. The aliphatic structures remaining in this surface layer are probably recalcitrant (poly)methylene units. At a depth of ∼100 nm, the aliphatic content significantly increases suggesting the presence of more labile structures. The presence of these more labile aliphatic compounds may be due to slow decomposition rates in the wet, often cool environments present and to the protective action of the more refractory components in the surface ∼10 nm of the organic matter. Drying of the wetlands, either by draining or as a result of climate change, is likely to result in the rapid decomposition of these labile organic structures releasing carbon dioxide. Our data indicate that the preservation of the organic carbon compounds in these soils is a result of their presence as surface adsorbed layers on the soil mineral particles. The soils contain three different classes of sulphur compounds: reduced organic sulphur such as sulphides, low valent oxidized sulphur such as sulphoxides, and high valent oxidized sulphur such as sulphonate and sulphate. Of these, reduced sulphur species constitute between one-third and two-thirds of the total. Sulphonate structures comprise between a fifth and a third of the total. Sulphates exhibit a wide variation in content, and sulphoxides are either not detected or are present to a lesser extent (<5%). Drying of the wetlands would cause oxidation of sulphides to sulphates.     

桂林毛村岩溶区不同土地利用方式土壤有机碳矿化及土壤碳结构比较

应用土壤培养法,比较分析了桂林毛村岩溶区不同土地利用方式(农田、灌丛和林地)土壤在25℃、黑暗条件下培养90d有机碳矿化速率的差异(以90d累计释放的CO2-C计)。农田土壤矿化释放的CO2-C含量分别比灌丛和林地少62.9%和56.6%。利用6mol/L的HCl酸解法得到惰性碳含量,并利用三库一级动力学方程在SAS8.2软件中通过非线性拟合得到三种土地利用方式的活性碳库、缓效性碳库的大小及其分解速率,计算得出各库驻留时间。结果表明各土地利用方式均为活性碳库含量最少,占总有机碳的比例在1.82%~2.71%之间,平均驻留时间在8.4~16.3d之间;缓效性碳库次之,占总有机碳的比例在33.91%~45.47%之间,平均驻留时间为4.8~7.7a之间;惰性碳库所占比例最大,在51.82%~64.01%之间,平均驻留时间为假定的1000a。通过固态13C交叉极化魔角自旋核磁共振(13CCPMASNMR)方法对土壤碳结构进行分析,结果表明:与灌丛和林地相比,受人类活动干扰较多的农田烷基C和芳香C的比例增加,烷氧C和羰基C的比例降低;烷基C/烷氧C和疏水C/亲水C的大小顺序均为农田>林地>灌丛,而脂族C/芳香C的大小顺序则相反,即灌丛>林地>农田。这说明农田土壤有机碳分解程度较高,难分解程度增加,难分解有机碳比例增加。      

高寒草甸土壤微生物功能多样性对积雪变化的响应

积雪是高寒地区不可忽视的生态因子,不仅直接影响土壤温度、水分,而且间接影响土壤微生物群落组成和多样性。为研究高寒草甸生态系统中土壤微生物对积雪变化的响应,于2013年11月至2014年7月在青藏高原东缘红原县高寒草甸,通过人工堆积的方法建立4个不同积雪梯度,以自然积雪量为对照（CK）,2倍于自然积雪量（S1）、3倍于自然积雪量（S2）、4倍于自然积雪量（S3）。运用Biolog-Eco板法研究不同积雪梯度下土壤微生物功能多样性,并测定积雪变化对土壤温度和土壤养分的影响。结果表明：积雪期内,0~10 cm土层土壤温度随着积雪量的增加而降低,而10~20 cm土层随积雪量增加先降低后升高。增加积雪量处理后0~10 cm土层全磷（TP）、有机碳（SOC）显著增加（P<0.05）;而10~20 cm土层仅S3下全氮（TN）、TP、SOC增加。每孔平均变化率值（Average well color development,AWCD）在0~10 cm土层表现为CK > S2 > S1 > S3,而10~20 cm表现为S2 > S1 > CK > S3。在0~10 cm土层,S3处理显著降低了土壤微生物多样性McIntosh指数、Shannon-Wiener指数和Pielou指数（P<0.05）; 10~20 cm土层,S1和S2处理下多样性指数显著增加（P<0.05）。主成分分析显示：氨基酸类和酚酸类是微生物利用的主要碳源类型。相关性分析表明：多样性指数与TP、SOC、碳氮比（C/N）显著负相关（P<0.05）,氨基酸类碳源与TP、C/N显著负相关（P<0.05）。因此,冬季积雪一定程度上影响着土壤温度和土壤养分,进而影响高寒草甸土壤微生物群落功能多样性。     

Effects of long-term irrigation with treated wastewater. Part II: Role of organic carbon on Cu,Pb and Cr behaviour

The long-term impact of irrigation on a Mediterranean sandy soil irrigated with treated wastewater (TWW) since 1980 was evaluated. The main soil properties (CEC, pH, size distribution, exchangeable cations and chloride, hydraulic conductivity) as well as the organic matter and Cu, Cr and Pb speciation in an irrigated soil and a non-irrigated control soil at various soil depths were monitored and compared during a 2 year experiment. In this second part, we focused on Cu, Cr and Pb behaviour in relation with soil organic carbon (SOC). Soil samples were collected every 3 months during 2 years at the depths 0–20, 20–40 and 40–60 cm and were analysed for exchangeable and total metals, organic carbon content, metal sequential extraction and humic substances – Humic Acids (HA), Fulvic Acids (FA) and Non-Humified Fraction (NHF). Long-term irrigation with a domestic treated wastewater (TWW) may be considered safe with regard to trace metal accumulation in soil. Irrigation lowered the HA and NHF fractions of SOC and made the FA fraction more mobile. Cu bound preferentially to the SOC fraction, Cr was found mainly in the reducible fraction and Pb was bound to all fractions indiscriminately. Cu exhibited a high affinity for the HA fraction, while Pb and Cr had a high affinity for the FA fraction, which indicates a greater mobility of the organically-bound Pb and Cr than of the organically-bound Cu. Evaluation of the potential metal mobility has to take into account not only the usual speciation between labile, reducible and oxidisable fractions, but also the nature of the SOC responsible for the oxidisable fraction.     

Identification and emission factors of molecular tracers in organic aerosols from biomass burning Part 1. Temperate climate conifers

Smoke particulate matter from conifers subjected to controlled burning, both under smoldering and flaming conditions, was sampled by high volume air filtration on precleaned quartz fiber filters. The filtered particles were extracted with dichloromethane and the crude extracts were methylated for separation by thin layer chromatography into hydrocarbon, carbonyl, carboxylic acid ester and polar fractions. Then, the total extract and individual fractions were analyzed by gas chromatography and gas chromatography–mass spectrometry. The major organic components directly emitted in smoke particles were straight chain aliphatic compounds from vegetation wax and diterpenoid acids (biomarkers) from resin. The major natural products altered by combustion included derivatives from phenolic (lignin) and monosaccharide (cellulose) biopolymers and oxygenated and aromatic products from diterpenoids. Other biomarkers present as minor components included phytosterols, both the natural and altered products, and unaltered high molecular weight wax esters. Polycyclic aromatic hydrocarbons (PAH) were also present, however, only as minor constituents. Although the concentrations of organic compounds in smoke aerosols are highly variable and dependent on combustion temperature, the biomarkers and their combustion alteration products are source specific. The major components are adsorbed or trapped on particulate matter and thus may be utilized as molecular tracers in the atmosphere for determining fuel type and source contributions from biomass burning.     

中国土壤有机碳库量与农业土壤碳固定动态的若干问题

在整理和统计国内对土壤有机碳及其变化的文献资料基础上,着重讨论中国土壤有机碳库及其分布、不同时期土壤有机碳的变化以及最近时期有机碳的固定趋势,分析我国不同土壤有机碳的保护机制的特点,期望对于我国当前土壤有机碳库与全球变化研究提供参考依据。我国总土壤有机碳库的估计在50—180Pg之间。估计我国表层土壤有机碳库为20Pg,它主要分配于几个与湿地和水成过程有关的土壤类型,且水稻土占有较大比例。因而我国人为土的管理在陆地生态系统碳循环与全球变化上有重要意义。     

NEXAFS and XPS characterisation of carbon functional groups of fresh and aged biochars

The oxidation of surface functional groups on biochar increases its reactivity and may contribute to the cation exchange capacity of soil. In this study, two Eucalyptus wood biochars, produced at 450 °C (B450) and 550 °C (B550), were incubated separately in each of the four contrasting soils for up to 2 years at 20 °C, 40 °C and 60 °C. Carbon functional groups of the light fraction (< 1.8 g/cm³) of the control and biochar amended soils (fresh and aged for 1 and 2 years at 20 °C, 40 °C and 60 °C) were investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The spectra of biochar and light fractions of the control and biochar amended soils showed two distinct peaks at ∼285.1 eV and 288.5 eV, which were attributed to the C1s-π¹_CC transitions of aromatic C and C1s-π¹_CO transitions of carboxylic C, carboxyamide C and carbonyl C. The proportion of aromatic C was substantially greater in the light fraction of the biochar amended soils than the corresponding light fraction of the control soils. Also, the proportion of aromatic C was much higher in the light fraction of the B550 amended soils than in the corresponding B450 amended soils. Neither NEXAFS nor XPS results show any consistent change in the proportion of aromatic C of biochar amended soils after 1 year ageing. However, XPS analysis of hand-picked biochar samples showed an increase in the proportion of carboxyl groups after ageing for 2 years, with an average value of 8.9% in the 2 year aged samples compared with 3.0% in the original biochar and 6.4% in the control soil. Our data suggest that much longer ageing time will be needed for the development of a significant amount of carboxyl groups on biochar surfaces.     

Characteristics of a paleosol and its implication for the Critical Zone development,Rocky Mountain Front Range of Colorado,USA

Activity and stability phases as well as geomorphic processes within the Critical Zone are well known. Erosion and deposition of sediments represent activity; soils represent geomorphic stability phases. Data are presented from a 4 m deep sediment section that was dated by luminescence techniques. Upslope erosion and resulting sedimentation started in the late Pleistocene around 18 ka until 12 ka. Conditions at the study site then changed, which led to the formation of a well-developed soil. Radiocarbon dating of the organic matter yielded ages between 8552 and 8995 cal. BP. From roughly 6.2 to 5.4 ka another activity phase accompanied by according sediment deposition buried the soil and a new soil, a Cambisol, was formed at the surface. The buried soil is a strongly developed Luvisol. The black colors in the upper part of the buried soil are not the result of pedogenic accumulation of normal organic matter within an A-horizon. Nuclear magnetic resonance spectroscopy clearly documents the high amount of aromatic components (charcoal), which is responsible for the dark color. This indicates severe burning events at the site and the smaller charcoal dust (black carbon) was transported to deeper parts of the profile during the process of clay translocation.     

Rayleigh distillation and the depth profile of C/C ratios of soil organic carbon from soils of disparate texture in Iron Range National Park, Far North Queensland, Australia

A depth- and particle size-specific analysis of soil organic carbon (SOC) and its isotopic composition was undertaken to investigate the effects of soil texture (or particle size) on the depth profile of stable carbon isotopic composition of SOC (δ¹³C_SOC) in two tropical soils. Depth-specific samples from two soil profiles of markedly different texture (coarse grained and fine grained) were separated into particle size classes and analyzed for the (mass/mass) concentration of SOC (C) and δ¹³C_SOC. Within 1 m of the soil surface, δ¹³C_SOC in the coarse-textured soil increases by 1.3 to 1.6‰, while δ¹³C_SOC from the fine-textured soil increase by as much as 3.8 to 5.5‰. This increasing depth trend in the coarse-textured soil is approximately linear with respect to normalized C, while the increase in the fine-textured soil follows a logarithmic function with respect to normalized C. A model of Rayleigh distillation describing isotope fractionation during decomposition of soil organic matter (SOM) accounts for the depth profile of δ¹³C_SOC in the fine-textured soil, but does not account for the depth profile observed in the coarse-textured soil despite their similar climate, vegetation, and topographic position. These results suggest that kinetic fractionation during humification of SOM leads to preferential accumulation of ¹³C in association with fine mineral particles, or aggregates of fine mineral particles in fine-textured soils. In contrast, the coarse-textured soil shows very little applicability of the Rayleigh distillation model. Rather, the depth profile of δ¹³C_SOC in the coarse-textured soil can be accounted for by mixing of soil carbon with different isotopic ratios.     

Composition and structure of microaggregates in soils of the Behr hillocks, southern Astrakhan district

Study of the composition, structure and morphology of microaggregates less than 1 mm in size in soils of one of the Behr hillocks revealed a high share of the clay-salt segregations (microaggregates and cutans). It was established that the segregation type depends on the mud content in soil, while the packing of particles, as well as the size and morphology of microaggregates, depend on the content and properties of salts. The clay component in the microaggregates is mainly represented by smectite. Carbonate (calcitic, dolomitic, and ferruginous) clay-salt microaggregates were found in all horizons of the studied soils, except for horizon Ap on the subhillock plain in the former rice paddy. Gypsum clay-salt microaggregates were identified in the salic horizon Bs of soils. The microaggregates in solonchaks were formed with the contribution of Na-Mg-Ca chlorides and sulfates. The formation of clay-salt aggregates promoted high microstructuring of soils and stability of the Behr hillocks under arid climatic conditions.     

The transformation and mobility of charcoal in a fire-impacted watershed

The incomplete combustion of fossil fuels and biomass has resulted in the global-scale distribution and accumulation of black carbon (BC) in the environment. Recently, the molecular identity of BC in the dissolved phase has been distinguished from that of natural organic matter. However, many of the processes that control BC cycling remain unidentified. We investigate changes in soil charcoal particle morphology and chemical composition using surface area analysis, scanning electron microscopy, energy dispersive X-ray spectroscopy, chemical oxidation, and ¹³C NMR spectroscopy. A comparison of soil charcoals differing in age by 100 years shows that aged charcoal has lower specific surface areas, higher BC/OC ratios, direct associations with soil minerals and microbial biomass, and a greater abundance of non-aromatic carbon. The water-soluble portion of soil charcoal and dissolved organic matter (DOM) from the watershed were also characterized by electrospray ionization mass spectrometry. Aqueous charcoal extracts are comprised mostly of condensed aromatic ring structures (CARS) which are also present in soil pore, river, and ground water samples. We present indirect evidence and a chemical rationale for a microbial BC dissolution mechanism. Furthermore, the speciation of CARS in the soil solution versus river and ground water provides molecular evidence of reactivity in the dissolved phase. The dissolution and export of soil BC are presently unmeasured fluxes with important implications for the global carbon cycle.     

1986—2019年黑龙江省松嫩平原表层土壤有机碳变化及固碳潜力估算

土壤有机碳(SOC)是评价土壤肥力和固碳能力的重要指标。因此,研究土壤有机碳的变化,对准确评价区域土壤固碳潜力,实现土壤资源的可持续利用具有重要的意义。利用黑龙江省第二次土壤普查数据和2019年实测土壤数据,运用GIS空间分析方法,分析了1986—2019年黑龙江省松嫩平原表层(0~20 cm)土壤有机碳密度(SOCD)的时空变化特征,运用土壤类型法估算了土壤有机碳储量,运用平衡法估算了土壤固碳潜力。结果表明:30多年来表层SOCD平均减少1.06 kg/m²,SOCD减少的地区主要分布在黑龙江省松嫩平原中部和东南部地区;表层SOC储量减少约143.99 Tg,SOC储量减少较多的土壤类型是草甸土、黑钙土和黑土,三者减少量占总SOC储量减少量的84.55%;当前黑龙江省松嫩平原表层土壤固碳潜力为-2.08 Tg,其中暗棕壤、白浆土、黑土为正潜力,其余土壤类型为负潜力。建议通过增施有机化肥、秸秆还田、推广免耕少耕等方法措施,以提高松嫩平原土壤固碳潜力。     

Comparison of soil organic carbon content,hydraulic conductivity,and particle size fractions between a grassland and a nearby black pine plantation of 40 years in two surface depths

In highlands of semiarid Turkey, ecosystems have been significantly transformed through human actions, and today changes are taking place very rapidly, causing harmful consequences such as soil degradation. This paper examines two neighboring land use types in Indagi Mountain Pass, Cankiri, Turkey, to determine effects of the conversion of Blackpine (Pinus nigra Arn. subsp. pallasiana) plantation from grassland 40 years ago on soil organic carbon (SOC) and soil erodibility (USLE-K). For this purpose, a total of 302 disturbed and undisturbed soil samples were taken at irregular intervals from two sites and from two soil depths of 0–10 cm (D₁) and 10–20 cm (D₂). In terms of SOC, conversion did not make any statistical difference between grassland and plantation; however, there were statistically significant differences with soil depth within each land use, and SOC contents significantly decreased with the soil depth (P < 0.05) and mostly accumulated in D₁. SOC values were 2.4 and 1.8% for grassland and 2.8 and 1.6% for plantation, respectively, at D₁ and D₂. USLE-K values also statistically differed significantly with the land use, and in contrast to the statistics of SOC, there was no change in USLE-K with the soil depth. Since USLE-K was estimated using SOC, hydraulic conductivity (HC) and soil textural composition––sand (S), silt (Si), and clay (C) contents of soils––as well as SOC did not change with the land use, we ascribed the changes of USLE-K with the land uses to the differences in the HC as strongly affected by the interactions between SOC and contents of S, Si, and C. On an average, the soil of the grassland (USLE-K = 0.161 t ha h ha⁻¹ MJ⁻¹ mm⁻¹) was more erodible than those of the plantation (USLE-K = 0.126 t ha h ha⁻¹ MJ⁻¹ mm⁻¹). Additionally, topographic factors, such as aspect and slope, were statistically effective on spatial distribution of the USLE-K and SOC.     

Organic carbon stocks and erosion in the soils of Guangdong,South China

Soil organic carbon (SOC) storage and erosion in South China at the regional scale in the past decades remains far from being understood. This paper calculated the SOC density, storage and erosion in 14 soil classes in Guangdong Province, South China, based on statistical data from the soil survey and soil erosion survey of Guangdong, which was performed in the 1990s. The purpose of this study is to understand the relationships between soil classes and SOC erosion at the regional scale. The results indicated that the SOC density in the soils of Guangdong varied from 12.7 to 144.9 Mg ha^?1 over the entire profile and from 12.6 to 68.4 Mg ha^?1 in the top 20-cm soil layer. The average area-weighted SOC density in the topsoil (0–20 cm) and the entire profile was 32 ± 3 and 86 ± 4 Mg ha^?1, respectively. The total SOC storage was 1.27 ± 0.06 Pg, with 35.6 % (0.46 ± 0.04 Pg) located in the topsoil. The average area-weighted strength of the SOC erosion in the 1990s was 20.6 ± 0.8 Mg km^?2 year^?1. The results indicated that SOC erosion was strongly related to soil class.     

广南县幅岩溶区与非岩溶区土壤碳氮磷生态化学计量比空间变异分析

借助1∶25万云南省广南县幅土壤地球化学调查数据,并利用单因素方差分析、多重比较法以及地统计学方法,对岩溶区和非岩溶区土壤碳氮磷生态化学计量特征及其空间分布进行了对比分析。结果显示:广南县幅岩溶区土壤中有机碳（SOC）、全氮（TN）、全磷（TP）含量显著高于非岩溶区,而碳氮比（C∶N）、碳磷比（C∶P）、氮磷比（N∶P）显著低于非岩溶区;无论是岩溶区还是非岩溶区,表层（0～20 cm）SOC、TN、C∶N、C∶P、N∶P均显著高于深层（＞100 cm）。克里格空间插值结果表明,研究区表层土壤中SOC、TN、TP含量具有东高西低的特征,而C∶N、C∶P、N∶P具有低值区集中于东部、高值区散布在西部的空间分布格局。成土母质和土壤类型等自然因素严重制约了研究区土壤碳氮磷的空间变异,同时土地利用变化等人为因素也起到了不可忽视的作用。      

Amelioration of soil acidity,Olsen-P,and phosphatase activity by manure- and peat-derived biochars in different acidic soils

To increase soil productivity, ameliorate nutrient scarcity, and reduce metal toxicity in highly weathered acidic soils usually requires fertilizer and lime application. Effects of three biochars on soil acidity, Olsen-phosphorus (P), phosphatase activities, and heavy metal availability were investigated to test potential of these biochars as soil amendments in highly weathered acidic soils. Incubation experiments were conducted for 6 weeks with three acidic soils: Alfisol, Ultisol, and Oxisol. Three biochars were derived from chicken manure (CMB), pig manure (PMB), and peat moss (PB) at 400 °C and applied at 1 or 2% (wt/wt). The addition of the three biochars increased Olsen-P in the three acidic soils in the following order: CMB?>?PMB?>?PB. Application of 2% CMB increased Olsen-P contents by 2.41-, 7.4-, and 1.78-fold in the Ultisol, Oxisol, and Alfisol compared with controls, respectively. Moreover, CMB increased the soil pH, electrical conductivity (EC), cation exchange capacity (CEC), and alkaline phosphatase activity, but reduced exchangeable acidity, acid phosphatase activity, and the availability of heavy metals—more effectively than PMB and PB. Addition of CMB increased soil pH by 0.90, 0.90, and 0.92 units for the Alfisol, Ultisol, and Oxisol, respectively, correspondingly followed by 0.80, 0.84, and 0.87 units for PMB and 0.15, 0.28, and 0.25 for PM. Changes in EC, CEC, and exchangeable acidity followed the same order for the three soils: CMB?>?PMB?>?PB. The results suggested that the magnitude of changes in soil properties and Olsen-P contents depended on biochar type and application rate. Application of CMB increased nutrient availability and reduced the availability of heavy metals more than other amendments. Due to higher pH, EC, and CEC, and greater concentrations of carbon, nitrogen, and exchangeable calcium and potassium, incorporation of CMB should be a better cost-effective method to correct soil acidity and improve fertility and Olsen-P contents in Ultisols and Oxisols from tropical and subtropical regions of the world.     

Experimental investigation on soil carbon, nitrogen, and their components under grazing and livestock exclusion in steppe and desert steppe grasslands, Northwestern China

Livestock grazing is one of the main causes for the change of soil organic carbon (SOC) and total nitrogen (TN) in the arid and semi-arid parts in northern China. This paper examined the SOC, TN, and their components of the local steppe and desert steppe, considering continuous grazing and 4-year livestock exclusion, respectively. In steppe where livestock is excluded, both SOC and TN in the topsoil (0–0.20 m) are found to remain unchanged; however, significant growths are found in microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), particulate organic carbon (POC), and particulate organic nitrogen (PON). On the contrary, both POC and PON progressively decrease at the continuous grazing sites, attributed to the reduction of the mass proportions of soil particulate fraction in the top 0.10 m soil. In the desert steppe where grazing is excluded, the SOC, TN, and their components of the topsoil increase. However, at the continuous grazing sites, POC and PON in the 0.10 m topsoil are reduced, caused by the decrease of C and N content in soil particulate fraction. Besides, microbial quotients were lower in the continuous grazing sites in the two grasslands. It is also found that both MBC and POC are more sensitive to human-induced activities than SOC, and thus could serve as earlier indicators of the soil-fertility variation caused by short-period grazing management.