Tillage-induced CO2 loss across an eroded landscape

Soil carbon (C) losses and soil translocation from tillage operations have been identified as causes of soil degradation and soil erosion. The objective of this work was to quantify the variability in tillage-induced carbon dioxide (CO₂) loss by moldboard (MP) and chisel (CP) plowing across an eroded landscape and relate the C loss to soil properties. The study site was a 4 ha wheat (Triticum aestivum L. cv. Marshall) field with rolling topography and five soil types in the Svea-Barnes complex in west central Minnesota (N. Latitude = 45°41′W, Longitude = 95°43′). Soil properties were measured at several depths at a 10 m spacing along north–south (N–S) and west–east (W–E) transects through severely eroded, moderately eroded and non-eroded sites. Conventional MP (25 cm deep) and CP (15 cm deep) equipment were used along the pre-marked transects. Gas exchange measurements were obtained with a large, portable chamber within 2 m of each sample site following tillage. The measured CO₂ fluxes were largest with the MP > CP > not tilled (before tillage). The variation in 24 h cumulative CO₂ flux from MP was nearly 3-fold on the N–S transect and 4-fold on the W–E transect. The surface soil organic C on the transects was lowest on the eroded knolls at 5.1 g C kg⁻¹ and increased to 19.6 g C kg⁻¹ in the depositional areas. The lowest CO₂ fluxes were measured from severely eroded sites which indicated that the variation in CO₂ loss was partially reflected by the degradation of soil properties caused by historic tillage-induced soil translocation with some wind and water erosion.

The spatial variation across the rolling landscape complicates the determination of non-point sources of soil C loss and suggests the need for improved conservation tillage methods to maintain soil and air quality in agricultural production systems.     

SOIL SEGMENT OVERLAP IN CHARACTER SPACE AND ITS IMPLICATIONS FOR SOIL CLASSIFICATION

A major problem in soil classification for soil survey is the lack, or uncertainty, of correspondence between mapping units in different localities. The problem is examined using multivariate soil data recorded at short regular intervals along transects in Oxfordshire and Aberdeenshire. The data for each transect were transformed to canonical variates, the first two of which were then used to locate soil boundaries and to show the relationships among the sampling points in that projection of the character space. With few exceptions, the sampling points in adjacent segments on all three transects lay in different parts of the canonical variate (CV) plane, showing that the boundaries were well defined. On the transect near Witney, Oxfordshire, distant segments either occupied distinct parts of the CV plane, or were superimposed on it. For the other two transects the portions of the CV plane occupied by some segments overlapped only partially the space occupied by one or more other segments that were not contiguous on the transects. The results show that on two of the three examples, lack of correspondence between mapping classes in different localities is already present in small tracts of country.     

OPTIMALLY PARTITIONING SOIL TRANSECTS

Two methods are described for locating soil boundaries on transects from multivariate data. One method (split moving-window, SMW) examines a transect through a window which is split at its mid point and the two halves compared. The window is moved along the transect and a boundary inferred wherever the difference between the two halves attains a local maximum. The other (maximum level-variance, MLV) examines all possible partitions of a transect that can be produced by a given number of boundaries or less and places boundaries in those positions that minimize the within-segment sum of squared deviations from the segment means (and hence maximize the between-segment sum of squares). The performances of the two methods are compared for two soil transects in Oxfordshire. MLV identified all major changes even where gradual, but was less reliable for small changes. SMW identified all sharp boundaries, even between soil types that are fairly similar. In its original form SMW did not identify gradual changes reliably, but its performance was improved both by omitting data near the centre of the window and by concentrating the discriminating power of the data beforehand as in MLV. SWM requires less computing than MLV.     

宁夏固原云雾山天然草坡土壤电阻率和含水率的关系及其空间变异

为了解黄土区坡面土壤水分的空间分布,于2010年5月初选择宁夏固原云雾山2个典型草坡上利用多电极电阻仪法(ERT)测定了土壤电阻率的坡面空间分布。分别在长约400m的东南坡向坡面和圆丘坡面的4个长约100m坡向上设置纵向样线,并在样线上多点同步地测定了土壤含水率和电阻率,结果表明:土壤电阻率沿纵向坡面的空间连续性和变异性总体较好,阴坡半阴坡(西北、东北)的土壤电阻率小于阳坡半阳坡(东南、西南),并随土层深度增加而逐渐增大。同时,土壤电阻率随着体积含水率增大而几乎线性降低,其相关关系较好,说明利用土壤电阻仪测定值计算土壤含水率的空间连续发布是定量了解黄土坡面土壤水分空间分布非破坏性的一个适合技术途径。     

Variability in topsoil texture and carbon content within soil map units and its implications in predicting soil water content for optimum workability

The ability to predict the timing of optimum soil workability depends on knowledge of the extent and structure of variability in main physical characteristics of the soil. Our objectives were to quantify the variability in texture and carbon content within soil map units in a small agriculture-dominated catchment in South-east Norway and to assess implications of variability in texture and carbon content on land management operations, using the predicted maximum water content for optimum workability as an example. Information from three different sources were used: a soil map (1:5000), a large sample grid (100 m spacing, 270 ha extent), and a small sample grid (10 m spacing, 2.25 ha extent). Readily available information on texture and organic matter content from the soil map was found to be of limited use for soil management due to broad textural classes together with deviations from the mapped main textural classes. There were significant differences in clay, silt and sand content between the different soil textural classes on the soil map. Statistical distributions within soil map units were generally either positively or negatively skewed and the coefficient of variation was intermediate, 15–50%. Most of the variation in both grids was spatially correlated. The large grid was dominated by a patchy structure, whilst the small grid showed a systematic trend with a gradual transition indicating fuzzy boundaries between map units in this catchment. The effective range for texture was 16 times larger in the large grid. Implications of variability in texture and carbon content on land management operations were assessed for the maximum water content for optimum workability (Wopt), predicted using pedotransfer functions. Wopt was usually in the same range as the water content at–100 kPa matric potential, indicating that considerable evaporation in addition to drainage is required for obtaining workable conditions in the field. The time required for obtaining the water content was estimated to about 5 days, which is longer than an average length of periods without precipitation in the area, median 3.7 days. Wopt predicted from the soil map deviated strongly from Wopt predicted from the sample grids. Comparing estimates of Wopt from the large grid with measurements in the small grid showed differences corresponding to ±2–3 days of evaporation.     

A multiscale study of silty soil structure

Dependency of soil properties on scale is a crucial issue in soil physics. In this paper, fractal approaches are used in two case studies in France and Australia, respectively, to study how measured physical soil properties change with the sample spacing and the scale of observation. At a scale of 10–1000 m (10⁴ to 10⁶ mm), fractals were applied to sample data from a linear transect, while at the 10^?6 to 10² mm scale, fractals were applied in two dimensions to analyse both soil micro‐ and macrostructure, based on thin section samples. Porosity was characterized by short‐range spatial variations using sample spacings of 0.5 and 5 m (from the transect data), and a sample spacing of 1 cm (from the thin section analysis). The size of the representative elementary volume (REV) or representative elementary area (REA), required to represent statistically the elementary soil structure, was identified in three ways: (i) by the correlation length of a representative interconnected pore network, (ii) by the upper limit of the non‐linear increase with observation scale of mean porosity (upper limit of the solid mass fractal domain), and (iii) by the non‐linear decrease with observation scale of the coefficient of variation, CV, of mean porosity. Two embedded REAs were identified: the first (0.1–0.4 mm) related to the soil microstructure whereas a second (11–44 mm) related to the soil macrostructure. The solid mass fractal dimensions of the two embedded structural domains showed that hierarchical heterogeneity of soil structure was more pronounced for microstructures than for macrostructures. The mean area ratio of microstructural matrix/total surface and the CV of mean microporosity both scale similarly at observation scales smaller than the REA size. Their scaling exponents were both related to the fractal dimension of microstructural matrix. This preliminary study shows that the theory of fractals applied to soil structures at a specific scale range cannot be directly applied to predict soil physical properties at another scale range. This is because there are different interdependent structuring processes operating at different scales resulting in fractal dimensions being consistent only over particular domain limits.     

The elucidation of soil pattern in the Wyre Forest of the West Midlands, England. II. Spatial distribution

The spatial distribution of soil in the Wyre Forest of England was analysed in two phases. In the first the soil was examined at sites chosen using a five-stage nested design with spacings increasing geometrically from 6 m to 600 m. Some 80% of the variance was contributed by components for the spacings between 6 m and 60 m. Measurements were then made on transects at 5 m intervals and semi-variograms estimated to 70 m. Except for those of pH the semi-variograms of the soil properties had the same general transitive form and a common effective range of about 40 m. This short range meant that very intensive sampling, approximately one point per 400 m², is needed to map the soil variation. A survey was made of a small portion of the forest on a 20m × 25 m grid to test the inference. Maps of clay and sand content were made successfully by kriging from the data. The mutual correlations between soil properties and the common range of their semi-variograms enabled a general purpose and spatially coherent soil classification to be created from the data. Its mapping confirmed the intricacy of the soil pattern in the Forest.     

The use of a water budget model and yield maps to characterize water availability in a landscape

Crop yield maps may contain substantial corollary information regarding the distribution of yield related soil properties across a landscape. One of these properties is water holding capacity (WHC). Since WHC is an important parameter for crop models and is also critical for crop yield, our objective was to determine if WHC could be estimated by matching simulated yield with yield map data. We collected soil cores for water retention measurements and recorded plant phenological stages from 60 plots on four transects over two growing seasons (1997 and 1998). Soil cores were also sampled on 40 other locations set out on a grid pattern. We utilized a simple water budget model that uses the relative transpiration ratio to calculate relative yield from available water in the soil profile. Rainfall, potential evapotranspiration and soil water holding capacity are input. An optimization program varies the WHC to produce a grain yield similar to the one from the yield map at a particular location. This analysis was carried out over several scales by averaging yields over 55 m×71 m, 27 m×35 m, and 11 m×14 m areas. Yield data from 2 years were used. Yields from the transects in both years were significantly related to measured WHC in the surface 0–10 cm of soil. The calculated stress indices from the water budget model and estimated available WHC calculated for the 1997 data were similar to those calculated for the 1998 data where data were aggregated in 27 by 35 m or larger blocks. The contour map of estimated WHC was similar to the map of measured WHC for some features though there were also some differences. Use of multiple years of yield data are required to give stable results for estimated water holding capacities.. This information could be used in a farm management plan by allowing a producer to classify a field into areas that are buffered against drought and areas more susceptible to drought.     

The development of detailed soil maps on the basis of interpolation of data on soil properties

A methodology for creating detailed soil maps on the basis of a dense grid of soil testing points and the numerical interpolation of experimental data on the soil properties is discussed. The study of the soil cover patterns combines regular sampling grids with equal spacing and additional sampling points chosen with due account for the soil cover specificity in particular areas. Soil diagnostics are performed at each of the points, and the diagnostic features of the soils are recorded in the field. In a laboratory, these data are arranged into a database, and a legend to the soil map is created. The necessary and sufficient set of the quantitative soil characteristics is selected, and quantitative criteria of the boundaries between the separate soil polygons are determined on the basis of numerical interpolation. Algorithms to delineate soil polygons on the basis of the selected indices are developed. Separate thematic map layers are produced for each of the selected soil characteristics. An integral soil map for the investigated area is obtained via the superposition of these layers. The thickness and/or the depths of the upper/lower boundaries of the soil layer with definite diagnostic characteristics making it possible to distinguish the given soil from its neighbors are used as the criteria for delineating the boundaries between soil polygons. Special criteria based on the proportions between the thicknesses or depths of several layers can also be applied for this purpose. The creation of a detailed soil map of a plot on the Kamennaya Steppe is discussed as an example of the practical application of this methodology.     

Spatial heterogeneity in the metabolism and dynamics of isoproturon degrading microbial communities in soil

The characteristics of isoproturon [3-(4-isopropylphenyl)-1,1-dimethylurea] metabolism were investigated in soil taken from two transects within a single field. Along transect 1, complete degradation of the parent compound occurred within 18 days, and over 40% of ring C had been metabolised after 65 days. In these soils, both side chain and ring metabolism had a short lag phase, followed by a period of rapid degradation. Along transect 2, the rate of side chain metabolism was highly variable, and 20% of ring C had been metabolised after 65 days. The dynamics of isoproturon ring C metabolism were typical of cometabolic degradation, even at sites in which enhanced side chain metabolism occurred. Isoproturon degrading organisms were found in similar numbers in soil from the two transects prior to isoproturon application. In soils from transect 1, there was considerable proliferation of degrader organisms during the lag phase, in which 40% of the isoproturon was degraded. In most soils from transect 2, there had been no proliferation of isoproturon metabolising organisms at the point of 40% metabolism. Before enhanced degradation could develop, there was clearly a requirement for the isoproturon metabolising community to reach a threshold size. Immobilisation of isoproturon ring C into the microbial biomass and formation of bound residues was lower in soil from transect 2 relative to soil from transect 1. We conclude that the in-field spatial heterogeneity of isoproturon side chain and ring metabolism, the formation of bound residues and the immobilisation of pesticide residues in the biomass, results from variation in the development and significance of growth linked and cometabolic degradation.     

Soil Aluminum Distribution in the Near-Stream Zone at the Bear Brook Watershed in Maine

Near-stream and upslope soil chemical properties were analyzed to infer linkages between soil and surface water chemistry atthe Bear Brook Watershed in Maine [BBWM]. Organic and mineral soil samples were collected along six 20 m transects perpendicular to the stream and one 200 m transect parallel tothe stream. O horizon soils immediately adjacent to the streamhad a significantly higher pH (4.20) and lower soil organic matter percentage (54%) than upslope O horizons (3.84 and 76%,respectively). Additionally, near-stream O horizon soils hadsignificantly higher concentrations of water-soluble Al (2.7 ×),exchangeable Al (2.3 ×), and organically-bound Al (3.9 ×) andsignificantly lower concentrations of exchangeable Ca (0.4 ×) than O horizons upslope. These results suggest that Al can accumulate in non-hydric near-stream zone soils at this site. Mobilization of labile Al from near-stream zone soils duringhydrologic events could play a key role in explaining controls on Al in stream water at BBWM.     

Sampling Strategies and Soil Characteristics for Estimation of Plant Patches Dimensions (Case Study: Inchebroun Rangelands,Golestan Province,Iran)

Survey and monitoring rangelands projects is one the important plans in rangelands management. For this purpose, the survey and monitoring methods could be time-consuming and costly. Sampling strategies of plant patches are effective in sampling procedures that cause minimizing sampling variance and time. In this study, for decreasing sampling time and cost, increasing speed, plant patches parameters (including length, width, height, area, and distance between the patches) were compared. Three transects with 50 m length at three aspect slopes were located randomly systematic at the selected site with different grazing intensities. Soil samples for nitrate (NO₃^?), total nitrogen (TN), ammonium (NH₄⁺), phosphorus (P), sodium absorption ration (SAR), soil acidity or soil reaction (pH), sodium (Na), electrical conductivity (EC), organic carbon (OC), and potassium (K) content were taken in these transects. The data were subjected to analysis of variance (ANOVA), and the average of the measured parameters in the transects was compared by Duncan Multiple Range test using SPSS₁₆ software. The soil characteristics compared between the regions were analyzed with Discriminant Function Analysis using STATISTICA₁₀ software. Results showed that the transect slope aspect and soil characteristics had an effect on measured parameters. It was suggested to withdraw the located transect at one of the North or South and West or East Slope aspects for increasing sampling efficiency. It was also suggested that to avoid time consumtion, sampling can be recorded on plant patches’ width instead of area. It is also suggested that it would be better to record the length of the plant patches instead of their area in the non-grazed zones and record the length and the width of the plant patches in the moderate- and the high-grazed zones.     

Variability of Water Quality in a Spruce Stand

The spatial variation of the chemical composition of soil water under a 75 year old spruce stand on a stagni-orthic Acrisol was investigated. Soil water was sampled with 72 suction probes from depths of 0.2 m and 0.7 m. Sampling points formed a grid of 9 × 8 nodes with an average spacing of 5 m. Sodium, potassium, magnesium and calcium concentrations and pH were measured on 12 dates. The statistical distributions of the properties were mainly lognormal. Coefficients of variation mostly ranged between 20 % and 60 %. Trees had a clear influence on sample volumes and element concentrations. The latter increased generally in the neighbourhood of the stem. The coefficient of variation of the properties was not constant through time. Maximum coefficients occurred at the beginning of the period of vegetation growth.     

Exploring the variation in soil saturated hydraulic conductivity under a tropical rainforest using the wavelet transform

Saturated hydraulic conductivity (K_s) of the soil is a key variable in the water cycle. For the humid tropics, information about spatial scales of K_s and their relation to soil types deduced from soil map units is of interest, as soil maps are often the only available data source for modelling. We examined the influence of soil map units on the mean and variation in K_s along a transect in a tropical rainforest using undisturbed soil cores at 0–6 and 6–12 cm depth. The K_s means were estimated with a linear mixed model fitted by residual maximum likelihood (REML), and the spatial variation in K_s was investigated with the maximum overlap discrete wavelet packet transform (MODWPT). The mean values of K_s did not differ between soil map units. The best wavelet packet basis for K_s at 0–6 cm showed stationarity at high frequencies, suggesting uniform small‐scale influences such as bioturbation. There were substantial contributions to wavelet packet variance over the range of spatial frequencies and a pronounced low frequency peak corresponding approximately to the scale of soil map units. However, in the relevant frequency intervals no significant changes in wavelet packet variance were detected. We conclude that near‐surface K_s is not dominated by static, soil‐inherent properties for the examined range of soils. Several indicators from the wavelet packet analysis hint at the more dominant dynamic influence of biotic processes, which should be kept in mind when modelling soil hydraulic properties on the basis of soil maps.     

六盘山香水河小流域典型坡面的土壤电阻率空间变异

为了解坡面土壤水文特征的空间分布,采用多电极电阻仪法(ERT),于2009年5月初在宁夏六盘山香水河小流域一个长约600 m的华北落叶松人工林典型坡面上,设置了从坡顶至坡底的1条纵向样线及处于不同坡位的水平分布的3条横向样线,多点测定了多层土壤的电阻率值并分析其空间变异特征。结果表明,土壤电阻率在纵向样线上总体有较好的空间连续性及一定的空间变异性,随着坡位下降,电阻率呈现出由坡上至坡中上逐渐减小和然后又恢复性增大的过程。由于局部特殊的土壤特征、地貌及植被分布差异,使土壤电阻率沿坡面纵向变异程度大于不同坡位的横向变异程度。在土层垂直方向上,由于石砾含量和岩石比例随深度逐渐增多,电阻率表现为随深度增加逐渐增大。电阻率与土壤总孔隙度和体积含水率相关最紧密,尤其与体积含水率相关较好,说明通过测定坡面电阻率推求土壤水分等土壤特性的坡面变化是可行的。     

Variations of Soil Nutrient and Organic Matter Content in a Subtropical Mangrove Ecosystem

Variations of soil nutrient and organic matter content along tidal gradients of a small mangrove forest in Hong Kong were investigated. Effects of sampling months on nutrient status were also analysed. Two transects, A and B, were established perpendicular to the shoreline. Transect A transverse a wider band of vegetation and was less disturbed by human activities; while Transect B was close to human settlement and all mangrove plants were more frequently flooded by incoming and outgoing tides. Surface soils at 5 and 10 m intervals along Transects A and B, respectively, were collected in December 1989, March 1990, July 1990 and September 1990. Concentrations of soil organic matter, total and extractable nitrogen, phosphorus and potassium were high in landward sites and decreased gradually along tidal gradients; while pH and salinity increased with distance from landward to seaward sites. These trends along tidal gradients indicate the importance of tidal frequency and amplitude on soil properties. Significant differences in soil properties were also found between two transects. Transect A had significantly higher organic matter and nutrient content than Transect B. In both transects, there were sites with extraordinarily high levels of nutrients and organic matter (5–10 times higher than the average values) suggesting local contamination. Most of these contaminated sites were located at the landward edges. Positive correlations between organic matter, N, P and K content were found suggesting that these nutrients were from similar input sources. Variations along tidal gradients and local contaminated sites at the landward edges suggest that human discharges, litter deposition and surface runoff were major nutrient inputs. In both transects, no definite seasonal periodicity was found and temporal variations existed in a more random manner than variations along tidal gradients.     

THE RELATION BETWEEN COST AND UTILITY IN SOIL SURVEY

This paper discusses how to compare the utility of soil maps. For three contrasting areas, in Southern England, it compares the utilities of a number of single-property and general purpose (series) soil maps, produced by free and grid survey, at map scales from 1 :20,000–1:70,000. The‘purity’of series mapping units increases with survey effort. Where the soil boundaries had fairly clear external expression free survey was superior to grid survey. On average the purity of maps by grid survey was higher than for maps by free survey at comparable survey effort. The uniformity of soil properties within mapping units was measured by relative variance (RV)= within-unit variance/total variance, (1–RV) indicates the increased precision achieved by the soil map. It increased with survey effort. For series maps by free or grid survey (1–RV) depended on how closely soil properties were associated with profile classes, and on the extent to which soil boundaries could be mapped on their external expression. Free survey was preferred at scales between 1: 100,000 and 1:30,000, when the soil boundaries could be mapped on their external expression, (1–RV) for the profile classes on which the mapping units are defined puts a ceiling to the utility of series maps (grid or free) which is already nearly achieved at map scales near 1:25,000. Except for properties very poorly associated with profile classes, single property maps do not have great advantage over series maps at medium map scales, but they attain higher values of (1–RV) at scales greater than 1: 25,000.     

Changes in variance and correlation of soil properties with scale and location: analysis using an adapted maximal overlap discrete wavelet transform

The magnitude of variation in soil properties can change from place to place, and this lack of stationarity can preclude conventional geostatistical and spectral analysis. In contrast, wavelets and their scaling functions, which take non‐zero values only over short intervals and are therefore local, enable us to handle such variation. Wavelets can be used to analyse scale‐dependence and spatial changes in the correlation of two variables where the linear model of coregionalization is inadmissible. We have adapted wavelet methods to analyse soil properties with non‐stationary variation and covariation in fairly small sets of data, such as we can expect in soil survey, and we have applied them to measurements of pH and the contents of clay and calcium carbonate on a 3‐km transect in Central England. Places on the transect where significant changes in the variance of the soil properties occur were identified. The scale‐dependence of the correlations of soil properties was investigated by calculating wavelet correlations for each spatial scale. We identified where the covariance of the properties appeared to change and then computed the wavelet correlations on each side of the change point and compared them. The correlation of topsoil and subsoil clay content was found to be uniform along the transect at one important scale, although there were significant changes in the variance. In contrast, carbonate content and pH of the topsoil were correlated only in parts of the transect.     

Variability in soil properties at different spatial scales (1 m-1 km) in a deciduous forest ecosystem

The purpose of this research was to test the hypothesis that variability in 11 soil properties, related to soil texture and soil C and N, would increase from small (1 m) to large (1 km) spatial scales in a temperate, mixed-hardwood forest ecosystem in east Tennessee, USA. The results were somewhat surprising and indicated that a fundamental assumption in geospatial analysis, namely that variability increases with increasing spatial scale, did not apply for at least five of the 11 soil properties measured over a 0.5-km² area. Composite mineral soil samples (15 cm deep) were collected at 1, 5, 10, 50, 250, and 500 m distances from a center point along transects in a north, south, east, and westerly direction. A null hypothesis of equal variance at different spatial scales was rejected (P?0.05) for mineral soil C concentration, silt content, and the C-to-N ratios in particulate organic matter (POM), mineral-associated organic matter (MOM), and whole surface soil. Results from different tests of spatial variation, based on coefficients of variation or a Mantel test, led to similar conclusions about measurement variability and geographic distance for eight of the 11 variables examined. Measurements of mineral soil C and N concentrations, C concentrations in MOM, extractable soil NH₄-N, and clay contents were just as variable at smaller scales (1-10 m) as they were at larger scales (50-500 m). On the other hand, measurement variation in mineral soil C-to-N ratios, MOM C-to-N ratios, and the fraction of soil C in POM clearly increased from smaller to larger spatial scales. With the exception of extractable soil NH₄-N, measured soil properties in the forest ecosystem could be estimated (with 95% confidence) to within 15% of their true mean with a relatively modest number of sampling points (n?25). For some variables, scaling up variation from smaller to larger spatial domains within the ecosystem could be relatively easy because small-scale variation may be indicative of variation at larger scales.