Key Physical Factors Affecting Spatial-temporal Variation of Labile Organic Carbon Fractions by Biochar Driven in Mollisols Region of Northeast China

Biochar is widely used to improve soil physical properties and carbon sequestration. However, few studies focuse on the impact of maize stalk biochar on labile organic carbon(LOC) pool and the relationship between physical properties and LOC fractions. A field positioning experiment was performed in Mollisols region of Northeast China to evaluate the influence of maize stalk biochar on the spatial distribution and temporal changes of physical properties and LOC fractions. Maize stalk biochar treatments included C1(1.5 kg·hm^(-2)), C2(3 kg·hm^(-2)), C3(15 kg·hm^(-2)), C4(30 kg·hm^(-2)), and CK(0). The results showed that maize stalk biochar increased soil water contents(SWC) and soil porosity(SP), but reduced bulk density(BD). Maize stalk biochar reduced dissolved organic carbon(DOC) contents in the 0-20 cm soil layer, ranging from 0.25 g·kg^(-1) to 0.31 g·kg^(-1) in harvest period, while increased in the 20-40 cm soil layer. In addition, the application of biochar had a significant impact on the spatial distribution and temporal change of SWC, BD, SP, DOC, hot-water extractable carbon(HWC), acid hydrolyzed organic carbon(AHC Ⅰ, Ⅱ), and readily oxidized organic carbon(ROC). High amounts of maize stalk biochar up-regulated the contents of soil organic carbon SOC, HWC, AHC Ⅰ, AHC Ⅱ, and ROC. In addition, SWC and SP were the key physical factors to affect LOC fractions. In conclusions, maize stalk biochar could improve physical properties, and then influence LOC fractions, and maize stalk biochar could be used as an organic amendment for restoring degraded soils governed by their rates of addition.

Subsoil tillage enhances wheat productivity,soil organic carbon and available nutrient status in dryland fields

Tillage practices during the fallow period benefit water storage and yield in dryland wheat crops.However,there is currently no clarity on the responses of soil organic carbon(SOC),total nitrogen(TN),and available nutrients to tillage practices within the growing season.This study evaluated the effects of three tillage practices(NT,no tillage;SS,subsoil tillage;DT,deep tillage)over five years on soil physicochemical properties.Soil samples at harvest stage from the fifth year were analyzed to determine the soil aggregate and aggregate-associated C and N fractions.The results indicated that SS and DT improved grain yield,straw biomass and straw carbon return of wheat compared with NT.In contrast to DT and NT,SS favored SOC and TN concentrations and stocks by increasing the soil organic carbon sequestration rate(SOCSR)and soil nitrogen sequestration rate(TNSR)in the 0-40 cm layer.Higher SOC levels under SS and NT were associated with greater aggregate-associated C fractions,while TN was positively associated with soluble organic nitrogen(SON).Compared with DT,the NT and SS treatments improved soil available nutrients in the 0-20 cm layer.These findings suggest that SS is an excellent practice for increasing soil carbon,nitrogen and nutrient availability in dryland wheat fields in North China.

Can soil organic carbon sequestration and the carbon management index be improved by changing the film mulching methods in the semiarid region?

Plastic film mulching has been widely used to increase maize yield in the semiarid area of China.However, whether long-term plastic film mulching is conducive to agricultural sustainability in this region remains controversial.A field experiment was initiated in 2013 with five different film mulching methods:(i) control method, flat planting without mulching (CK),(ii) flat planting with half film mulching (P),(iii) film mulching on ridges and planting in narrow furrows(S),(iv) full film mulching on double ridges (D), and (v) film mulching on ridges and planting in wide furrows (R).The effects on soil organic carbon (SOC) content, storage, and fractions, and on the carbon management index (CMI)were evaluated after nine consecutive years of plastic film mulching.The results showed that long-term plastic film mulching generally maintained the initial SOC level.Compared with no mulching, plastic film mulching increased the average crop yield, biomass yield, and root biomass by 48.38, 35.06, and 37.32%, respectively, which led to the improvement of SOC sequestration.Specifically, plastic film mulching significantly improved CMI, and increased the SOC content by 13.59%, SOC storage by 7.47%and easily oxidizable organic carbon (EOC) by 13.78%on average,but it reduced the other labile fractions.SOC sequestration and CMI were improved by refining the plastic film mulching methods.The S treatment had the best effect among the four mulching methods, so it can be used as a reasonable film mulching method for sustainable agricultural development in the semiarid area.

Fertilization and Soil Ploughing Practices under Changing Physical Environment Lead to Soil Organic Carbon Dynamics under Conservation Agriculture in Rice-Wheat Cropping System: A Scoping Review

Ploughing and fertilization practices in rice-wheat system have deteriorated the soil carbon (C) pools. Conservation agriculture (CA) based management approaches have proven to enhance C sequestration and reverse the loss of soil-organic-carbon (SOC), which further enhances soil fertility. Different fractions of SOC pools react to the alterations in management practices and indicate changes in SOC dynamics as compared to total C in the soil. Higher SOC levels in soil have been observed in case of reduced/no-till (NT) practices than conventional tillage (CT). However, between CT and zero tillage/NT, total SOC stocks diminished with an increase in soil depth, which demonstrated that the benefits of SOC are more pronounced in the topsoil under NT. Soil aggregation provides physical protection to C associated with different-sized particles, thus, the improvement in soil aggregation through CA is an effective way to mitigate soil C loss. Along with less soil disturbance, residual management, suitable crop rotation, rational application of manures and fertilizers, and integrated nutrient management have been found to be effective in not only improving soil C stock but also enhancing the soil health and productivity. Thus, CA can be considered as a potential method in the build-up of SOC of soil in rice-wheat system.

Determination of Atomic Fractions of Isotopes Carbon-13 and Nitrogen-15 Directly in Glicine, L-Leucine, Isoleucine and Alanine

Using compounds modified by the isotopes carbon-13 and nitrogen-15 helps conduct research in various fields of science, such as medicine, pharmacology, pharmacokinetics, metabolism, agriculture, and others. In the case of the availability of reliable, express, and cheap methods, the area of their use will gradually expand. A determination of the atomic fraction of the isotopes carbon-13 and nitrogen-15 directly in glycine, leucine, isoleucine, and alanine is proposed;the modification concerns all centers or one or more identical carbon and nitrogen centers separately, as well as both isotopes at the same time. There are defined mass lines of the mass spectrum of each amino acid, through which the isotopic content of carbon and nitrogen is calculated. The processes that must be taken into account for the determination of the isotopic content are also established. Isotopic analysis of these compounds until now was carried out by transforming them into carbon oxide, dioxide, and molecular nitrogen, and determination of their content in individual centers was impossible.

Soil organic carbon associated with aggregate-size and density fractions in a Mollisol amended with charred and uncharred maize straw

Straw return has been strongly recommended in China,whereas applying biochar into soil is considered to provide more benefits for agriculture as well as the environment.In this study,a five-year(2011-2015) field experiment was conducted to evaluate the effects of uncharred maize straw amendment(MS) and charred maize straw amendment(charred MS) on organic carbon(C) contents in bulk soil and in various soil aggregate-size and density fractions.Compared to no amendment(CK),the bulk soil organic C content significantly improved by 9.30% for MS and by 23.4% for charred MS.Uncharred and charred maize straw applied annually at a consistent equal-C dosage resulted in 19.7 and 58.2% organic C sequestration efficiency in soil,respectively,after the five years of the field experiment.The percentages of macroaggregates(>0.25 mm) and occluded microaggregates(0.25-0.053 mm) obviously increased by 7.73 and 18.1% for MS and by 10.7 and 19.6% for charred MS,respectively.Moreover,significant incremental increases of 19.4 and 35.0% in macroaggregate-associated organic C occurred in MS and charred MS,respectively.The occluded microaggregates associated organic C significantly increased by 21.7% for MS and 25.1% for charred MS.Mineral-associated organic C(<0.053 mm) inside the macroaggregates and the occluded microaggregates obviously improved by 24.7 and 33.3% for MS and by 18.4 and 44.9% for charred MS.Organic C associated with coarse particulate organic matter(POM) within the macroaggregates markedly increased by 65.1 and 41.2% for MS and charred MS,respectively.Charred MS resulted in a noteworthy increment of 50.4% for organic C associated with heavy POM inside the occluded microaggregates,whereas charred MS and MS observably improved organic C associated with heavy POM inside the free microaggregates by 36.3 and 20.0%,respectively.These results demonstrate that uncharred and charred maize straw amendments improve C sequestration by physically protecting more organic C in the macroaggregates and the occluded microaggregates.Compared to the feedstock straw amendment,charred maize straw amendment is more advantageous to C sequestration.

Soil Organic Carbon,Carbon Fractions and Nutrients as Affected by Land Use in Semi-Arid Region of Loess Plateau of China

Cropland (CP),native grassland (NG) and two shrub land treatments which were converted from cropland in 1985:seabuckthorn (Hippophae rhamnoides L.) (ST),and branchytamarisk (Tamarix ramosissima) (BT) were investigated to evaluate effects of land use conversion on soil organic carbon (SOC) and soil nutrients in the semi-arid region of the Loess Plateau of China.Total organic carbon (TOC),light fraction organic carbon (LFOC),heavy fraction organic carbon (HFOC),total N (TN),nitrate nitrogen (NO 3-N) and nitrite nitrogen (NO 2-N),ammonium nitrogen (NH + 4-N),total P,and available P (AP) were measured.The results showed that SOC in NG,ST and BT were 12.7%,27.7% and 34.8% higher than that of the cropland,respectively.LFOC,light fraction (LF) dry matter,ratio of TOC to TN (C/N) and the ratio of TOC to AP (C/P) were higher in the shrub land or native grassland than in the cropland.Cropland had the highest TN,the sum of NO 3-N and NO 2-N,TP and AP due to the use of chemical fertilizers.TOC significantly correlated with LFOC,HFOC and C/N.LFOC significantly correlated with dry matter of the LF and C/N.TN,the sum of NO 3-N and NO 2-N and AP were significantly negatively correlated with TOC and LFOC.Therefore,land use conversion from cropland to shrub land,or maybe grassland,contributed to SOC sequestration and improved soil nutrients stabilization.

Long-Term Effect of No-Tillage on Soil Organic Carbon Fractions in a Continuous Maize Cropping System of Northeast China

Increasing evidence has shown that conservation tillage is an effective agricultural practice to increase carbon （C） sequestration in soils. In order to understand the mechanisms underlying the responses of soil organic carbon （SOC） to tillage regimes, physical fractionation techniques were employed to evaluate the effect of long-term no-tillage （NT） on soil aggregation and SOC fractions. Results showed that NT increased the concentration of total SOC by 18.1% compared with conventional tillage （CT） under a long-term maize （Zea mays L.） cropping system in Northeast China. The proportion of soil large macroaggregates （〉 2 000 μm） was higher in NT than that in CT, while small macroaggregates （250-2 000μm） showed an opposite trend. Therefore, the total proportion of macroaggregates （〉 2 000 and 250-2 000μm） was not affected by tillage management. However, C concentrations of macroaggregates on a whole soil basis were higher under NT relative to CT, indicating that both the amount of aggregation and aggregate turnover affected C stabilization. Carbon concentrations of intra-aggregate particulate organic matter associated with microaggregates （iPOM-m） and microaggregates occluded within macroaggregates （iPOM-mM） in NT were 1.6 and 1.8 times greater than those in CT, respectively. Carbon proportions of iPOM-n and iPOM-mM in the total SOC increased from 5.4% and 6.3% in CT to 7.2% and 9.7% in NT, respectively. Furthermore, the difference in the microaggregate protected C （i. e., iPOM-m and iPOM-mM） between NT and CT could explain 45.4% of the difference in the whole SOC. The above results indicate that NT stimulates C accumulation within microaggregates which then are further acted upon in the soil to form macroaggregates. The shift of SOC within microaggregates is beneficial for long-term C sequestration in soil. We also corroborate that the microaggregate protected C is useful as a pool for assessing the impact of tillage management on SOC storage.

Effects of long-term fertilization on oxidizable organic carbon fractions on the Loess Plateau,China

The effects of long-term fertilization on pools of soil organic carbon （SOC） have been well studied, but limited information is available on the oxidizable organic carbon （OOC） fractions, especially for the Loess Plateau in China. We evaluated the effects of a 15-year fertilization on the OOC fractions （F1, F2, F3 and F4） in the 0-20 and 20-40 cm soil layers in flat farmland under nine treatments （N （nitrogen, urea）, P （phosphorus, monocalcium phosphate）, M （organic fertilizer, composted sheep manure）, N＋P （NP）, M＋N （MN）, M＋P （MP）, M＋N＋P （MNP）, CK （control, no fertilizer） and bare land （BL, no crops or fertilizer））. SOC content increased more markedly in the treatment containing manure than in those with inorganic fertilizers alone. F1, F2, F4 and F3 accounted for 47%, 27%, 18% and 8% of total organic carbon, respectively. F1 was a more sensitive index than the other C fractions in the sensitivity index （SI） analysis. F1 and F2 were highly correlated with total nitrogen （TN） and available nitrogen （AN）, F3 was negatively correlated with pH and F4 was correlated with TN. A cluster analysis showed that the treatments containing manure formed one group, and the other treatments formed another group, which indicated the different effects of fertilization on soil properties. Long-term fertilization with inorganic fertilizer increased the F4 fraction while manure fertilizer not only increased labile fractions （F1） in a short time, but also increased passive fraction （F4） over a longer term. The mixed fertilizer mainly affected F3 fraction. The study demonstrated that manure fertilizer was recommended to use in the farmland on the Loess Plateau for the long-term sustainability of agriculture.

Effects of Land Management Practices on Labile Organic Carbon Fractions in Rice Cultivation

A research trial with four land management practices, i.e., traditional tillage-fallow (TTF), traditional tillage-wheat (TTW), conservation tillage-fallow (CTF) and conservation tillage-wheat (CTW), was sampled in the 15th year after its establishment to assess the effects of different management practices on labile organic carbon fractions (LOCFs), such as easily oxidizable organic carbon (EOC), dissolved organic carbon (DOC), particulate organic carbon (POC) and microbial biomass carbon (MBC) in a typical paddy soil, Chongqing, Southwest China. The results indicated that LOCFs were significantly influenced by the combination of no-tillage, ridge culture and crop rotation. And, different combination patterns showed different effectiveness on soil LOCFs. The effects of no-tillage, ridge culture and wheat cultivation on EOC, DOC, POC and MBC mainly happened at 0-10cm. At this depth, soil under CTW had higher EOC, DOC, POC and MBC contents, compared to TTF, TTW and CTF, respectively. Moreover, the contents of LOCFs for different practices generally decreased when the soil depth increased. Our findings suggest that the paddy soil in Southwest China could be managed to concentrate greater quantities of EOC, DOC, POC and MBC.

Effects of afforestation on soil carbon and its fractions:a case study from the Loess Plateau,China

Afforestation has been implemented to reduce soil erosion and improve the environment of the Loess Plateau,China.Although it increased soil organic carbon（SOC）,the stability of the increase is unknown.Additionally,the variations of soil inorganic carbon（SIC） following afforestation needs to be reconfirmed.After planting Robinia pseudoacacia,Pinus tabuliformis,and Hippophae rhamnoides on bare land on the Loess Plateau,total soil carbon（TSC） was measured and its two components,SIC and SOC,as well as the light and heavy fractions within SOC under bare lands and woodlands at the soil surface（0–20 cm）.The results show that TSC on bare land was 24.5 Mg ha^（-1） and significantly increased to 51.6 Mg ha^（-1） for R.pseudoacacia,47.0 Mg ha^（-1） for P.tabuliformis and 39.9 Mg ha^（-1） for H.rhamnoides.The accumulated total soil carbon under R.pseudoacacia,P.tabuliformis,and H.rhamnoides,the heavy fraction（HFSOC） accounted for 65.2,31.7 and 76.2%,respectively; the light fraction（LF-SOC） accounted for 18.0,52.0 and 4.0%,respectively; SIC occupied 15.6,15.3 and 19.7%,respectively.The accumulation rates of TSC under R.pseudoacacia,P.tabuliformis,and H.rhamnoides reached159.5,112.4 and 102.5 g m^（-2） a^（-1）,respectively.The results demonstrate that afforestation on bare land has high potential for soil carbon accumulation on the Loess Plateau.Among the newly sequestrated total soil carbon,the heavy fraction（HF-SOC） with a slow turnover rate accounted for a considerably high percentage,suggesting that significant sequestrated carbon can be stored in soils following afforestation.Furthermore,afforestation induces SIC sequestration.Although its contribution to TSC accumulation was less than SOC,overlooking it may substantially underestimate the capacity of carbon sequestration after afforestation on the Loess Plateau.

Soil Organic Carbon and Its Fractions Across Vegetation Types:Effects of Soil Mineral Surface Area and Microaggregates

Soil organic carbon(SOC)can act as a sink or source of atmospheric carbon dioxide;therefore,it is important to understand the amount and composition of SOC in terrestrial ecosystems,the spatial variation in SOC,and the underlying mechanisms that stabilize SOC.In this study,density fractionation and acid hydrolysis were used to assess the spatial variation in SOC,the heavy fraction of organic carbon(HFOC),and the resistant organic carbon(ROC)in soils of the southern Hulun Buir region,northeastern China,and to identify the major factors that contribute to this variation.The results showed that as the contents of clay and silt particles(0–50μm)increased,both methylene blue(MB)adsorption by soil minerals and microaggregate contents increased in the 0–20 and 20–40 cm soil layers(P<0.05).Although varying with vegetation types,SOC,HFOC,and ROC contents increased significantly with the content of clay and silt particles, MB adsorption by soil minerals,and microaggregate content(P<0.05),suggesting that soil texture,the MB adsorption by soil minerals,and microaggregate abundance might be important factors influencing the spatial heterogeneity of carbon contents in soils of the southern Hulun Buir region.

Simulation Experiments on the Reaction of CH_4-CaSO_4 and Its Carbon Kinetic Isotope Fractionation

Thermochemical sulfate reduction (TSR) in geological deposits can account for the accumulation of H2S in deep sour gas reservoirs. In this paper, thermal simulation experiments on the reaction of CH4-CaSO4 were carried out using an autoclave at high temperatures and high pressures. The products were characterized with analytical methods including carbon isotope analysis. It is found that the reaction can proceed to produce H2S, H2O and CaCO3 as the main products. Based on the experimental results, the carbon kinetic isotope fractionation was investigated, and the value of Ki (kinetic isotope effect) was calculated. The results obtained in this paper can provide useful information to explain the occurrence of H2S in deep carbonate gas reservoirs.

Difference in Organic Carbon Contents and Distributions in Particle-size Fractions between Soil and Sediment on the Southern Loess Plateau, China

The purpose of this study was to assess the effect of long-term cultivation and water erosion on the soil organic carbon （OC） in particle-size fractions. The study site is located at Nihegou Watershed in the Southern Loess Plateau, China. The soil at this site is loess with loose and silty structure, and contains macropores. The results showed that the OC concentrations in sediments and in the particle-size fractions of sediments were higher than those in soils and in the particle-size fractions of soils. The OC concentration was highest in the clay particles and was lowest in the sand particles. Clay particles possessed higher OC enrichment ability than silt and sand particles. The proportions of OC in the silt fractions of soil and sediment were the highest （mean value of 53.87% and 58.48%, respectively）, and the total proportion of OC in the clay and silt fractions accounted for 96% and 98% of the total OC in the soil and sediment, respectively. The loss of OC was highest in silt particles, with an average value of 0.16 Mg ha^-1 y^-1, and was lowest in the sand （0.003 Mg ha^-1 y^-l）. This result suggests that the fine particle-size fraction in the removed sediment may be an important indicator to assess soil OC losses.

Analysis of the carbon dioxide mole fraction variation and its transmission characteristics in Taiyuan

Based on the concentrations of CO2,PM2.5 and PM1.0,and conventional meteorological observation data from 2016 to 2018 at Taiyuan station,which belongs to the Shanxi greenhouse gas observation network,the CO2 concentration monthly and daily distribution characteristics,the weekend effect,and the variation characteristics on haze days and non-haze days,are analyzed.By using the Hybrid Single-Particle Lagrangian Integrated Trajectorymodel(backward trajectory model)and surface wind data,the transmission characteristics of atmospheric CO2 in Taiyuan are studied in various seasons.The results show that,in Taiyuan,the CO2 mole fraction in autumn and winter is higher than that in spring and summer,and on haze days is higher than that on non-haze days.The diurnal variation characteristic of CO2mole fraction in each season is‘single peak and single valley’with the peak value around 0700(hereafter refers to Beijing Time)and the valley value around 1600.The CO2 mole fraction on workdays is slightly higher than that on non-workdays and obviously different around 0800 of the early peak.Horizontal diffusion can reduce the CO2 mole fraction,while breezy weather is not beneficial to CO2 diffusion.The wind direction and speed in the upper levels are different from those near the surface,and the close air masses in the southwest–west–northwest sector raise the CO2 concentration in Taiyuan obviously.This indicates that the CO2 in Taiyuan is mainly contributed by local sources.

Light Fraction Carbon and Water-Stable Aggregates in Black Soils

The distribution of light fraction carbon （LF-C） in the various size classes of aggregates and its relationship to water- stable aggregates as well as the influence of cultivation on the organic components in virgin and cultivated black soils were studied by wet sieving and density separation methods. The total organic carbon （TOC） and LF-C were significantly higher （P≤ 0.05） in the virgin soils than in the cultivated soils. The LF-C in aggregates of different size classes varied from 0.9 to 2.5 g kg^-1 in the cultivated soils and from 2.5 to 7.1 g kg^-1 in the virgin soils, whereas the ratio of LF-C to TOC varied from 1.9% to 7.3% and from 5.0% to 12.2%, respectively. After being incubated under constant temperature and controlled humidity for three months, the contribution of LF-C to TOC sharply decreased to an amount （1.7%4.5%） close to the level in soils that had been cultivated for 20 to 25 years （1.3%-8.8%）. As a result, the larger water-stable macro-aggregates （especially 〉 1 mm） decreased sharply, indicating that the LF-C pool in virgin soils declined quickly after cultivation, which reduced the water stability of soil aggregates.

Corn straw return can increase labile soil organic carbon fractions and improve water-stable aggregates in Haplic Cambisol

Corn straw return to the field is a vital agronomic practice for increasing soil organic carbon(SOC)and its labile fractions,as well as soil aggregates and organic carbon(OC)associated with water-stable aggregates(WSA).Moreover,the labile SOC fractions play an important role in OC turnover and sequestration.The aims of this study were to determine how different corn straw returning modes affect the contents of labile SOC fractions and OC associated with WSA.Corn straw was returned in the following depths:(1)on undisturbed soil surface(NTS),(2)in the 0–10 cm soil depth(MTS),(3)in the 0–20 cm soil depth(CTS),and(4)no corn straw applied(CK).After five years(2014–2018),soil was sampled in the 0–20 and 20–40 cm depths to measure the water-extractable organic C(WEOC),permanganate oxidizable C(KMnO4-C),light fraction organic C(LFOC),and WSA fractions.The results showed that compared with CK,corn straw amended soils(NTS,MTS and CTS)increased SOC content by 11.55%–16.58%,WEOC by 41.38%–51.42%,KMnO4-C and LFOC by 29.84%–34.09%and 56.68%–65.36%in the 0–40 cm soil depth.The LFOC and KMnO4-C were proved to be the most sensitive fractions to different corn straw returning modes.Compared with CK,soils amended with corn straw increased mean weight diameter by 24.24%–40.48%in the 0–20 cm soil depth.The NTS and MTS preserved more than 60.00%of OC in macro-aggregates compared with CK.No significant difference was found in corn yield across all corn straw returning modes throughout the study period,indicating that adoption of NTS and MTS would increase SOC content and improve soil structure,and would not decline crop production.

Conventional tillage improves the storage of soil organic carbon in heavy fractions in the Loess Plateau, China

Soil labile organic carbon （C） plays an important role in improving soil quality. The relatively stable fractions of soil organic C （SOC） represent the bulk of SOC, and are also the primary determinant of the long-term C balance of terrestrial ecosystems. Different land use types can influence the distribution patterns of different SOC fractions. However, the underlying mechanisms are not well understood. In the present study, different fractions of SOC were determined in two land use types： a grazed grassland （established on previously cultivated cropland 25 years ago, GG） and a long-term cultivated millet cropland （MC）. The results showed that C concentration and C storage of light fractions （LF） and heavy fractions （HF） presented different patterns along the soil profiles in the two sites. More plant residues in GG resulted in 91.9% higher LF storage at the 0-10 cm soil depth, further contributed to 21.9% higher SOC storage at this soil depth; SOC storage at 20-60 cm soil depth in MC was 98.8% higher than that in GG, which could be mainly attributed to the HF storage 104.5% higher than in GG. This might be caused by the long-term application of organic manure, as well as the protection from plough pan and silt- and clay-sized particles. The study indicated that different soil management practices in this region can greatly influence the variations of different SOC fractions, while the conventional tillage can greatly improve the storage of SOC by in- creasing heavy fractions.

Labile and stabile soil organic carbon fractions in surface horizons of mountain soils–relationships with vegetation and altitude

Global and local climate changes could disturb carbon sequestration and carbon stocks in forest soils. Thus, it is important to characterize the stability of soil organic matter and the dynamics of soil organic carbon(SOC) fractions in forest ecosystems. This study had two aims:(1) to evaluate the effects of altitude and vegetation on the content of labile and stabile forms of organic carbon in the mountain soils; and(2) to assess the impact of the properties of soil organic matter on the SOC pools under changing environmental conditions. The studies were conducted in the Karkonosze Mountains(SW Poland, Central Europe). The content of the most labile fraction of carbon(dissolved organic carbon,DOC) decreases with altitude, but the content of fulvic acids(FA), clearly increases in the zone above 1000 m asl, while the stabile fraction(humins, nonhydrolyzing carbon) significantly decreases. A higher contribution of stabile forms was found in soils under coniferous forests(Norway spruce), while a smaller-under deciduous forests(European beech) and on grasslands. The expected climate change and the ongoing land use transformations in the zone above1000 m asl may lead to a substantial increase in the stable humus fraction(mainly of a non-hydrolyzing carbon) and an increase in the SOC pools, even if humus acids are characterized by a lower maturity and greater mobility favorable to soil podzolization.In the lower zone(below 1000 m asl), a decrease in the most stable humus forms can be expected,accompanied by an increase of DOC contribution,which will result in a reduction in SOC pools. Overall,the expected prevailing(spatial) effect is a decreasing contribution of the most stable humus fractions,which will be associated with a reduction in the SOC pools in medium-high mountains of temperate zone of Central Europe.

Effects of Caragana microphylla plantations on organic carbon sequestration in total and labile soil organic carbon fractions in the Horqin Sandy Land, northern China

Afforestation is conducive to soil carbon（C） sequestration in semi-arid regions. However, little is known about the effects of afforestation on sequestrations of total and labile soil organic carbon（SOC） fractions in semi-arid sandy lands. In the present study, we examined the effects of Caragana microphylla Lam. plantations with different ages（12-and 25-year-old） on sequestrations of total SOC as well as labile SOC fractions such as light fraction organic carbon（LFOC） and microbial biomass carbon（MBC）. The analyzed samples were taken from soil depths of 0–5 and 5–15 cm under two shrub-related scenarios： under shrubs and between shrubs with moving sand dunes as control sites in the Horqin Sandy Land of northern China. The results showed that the concentrations and storages of total SOC at soil depths of 0–5 and 5–15 cm were higher in 12-and 25-year-old C. microphylla plantations than in moving sand dunes（i.e., control sites）, with the highest value observed under shrubs in 25-year-old C. microphylla plantations. Furthermore, the concentrations and storages of LFOC and MBC showed similar patterns with those of total SOC at the same soil depth. The 12-year-old C. microphylla plantations had higher percentages of LFOC concentration to SOC concentration and MBC concentration to SOC concentration than the 25-year-old C. microphylla plantations and moving sand dunes at both soil depths. A significant positive correlation existed among SOC, LFOC, and MBC, implying that restoring the total and labile SOC fractions is possible by afforestation with C. microphylla shrubs in the Horqin Sandy Land. At soil depth of 0–15 cm, the accumulation rate of total SOC under shrubs was higher in young C. microphylla plantations（18.53 g C/（m^2·a）; 0–12 years） than in old C. microphylla plantations（16.24 g C/（m^2·a）; 12–25 years）, and the accumulation rates of LFOC and MBC under shrubs and between shrubs were also higher in young C. microphylla plantations than in old C. microphylla plantations. It can be concluded that the establishment of C. microphylla in the Horqin Sandy Land may be a good mitigation strategy for SOC sequestration in the surface soils.