Soil Surface Roughness as the Result of Aggregate Size Distribution 1. Report: Measuring and Evaluation Method

It is assumed that the soil surface roughness is related to single aggregate size classes and the evaluation procedure was developed. If is based on the high resolution measurements with a noncontact laser scanner. Single harmonic ozcillations corresponding to aggregate size classes are filtered out from the surface profile using the Fourier analysis. By means of two examples, the advantage of this method over the simple profile length ratio is demonstrated.     

Clay mineralogy differs qualitatively in aggregate‐size classes: clay‐mineral‐based evidence for aggregate hierarchy in temperate soils

Clay minerals have a major role in soil aggregation because of their large specific surface area and surface charges, which stimulate interactions with other mineral particles and organic matter. Soils usually contain a mixture of clay minerals with contrasting surface properties. Although these differences should result in different abilities of clay minerals regarding aggregate formation and stabilization, the role of different clay minerals in aggregation has been seldom evaluated. In this study, we took advantage of the intrinsic mineral heterogeneity of a temperate Luvisol to compare the role of clay minerals in aggregation. First, grassland and tilled soil samples were separated in water into aggregate‐size classes based on the aggregate hierarchy model. Then, clay mineralogy and organic C in the aggregate‐size classes were analysed. Interstratified minerals containing swelling phases accumulated in aggregated fractions compared with free clay fractions under the two land‐uses. The accumulation increased with decreasing aggregate size from large macroaggregates (> 500 µm) to microaggregates (50–250 µm). Carbon content and carbon‐to‐nitrogen ratio followed the opposite trend. This fully supports the aggregate hierarchy model, which postulates an increasing importance of mineral reactivity in smaller aggregates than in larger aggregates in which the cohesion relies mostly on physical enmeshment by fungal hyphae or small roots. Consequently, differences in the proportion of the different 2:1 clay minerals in soils can influence their structure development. Further research on the links between clay mineralogy and aggregation can improve our understanding of mechanisms of soil resistance to erosion and organic matter stabilization.     

Eubacterial communities in different soil macroaggregate environments and cropping systems

Different positions within soil macroaggregates, and macroaggregates of different sizes, have different chemical and physical properties which could affect microbial growth and interactions among taxa. The hypothesis that these soil aggregate fractions contain different eubacterial communities was tested using terminal restriction fragment length polymorphism (T-RFLP) of the 16S ribosomal gene. Communities were characterized from two field experiments, located at the Kellogg Biological Station (KBS), MI, USA and the Ohio Agricultural Research and Development Center (OARDC), Wooster, OH, USA. Three soil management regimes at each site were sampled and management was found to significantly affect T-RFLP profiles. The soil aggregate erosion (SAE) method was used to isolate aggregate regions (external and internal regions). Differences between eubacterial T-RFLP profiles of aggregate exteriors and interiors were marginally significant at KBS (accounting for 12.5% of total profile variance), and not significant at OARDC. There were no significant differences among macroaggregate size classes at either site. These results are in general agreement with previous studies using molecular methods to examine microbial communities among different soil macroaggregate size fractions, although further study of communities within different aggregate regions is warranted. Analysis of individual macroaggregates revealed large inter-aggregate variability in community structure. Hence the tertiary components of soil structure, e.g. arrangement of aggregates in relation to shoot residue, roots, macropores, etc., may be more important than aggregate size or intra-aggregate regions in the determination of the types of microbial communities present in aggregates. Direct microscopic counts were also used to examine the bacterial population size in aggregate regions at KBS. The proportion of bacterial cells with biovolumes >0.18 μm³ was higher in aggregate interiors than in exteriors, indicating potentially higher activity in that environment. This proportion was significantly related to percent C of the samples, while total bacterial cell counts were not.     

Prediction of carbon mineralization rates from different soil physical fractions using diffuse reflectance spectroscopy

Soil carbon (C) mineralization rate is a key indicator of soil functional capacity but it is time consuming to measure using conventional laboratory incubation methods. Recent studies have demonstrated the ability of visible-near infrared spectroscopy (NIRS) for rapid non-destructive determination of soil organic carbon (SOC) and nitrogen (N) concentration. We investigated whether NIRS (350-2500 nm) can predict C mineralization rates in physically fractionated soil aggregates (bulk soil and 6 size fractions, n=108) and free organic matter (2 size fractions, n=27) in aerobically incubated samples from a clayey soil (Ferralsol) and a sandy soil (Arenosol). Incubation reference values were calibrated to first derivative reflectance spectra using partial least-squares regression. Prediction accuracy was assessed by comparing laboratory reference values with NIRS values predicted using full hold-out-one cross-validation. Cross-validated prediction for C respired (500 days) in soil aggregate fractions had an R² of 0.82 while that of C mineralized (300 days) in organic matter fractions was 0.71. Major soil aggregate fractions could be perfectly spectrally discriminated using a 50% random holdout validation sample. NIRS is a promising technique for rapid characterization of potential C mineralization in soils and aggregate fractions. Further work should test the robustness of NIRS prediction of mineralization rates of aggregate fractions across a wide range of soils and spectral mixture models for predicting mass fractions of aggregate size classes.     

贺兰山不同海拔植被下土壤团聚体分布及其稳定性研究

以贺兰山不同海拔植被下0—20,20—40 cm土层土壤为研究对象,分析不同粒径团聚体含量及团聚体稳定性随海拔升高的变化特征,探讨其与土壤理化性质之间的相关关系。结果表明:0—20 cm土层,0.25~0.053 mm团聚体为主要团聚体类型,随海拔增加,水稳性大团聚体(>0.25 mm)含量增加,<0.053 mm团聚体含量减少,表明土壤团聚体随海拔增加呈现由小粒径向大团聚体转变的趋势。20—40 cm土层,水稳性大团聚体含量在中海拔(2139 m)达到最高,占比为65.73%。平均重量直径(MWD)和几何平均直径(GMD)在0—20,20—40 cm土层均呈现随海拔升高先增加后减小的趋势,并在2139 m处达到峰值。不同海拔梯度土壤团聚体MWD和GMD与土壤有机碳(SOC)、全氮(TN)、全磷(TP)粉粒以及砂粒含量呈正相关,与黏粒含量、pH呈负相关。贺兰山不同海拔植被下土壤团聚体稳定性总体表现为中海拔>高海拔>低海拔,较高含量的大团聚体和土壤养分是团聚体稳定的关键要素。     

川南坡地不同退耕模式对土壤腐殖质及团聚体碳和氮的影响

通过对川南坡地进行退耕试验,研究坡地退耕成慈竹林、杂交竹林、桤木+慈竹林和弃耕地对土壤腐殖质及团聚体碳和氮的影响。结果表明,坡地退耕5年后土壤腐殖质(胡敏酸、富里酸和胡敏素)、活性腐殖质(活性胡敏酸和活性富里酸)及团聚体碳和氮含量、胡敏酸与富里酸比值和胡敏酸E4/E6值,以及可浸提腐殖质(胡敏酸和富里酸)、活性腐殖质及>0.25mm各粒径团聚体碳和氮分配比例均增加,并呈现出慈竹林>杂交竹林>桤木+慈竹林>弃耕地>农耕地的变化规律。土壤团聚体有机碳(氮)含量及其分配比例随土壤团聚体粒径的增加呈现出"V"形变化,其最小值分别出现在2～1mm和0.5～0.25mm粒径。说明川南坡地退耕对增加土壤腐殖质及团聚体碳和氮含量、改善土壤肥力状况和促进土壤碳固定具有重要的作用和意义。     

Phosphorus forms as affected by abandoned anthills (Formica polyctena Förster) in forest soils: sequential extraction and liquid-state 31P-NMR spectroscopy

Ants are known to concentrate phosphorus (P) inside their nests via collection of food and litter. To elucidate the possible effects on long-term availability of soil P, five anthills abandoned by Red wood ant (Formica polyctena Förster) > 5—20 years ago were characterized for soil P forms in a temperate Danish deciduous forest. Sequentially extracted P fractions and liquid-state ³¹P nuclear magnetic resonance (NMR) spectra were obtained on surface samples (0—10 cm) from abandoned anthills and adjacent topsoil; in addition one representative soil profile in an abandoned anthill was investigated. The results show that different inorganic and all organic P fractions were enriched by a factor of 2.0—3.3 inside anthills relative to the surrounding soil. The soil underneath the abandoned anthill had higher P contents until 50-cm depth. Phosphorus composition was less affected by former anthill construction. Only the younger anthills revealed a preferential accumulation of labile organic P forms such as Na-HCO₃ extractable P or diester P. The accumulation of the stable and moderate labile P forms, however, persisted for ≥ 20 years after abandonment. We concluded that former ant activity enhanced long-term P availability of soil due to high local P inputs, whereas changes of the P form distribution lasted 5—10 years after nest abandonment.     

The effect of Amazonian Eucalyptus plantations on soil aggregates and organic matter density fractions

Afforestation with Eucalyptus species is increasing in Brazil, but there is little information on the impacts of intensively managed short-rotation forestry on soil aggregate dynamics and labile organic matter fractions in these tropical ecosystems. This study investigated soil aggregate dynamics in a clay and sandy soil, each with a Eucalyptus plantation and an adjacent primary forest. It is shown that silviculture alters the processes of soil aggregate formation on both soil types. Micro-aggregates at 0–20 cm depth in the planted clay soil were 40% greater in mass than under native forest, and C and N were reduced by 87 and 75%, respectively. In plantations with a sandy soil, micro-aggregates had equal mass compared with native forest, but increased in C and N by 20 and 67%, respectively. The results from the sandy soil indicate that C and N increased in micro-aggregates following afforestation. Macro-organic matter fractions separated by density had lower mass, C, and N concentrations, and higher C:N ratios only in lower soil profiles, with native forest having greater values in all comparisons with light and medium fractions. The differences in micro-aggregate C and N and in the light and medium macro-organic matter fractions between the upper and lower soil profiles in both soils, indicate that silvicultural management had contrasting effects on different soil textures and at different depths. Increased micro-aggregate protection of C and N in the sandy plantation soil could negatively affect long-term nutrient cycling although the quantity and quality of light and medium macro-organic matter fractions did not change between plantation and native forest in the upper soil profile; this indicates that labile OM availability and quality had not been diminished in plantation soils at this depth.     

Organic‐carbon fractions in an agricultural topsoil assessed by the determination of the soil mineral surface area

According to recent conceptual models, the organic carbon (OC) of soils can be divided into OC fractions of increasing stability from labile free OC to resistant OC associated with the soil mineral phase. In this study, we present a method for quantifying two OC fractions based on soil aggregate–size fractionation and the N₂ gas–adsorption method. For this purpose, we analyzed soil material of the plow layer of a Haplic Chernozem subjected to different fertilizer treatments (no fertilizer, mineral fertilizer, mineral and organic fertilizer). The total organic‐C concentration (TOC) and the clay content of the different size fractions were determined as well as the specific surface area (SSA_mineral) and the sample pore volume after thermal oxidation (OC‐free). The TOC of the different soil‐aggregate fractions was linearly related to SSA_mineral. Clay‐associated OC and nonassociated OC fractions of the different soil samples were quantified using two methods based on the OC surface loading at the clay fraction. The application of organic fertilizer increased the amount of nonassociated OC but hardly affected the concentration of clay‐associated OC. This finding agrees with previous studies on C dynamics in soils and indicates a finite capacity of soil materials to sequester OC. Even without any addition of organic fertilizer, the mineral phase of the analyzed soil material appears to be C‐saturated.     

黄河三角洲盐渍化荒地种植植物对土壤改良、磷形态转化及有效性的影响

为了明确种植不同植物对黄河三角洲盐渍化荒地改良效果、土壤磷形态转化及增效调控机制,研究了在生长芦苇的盐渍化荒地改变种植植物(柽柳、白蜡、苜蓿)的改良措施对土壤磷形态转化及有效含量的影响。结果表明:与荒地芦苇相比,3种植物种植均能显著提高0—20 cm土壤磷酸酶活性,柽柳种植显著降低0—20 cm土壤pH,促进0—20 cm土壤大颗粒团聚体形成,提高0—40 cm土壤有机碳含量,显著增加0—100 cm土壤盐渍化程度,促使土壤速效磷(Ca2—P)和缓效磷(Ca8—P和Al—P)向难溶性磷(Ca10—P)的转化,导致土壤磷有效性降低;白蜡种植显著降低0—100 cm土壤盐渍化程度及60—100 cm土壤pH,0—20 cm土壤小颗粒团聚体比例增加,0—40 cm土壤有机碳含量显著降低,促进土壤难溶性磷(O—P)向缓效磷(Fe—P)转化,有利于土壤磷有效性提高;苜蓿种植显著增加0—20 cm土壤有效磷含量,提高0—20 cm土壤磷酸酶活性,促进难溶性磷(O—P)、缓效磷(Ca8—P和Al—P)向速效磷的转化,0—20 cm土壤大颗粒团聚体比例增加有利于提升土壤磷有效性,其60—100 cm土壤pH增加不利于土壤磷的有效性提高;3种植物种植后土壤微生物量碳和微生物量磷显著降低,微生物量氮显著增加,土壤呼吸强度及脲酶活性均无显著变化。综上所述,白蜡种植最有利于降低土壤盐渍化程度和pH,苜蓿种植最有利于0—20 cm土壤磷有效性的提高,3种植物种植对20—100 cm土壤磷有效性无显著促进作用。由于3种植物对盐渍荒地改良时间较短,土壤微生物种群动态变化及对土壤磷形态转化过程及调控机制尚需深入研究。     

广东某硫酸厂含铊冶炼废渣堆放场土壤铊剖面特征及迁移行为

重点研究了广东某硫酸工业废渣堆放场周围土壤剖面中铊的污染特征及迁移影响因素。研究表明,铊污染物主要集中在土壤0~16.5cm范围,并且表现出沿垂直方向向下迅速降低的特点。土壤中铊污染物活动态的比例很高,主要以铊的铁锰氧化物结合态(TlFe/MnOx)存在,铊的可交换态(Tlexc)、铊的碳酸盐结合态(Tlcar)、TlFe/MnOx在土壤中迁移性强,而铊的有机结合态(Tlorg)和铊的残余态(Tlres)迁移性相对较弱。土壤pH值是影响铊污染物在土壤中迁移的主要因素,铁锰氧化物/氢氧化物、有机质、土壤粒度组成、CEC(阳离子交换容量)是影响铊迁移的次要因素。     

Soil phosphorus fractions in aggregate size classes in southwestern China

The build‐up of topsoil phosphorus (P) through excess fertilizer application can increase P losses in run‐off leading to negative impacts on aquatic ecosystems. To better understand the risk of P losses, the fractions of soil P in four aggregate size classes were quantified for two vegetable production sites (<10 and >25 yrs) and a conservation buffer site (8 yrs) in southwestern China. Sequential extraction methods of inorganic P (Pi) and organic P (Po) were carried out on samples from Nitisol and Gleysol soils from 0 to 5 cm and 5 to 10 cm depths. On average, soil Pi concentrations exceeded Po concentrations threefold, primarily in the bioavailable Pi fractions (labile Pi, loosely bound Pi and non‐occluded Pi). Soil Po fractions and bioavailable Pi fractions were significantly greater under the >25 yrs field than in the <10 yrs field. The conversion of fields under vegetable production to forested buffer substantially decreased the levels of the bioavailable Pi and labile Po in the Gleysol after 8 yrs. Soil macro‐aggregates (>0.25 mm) had greater concentrations of bioavailable Pi fractions and of labile and moderately labile Po than did micro‐aggregates and silt and clay size components. Although more P was stored in recalcitrant P forms, a larger percentage of all P fractions was found in macro‐aggregates in these soils. Small active P‐enriched aggregates potentially intensify export of P from the vegetable soils by run‐off, and therefore, management practices must be optimized to enhance agricultural P efficiencies.     

套作及秸秆还田对西兰花连作田土壤团聚体分布的影响

随着蔬菜的规模化与集约化发展,菜田连作障碍日趋严重,合理间套作与秸秆还田对恢复菜田土壤生态环境有很大的潜力。该文基于6a的田间长期定位试验,以西兰花单作为对照,设置西兰花套作糯玉米且糯玉米秸秆还田-西兰花(B/MR-B)、西兰花套作糯玉米且糯玉米秸秆不还田-西兰花(B/M-B)、西兰花-西兰花(B-B),共3个处理。采用湿筛法探索了套作糯玉米(Zea mays L.sinensis Kulesh)并秸秆还田对连作西兰花(Broccoli)田土壤固碳能力以及土壤水稳定性团聚体分布的影响。结果表明:随着土层的增加,不同处理的水稳定团聚体分布呈粒径逐渐减小、分布范围呈逐渐集中趋势。随着生育时期的推进,幼苗期和花球生长期水稳性团聚体多集中于0.25~0.5 mm与0.25 mm粒径范围;营养生长期多集中于0.5~1 mm与0.25~0.5 mm粒径范围;而秋菜收获期则多分布于0.5~1 mm、0.25~0.5 mm与0.25 mm粒径范围。B-B处理下各个时期0~40 cm土层的粒径0.5~1 mm和0.25~0.5 mm的水稳性团聚体基本呈现低于其他处理,相反B-B处理下的粒径0.25 mm水稳性团聚体均呈现显著高于其他处理。秋季西兰花整个生育期水稳定性大团聚体均表现为B/MR-BB/M-BB-B。B/MR-B处理的土壤有机碳(SOC)含量显著高于其他处理。多元回归分析表明土壤水稳定性大团聚体与土壤有机碳含量在0~10 cm与10~20 cm土层中呈极显著(P0.01)正相关,20~40 cm土层呈显著(P0.05)正相关。综上,套作糯玉米并秸秆还田能有效提高连作西兰花田耕作层土壤水稳定性大团聚体的比例,改善连作导致的土壤理化性状的恶化。     

SOIL ORGANIC CARBON STOCK AND FRACTIONS IN RELATION TO LAND USE AND SOIL DEPTH IN THE DEGRADED SHIWALIKS HILLS OF LOWER HIMALAYAS

The proportional differences in soil organic carbon (SOC) and its fractions under different land uses are of significance for understanding the process of aggregation and soil carbon sequestration mechanisms. A study was conducted in a mixed vegetation cover watershed with forest, grass, cultivated and eroded lands in the degraded Shiwaliks of the lower Himalayas to assess land‐use effects on profile SOC distribution and storage and to quantify the SOC fractions in water‐stable aggregates (WSA) and bulk soils. The soil samples were collected from eroded, cultivated, forest and grassland soils for the analysis of SOC fractions and aggregate stability. The SOC in eroded surface soils was lower than in less disturbed grassland, cultivated and forest soils. The surface and subsurface soils of grassland and forest lands differentially contributed to the total profile carbon stock. The SOC stock in the 1.05‐m soil profile was highest (83.5 Mg ha⁻¹) under forest and lowest (55.6 Mg ha⁻¹) in eroded lands. The SOC stock in the surface (0–15 cm) soil constituted 6.95, 27.6, 27 and 42.4 per cent of the total stock in the 1.05‐m profile of eroded, cultivated, forest and grassland soils, respectively. The forest soils were found to sequester 22.4 Mg ha⁻¹ more SOC than the cultivated soils as measured in the 1.05‐m soil profiles. The differences in aggregate SOC content among the land uses were more conspicuous in bigger water‐stable macro‐aggregates (WSA > 2 mm) than in water‐stable micro‐aggregates (WSA < 0.25 mm). The SOC in micro‐aggregates (WSA < 0.25 mm) was found to be less vulnerable to changes in land use. The hot water soluble and labile carbon fractions were higher in the bulk soils of grasslands than in the individual aggregates, whereas particulate organic carbon was higher in the aggregates than in bulk soils. Copyright © 2012 John Wiley & Sons, Ltd.     

基于神经网络与决策树的土壤粗糙度测量

利用野外拍摄照片判读土壤粗糙度是一种简单快速的测算方法。为了克服人工判读处理效率低、结果易受人为因素影响和目前的计算机自动读取结果易受野外杂草影响、自动化程度有待提高的缺点,提出了一种基于神经网络和决策树的土壤粗糙度测量方法。该方法在建立神经网络时充分考虑土壤边界线、白板边缘及参考方框的特征选取输入特征向量,消除噪声影响,实现图像边缘检测;并在此基础上建立决策树区分白板、土壤、参考方框等信息,完成土壤边界线和比例尺的获取。野外试验中不可避免有杂草和光照的影响,考虑到阴影、土壤和杂草色彩的差异性,决策树判决准则的选择不仅包含了纹理信息而且考虑了色彩信息。试验表明,基于神经网络与决策树的土壤粗糙度测量采用带有简单方框的参考白板能快速高效地从复杂的野外照片中获取土壤粗糙度信息,降低了拍摄要求。自动提取结果与人工判读相比,所得均方根高度的误差在5%以内,相关长度的计算误差在1%以内,具有较高的精度和可靠性。该研究方法为土壤粗糙度实时在线测算提供了有效的解决方案。     

茶园土壤团聚体分布特征及其对有机碳含量影响的研究

通过野外调查与室内分析相结合的方法,对茶园土壤团聚体的分布及其有机碳的含量及分布进行了研究.结果表明:茶园0-20 cm,20-40 cm土层土壤团聚体的分布均以>2.00mm和2～5 mm团聚体为主,分别占总团聚体的比例为56.57%和69.53%.茶园土壤团聚体平均重量直径平均值为1.02 mm,并且随着土壤层次的增加有增加的趋势.茶园0-20 cm土层0.25～0.5 mm粒径土壤团聚体有机碳含量最高,20-40 cm土层<0.25 mm粒径土壤团聚体有机碳含量最高,而2～5 mm粒径土壤团聚体有机碳含量在0-20 cm和20-40 cm土层均最低.茶园0-20 cm土层.各粒径团聚体中的有机碳分配比例均高于20-40 cm土层土壤.     

Soil surface roughness as influenced by selected soil physical properties

Soil surface roughness affects infiltration, the storage of water in depressions on the soil surface, runoff and other processess. Roughness of soil after tillage or cultivation is affected by soil factors such as soil type, soil aggregation, water content and others. Specific soil properties that determine a soil's physical reaction to tillage should be identified, so that mechanistic relationships between those properties and the resultant roughness can be developed. The objective of this study was to determine relationships between soil surface roughness, measured using an MIF parameter (the product of a microrelief index and peak frequency), and water content, bulk density, soil texture, wet and dry aggregate size distributions, aggregate stability, organic matter content and other soil properties, measured after each of3 cultivations throughout a growing season. During the summer of 1984, soil physical properties at depths of 10.8 and 30.5 cm were measured prior to primary tillage, and at the surface immediately before 3 cultivations of soya beans, Glycine max (L.) Merr. An automated, non-contact profiler measured surface profiles along transects, 5 cm apart, of 1 × 1 m plots after each cultivation. With water content and dry bulk density at the soil surface ranging from 0.06 to 0.21 kg kg⁻¹ and from 1.05 to 1.26 Mg m⁻³, respectively, roughness, as the common logarithm of MIF, ranged from −0.758 to −0.788. Dry and wet bulk density were found to account for 64 and 52% of the variation in the MIF parameter, respectively. Water content at cultivation, and at −33 kPa, accounted for 21 and 22%, respectively, of the variation in surface roughness.     

Soil Aggregate Size and Mycorrhizal Colonization Effect on Root Growth and Phosphorus Accumulation by Berseem Clover

The effects of soil aggregate size and mycorrhizal colonization on phosphorus (P) accumulation and root growth of Berseem clover (Trifolium alexandrinum L.) were studied. Root length and dry weight decreased with increasing aggregate diameter. Colonization of clover plants by arbuscular mycorrhizae (Glomus intraradices Schenck and Smith) improved root growth and P accumulation in all aggregate‐size classes. Although total root length of either mycorrhizal or nonmycorrhizal plants decreased with increasing aggregate diameter, the length of living external hyphae was not affected by aggregate size. Thus, colonized root length was improved by 20% as soil aggregate diameter increased. Total P accumulation per plant decreased with increasing aggregate size. However, total P accumulation per unit root length improved as the size of soil aggregate increased. In our study, mycorrhizal colonization improved total P accumulation and root growth in soil with large aggregates and compensated, in part, for the effect of soil strength.     

Sources and composition of soil organic matter fractions between and within soil aggregates

It is generally accepted that particulate organic matter derives from plants. In contrast, the enriched labile fraction is thought by many to derive from microbes, especially fungi. However, no detailed chemical characterization of these fractions has been done. In this study, we wanted to assess the sources (plants or microbes; fungi or bacteria) and degree of microbial alteration of (i) three particulate organic matter fractions – namely the free light fraction (1.85 g cm^?3), the coarse (250–2000 μm) and the fine (53–250 μm) intra‐aggregate particulate organic matter fractions – and of (ii) three density fractions of fine‐silt associated carbon – namely < 2.0, 2.0–2.2 (i.e. enriched labile fraction) and > 2.2 g cm^?3– by analysing the amino sugars, by CuO oxidation analyses, and by ¹³C‐, ¹H‐ and ³¹P‐NMR analyses. Macroaggregates (250–2000 μm) were separated by wet‐sieving from a former grassland soil now under a no‐tillage arable regime. The three particulate organic matter fractions and the three density fractions were isolated from the macroaggregates by a combination of density flotation, sonication and sieving techniques. Proton NMR spectroscopy on alkaline extracts showed that the enriched labile fraction is not of microbial origin but is strongly degraded plant material that is enriched in aliphatic moieties partly bound to aromatics. In addition, the enriched labile fraction had a glucosamine content less than the whole soil, indicating that it is not enriched in carbon derived from fungi. Decreasing yields of phenolic CuO oxidation products and increasing side‐chain oxidation in the order coarse intra‐aggregate particulate organic matter < fine inter‐aggregate particulate organic matter < fine‐silt fractions indicate progressive alteration of lignin as particle size decreases. The light fraction was more decomposed than the coarse inter‐aggregate particulate organic matter, as indicated by (i) its larger ratio of acid‐to‐aldehyde of the vanillyl units released by CuO oxidation, (ii) the smaller contribution of H in carbohydrates to total extractable H as estimated by ¹H‐NMR spectroscopy, and (iii) a larger contribution of monoester P to total extractable P in the ³¹P‐NMR spectra. In conclusion, the four fractions are derived predominantly from plants, but microbial alteration increased as follows: coarse inter‐aggregate particulate organic matter < light fraction ≈ fine inter‐aggregate particulate organic matter < enriched labile fraction.     

Characterizing soil surface roughness using a combined structural and spectral approach

The ability to quantitatively and spatially assess soil surface roughness is important in geomorphology and land degradation studies. This paper describes the results of an experiment designed to investigate whether hyperspectral directional reflectance factors can describe fine‐scale variations in soil surface roughness. A Canadian silt loam soil was sieved to an aggregate size range of 1–4.75 mm and exposed to five different artificial rainfall durations to produce soils displaying progressively decreasing levels of surface roughness. Each soil state was measured using a point laser profiling instrument at 2 mm spatial resolution, in order to provide information on the structure and spatial arrangement of soil particles. Hyperspectral directional reflectance factors were measured using an Analytical Spectral Devices FieldSpec Pro Spectroradiometer (range 350–2500 nm), at a range of measurement angles (θ_r=?60° to +60°) and illumination angle conditions (θ_i= 28°–74°). Directional reflectance factors varied with illumination and view angles, and with soil structure. Geostatistically‐derived indicators of soil surface roughness (sill variance) were regressed with directional reflectance factors. The results showed a strong relationship between directional reflectance and surface roughness (R²= 0.94 where θ_r=?60°, θ_i= 67°–74°). This fine‐scale quasi‐natural experiment allowed the control of slope, initial aggregate size and rainfall exposure, permitting an investigation into factors affecting a soil’s bidirectional reflectance response. This has highlighted the relationship between fine‐scale variations in surface roughness, illumination angle and reflectance response. The results show how the technique could provide a quantitative measure of surface roughness at fine spatial scales.