Composition of organic matter in particle-size fractions of an agricultural soil

The composition of organic matter was studied in clay (< 2 μm), fine silt (2-6.3 μm), medium silt (6.3-20 μm), coarse silt (20-63 μm) and sand (63-2000 μm) fractions of the Ap-horizon of a clay loam (Orthic Humic Gleysol) from Bainsville (Ottawa, Canada) by organic C and total N analyses and pyrolysis-field ionization mass spectrometry (Py-FIMS). The C and N contents were largest in fine silt and medium silt and smaller in coarse silt and sand. Differences in the contents of organic matter and absorbed water were significantly (r= 0.945***) reflected by the amounts of volatilized matter during Py-FIMS. The Py-FI therniograms and mass spectra showed clear differences in thermal stability and molecular composition of organic matter between the organo-mineral size-fractions. Abundances of carbohydrates, phenols and lignin monomers, alkylaromatics and N-containing compounds decreased, whereas abundances of lignin dimers and lipids increased with increasing equivalent diameters. An exception was the sand fraction which was dominated by the characteristic features of plant residues. The six compound classes, calculated using signals of biomarkers, accounted for 35% to 60% of the recorded total ion intensity. The thermal evolution of the selected compound classes, which are important constituents of soil organic matter (SOM), indicated the stability of humic and organo-mineral bonds in particle-size fractions, Moreover, the influence of mineral matrix on organic matter composition was shown by significant correlations between relative abundances of carbohydrates, N-containing compounds, lipids, lignin dimers, and proportions of phyllosilicates.     

Estimating particle-size fractions of soil dominated by silicate minerals from geochemistry

Measuring particle-size distribution (psd) is so time-consuming and expensive that it is rarely affordable for systematic survey. In young soil dominated by silicate minerals the size fractions have characteristic elemental compositions, and so they might be predicted from geochemical analysis. We tested the feasibility of predicting the psd from concentrations of elements in samples of topsoil taken in surveys of two large regions in eastern England and on which the psd had been determined. Of the 35 elements measured we chose eight (a) from our general knowledge of mineral composition, (b) after a principal component analysis to avoid redundancy and (c) experience elsewhere. Of the eight elements, five were used (Al, Fe, Ni, Ti and Zr) to build multiple linear regression models for the predictions. The equations were assessed by their coefficients of determination, R ², and the effectiveness of their predictions, expressed as root-mean-square errors (RMSEs) on validation sets of data of known psd. The models accounted on average for 89% of the variance in the clay size-fraction and 82% for sand. The corresponding median RMSEs were 4.9% and 8.8% on medians of 17% and 58%, respectively. The silt size-fraction was less well predicted; R ² was only 0.58, and the median RMSE was 10.6% on a median of 22%. We judge our approach, in which the regression models may be regarded as pedotransfer functions, to have been moderately successful and to merit attention in similar circumstances in other regions.     

Exploring the enzymatic landscape: distribution and kinetics of hydrolytic enzymes in soil particle-size fractions

The location of extracellular enzymes within the soil architecture and their association with the various soil components affects their catalytic potential. A soil fractionation study was carried out to investigate: (a) the distribution of a range of hydrolytic enzymes involved in C, N and P transformations, (b) the effect of the location on their respective kinetics, (c) the effect of long-term N fertilizer management on enzyme distribution and kinetic parameters. Soil (silty clay loam) from grassland which had received 0 or 200 kg N ha⁻¹ yr⁻¹ was fractionated, and four particle-size fractions (>200, 200-63, 63-2 and 0.1-2 μm) were obtained by a combination of wet-sieving and centrifugation, after low-energy ultrasonication. All fractions were assayed for four carbohydrases (β-cellobiohydrolase, N-acetyl-β-glucosaminidase, β-glucosidase and β-xylosidase), acid phosphatase and leucine-aminopeptidase using a microplate fluorimetric assay based on MUB-substrates. Enzyme kinetics (V_max and K_m) were estimated in three particle-size fractions and the unfractionated soil. The results showed that not all particle-size fractions were equally enzymatically active and that the distribution of enzymes between fractions depended on the enzyme. Carbohydrases predominated in the coarser fractions while phosphatase and leucine-aminopeptidase were predominant in the clay-size fraction. The Michaelis constant (K_m) varied among fractions, indicating that the association of the same enzyme with different particle-size fractions affected its substrate affinity. The same values of K_m were found in the same fractions from the soil under two contrasting fertilizer management regimes, indicating that the Michaelis constant was unaffected by soil changes caused by N fertilizer management.     

Dynamics of soil organic matter associated with particle-size fractions of water-stable aggregates

Stable macroaggregates (> 200 μm) of cultivated soils are reported in the literature to be richer in organic carbon, and in young organic carbon in particular, than microaggregates (< 200 μm). However, the nature of this additional carbon is not yet known. To determine it, we compared the composition of organic matter in stable macroaggregates with that in unstable ones. Macroaggregates 2–3 mm in diameter were separated from two silty cultivated soils from the Paris basin. They were slaked, and the primary particle composition of the resulting fractions was analysed. We used the natural abundance of ¹³C to quantify the amount of young carbon, derived from a maize crop, in the various size fractions. The stable macroaggregates were richer in total C and in young C (younger than 6 and 23 years respectively in the two soils studied) than the unstable ones. This young C comprised 50% particulate organic matter, 20% associated with silt and 30% with clay particles. We propose a schematic composition of aggregates in these soils in which stable aggregates are formed by the binding of microaggregates by additional young organic matter, predominantly plant debris. Young organic matter is preferentially incorporated and is responsible for aggregation, though it is eventually redistributed among aggregate classes through the destruction and re‐formation of the aggregates. We have developed a model to simulate this redistribution. The model shows that stable macroaggregates have a life of a few years, but that microaggregates may exist for decades. We suggest that the stabilization and de‐stabilization of macroaggregates in soils is linked to the incorporation and biodegradation of plant debris.     

长期不同施肥对土壤各粒级组分中氮含量及分配比例的影响

土壤培肥过程中氮素的积累和在各级土壤颗粒中的分配对了解土壤氮素积累机制和氮素的可持续管理具有重要意义。本研究以红壤旱地长期定位试验为基础,采用离心法分离土壤颗粒,通过分析氮素在土壤不同颗粒中的含量,研究长期施肥对土壤中不同粒级土壤氮的分配的影响。结果表明,施肥二十年后,在020 cm土层,施肥处理的全氮含量在1.19~1.98 g/kg 之间,施肥处理全氮含量显著高于不施肥处理（CK）。在2040 cm土层,土壤全氮含量分布在0.59~0.84 g/kg之间,显著低于表层土壤氮含量; 在020 cm土层,与氮磷钾化肥配施处理（NPK）相比,单施或者配施有机肥的土壤与砂粒、 粗粉粒、 细粉粒、 粘粒结合的氮含量分别提高220%、 84%、 46%和26%。而在2040 cm土层,与NPK处理相比,有机肥的施用仅提高了与细粉粒、 粘粒结合的氮含量,降低了与粗粉粒结合的氮含量; 在020 cm土层中,红壤氮素主要分布在粘粒中。2040 cm土层CK、 NPK和氮磷钾配施加秸秆还田处理（NPKS）表现为与粗粉粒结合的氮的分配比例最高,而施用有机肥的处理氮素分布仍以粘粒占比例最高。相关分析表明,土壤表层＜5 m粒级的土壤颗粒中氮含量和作物的经济产量呈显著性正相关。长期施用有机肥可以显著提高表层土壤全氮及与各个颗粒结合氮的含量,不同处理的氮主要保存在粘粒中。而亚表层土壤中,施化肥或者秸秆还田加速了粘粒结合态氮的耗竭,配施有机肥促进了氮在粘粒中的积累,也更有利于土壤全氮的积累,是可持续的氮素管理措施。     

长期施肥下红壤有机碳及其颗粒组分对不同施肥模式的响应

采集不同施肥24年的红壤,采用物理分组的方法,观测了长期不同施肥下红壤有机碳及其组分变化,并结合历史资料分析了不同施肥模式对红壤有机碳及其颗粒组分的影响。结果表明,化肥配施有机肥（NPKM）处理下红壤总有机碳含量（10.33 g/kg）,砂粒（2000～53 m）、细粉粒（5～2 m）和粘粒（2 m）组分中的有机碳含量显著高于其他处理。与不施肥（CK）相比,施用化肥（NPK、2NPK）和有机肥（NPKM、M）显著地提高了红壤有机碳在砂粒和粘粒中的分配比例,而降低了其在粗粉粒和细粉粒的分配比例。施化肥（NPK、2NPK）、单施有机肥（M）、化肥配施有机肥（NPKM）处理,土壤有机碳的平均固定速率分别为0.05 t/(hm2?a)、0.18 t/(hm2?a)、0.26 t/(hm2?a)。相关分析表明,不同施肥模式下红壤有机碳的固定量与碳投入量之间存在着极显著的线性相关关系（R2=0.909, P0.01）,土壤的固碳效率为8.1%;随着碳投入的增加,粗粉粒和细粉粒有机碳储量逐渐下降,而砂粒和粘粒中碳储量逐渐增加,并且粘粒增加速率要远远高于砂粒。以上结果说明,红壤中有机碳还没有达到饱和,还具有一定的固碳潜力,增加的有机碳主要固持在粘粒中,粘粒是红壤有机碳的主要固持组分。     

A mechanistic model for describing the sorption and desorption of phosphate by soil

A model of phosphate reaction is constructed and its output compared with observations for the sorption and desorption of phosphate by soil. The model has three components: first, the reaction between divalent phosphate ions and a variable-charge surface; second, the assumption that there is a range of values of surface properties and that these are normally distributed; third, the assumption that the initial adsorption induces a diffusion gradient towards the interior of the particle which begins a solid-state diffusion process. The model closely describes the effects on sorption of phosphate of: concentration of phosphate, pH, temperature, and time of contact. It also reproduces the effects on desorption of phosphate of: period of prior contact, period and temperature of desorption, and soil: solution ratio. The model is general and should apply to other specifically adsorbed anions and cations. It suggests that phosphate that has reacted with soil for a long period is not ‘fixed’ but has mostly penetrated into the soil particles. The phosphorus can be recovered slowly if a low enough surface activity is induced.     

长期施肥对白浆土磷吸附与解吸的影响

对白浆土在长期施肥条件下磷的吸附与解吸进行了研究。试验结果表明，供试白浆土磷的等温吸附曲线符合langmuir方程，在低磷浓度下其吸磷能力较强，随着磷浓度的增大，吸磷能力减弱。在不同施肥处理中，CK处理的最大吸附量（Xm）最大，OM处理最小，各处理吸附常数（K）值变化较小，均在0．2左右，而Xm变化较大；不同施肥处理总的解吸趋势是一致的，即解吸量随着浓度的增加而增加，而后逐步趋于平稳。施有机肥的处理土壤吸附磷能力降低，但解吸磷能力增强；长期不施肥土壤固磷能力增强。     

Some considerations on the sorption and desorption phenomena of iodide and iodate on soil

Radiotracer experiments on the sorption of I^? (iodide) and IO _inf ^{3 p?} (iodate) from water by soils such as field soil, rice paddy soil and sandy soil, as well as by some soil components, have been carried out with special reference to the effects of heating and gamma-irradiating the soil. Desorption phenomena of I from soil to various solutions were also studied. The sorption of I^? by soil was markedly reduced through treatments of air-drying and heating the soil prior to its equilibration with water. The results indicated that I^? sorption was by the soil fraction which was unstable at about 150 °C, while IO _inf ^{3 p?} sorption was by the soil fraction which was relatively stable to heating. Gamma-irradiation at 27 kGy affected the sorption to a smaller extent than heating at 150 °C. A very high sorption (or soil-water distribution coefficient, Kd) was found in untreated field soil (andosol) with a low organic C (humus) content, while the sorption by sandy soil was considerably smaller than the other soils. Neither I^? or IO _inf ^{3 p?} were well sorbed by clay minerals, Al₂O₃ and quartz sand, while the sorption by Fe₂O₃ was IO _inf ^{3 p?} were desorbed by 1N NaOH solution. By acidifying this solution, only a part of the desorbed I was re-precipitated with humic acid. The desorption by solutions containing K₂SO₃ or KI was also high, while that by solutions containing HCI, CH₃COONH₄ or chemical fertilizer was considerably lower. These findings suggested the possibility that I was not directly associated with humic acid itself.     

Contrast effect of long-term fertilization on SOC and SIC stocks and distribution in different soil particle-size fractions

Purpose

The objectives of the study were (1) to quantify the long-term effects of nitrogen-phosphorus fertilizer (NP) and a combination of nitrogen-phosphorus with organic manure (NPM) on total soil organic carbon (SOC) and total soil inorganic carbon (SIC), (2) to identify the changes of SOC and SIC in soil particle-size fractions, and (3) to investigate the relationship between SOC and SIC.

Materials and methods

Two long-term field experiments (sites A and B) were performed in 1984 (site A) and 1979 (site B) in the North China Plain. The soil samples were collected in 2006 and separated for clay, silt and sand size particle fractions and then determined for SOC and SIC.

Results and discussion

The long-term fertilization significantly increased SOC in 0–20 cm soil layer by 9–68% but significantly decreased or had no effect on SIC. In total, soil carbon storage was little affected by NP, but significantly increased by NPM application (p < 0.05). Fertilization affected both SOC and SIC in sand- and silt-sized particles but not in clay-size fraction. Both NP and NPM increased SOC in sand- and silt-sized particles by 8.7–123.9% in the 0–20 cm layer but decreased SIC up to 80.4% in the 40–60 cm layer. The SOC concentration in the particle-size fractions was negatively correlated with SIC concentration, which may imply an antagonistic interaction between organic and inorganic carbon levels.

Conclusions

These results illustrate the importance of soil inorganic carbon pool in evaluating soil total carbon pool in semi-arid farmlands. Previous assessments of the effects of fertilizers on the soil carbon pool, using only SOC determinations, require re-evaluation with the inclusion of SIC determinations.

     

Influences of organic fertilization and solarization in a greenhouse on particle-size fractions of a Mediterranean sandy soil

 The effects of a composted organic amendment and solarization on the organic matter (OM) of a sandy soil were determined by means of particle-size fractionation and analysis of carbon and nitrogen contents. After 2 years, total soil carbon increased under organic fertilization but did not significantly change with solarization. As a consequence of the climatic conditions in the greenhouse, the carbon concentrations (g kg^–1 fraction) of the particle-size fractions were lower than those found for temperate soils and closer to those for tropical soils. The carbon amounts (g kg^–1 soil) and carbon:nitrogen ratios, which were highest in fractions >200 μm, reflected the short-term influence of the industrially processed organic amendment, rich in composted coarse plant debris. In contrast, the characteristics of the OM associated with each fraction were not significantly affected by solarization. In comparison with other coarse-textured temperate or tropical soils, carbon concentrations in fine silt (2–20 μm) and clay (0–2 μm) fractions were very low. This suggests a "greenhouse effect", together with a high rate of carbon mineralization affecting fine silt and clay fractions. Received: 19 November 1999     

长期添加外源有机物料对华北平原不同粒级土壤氮素和氨基糖的影响

华北平原是我国主要的粮食生产基地之一,农民为了追求高产,过量施用化肥的弊端日益凸显。本研究依托中国科学院栾城农业生态系统试验站有机养分循环再利用长期定位试验,开展不同外源有机物料对土壤氮素和氨基糖在不同粒级土壤库中分布的影响研究,为阐释不同农业管理措施下土壤氮素的物理保护机制和生物保护机制提供依据。定位试验设6个处理：无肥无秸秆处理（对照,CK）、单施猪圈肥（M）、单施化肥（NPK）、单施秸秆（SCK）、化肥配施猪圈肥（MNPK）和化肥配施秸秆（SNPK）。通过超声波分散-离心分离得到3种粒径土壤——砂粒级（2 000~53 μm）、粉粒级（53~2 μm）和黏粒级（<2 μm）,分析全土及各粒级土壤中全氮和3种土壤氨基糖（氨基葡萄糖、胞壁酸和氨基半乳糖）的含量及变化;基于这3种土壤氨基糖的稳定性和异源性,以氨基糖作为微生物残留物标识物,了解真菌和细菌残留物的积累和转化,阐释真菌和细菌在养分转化中的作用。结果表明：添加有机物料（秸秆、猪圈肥）明显提升了土壤全氮和氨基糖含量,粒级间土壤氮素和氨基糖含量顺序均为黏粒级>砂粒级>粉粒级。添加有机物料对砂粒级土壤氮素影响最大,长期化肥配施猪圈肥中氮素主要在砂粒级中富集,长期化肥配施秸秆的氮素主要在黏粒级中富集。添加秸秆主要提高了真菌来源的氨基葡萄糖的含量,而添加猪圈肥主要提高了土壤中细菌来源的胞壁酸含量,表明添加不同有机物料可影响土壤微生物的群落结构。从各粒级中氨基葡萄糖/胞壁酸的比值来看,添加不同类型外源有机物料对砂粒级土壤微生物群落结构影响最为明显。由此可见,在长期秸秆还田措施下实施有机粪肥部分替代化肥不仅可以减少化肥用量,还可提升土壤养分含量和微生物多样性,改善土壤质量。     

塿土表面铜吸附解吸的动力学特征及滞后效应

采用间歇法发现土娄土及不同处理土样对铜离子的吸附过程遵循二级反应动力学方程 ,吸附速率常数ka 呈 :土娄土 >去OM土娄土 >去Ca土娄土 >去Ca、OM土娄土 ;解吸过程符合两点位一级动力学方程 ,第一点位解吸速率常数kd1大于第二点位解吸速率常数kd2 ;各土样解吸速率常数的大小顺序与ka相反 ;吸附 解吸之间的滞后效应用ka/kd来衡量 ,滞后程度大小与ka大小顺序一致     

Influence of long-term fertilization with farmyard manure on soil organic matter: Characteristics of particle-size fractions

Summary The influence of more than 100 years of fertilization with farmyard manure on soil organic matter in comparison to unfertilized soil was studied in particle-size fractions using elemental (C and N) analyses and pyrolysis-field ionization mass spectrometry. Distinct differences in C and N concentrations and distribution and in the quality of organic matter between the size fractions and the fertilization treatments were observed. Clay-associated C and N were relatively higher in the unfertilized treatment, whereas the application of farmyard manure preferentially increased soil organic matter associated with the fine and medium silt fractions. Pyrolysis-field ionization mass spectrometry of soil fractions <20 m showed increasing values for lignin monomers and dimers and fatty acids with larger equivalent diameters, whereas the proportion of N compounds, mono- and polysaccharides and phenolics decreased in the larger size fractions. Sand fractions were particularly rich in lignin fragments, mono- and polysaccharides, and alkanes/alkenes. These relationships seemed to be independent of management practices. In the same size fractions of the different treatments, however, a higher relative abundance of N-compounds, mono- and polysaccharides, phenolics, lignin monomers, and alkanes/alkenes was observed in the unfertilized variant. Lignin dimers and fatty acids were more abundant in the farmyard manure treatment. Both trends together imply that soil enrichment in organic matter due to the application of farmyard manure largely reflects an increase in lignin building blocks and partly reflects an increase in lipids such as fatty acids in the silt fractions. Therefore these constituents are of particular importance in assessing the positive effects of farmyard manure on soil fertility.     

Response of the bacterial diversity and soil enzyme activity in particle-size fractions of Mollisol after different fertilization in a long-term experiment

Particle-size soils were fractionated for evaluating changes in the composition of bacterial community and enzyme activity in response to 13 years of fertilization. This study focused on Mollisol and its particle-size fractions of 200–2,000 μm (coarse sand sized), 63 to 200 μm (fine sand sized), 2 to 63 μm (silt sized), and 0.1 to 2 to μm (clay-sized). Long-term chemical fertilization lowered the pH of all particle fractions, whereas organic fertilizer application mitigated soil acidification. Nutrient concentrations depended on both fertilizer treatment and particle fractions and enzymes were unevenly active throughout the soil. Generally, the highest enzyme activities were observed in the silt and clay fractions of control soil and the soil treated with chemical fertilizer (N, P, and K (NPK)) and in the sand-sized fraction of soil treated with manure and chemical fertilizer (MNPK). Except for acid phosphomonoesterase, the other tested enzyme activities in coarse-sized fractions of MNPK soil were significantly higher than those of the control and NPK soils. Fertilization and soil fraction interactively (p?<?0.05) affected the enzyme activity. Denaturing gradient gel electrophoresis analysis showed that the bacterial community structure significantly differed in different particle sizes with a higher bacterial diversity in small-sized than in coarse-sized fractions. Dominant bands were excised and sequenced. We have found the following bacterial groups: Actinobacteria, γ-proteobacteria, and Acidobacteria. In addition, enrichment of organic matter in coarser fractions was related to greater bacterial diversity than any other treatment. Principal component analysis showed a smaller variability among fractions of the organic amended treatment. Redundancy analysis showed that the tested properties significantly affected the composition of bacterial community with the exception of C/N and available P. No significant correlation between enzyme activity and bacterial community composition was detected, whereas positive correlations between other soil properties and enzyme activities were observed to various extents. Probably, enzyme activities might be affected by specific functional bacterial communities rather than by the overall bacterial community. We concluded that the long-term application of organic manures contributed to the increase of soil organic matter content of particles higher than 200 mm, with higher bacterial diversity and increases in most of the enzyme activities.     

Natural nitrogen-15 abundance and carbohydrate content and composition of organic matter particle-size fractions from a sandy soil

Sandy soil samples collected from under a woody/grass savanna in the Lamto experimental area (6°13N, 5°20W; Côte dIvoire, West Africa), were fractionated according to particle size with the aim of measuring the natural abundance of ¹⁵N and determining the contents and composition of hydrolysable carbohydrates of soil organo-mineral particles for a better understanding of the contribution of each individual fraction to the soil function. The contributions of the fractions <20 m to the total pool of organic matter were 77% for C and 84% for N. Larger amounts of carbohydrates were found in the clay and silt fractions (3,784–6,043 g g^–1 soil). The carbohydrate composition indicated that microbe-derived carbohydrates [e.g. galactose (Gal) and mannose (Man)] accumulated preferentially in the fine fractions while plant-derived sugars [e.g. arabinose (Ara) and xylose (Xyl)] were dominant in coarse fractions. A negative relationship was observed between C:N ratio and ¹⁵N natural abundance on the one hand, and on the other hand between C:N and (Gal+Man):(Ara+Xyl), Man:(Ara+Xyl) and Man:Xyl ratios, clearly indicating that the chemistry of the organic materials of the particle-size fractions reflects a change from soil chemistry dominated by plant materials to that dominated by microbial biomass and metabolites. The contribution of a given fraction to soil microbial activity is controlled by the quality or quantity of associated soil organic matter, its microbial biomass and also by the accumulation of microbial-derived carbohydrates which can be resynthesized or recycled.     

Distribution of the particle-size fractions in soddy-podzolic soils subjected to sheet erosion

Tendencies in the formation of organomineral profiles of arable agrosoddy-podzolic soils on slopes in different denudation and accumulation zones were characterized on the basis of an integrated study of the humus status of the soils (using the granulodensimetric fractionation method), the content of clay, and the composition of clay minerals. It was shown that the organomineral and mineralogical parameters of the surface-horizon transformation could be used for improving the diagnostic accuracy of soils on the slopes basing on their conventional characteristics.     

Investigating sulfate sorption and desorption of acid forest soils with special consideration of soil structure

When investigating the reversibility of soil and water acidification due to a reduction of SO₄^2? deposition, the size and stability of the reversibly bound SO₄^2? fraction in soils are important parameters. The desorption behaviour of SO₄^2? in three acid forest soils was investigated using columns with undisturbed and disturbed (< 5 mm sieved) soil material. The results were compared to batch experiments. A comparison of the undisturbed and the disturbed soil samples showed that the soil structure had no effect on the chemistry of the soil solution, the S-mineralisation rates or the SO₄^2? desorption rates. A comparison of the batch and the column method showed only minor differences in desorption rates. However, fitting the measured desorption rates to a modified Langmuir equation showed a more distinct difference between both methods. It was concluded that the batch method was more suitable to establish SO₄^2? desorption isotherms. When investigating SO₄^2? dynamics of soils, the heterogeneity of the soils has to be considered because the spatial variability of isotherm parameters was found to be greater than differences between the investigated methods. Furthermore, SO₄^2? sorption showed a distinct hysteresis. While most of the sorbed SO₄^2? was desorbed at concentrations < 5–10 mg SO₄² ?1^?1, a sorption of SO₄^2? was observed only at concentrations > 20–30 mg SO₄^2? ·1^?1.     

源于松针的可溶性有机物对土壤多环芳烃吸附和解吸行为的影响研究

Study of the relationship between plant litter-derived dissolved organic matter（DOM） and organic pollutant transport in soil is important for understanding the role of forest litter carbon cycling in influencing pollutant behaviour and fate in forest soil.With the aim of providing insight into the capacity of plant litter-derived DOM to influence sorption and desorption of selected polycyclic aromatic hydrocarbons（PAHs） on soil, batch experiments were carried out with application of a sorption-desorption model incorporating DOM effects. Freshly fallen pine（Pinus elliottii） needles were used as the source of organic matter. Input of the pine needle litter-derived DOM was found to significantly decrease desorption hysteresis as well as soil adsorption capacity of phenanthrene（PHE） and fluoranthene（FLA）. Addition of 1 728 mg L-1dissolved organic carbon（DOC） lowered the organic carbon-normalized sorption distribution coefficient of PHE from 7 776 to 2 541 L kg-1C and of FLA from 11 503 to 4 368 L kg-1C. Decreases of the apparent sorption-desorption distribution coefficients of PHE and FLA with increased DOC concentration indicated that DOM favored desorption of PAHs from soil. Increases in the fraction of apparently dissolved PAHs were attributable to the dissolved PAH-DOM complexes, accounting for the dissolved proportions of 39% to 69% for PHE and 26% to 72% for FLA in the sorption and desorption processes as the concentration of the added DOM solution rose from 0 to 1 728 mg L-1. Our results suggest that pine needle litterderived DOM can have a substantial effect of inhibiting PAHs sorption and promoting PAHs desorption, thus leading to enhanced leaching in soil, which should be taken into account in risk assessment of PAHs accumulated in forest soil.     

The effect of soil type on phosphorus sorption capacity and desorption dynamics in Irish grassland soils

Abstract. The phosphorus (P) sorption and desorption dynamics of eleven major agricultural grassland soil types in Ireland were examined using laboratory techniques, so that soils vulnerable to P loss might be identified. Desorption of P from soil using the iron-oxide paper strip test (Pfeo), water extractable P (Pw) and calcium chloride extractable P (Pcacl₂) depended on soil P status in all soils. However, soil types with high organic matter levels (OM), namely peat soils (%OM >30), had lower Pfeo and Pw but higher Pcacl₂ values compared to mineral soils at similar soil test P levels. Phosphorus sorption capacity remaining (PSCr) was measured using a single addition of P to soils and used to calculate total P sorption capacities (PSCt) and degree of P saturation (DPS). Phosphorus sorption capacities correlated negatively with % OM in soils indicating that OM may inhibit P sorption from solution to soil. High organic matter soils exhibited low P sorption capacities and poor P reserves (total P, oxalate extractable P) compared to mineral soils. Low P sorption capacities (PSCt) in peat soils were attributed to OM, which blocked or eliminated sorption sites with organic acids, therefore, P remained in the soil solution phase (Pcacl₂). In this work, peat and high organic matter soils exhibited P sorption and desorption characteristics which suggest that these soils may not be suitable for heavy applications of manure or fertilizer P owing to their low capacities for P sorption and storage.