A lysimeter study of the fate of fertilizer nitrogen in spring barley crops grown on a shallow soil overlying Chalk: denitrification losses and the nitrogen balance

Four successive spring barley crops were grown in monoliths of a shallow soil overlying Chalk, contained in lysimeters. After harvest of the fourth crop, 25% of the nitrogen-15 labelled fertilizer applied 4 years earlier was found remaining in the roots and soil. Of this, 73% was present in the upper 30cm of the profile. From the amounts of fertilizer derived nitrogen that remained at the beginning of each cropping season we estimate that 5–6% of the residual nitrogen-15 turned over each year, representing a net release of 20% of the labelled nitrogen contained in the microbial biomass. Mineralization of the total biomass at the same fractional rate would release 120 kg N ha^?1 a^?1. This estimate is supported by the difference between input and outputs of total nitrogen during the experiment of 76–94 kg N ha^?1 a^?1 in fertilized lysimeters and 129kg N ha^?1 a^?1 in unfertilized control lysimeters. The total recovery of the applied labelled nitrogen was 81–87%. The nitrogen not accounted for was taken to be lost by denitrification of nitrate to dinitrogen, as no nitrous oxide emissions were detected during the experiment. Laboratory studies in aerobic and anaerobic conditions in presence of acetylene confirmed that 10–20% of the applied nitrogen-15 could have been transformed to dinitrogen.     

Geostatistical analysis of cyst nematodes in soil

The numbers of cyst nematodes that live in agricultural soil vary both in time and space with important consequences when devising procedures for inspection and strategies for their control. We have examined the distributions of two species, Globodera rostochiensis and Hererodera avenue, in fields in the east of Scotland by counting their live cysts in the topsoil. Nested sampling and analysis showed that most of the spatial variation occurs within distances of 5 m to 50 m. Experimental variograms computed from transects and grid surveys appeared to be isotropic and bounded with a range of approximately 60 m. Both species are strongly aggregated. H. avenue occurred in large patches. Indicators for cyst counts of 1, 4 and 10 were created from the data. The experimental auto and cross variograms of these indicators were computed and models fitted to them. All of these variograms had the same spherical form as that of the original counts but shorter ranges, and the cross variograms had more structure, suggesting a border effect. Biologically, the distribution can be explained by the nematodes being spread by cultivation from initial fortuitous foci of infestation or control by nematophagous fungi or both.     

COMPUTER-BASED SOIL MAPPING OF SMALL AREAS FROM SAMPLE DATA

A 1400 m × 600 m rectangular area of north Berkshire had been sampled at the intersections of a 100 m square grid, and seventeen properties of the soil profile measured. The eighty-four sampling sites were classified numerically to produce a hierarchy, and the classes of the upper part of the hierarchy mapped. The mapped classes became increasingly fragmented as the number of classes increased. At the 3-class level, the classes corresponded to character space clusters and class fragmentation was not serious. Principal-component analysis of the sample data yielded a first component that accounted for 40 per cent of the total variance and well represented the field characters used for soil classification. An isarithm map of the first component shows how the soil changes gradually over the landscape in good agreement with a soil-series map made by free survey.     

An analysis of crack pattern in clay soil: its density and orientation

The structure of much clay soil is angular blocky, and the pore space consists of a three-dimensional network of narrow, approximately planar cracks. This paper presents a means of describing such a network in terms of its density and orientation. Undisturbed samples are impregnated with resin containing a fluorescent dye. Plane sections of known azimuth and elevation are cut through them and photographed under ultra-violet light. The photographs are projected onto a screen and the spacings between successive cracks along linear probes are measured in several directions. The sets of distances obtained are then analysed. The method was applied to subsoil of the Windsor series. The distribution of inter-crack distances appeared to be exponential but with a dearth of distances less than about 1 mm. The crack pattern was judged to be the outcome of a Poisson process, but one in which some of the very narrow wedges of soil had collapsed. The crack network was perceptibly anisotropic, and was modelled as two superimposed patterns, one isotropic and the other a set of approximately parallel cracks. The orientation of the parallel component was estimated from the sections. The cracks were found to be aligned parallel to the contour of the land and to dip into the hill slope at angles ranging from about 25° to 75° to the horizontal. The intensities in the soil of the two components were estimated as 0.18 mm^?1 for the isotropic component and 0.2 mm^?1 normal to the parallel component, equivalent to 0.1 mm^?1 for a random direction.     

An amended functional leaching model applicable to structured soils. II. Model application

The performance of a functional leaching model (Hall, 1993) is compared with leaching data from two lysimeter experiments with soils of contrasting texture using sodium bromide and potassium chloride as the non-reactive solutes. The model parameters are optimized using the solute elution curves as standards and compared with the physical properties of the soil. A good match with the measured discharge of both water and solute was achieved for both soils using the moisture release characteristics to define the pore volume available for mobile and immobile water. The results indicate that preferential flow takes place through even coarse-textured soils but that there is negligible diffusive exchange of solute between water passing through the macropores and the rest of the soil.     

Optimal interpolation and isarithmic mapping of soil properties. VI. Disjunctive kriging and mapping the conditional porbability

Disjunctive kriging is a technique for estimating values of spatially distributed random variables by combining data non-linearly and with minimum variance. Data are transformed using Hermite polynomials to a normal distribution, and the resultant transformation is assumed to produce a stationary bivariate normal distribution for all pairs of data. Conditional probabilities that the true values exceed or are less than a specified critical threshold can then be calculated. For soil survey both the estimated values of soil properties and their associated probabilities can be mapped isarithmically as aids to decisions on land management.
The technique is described and illustrated from two case studies. The pH, exchangeable potassium and available phosphorus are mapped over 77 ha of the Broom's Barn Farm, and in the eastern Border Region of Scotland the available copper and cobalt in the topsoil are mapped. The estimates are similar to those obtained by simple (linear) kriging. The conditional probabilities were estimated for values falling short of the minimum recommended thresholds, and maps of them showed where farmers would be advised to remedy or forestall deficiencies in lime and plant nutrients and to take prophylactic measures for their livestock.     

Scale-dependent correlation between topsoil copper and cobalt concentrations in Scotland

The soil of south-east Scotland is locally deficient in copper and cobalt. Measurements from nearly 3000 fields for which the soil association is known were analysed to study the coregionalization of the two elements and to assess the influence of parent material on the metals' concentrations. The experimental auto- and cross-variograms revealed distinct local (1.5 km) and regional (20 km) scales of spatial variation. A combination of indicator variograms of the soil associations had the same spatial structures, suggesting that parent material influences the concentrations of the metals. The coregionalization between copper and cobalt was modelled as a linear combination of three spatial structures. The resulting structural correlation coefficients showed the two elements to be fairly strongly positively correlated at the regional scale. Kriging allowed determination and mapping of each spatial component; these maps were then compared with the spatial distribution of soil associations in the region. An analysis of variance was performed before and after filtering out the nugget and short-range spatial components. Classification by soil association (parent material) accounted for a large proportion of the variance at the regional scale, suggesting that the parent material contributes substantially more to the trace element content of the soil than had been thought earlier.