Influence of prolonged arable cropping on lignin compounds in sandy soils of the South African Highveld

The preservation of plant residues is important for sustainable arable cropping. Lignin is a marker for plant residues in soils. We have investigated influences of the length of cultivation on the dynamics of lignin. Composite samples were taken from the top 20 cm of soils that have been cropped for periods varying from 0 to 98 years in each of three different agro‐ecosystems in the Free State Province of South Africa. Lignin‐derived phenols were determined in the <2 µm (clay), 2–20 µm (silt), 20–250 µm (fine sand) and 250– 2000 µm (coarse sand) size separates. With increasing length of cultivation, the concentration of such phenols decreased to 36% of that in the grassland. The lignin contents as proportions of the total carbon did not change during cultivation, suggesting that there was no selective enrichment of lignin moieties as C was lost as a result of cultivation. The loss rate constants of lignin concentrations in particle‐size fractions increased in the order clay (0.17 year^?1) ≤ silt (0.18 year^?1) < fine sand (0.20 year^?1) < coarse sand (0.22 year^?1). Increasing ratios of phenolic acids to aldehydes in bulk soil, silt and fine sand fractions with increasing length of cultivation indicated that side chains were being oxidized. The ratios in the silt fraction, however, decreased after 10–20 years. We attribute this to a loss of lignin together with silt by wind erosion, resulting in a rejuvenation of lignin compounds in the remaining silt‐sized pools of C.     

Characteristics of Water Infiltration in Layered Water-Repellent Soils

Water-repellent(WR) soil greatly influences infiltration behavior. This research determined the impacts of WR levels of silt loam soil layer during infiltration. Three column scenarios were utilized, including homogeneous wettable silt loam or sand, silt loam over sand(silt loam/sand), and sand over silt loam(sand/silt loam). A 5-cm thick silt loam soil layer was placed either at the soil surface or 5 cm below the soil surface. The silt loam soil used had been treated to produce different WR levels, wettable, slightly WR, strongly WR, and severely WR. As the WR level increased from wettable to severely WR, the cumulative infiltration decreased. Traditional wetting front-related equations did not adequately describe the infiltration rate and time relationships for layered WR soils. The Kostiakov equation provided a good fit for the first infiltration stage. Average infiltration rates for wettable, slightly WR, strongly WR, and severely WR during the 2 nd infiltration stage were 0.126, 0.021, 0.002, and 0.001 mm min~(-1) for the silt loam/sand scenario,respectively, and 0.112, 0.003, 0.002, and 0.000 5 mm min~(-1) for the sand/silt loam scenario, respectively. Pseudo-saturation phenomena occurred when visually examining the wetting fronts and from the apparent changes in water content(?θ_(AP)) at the slightly WR,strongly WR, and severely WR levels for the silt loam/sand scenario. Much larger ?θAPvalues indicated the possible existence of finger flow. Delayed water penetration into the surface soil for the strongly WR level in the silt loam/sand scenario suggested negative water heads with infiltration times longer than 10 min. The silt loam/sand soil layers produced sharp transition zones of water content. The WR level of the silt loam soil layer had greater effects on infiltration than the layer position in the column.     

Nitrogen mineralization in relation to site history and soil properties for a range of Australian forest soils

Rates of N mineralization were measured in 27 forest soils encompassing a wide range of forest types and management treatments in south-east Australia. Undisturbed soil columns were incubated at 20°C for 68 days at near field-capacity water content, and N mineralization was measured in 5-cm depth increments to 30 cm. The soils represented three primary profile forms: gradational, uniform and duplex. They were sampled beneath mature native Eucalyptus sp. forest and from plantations of Pinus radiata of varying age (<1 to 37 years). Several sites had been fertilized, irrigated, or intercropped with lupins. The soils ranged greatly in total soil N concentrations, C:N ratios, total P, and sand, silt, and clay contents. Net N mineralization for individual soil profiles (0–30 cm depth) varied from 2.0 to 66.6 kg ha^-1 over 68 days, with soils from individual depths mineralizing from <0 (immobilization) to 19.3 kg ha^-1 per 5 cm soil depth. Only 0.1–3.1% of the total N present at 0–30 cm in depth was mineralized during the incubation, and both the amount and the percentage of total N mineralized decreased with increasing soil depth. N fertilization, addition of slash residues, or intercropping with lupins in the years prior to sampling increased N mineralization. Several years of irrigation of a sandy soil reduced levels of total N and C, and lowered rates of N mineralization. Considuring all soil depths, the simple linear correlations between soil parameters (C, N, P, C:N, C:P, N:P, coarse sand, fine sand, silt, clay) and N mineralization rates were generally low (r<0.53), but these improved for total N (r=0.82) and organic C (r=0.79) when the soils were grouped into primary profile forms. Prediction of field N-mineralization rates was complicated by the poor correlations between soil properties and N mineralization, and temporal changes in the pools of labile organic-N substrates in the field.     

Sand mineralogy and related properties of some soils in South‐Western Nigeria

Abstract

Five representative soil profiles were excavated along a toposequence selected in the Itagunmodi area of South‐Western Nigeria. The soils were subjected to physical, chemical, and mineralogical analyses. The results indicated soils with high fine sand and clay contents, but low silt content. The soils were found to vary from slightly acid to strongly acid (pHH₂O = 4.0 to 6.2). Organic C, available P, and Kjeldahl N contents decreased with increasing depth. Cation exchange capacity (CEC) ranged from 3.11 to 28.75 cmol(+)/kg soil. Base saturation was low (<51%). From a total elemental analysis, Si was found to be the dominant element, followed by Al, and then Fe. Extractable P, and exchangeable K, Mg, and Ca were quite low. The dominant minerals in the fine sand fraction were quartz, feldspar, zircon, hornblende, tourmaline, and opaque ores. The variation in the zircon/tourmaline (two resistant minerals) ratios with depth suggests a stratification of the parent material. The change in the quartz/feldspar ratios was an indication that the degree of weathering in the soil profile is not uniform.     

有机肥对棕壤不同粒级有机碳和氮的影响

采集棕壤长期肥料定位试验站不施肥和施用不同用量有机肥的土壤,通过超声波分散—离心分离得到细黏粒(<0.2μm)、粗黏粒(0.2～2μm)、粉粒(2～53μm)、细砂粒(53～250μm)和粗砂粒(250～2000μm)5个颗粒级别后,分析全土及不同粒级中土壤有机碳和氮并进行含量与分布的比较。结果表明,有机质主要分布于黏粒级中,其含量占全土有机碳的42.8%、全氮的58.3%,碳氮比随着粒级的增加而逐渐增大,表明氮易于在小粒级中富集。长期施用有机肥后,全土及各粒级有机碳和氮含量均有显著增加;砂粒级中有机碳和氮的富集系数升高,黏粒级中富集系数降低,粉粒级和砂粒级中的碳氮比降低。增加有机肥的用量加强了全土和各粒级对有机碳和氮的积累,同时加强了粉粒级和砂粒级碳氮比降低的程度。     

Does Particulate Organic Matter Fraction Meet the Criteria for a Model Soil Organic Matter Pool?

There is a well-recognized need for improved fractionation methods to partition soil organic matter into functional pools. Physical separation based on particle size is widely used, yielding particulate organic matter(POM, i.e., free or "uncomplexed" organic matter 50 μm) as the most labile fraction. To evaluate whether POM meets criteria for an ideal model pool, we examined whether it is:1) unique, i.e., found only in the 50 μm fraction and 2) homogeneous, rather than a composite of different subfractions. Following ultrasonic dispersion, sand( 50 μm) along with coarse(20–50 μm) and fine(5–20 μm) silt fractions were isolated from a silt loam soil under long-term pasture at Lincoln, New Zealand. The sand and silt fractions contained 20% and 21% of total soil C, respectively.We adopted a sequential density separation procedure using sodium polytungstate with density increasing step-wise from 1.7 to 2.4 g cm~(-3) to recover organic matter(light fractions) from the sand and silt fractions. Almost all(ca. 90%) the organic matter in the sand fraction and a large proportion(ca. 60%–70%) in the silt fractions was recovered by sequential density separation. The results suggested that POM is a composite of organo-mineral complexes with varying proportions of organic and mineral materials. Part of the organic matter associated with the silt fractions shared features in common with POM. In a laboratory bio-assay, biodegradability of POM varied depending on land use(pasture arable cropping). We concluded that POM is neither homogeneous nor unique.     

砒砂岩区典型坡面土壤颗粒组成空间分布特征研究

为揭示土壤颗粒组成在垂直和水平方向上的分布规律,采集砒砂岩区典型坡面0—100 cm剖面的土壤,运用经典统计学测定其颗粒组成。结果表明：坡面0—100 cm土壤质地主要为砂壤土(59.21%)和壤砂土(36.40%)。随土层深度增加,砂粒含量增加,粉粒和黏粒含量减少,土壤逐渐呈现粗粒化的趋势;坡面尺度上,表层0—10 cm砂粒为弱变异,其余各层各粒级均为中等程度变异,且随土层深度增加变异性增强,不同粒级的变异系数表现为砂粒<粉粒<黏粒。沿坡面等高线方向,样带B粗粒化程度最弱,10 cm土层以下,样带B粗粒化程度最强。沿垂直坡面等高线方向,坡中砂粒含量(73.60%)相比于坡上和坡下分别增加6.90%(p<0.05)和11.66%(p<0.05),坡下粉粒(31.85%)和黏粒(3.10%)含量相比于坡上和坡中分别增加13.13%(p<0.05),23.59%(p<0.05)和4.36%,51.70%(p<0.05),坡中粗粒化程度最强,坡下是细颗粒堆积的主要区域。研究结果可加强对砒砂岩区坡面土壤颗粒空间分布规律的认识。     

Effects of animal manure and mineral fertilizer on the total carbon and nitrogen contents of soil size fractions

Summary Soil was sampled in autumn 1984 in the 132 field (sandy loam soil) of the Askov long-term experiments (started in 1894) and fractionated according to particle size using ultrasonic dispersion and sedimentation in water. The unmanured plot and plots given equivalent amounts of N (1923–1984 annual average, 121 kg N/ha) in either animal manure or mineral fertilizer were sampled to a depth of 15 cm, fractionated and analysed for C and N. Mineral fertilizer and animal manure increased the C and N content of whole soil, clay (<2 m) and silt (2–20 m) size fractions relative to unmanured samples, while the C content of the sand size fractions (fine sand 1, 20–63 m; fine sand 2, 63–200 m; coarse sand, 200–2000 m) was less affected. Clay contained 58% and 65°70 of the soil C and N, respectively. Corresponding values for silt were 30% and 26%, while sand accounted for 10% of the soil C. Fertilization did not influence this distribution pattern. The C : N ratio of the silt organic matter (14.3) was higher and that of clay (10.6) lower than whole-soil C:N ratios (12.0). Fertilization did not influence clay and silt C : N ratios. Animal manure caused similar relative increases in the organic matter content of clay and silt size fractions (36%). In contrast, mineral fertilizer only increased the organic matter content of silt by 21% and that of clay by 14%.     

Nitrogen in particle size fractions of soils incubated for five years with 15N-ammonium and 14C-hemicellulose

Four soils with a range of clay and silt contents were incubated for 5 a with ¹⁵N-labelled (NH₄)SO₄ and ¹⁴C-labelled hemicellulose and then fractionated according to particle size by ultrasonic dispersion and sedimentation. The distribution of labelled and native N between clay, silt and sand fractions was determined and elated to previous results on the C distributions. Between 29% and 48% of the added N was found in organic form. The ¹⁵N atom percentage excess decreased in the order: clay > whole soil > silt > sand. For both clay and silt, the enrichment factor for labelled and native N decreased with increasing fraction weight. Clay enrichment was higher for labelled than for native N, the converse being true for silt. The distribution of whole soil labelled organic N was: clay 77–91%, silt 4–11%, and sand <0.5%. Corresponding values for native N were 69–74%, 16–22%, and 1–2%, respectively. All soils had higher proportions of labelled than of native N in the clay, the converse was true for the silt. The C/N ratio of the native silt organic matter was higher and that of clay organic matter lower than whole soil C/N ratios. Differences between the C/N ratio distributions of native and labelled organic matter were small. The relative distribution of labelled N and C was very similar confirming that the turnover of C and N in soil organic matter is closely interrelated.     

流动沙丘造林对土壤物理组分有机碳分配的影响

(1)根据中国知网(CNKI)的《中国学术期刊影响因子年报(自然科学与工程技术.2010版)计量指标统计表》,《水土保持通报》综合统计源统计的总被引频次为3 446次(2009年版中为1 358次);复合影响因子为0.955;期刊综合影响因子为0.568(2008年为0.493),在所统计     

Lasting influence of biochemically contrasting organic inputs on abundance and community structure of total and proteolytic bacteria in tropical soils

The SOM field experiments in Kenya, which have been initiated in 2002 on two contrasting soils (clayey Humic Nitisol (sand: 17%; silt: 18%; clay: 65%) at Embu, sandy Ferric Alisol (sand: 66%; silt: 11%; clay: 22%) at Machanga), were used for exploring the effect of nine year annual application of biochemically contrasting organic inputs (i.e., Zea mays (ZM; C/N ratio: 59; (lignin + polyphenols)-to-N ratio: 9.8); Tithonia diversifolia (TD; 13; 3.5); Calliandra calothyrsus (CC; 13; 6.7)) on the soil bacterial decomposer community. Soil samples were taken at the onset of the rainy season before application of fresh organic inputs in March 2011. We studied the abundance (quantitative PCR) and community structure (T-RFLP analysis) of the total (i.e., 16S rRNA gene) and specifically proteolytic (i.e., npr gene encoding neutral metalloproteases) bacteria. Alterations of the soil microbial decomposer community were related to differences of quantity (i.e., soil carbon (TC)) and particularly composition of SOC, where mid-infrared spectroscopic (DRIFTS) information, and contents of extractable soil polyphenol (PP) and the newly introduced PP-to-TC ratio served as SOC quality indicators. For total bacteria, effect of organic input quality was minor in comparison to the predominant influence of soil texture. Elevated soil PP content, driven by polypheneol rich organic inputs, was not suppressive for overall bacterial proliferation, unless additional decomposable C substrates were available as indicated by PP-to-TC ratios. In contrast to the total bacterial community, biochemical quality of organic inputs exposed a stronger effect on functionally specialized bacterial decomposers, i.e., proteolytic bacteria. The npr gene abundance was depressed in the TD treated soils as opposed to soils receiving CC, and showed a positive correlation with soil PP. It was suggested that the high presence of lignin and polyphenol relative to the N content in organic inputs was increasing the npr gene abundance to counteract most likely the existence of polyphenol–protein complexes aggravating protein degradation. We concluded from our study that integration of spectroscopic, geochemical (i.e., soil PP) and molecular soil data provides a novel pathway to enhance our understanding of the lasting effect of organic input quality induced SOC quality changes on bacterial decomposers and particularly proteolytic bacteria driving soil organic N cycling.     

黄土丘陵区典型植被土壤剖面的颗粒分形特征

运用分形理论研究陕北安塞五里湾流域5种典型植被0～200 cm土壤剖面土壤颗粒大小分布(particle size distribution,PSD)及其体积分形维数分布特征,并进一步分析土壤PSD的分形维数与土壤有机碳、全氮和含水量的相关性。结果表明:研究区典型植被群落土壤颗粒组成主要为细颗粒(黏粒和粉粒),其中粉粒体积分数占总颗粒的56.82%～71.99%;铁杆蒿草地的细颗粒平均体积分数最大(78.86%),乔木林的最小(65.77%)。5种典型植被群落土壤PSD的体积分形维数介于2.498～2.599,均表现出随着土层深度的增加呈递增趋势;相同土层深度的分形维数呈现出铁杆蒿草地黄芪草地农田灌木林乔木林的趋势,灌木林和农田间差异不显著,其他植被群落间差异显著。典型植被土壤PSD的体积分形维数与黏粒、粉粒的体积分数和含水量呈极显著正相关(P0.01),但与砂粒的体积分数呈极显著负相关(P 0.01),与有机碳含量呈显著负相关(P 0.05)。     

河北坝上低山区坡地灌丛沙堆表面土壤理化性质

[目的]揭示河北省坝上地区低山坡地不同坡向和坡位小叶锦鸡灌丛沙堆土壤理化性质变化规律,为坡地草地生态环境稳定提供理论依据。[方法]基于对山坡638个灌丛沙堆野外测量和86个室内土样分析的数据,运用地统计学和样品室内分析的方法。[结果](1)从低山坡脚到坡顶,灌丛沙堆pH值、粗砂、中砂和细砂总含量逐渐减少,粉砂、极细砂逐渐增加;有机质、全氮和全磷含量逐渐增加。(2)迎风坡不同坡位上灌丛沙堆表面颗粒中粗砂、中砂、细砂总含量大于背风坡相同坡位,极细砂和粉砂小于背风坡相同坡位。表面的土壤养分(有机质、全氮和全磷)为迎风坡坡脚大于背风坡坡脚,迎风坡坡中和坡顶的灌丛沙堆土壤养分含量小于背风坡坡中和坡顶灌丛沙堆,但是pH值变化与灌丛沙堆养分变化趋势相反。[结论]受坡位和坡向的影响,灌丛沙堆土壤理化性质空间分布具有一定的差异性。     

The effect of cultivation on sediment composition and deposition in prairie pothole wetlands

Texture, major nutrient content, and deposition rate of sediments were compared for five prairie pothole wetlands surrounded by native grassland and seven otherwise similar wetlands surrounded by row crop and small grain farmland. Specific differences in the nature of the sedimentation cycle of cultivated and noncultivated watersheds were indicated. Flux of total inorganic material into sediments averaged 80 and 43 mg cm^?2 yr^?1 in cultivated and grassland wetlands, respectively. Cultivated sediments contained significantly higher clay percentages, but lower percentages of silt and sand than grassland sediments. Deposition rates of clay at cultivated sites averaged five times that of grassland locations. Enrichment ratios (the quotient of sediment concentration divided by upland soil concentrations) suggested that sand was selectively retained in equal proportions on uplands in both types of watersheds, that silt was selectively removed (although in different proportions) from uplands in both types of watersheds, and that clay was selectively retained only on grasslands. Total N and organic matter concentrations were significantly higher in both the soils and sediments of grassland watersheds, but there were no differences in total P concentrations with respect to land use. Sediment flux rates for total N and organic matter were similar in the two land use types; however, P was transported at nearly twice the rate to cultivated wetlands. Enrichment ratios indicated that N and P were selectively removed in similar proportions from upland soils in both types of watersheds.     

Effects of silicate weathering and lessivage on K‐content in forest soils derived from Pleistocene sediments

The potassium (K) content of soils developed from Pleistocene calcareous till, glacial sand and loess in NW Germany was investigated in order to characterize stores of K in feldspars (K_feldspar) and mica/illite (K_mica/illite) as well as changes as a function of soil depth. From each horizon, up to seven sand, six silt and three clay fractions were separated. K_feldspar and K_mica/illite were quantified by means of chemical composition and estimation by IR‐spectroscopy. On account of distinct differences in mineralogical composition between different particle size fractions, K‐content of the bulk soil < 2000 μm and the proportion of K_mica/illite and K_feldspar are clearly related to grain size distribution of the sample. Generally, the K‐content of particle size fractions of a soil derived from calcareous till is significantly higher than that of a soil from glacial sands. K_mica/illite of clay and silt fractions increases with depth, reflecting greater mica/illite weathering at the soil surface, whereas K_feldspar shows no noticeable change. Illite accumulates by lessivage in Bt horizons. On a whole‐soil basis, the Bt horizons of Luvisols derived from loess and calcareous till contain more K_mica/illite than either the A or the C horizons. By comparing the K‐content in the different particle size fractions with soil depth, the highest rate of change is found for soils derived from glacial sand. Gains in K in the silt fractions of soils from calcareous till and glacial sand result from weathering of feldspar sand grains. Additionally, decomposition of feldspar‐containing rock fragments of gravel size, and aeolian sedimentation, may also have contributed to these gains.     

Straw incorporation and soil organic matter in macro-aggregates and particle size separates

Two field experiments in which straw has been removed or incorporated for 17 yr (loamy sand) and 10 yr (sandy clay loam) were sampled to examine the effect of straw on the C and N contents in whole soil samples, macro-aggregate fractions and primary particle-size separates. The particle size composition of the aggregate fractions was determined. Aggregates were isolated by dry sieving. Straw incorporation increased the number of 1–20 mm aggregates in the loamy sand but no effect was noted in the sandy clay loam. Straw had no effect on the particle size composition of the various aggregate fractions. After correction for loose sand that accumulated in the aggregate fractions during dry sieving, macro-aggregates appeared to be enriched in clay and silt compared with whole soil samples. Because of the possible detachment of sand particles from the exterior surface of aggregates during sieving operations, it was inferred that the particle size composition of macro-aggregates is similar to that of the bulk soil. The organic matter contents of the aggregate fractions were closely correlated with their clay + silt contents. Differences in the organic matter content of clay isolated from whole soil samples and aggregate fractions were generally small. This was also true for the silt-size separates. In both soils, straw incorporation increased the organic matter content of nearly all clay and silt separates; for silt this was generally twice that observed for clay. The amounts of soil C, derived from straw, left in the loamy sand and sandy clay loam at the time of sampling were 4.4 and 4.5 t ha^?1, corresponding to 12 and 21% of the straw C added. The C/N ratios of the straw-derived soil organic matter were 11 and 12 for the loamy sand and sandy clay loam, respectively.     

Mulching effects on phosphorus and sulfur concentrations in a Miamian soil in central Ohio,USA

In addition to nitrogen (N), phosphorus (P) and sulfur (S) elements are also essential to conversion of biomass carbon into soil humus. Therefore, soil analyses were done on two long‐term mulching experiments initiated in 1989 and 1996 on a Crosby silt loam (Aeric Ochraqualf or Stagnic Luvisol) soil in central Ohio to assess P and S dynamics in soil for different rates of mulching. Mulch treatments were 0, 8 and 16 Mg ha⁻¹ y⁻¹ without any crop cultivation. Our objectives were to assess: (i) the effect of different mulch rates on P and S concentrations, and soil organic carbon sequestration; (ii) association of available and total P and S with different particle size fractions; and (iii) temporal changes in available and total P and total S concentrations within aggregate and particle size fractions with duration of mulching. Soil samples from 0 to 5 and 5 to 10 cm depths were obtained in November 2000. Mulch rate significantly increased Bray‐P in 0 to 5 and 5 to 10 cm depths but had no significant effect on total P after 4 years of mulching. Total P concentration in the 5 to 10 cm layer increased significantly with mulch application after 11 years, but the total S concentration was not affected. Total P in aggregates>2 mm size at 5 to 10 cm depth was significantly higher than whole soil after 11 years of mulching. More than 50 per cent of the total P was associated with clay fraction, and P concentration increased with duration of mulching. The enrichment factor for total P was in the order: clay>sand>silt. Total S concentration in aggregates increased with increase in aggregate size for both depths, and was in the order: clay>sand>silt. The clay fraction accounted for 48 per cent of total S after 4 years of mulching and 50 per cent after 11 years of mulching. The enrichment factor of S in clay and sand fractions increased with duration of mulching and with depth for clay, and decreased for sand. The C:P and C:S ratios decreased both with duration of mulching and particle size. Availability of P and S is essential for humification of carbon input in crop residue mulch. Copyright © 2004 John Wiley & Sons, Ltd.     

Long-term changes in organic matter of woodland soils cleared for arable cropping in Zimbabwe

Subsistence farmers in Africa depend largely on the soil organic matter to sustain crop productivity. Long-term changes in soil organic carbon and nitrogen were measured after woodland clearance for smallholder subsistence farming or for commercial farming. The contents of organic carbon and nitrogen in soil under reference woodlands were largest (53.3 t C ha⁻¹, 4.88 t N ha⁻¹) in a red clay soil (∼ 50% clay + silt), followed by a granitic sand (∼ 12% clay + silt; 22.8 t C ha⁻¹, 1.47 t N ha⁻¹) and least (19.5 t C ha⁻¹, 0.88 t N ha⁻¹) in a Kalahari sand (∼ 5% clay + silt). Organic carbon declined rapidly under cultivation to attain new equilibria within 10 years on all smallholdings. Greatest losses occurred in soils that initially contained most carbon and nitrogen in the order: red clay (22.4 t C ha⁻¹ and 1.0 t N ha⁻¹) > granitic sand (13.2 t C ha⁻¹ and 0.8 t N ha⁻¹) > Kalahari sand (10.6 t C ha⁻¹ and 0.5 t N ha⁻¹). On the clay soil, commercial farming with intensive use of mineral fertilizers and incorporation of maize stover led to more gradual decline: at equilibrium, contents of carbon and nitrogen were 15 t C ha⁻¹ and 1.7 t N ha⁻¹ greater than on smallholdings with similar soil and climate. In the Kalahari sand the δ¹³C of organic C remained constant after woodland clearance, and maize contributed less than 10% of the total C even after 55 years. The δ¹³C signature increased slightly with increasing duration of cultivation by smallholders in the granitic sands and red clay soil where maize contributed 29% and 35% of the C at equilibrium. Under more productive commercial farming, the carbon derived from maize accounted for 50% of the total after 10 years of cultivation and 67% at equilibrium. The persistence of woodland carbon in the sandy soil is attributed to chemical stabilization resulting from large concentrations of lignin and polyphenols in the tree litter, or as charcoal.     

Improving on the Prediction of Cation Exchange Capacity for Highly Weathered and Structurally Contrasting Tropical Soils from Their Fine-Earth Fractions

The roles of fine-earth materials in the cation exchange capacity (CEC) of especially homogenous units of the kaolinitic and oxyhydroxidic tropical soils are still unclear. The CEC (pH 7) of some coarse-textured soils from southeastern Nigeria were related to their total sand, coarse sand (CS), fine sand (FS), silt, clay, and organic-matter (OM) contents before and after partitioning the dataset into topsoils and subsoils and into very-low-, low-, and moderate-/high-stability soils. The soil-layer categories showed similar CEC values; the stability categories did not. The CEC increased with decreasing CS but with increasing FS. Silt correlated negatively with the CEC, except in the moderate- to high-stability soils. Conversely, clay and OM generally impacted positively on the CEC. The best-fitting linear CEC function (R², 68%) was attained with FS, clay, and OM with relative contributions of 26, 38, and 36%, respectively. However, more reliable models were attained after partitioning by soil layer (R², 71–76%) and by soil stability (R², 81–86%). Notably FS's contribution to CEC increased while clay's decreased with increasing soil stability. Clay alone satisfactorily modeled the CEC for the very-low-stability soils, whereas silt contributed more than OM to the CEC of the moderate- to high-stability soils. These results provide new evidence about the cation exchange behavior of FS, silt, and clay in structurally contrasting tropical soils.     

Heavy Metals and Polycyclic Aromatic Hydrocarbons (PAHs) in a Rural Community Leewards of a Waste Incineration Plant

In a rural community (Stephanskirchen, Southern Germany) near a waste incineration plant 7 soils, sewage sludge, waste incineration residues, the gutter sediment of a family home, and mosses were sampled to determine the total concentrations of Cd, Pb, Zn and 20 PAHs. Representative samples were used to measure NH₄NO₃- and EDTA-extractable Cd, Pb, and Zn as well as 20 PAHs in particle size separates (clay, silt, fine and coarse sand). Sites near the main road, hill top, and forested sites contain up to 1.24 mg Cd, 888 mg Pb, and 279 mg Zn per kg. The heavy metal concentrations of the sewage sludge, the gutter sediment, and especially the waste incineration residues are extremely high (up to 57 mg Cd, 3300 mg Pb, and 5700 mg Zn per kg). The extractability of Pb and Zn with NH₄NO₃ is low (< 5%), that with EDTA is high (up to 71.2% of total Cd, 82.5% of total Pb, and 47.2% of total Zn). The sum concentrations of PAHs range between 0.4 and 470 mg kg^?1. The silt has the highest PAH concentrations of the particle size separates. High saturation of organic matter with PAHs in the sand indicates high recent PAH deposition. Selected ratios of single PAHs reveal diesel and gasoline exhausts as main sources for PAH. Principal component and cluster analysis show that the pollutant pattern depends on the C_org concentration and on the time passed since deposition. There is no significant influence of the waste incineration emissions on the heavy metal and PAH concentrations.