Soil organic matter and its lignin component in surface horizons of salt-affected soils of the Argentinian Pampa

Salt-affected soils differ in their chemical properties to all other soils. Sodicity and salinity may affect the soil organic matter component of these soils. In a field experiment, we investigated organic matter decomposition in nonsaline nonsodic Aquic Argiudoll, a nonsaline sodic Typic Natraquoll, nonsaline nonsodic Petrocalcic Paleudoll and a saline sodic Typic Natralboll in the Pampa Deprimida, Argentina. The objectives were to identify the degree of stabilization of organic matter by association with mineral particles in these soils and to follow in particular the fate of lignin in these soils. We measured organic carbon, total nitrogen and the extent of lignin alteration with soil depth and in various particle size fractions. The salt-affected soils contained much less organic carbon and nitrogen in their mollic epipedons than the nonsaline nonsodic soils, and bioturbation into deeper layers was restrained. In the salt-affected soils most of the organic matter was in sand-sized particles. Retarded degradation of plant residues was indicated by the pattern of lignin-derived phenols, suggesting less alteration of lignin in the salt-affected soils than in the nonsaline nonsodic soils. We suggest that this results from the effects of high pH, high sodicity, and high salinity on the microorganisms and their enzymatic activities. The high pH and high concentrations of monovalent cations decreased formation of solid organo-mineral complexes. We conclude that in the salt-affected soils oxidatively altered organic compounds are susceptible to losses in dissolved or colloidal forms, because these compounds are not stabilized against leaching and mineralization by chemical bonding to soil minerals.     

盐化和有机质对土壤结构稳定性及阿特伯格极限的影响

对甘肃景电灌区盐化和有机质对土壤团聚体的稳定性、黏粒的分散性及阿特伯格极限进行了调查研究。研究结果表明该灌区盐化土壤中水溶性盐以钠盐为主,土壤具有钠质现象。随含盐量和交换性钠离子百分率（ESP）的增加土壤团聚体的稳定性显著降低,黏粒的分散性显著增加,阿特伯格极限具有降低的趋势,明确地说明盐化和伴随着的钠质化是土壤结构性能退化的主要原因。随有机质含量的增加,土壤团聚体的稳定性显著增加,黏粒的分散性显著降低,阿特伯格极限显著增加,说明增加有机质含量可以显著改善盐化土壤的结构性能。可以根据如下公式利用有机质含量（OM）和ESP预测团聚体的稳定性:WSAR=19.4 0.98OM-1.43 ESP（R2=0.5741,n=67）。团聚体稳定性、黏粒分散性、流限和塑限互相之间显著相关,说明流限和塑限可以作为反映盐化土壤结构状况的指标。     

Land-use management for sustainable rice production and carbon sequestration in reclaimed coastal tideland soils of South Korea: a review

ABSTRACT

The properties of secondary salt-affected soils developed from improper irrigation and drainage management and their effects on rice growth and yield are well documented. However, relevant information on coastal reclaimed tideland (RTL) soils, which are classified as primary salt-affected soils developed through salt-accumulated sediments is lacking. In this paper, we reviewed the physical and chemical properties of RTL soils in comparison with non-RTL soils and analyzed the relationship between rice production and soil salinity in RTL to suggest agricultural management practices for sustainable rice production and soil carbon sequestration in RTL. Similar to the secondary salt-affected soils, RTL soils were characterized by high alkalinity, salinity, and sodicity, and rice yield was negatively correlated with salinity. However, it was also found that lower fertility (e.g., organic matter and phosphorus) of RTL soils than non-RTL soils might also hamper rice growth and thus carbon input via plant residues in RTL soils. Correlation between years after reclamation and soil properties of RTL showed that cultivation of rice with annual fertilization and organic matter inputs increased soil fertility but salinity and sodicity did not show a significant tendency of change, suggesting that natural desalinization in RTL soils is hard to be achieved with conventional rice cultivation. Therefore, it is suggested that fertilization management as well as salinity management via drainage, gypsum application, tillage, and proper irrigation may be necessary to improve rice production and carbon sequestration in RTL soils.     

Carbon dynamics of sodic and saline soils following gypsum and organic material additions: A laboratory incubation

Carbon fluxes in sodic and saline soils were investigated by measuring the soil microbial biomass (SMB) and soil respiration rates under controlled conditions over 12 weeks. Gypsum (10 t/ha) and organic material, as kangaroo grass (10 t/ha), were incorporated in an acidic and an alkaline saline–sodic soils. Cumulative soil respiration rates were lowest in the sodic and saline soils without amendment, while the highest rates were found in those soils that had organic material addition. The addition of gypsum decreased the cumulative respiration rates in the 0–5 cm layer compared to the addition of organic material and the addition of organic material and gypsum. Similarly, the SMB was lowest in the sodic and saline soils without amendment and highest in the soils which had organic material addition, while the effects of gypsum addition were not significant. The low levels of respiration and SMB were attributed to the low soil organic carbon (SOC) levels that result from little or no C input into the soils of these highly degraded landscapes as the high salinity and high sodicity levels have resulted in scarcity or absence of vegetation. Following the addition of organic material to the sodic and saline soils, SMB levels and respiration rates increased despite adverse soil environmental conditions. This suggests that a dormant population of salt-tolerant SMB is present in these soils, which has become adapted to such environmental conditions over time and multiplies rapidly when substrate is available.     

盐化及钠质化对土壤物理性质的影响

对盐化土壤、钠质盐化土壤的概念及其参数进行了讨论 ,对盐化土壤的水分物理性质、渗透胁迫以及钠质化对土壤结构性能的影响 ,有机质对钠质化土壤结构性能的影响等方面的研究进行了综述     

Salinity effects on carbon mineralization in soils of varying texture

In salt-affected soils, soil organic carbon (SOC) levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter (SOM) may be negatively affected. Soil organic carbon models, such as the Rothamsted Carbon Model (RothC), that are used to estimate carbon dioxide (CO₂) emission and SOC stocks at various spatial scales, do not consider the effect of salinity on CO₂ emissions and may therefore over-estimate CO₂ release from saline soils. Two laboratory incubation experiments were conducted to assess the effect of soil texture on the response of CO₂ release to salinity, and to calculate a rate modifier for salinity to be introduced into the RothC model. The soils used were a sandy loam (18.7% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3% clay) and a clay (42% clay) in another experiment. The water content was adjusted to 75%, 55%, 50% and 45% water holding capacity (WHC) for the loamy sand, sandy loam, sandy clay loam and the clay, respectively to ensure optimal soil moisture for decomposition. Sodium chloride (NaCl) was used to develop a range of salinities: electrical conductivity of the 1:5 soil: water extract (EC_1:5) 1, 2, 3, 4 and 5 dS m⁻¹. The soils were amended with 2% (w/w) wheat residues and CO₂ emission was measured over 4 months. Carbon dioxide release was also measured from five salt-affected soils from the field for model evaluation. In all soils, cumulative CO₂-C g⁻¹ soil significantly decreased with increasing EC_1:5 developed by addition of NaCl, but the relative decrease differed among the soils. In the salt-amended soils, the reduction in normalised cumulative respiration (in percentage for the control) at EC_1:5 > 1.0 dS m⁻¹ was most pronounced in the loamy sand. This is due to the differential water content of the soils, at the same EC_1:5; the salt concentration in the soil solution is higher in the coarser textured soils than in fine textured soils because in the former soils, the water content for optimal decomposition is lower. When salinity was expressed as osmotic potential, the decrease in normalised cumulative respiration with increasing salinity was less than with EC_1:5. The osmotic potential of the soil solution is a more appropriate parameter for estimating the salinity effect on microbial activity than the electrical conductivity (EC) because osmotic potential, unlike EC, takes account into salt concentration in the soil solution as a function of the water content. The decrease in particulate organic carbon (POC) was smaller in soils with low osmotic potential whereas total organic carbon, humus-C and charcoal-C did not change over time, and were not significantly affected by salinity. The modelling of cumulative respiration data using a two compartment model showed that the decomposition of labile carbon (C) pool is more sensitive to salinity than that of the slow C pool. The evaluation of RothC, modified to include the decomposition rate modifier for salinity developed from the salt-amended soils, against saline soils from the field, suggested that salinity had a greater effect on cumulative respiration in the salt-amended soils. The results of this study show (i) salinity needs to be taken into account when modelling CO₂ release and SOC turnover in salt-affected soils, and (ii) a decomposition rate modifier developed from salt-amended soils may overestimate the effect of salinity on CO₂ release.     

Characterization of Slightly and Moderately Saline and Sodic Soils in Irrigated Agricultural Land using Simulated Data of Advanced Land Imaging (EO‐1) Sensor

Around the world, especially in semi‐arid regions, millions of hectares of irrigated agricultural land are abandoned each year because of the adverse effects of irrigation, mainly secondary salinity and sodicity. Accurate information about the extent, magnitude, and spatial distribution of salinity and sodicity will help create sustainable development of agricultural resources. In Morocco, south of the Mediterranean region, the growth of the vegetation and potential yield are limited by the joint influence of high temperatures and water deficit. Consequently, the overuse of surface and groundwater, coupled with agricultural intensification, generates secondary soils salinity and sodicity. This research focuses on the potential and limits of the advance land imaging (EO‐1 ALI) sensor spectral bands for the discrimination of slight and moderate soil salinity and sodicity in the Tadla's irrigated agricultural perimeter, Morocco. To detect affected soils, empirical relationships (second‐order regression analysis) were calculated between the electrical conductivity (EC) and different spectral salinity indices. To achieve our goal, spectroradiometric measurements (350 to 2500 nm), field observation, and laboratory analysis (EC of a solution extracted from a water‐saturated soil), and soil reaction (pH) were used. The spectroradiometric data were acquired using the ASD (analytical spectral device) above 28 bare soil samples with various degrees of soil salinity and sodicity, as well as unaffected soils. All of the spectroradiometric data were resampled and convolved in the solar‐reflective spectral bands of EO‐1 ALI sensor. The results show that the SWIR region is a good indicator of and is more sensitive to different degrees of slight and moderate soil salinity and sodicity. In general, relatively high salinity soils show higher spectral signatures than do sodic soils and unaffected soils. Also, strongly sodic soils present higher spectral responses than moderately sodic soils. However, in spite of the improvement of EO‐1 ALI spectral bands by comparison to Landsat‐ETM+, this research shows the weakness of multispectral systems for the discrimination of slight and moderate soil salinity and sodicity. Although remote sensing offers good potential for mapping strongly saline soils (dry surface crust), slight and moderately saline and sodic soils are not easily identified, because the optical properties of the soil surfaces (color, brightness, roughness, etc.) could mask the salinity and sodicity effects. Consequently, their spatial distribution will probably be underestimated. According to the laboratory results, the proposed Soils Salinity and Sodicity Indices (SSSI) using EO‐1 ALI 9 and 10 spectral bands offers the most significant correlation (52.91%) with the ground reference (EC). They could help to predict different spatial distribution classes of slight and moderate saline and sodic soils using EO‐1 ALI imagery data.     

Predicting the Classes and Distribution of Salt-Affected Soils in Northwest Libya

Sodicity and salinity can adversely affect soil structure and are common constraints to plant growth in arid regions. Current remote sensing techniques cannot distinguish between the various classes of salt-affected soils. Field and laboratory measurements of salt-affected soils are time-consuming and expensive. Mapping of the salt-affected soils can be used in soil conservation planning to identify regions with different degrees of limitations. There is a need to use existing field and laboratory measurements to create maps of classes of salt-affected soils. The objectives of this study are to classify salt-affected soils, use existing field data to interpolate and validate geospatial predictions of the classes of salt-affected soils using Geographic Information Systems (GIS), and create maps showing the different classes and distribution of salt-affected soils. The classification framework for salt-affected soils is based on electrical conductivity (ECe), soil pH and the sodium adsorption ratio (SAR), and provides four degrees of limitations to salt-affected soils: slight (normal soils), moderate (saline soils), severe (sodic soils), and extreme (saline-sodic soils). Spatial interpolation of the field data from northwestern Libya was verified by cross-validation, and maps of the salt-affected soils in the region were created. The majority of soils in this region of Libya are normal (slight degree of limitation). Twenty percent of the topsoil is saline-sodic (extreme degree of limitation). Land use recommendations and rehabilitation strategies can be developed from such maps of salt-affected soil classes. The methodology followed in this study can be applied to other arid regions around the world, particularly in developing countries where budgetary constraints limit detailed field and laboratory measurements of sodicity and salinity.     

Short-term carbon mineralization in saline–sodic soils

Previous studies have shown that carbon (C) mineralization in saline or sodic soils is affected by various factors including organic C content, salt concentration and water content in saline soils and soil structure in sodic soils, but there is little information about which soil properties control carbon dioxide (CO₂) emission from saline-sodic soils. In this study, eight field-collected saline–sodic soils, varying in electrical conductivity (EC_e, a measure of salinity, ranging from 3 to 262 dS m⁻¹) and sodium adsorption ratio (SAR_e, a measure of sodicity, ranging from 11 to 62), were left unamended or amended with mature wheat or vetch residues (2% w/w). Carbon dioxide release was measured over 42 days at constant temperature and soil water content. Cumulative respiration expressed per gram SOC increased in the following order: unamended soil<soil amended with wheat residues (C/N ratio 122)<soil with vetch residue (C/N ratio 18). Cumulative respiration was significantly (p < 0.05) negatively correlated with EC_e but not with SAR_e. Our results show that the response to EC_e and SAR_e of the microbial community activated by addition of organic C does not differ from that of the less active microbial community in unamended soils and that salinity is the main influential factor for C mineralization in saline–sodic soils.     

红壤侵蚀区植被恢复过程中土壤有机碳组分变化

为了解土壤有机碳组分在植被恢复过程中的变化规律,选取了红壤区本底条件基本一致的不同恢复年限马尾松林为研究对象,以未治理的侵蚀裸地(CK1)和恢复后的次生林(CK2)为对照,采用物理化学分组法,将土壤有机碳分为由溶解性有机碳(DOC)和颗粒有机碳(POM)组成的活性碳库、物理保护态的团聚体与粉粒和黏粒组合成的缓效性碳库以及化学结构稳定的惰性碳库。结果表明:在植被恢复过程中(0~30年)活性碳库储量及其分配比例在植被恢复7~10年显著提高(P0.05),并在植被恢复27~30年保持较稳定水平,缓效性碳库储量及其分配比例在27~30年呈显著变化(P0.05),而活性碳库分配比例有所降低,且POM、DOC与缓效性碳库均达显著相关(P0.01),说明活性碳库在恢复7~10年后逐渐向缓效性碳库转化;惰性碳库储量随恢复年限不断增加,但其分配比例保持较稳定水平。相关性分析显示,恢复年限、不同组分与不同碳库均达显著相关(P0.01),且缓效性碳库随植被恢复最敏感,说明在马尾松恢复过程中土壤有机碳以活性碳库积累逐渐转化为缓效性碳库积累为主,进而影响惰性碳库的积累,有利于土壤有机碳的长期保持。     

盐分对土壤呼吸和有机质动力学的影响与土壤电导率相关，而与渗透势的关系更为密切

Osmotic potential (OP) of soil solution may be a more appropriate parameter than electrical conductivity (EC) to evaluate the effect of salts on plant growth and soil biomass.However,this has not been examined in detail with respect to microbial activity and dissolved organic matter in soils with different texture.This study evaluated the effect of salinity and sodicity on respiration and dissolved organic matter dynamics in salt-affected soils with different texture.Four non-saline and non-sodic soils differing in texture (S-4,S-13,S-24 and S-40 with 4％,13％,24％ and 40％ clay,respectively) were leached using combinations of 1 mol L-1 NaC1 and 1 mol L-1 CaC12 stock solutions,resulting in EC (1:5 soil:water ratio) between 0.4 and 5.0 dS m-1 with two levels of sodicity (sodium absorption ratio (SAR) ＜ 3 (non-sodic) and 20 (sodic),1:5 soil:water ratio).Adjusting the water content to levels optimal for microbial activity,which differed among the soils,resulted in four ranges of OP in all the soils:from-0.06 to--0.24 (controls,without salt added),-0.55 to-0.92,-1.25 to-1.62 and-2.77 to-3.00 Mpa.Finely ground mature wheat straw (20 g kg-1) was added to stimulate microbial activity.At a given EC,cumulative soil respiration was lower in the lighter-textured soils (S-4 and S-13) than in the heavier-textured soils (S-24 and S-40).Cumulative soil respiration decreased with decreasing OP to a similar extent in all the soils,with a greater decrease on Day 40 than on Day 10.Cumulative soil respiration was greater at SAR =20 than at SAR ＜ 3 only at the OP levels between-0.62 and-1.62 MPa on Day 40.In all the soils and at both sampling times,concentrations of dissolved organic C and N were higher at the lowest OP levels (from-2.74 to-3.0 MPa) than in the controls (from-0.06 to-0.24 MPa).Thus,OP is a better parameter than EC to evaluate the effect of salinity on dissolved organic matter and microbial activity in different textured soils.     

Organic carbon in soils of Germany: Status quo and the need for new data to evaluate potentials and trends of soil carbon sequestration

Uncertainties in estimates of soil carbon (C) stocks and sequestration result from major gaps in knowledge of C storage in soils, land‐use history, the variability of field measurements, and different analytical approaches applied. In addition, there is a lack of long‐term datasets from relevant land‐use systems. As in many European countries, a national database on soil organic carbon (SOC) including all relevant information for the determination of soil C stocks is likewise missing in Germany. In this paper, we summarize and evaluate the present state of knowledge on organic‐C contents/pools in soils of Germany and discuss the need for the acquisition and access to new data on soil organic carbon. Despite the number of agricultural sites under permanent soil monitoring, regional surveys on SOC, comprehensive ecosystem studies, and long‐term field experiments, there is a striking lack of data in Germany particularly with regard to agricultural soils. Apart from a missing standardization of methods and homogeneous baseline values, the implementation of a periodic, nation‐wide soil inventory on agricultural soils is required in order to simultaneously record information on land use, land‐use change, and agricultural practice. In contrast, the existing national inventory of forest soils provides information on C‐stock changes in forest soils, although there is some concern with regard to the representativeness of the sampling design to adequately address the problem of spatial heterogeneity and temporal variability. It is concluded that the lack of comprehensiveness, completeness, actuality, data harmonization, and standardized sampling procedures will further prevent the establishment of a SOC database in Germany with regard to the monitoring of trends in soil C pools and fluxes and the assessment of long‐term C‐sequestration potentials of soils under different land use. A future soil inventory should represent the heterogeneity of organic matter through functionally different SOC pools, topsoil characteristics as well as content, pool, and flux data for the deeper mineral‐soil compartments.     

Effect of soil salinity and sodicity on Matricaria chamomilla,L. Growth

Abstract

Growth response of Matricaria chamomilla, L. was investigated on a range of soil salinity and sodicity levels using fine and coarse‐textured soil types. Twenty treatments including 4 levels of salinity and 4 levels of sodicity on each soil type were examined in addition to control. On the coarse‐textured soils, chamomile responded best under relatively low saline and sodic conditions. Plant growth decreased with increase in salinity and sodicity. On the fine‐textured soils, plants endured saline conditions up to 13 ECe and grew better under sodic conditions. The best growth of plants was achieved on fine‐textured soils with sodicity level of 31.8 Esp.     

Soil organic carbon in the rocky desert of northern Negev (Israel)

Purpose

So far, the soil organic carbon (SOC) literature is dominated by studies in the humid environments with huge stocks of vulnerable carbon. Limited attention has been given to dryland ecosystems despite being often considered to be highly sensitive to environmental change. Thus, there is insufficient research about the spatial patterns of SOC stocks and the interaction between soil depth, ecohydrology, geomorphic processes, and SOC stocks. This study aimed at identifying the relationship between surface characteristics, vegetation coverage, SOC, and SOC stocks in the arid northern Negev in Israel.

Materials and methods

The study site Sede Boker is ideally suited because of well-researched but variable ecohydrology. For this purpose, we sampled five slope sections with different ecohydrologic characteristics (e.g., soil and vegetation) and calculate SOC stocks. To identify controlling factors of SOC stocks on rocky desert slopes, we compared soil properties, vegetation coverage, SOC concentration, and stocks between the five ecohydrologic units.

Results and discussion

The results show that in Sede Boker, rocky desert slopes represent a significant SOC pool with a mean SOC stock of 0.58?kg?C?m^?2 averaged over the entire study area. The spatial variability of the soil coverage represents a strong control on SOC stocks, which varies between zero in uncovered areas and 1.54?kg?C?m^?2 on average in the soil-covered areas. Aspect-driven changes of solar radiation and thus of water availability are the dominant control of vegetation coverage and SOC stock in the study area.

Conclusions

The data indicate that dryland soils contain a significant amount of SOC. The SOC varies between the ecohydrologic units, which reflect (1) aspect-driven differences, (2) microscale topography, (3) soil formation, and (4) vegetation coverage, which are of greatest importance for estimating SOC stocks in drylands.     

Effect of sodicity and salinity on disaggregation and tensile strength of an Alfisol under different cropping systems

The influence of electrolyte concentration (EC) and sodium adsorption ratio (SAR) on the tensile strength and aggregate stability via flocculation and dispersion behaviour of an Alfisol varying in organic carbon content due to different cropping systems was assessed using a split-split plot experiment involving eight soils, three levels of EC and seven levels of SAR.

Generally, at a given SAR value, mean weight diameter (MWD) increased with organic matter status of the soil in the following order: virgin pasture>wheat>wheat-fallow. As MWD decreased, the amount of dispersible clay increased at a given SAR indicating that more surfaces exposed due to slaking of aggregates led to more clay dispersion. Statistical analysis of changes in tensile strength with various factors showed that an increase in organic matter decreased the magnitude of changes in strength induced by sodicity because organic matter tends to increase aggregate stability (higher MWD). While individual soils had significant relationships between the tensile strength of the aggregates and the amount of spontaneously dispersible clay, this relationship was poor when the results of all soils were pooled together. The amounts of dispersible clay from dry aggregates were higher than from wet aggregates and dispersive breakdown of the aggregates of sodic soils occured irrespective of the mode of wetting. The most important factor in determining the soil strength was the amount of clay dispersed during wet-sieving analysis followed by MWD.     

Effect of NaCl-induced saline sodicity on the interpretation of soil potassium dynamics

Excess of exchangeable sodium (Na) in salt-affected soils causes ion toxicity and decrease in nutrient uptake by plants, particularly potassium (K). A number of studies have been conducted to investigate the effect of K-fertilization on plant growth under sodic and saline-sodic conditions but the results are much diverse to process for concrete recommendations. To explore the possible reasons, it was hypothesized that Na applied as NaCl to produce salinity/sodicity in the soil may release non-exchangeable K, minimizing the effect of K-fertilization. Incubation studies were conducted for 2, 4 and 6 days in the light (sandy loam) and heavy (clay loam) textured soils producing two saline/sodic levels, i.e. 20 and 30 sodium adsorption ratio (SAR) along with control (SAR 3). Potassium fertilizer applied was calculated according to 40 (general recommendations based on soil-nutrient status), 80 and 160 kg K ha^?1. Interestingly, it was observed that addition of NaCl possibly released non-exchangeable K from the soil minerals and increased the K concentration in soil solution. Total K release was more in heavy textured soil but initial release was more in light textured soil. This release may eliminate the effect of K-fertilization applied under salt stress induced by NaCl. Therefore, it is suggested that while studying Na–K interaction in salt-affected soils, NaCl should be avoided to produce salinity, and naturally occurring saline-sodic soils may be used. Soil Na–K interaction studies including ameliorating effect of K under sodic or saline-sodic conditions should be conducted carefully considering the above-stated argument.     

Overcoming the Confounding Effects of Salinity on Sodic Soil Research

The adverse effects of sodicity on plant growth are difficult to quantify using naturally occurring soils because of the confounding variation in other soil properties, particularly salinity, pH, organic matter, soil nutrients, mineralogy, and texture. We applied a method involving the equilibration of large soil volumes with solutions varying in sodium adsorption ratio (SAR), followed by excess salt removal with solutions of similar SAR but lower ionic strength. Application of this method to a calcareous nonsodic, nonsaline Vertosol from Narrabri, New South Wales, resulted in soils with exchangeable sodium percentage (ESP) values between 2% and 25% but with similar magnesium and potassium concentrations and constant electrical conductivity (～2.7 dS/m). Soil pH and solution phosphorus concentrations automatically increased as the ESP of the soil rose, which is important to consider when addressing plant growth results. This method can successfully minimize the confounding of sodicity with other soil properties that so often plagues sodic soil research.     

Distribution of organic carbon in physical fractions of soils as affected by agricultural management

Soil organic carbon (SOC) is distributed heterogeneously among different-sized primary particles and aggregates. Further, the SOC associated with different physical fractions respond differently to managements. Therefore, this study was conducted with the objective to quantify the SOC associated with all the three structural levels of SOC (particulate organic matter, soil separates and aggregate-size fractions) as influenced by long-term change in management. The study also aims at reevaluating the concept that the SOC sink capacity of individual size-fractions is limited. Long-term tillage and crop rotation effects on distribution of SOC among fractions were compared with soil from adjacent undisturbed area under native vegetation for the mixed, mesic, Typic Fragiudalf of Wooster, OH. Forty five years of no-till (NT) management resulted in more SOC accumulation in soil surface (0–7.5 cm) than in chisel tillage and plow tillage (PT) treatments. However, PT at this site resulted in a redistribution of SOC from surface to deeper soil layers. The soils under continuous corn accumulated significantly more SOC than those under corn–soybean rotation at 7.5–45 cm depth. Although soil texture was dominated by the silt-sized particles, most of the SOC pool was associated with the clay fraction. Compared to PT, the NT treatment resulted in (i) significantly higher proportion of large macroaggregates (>2,000 μm) and (ii) 1.5–2.8 times higher SOC concentrations in all aggregate-size classes. A comparative evaluation using radar graphs indicated that among the physical fractions, the SOC associated with sand and silt fractions quickly changed with a land use conversion from native vegetation to agricultural crops. A key finding of this study is the assessment of SOC sink capacity of individual fractions, which revealed that the clay fraction of agricultural soils continues to accumulate more SOC, albeit at a slower rate, with progressive increase in total SOC concentration. However, the clay fraction of soil under native woodlot showed an indication for SOC saturation. The data presented in this study from all the three structural levels of SOC would be helpful for refining the conceptual pool definitions of the current soil organic matter prediction models.     

Organic matter decomposition and carbon content in soil fractions as affected by a gradient of labile carbon input to a temperate forest soil

Labile carbon (C) input to soils is expected to affect soil organic matter (SOM) decomposition and soil organic C (SOC) stocks in temperate coniferous forests. We hypothesized that the SOM...     

Organic C levels in intensively managed arable soils – long-term regional trends and characterization of fractions

Substantial losses of soil organic carbon (SOC) from the plough layer of intensively managed arable soils in western Europe have recently been reported, but these estimates are associated with very large uncertainties. Following soil surveys in 1952 and 1990 of arable soils in West Flanders (Belgium), we resampled 116 sites in 2003 and thus obtained three paired measurements of the OC stocks in these soils. Ten soils were selected for detailed physical fractionation to obtain possible further explanations for changes in SOC stocks. Between 1990 and 2003, the SOC stocks decreased at an average rate of ?0.19 t OC ha^?1 year^?1. This loss is significant but is still less than half the rate of SOC decrease that was estimated previously for the whole region of Flanders, which includes the study area. Variation in SOC stocks or in the magnitude of SOC stock losses could not be related to soil texture, to changes in ploughing depth, or to recent land‐use changes. A good relationship, however, was found between the SOC losses and organic matter (OM) inputs. The results of the physical fractionation also suggested management to be the predominant factor determining variation in SOC stocks because no correlation was found between soil texture and the absolute amounts of OC present in the largest OM fractions, that is, the OC in free particulate organic matter (POM), and OC associated with the silt + clay size fraction. The proportion of OC in free POM was up to 40% of the total OC, which indicates the important impact of management on SOC and also indicates that a substantial part of the SOC still present, may in the future be lost at a time scale of years to decades assuming that the intensive management continues.