Keywords: Mecanique des sols ; Drainage ; Ecoulement souterrain Reference Record created on 2004-09-07, modified on 2016-08-08

Applications of soil physics.

Improved methods are needed to measure the mineralization and uptake of soil N under field conditions. Because both soil disturbance (e.g. sieving, drying and rewetting) and environmental conditions markedly affect rates of soil N mineralization, rates measured in disturbed soils in the laboratory are unlikely to be quantitatively similar to, and may not be a reliable index of those in the field. It is generally impossible to assess the usefulness of most estimates of N mineralization as predictors of field rates because of the unknown effects of assay conditions (especially soil mixing). We show that depending on soil type, sieving can either increase net N mineralization, cause immobilization of N, alter the proportion of nitrate nitrogen produced during incubation, or induce or obscure the effects of previous fertilization on rates of N mineralization.

A methodology for studying fluxes of mineral-N in soils, based on sequential soil coring and in situ exposure of largely undisturbed soil columns confined within metal or PVC tubes is described and evaluated. The sequential measurement of changes in the quantity of soil mineral-N in confined (no N uptake by roots) and unconfined soil allows rates of net N mineralization (or immobilization), plant uptake of N, and maximum N leaching to be calculated. It is argued that the method causes less disturbance to natural processes than others, and that it provides reliable quantitative estimates of fluxes of mineral-N in field soils. The severing of roots appeared to have no effect on the pattern of accumulation of mineral-N in several undisturbed forest soils exposed in situ for up to 130 days.

Methodology for studying fluxes of soil mineral-N in situ

Publisher Summary Soils form a major repository of nitrogen (N) within both natural and agricultural terrestrial ecosystems, containing, on a global basis, an estimated 2.4 x 10 11 tons of N. The soil receives N inputs through fertilizer additions and from the atmosphere in precipitation and dry deposition or via biological fixation; inputs are also made in plant and animal residues. N is removed in the harvested crop and is lost by leaching and surface run-off of soluble forms, by gaseous transfer as N gas and N oxides (during nitrification and denitrification processes), and by ammonia volatilization. In some circumstances, erosion may also be important. In addition to these interactions with the total ecosystem, internal cycles also operate within the soil, so that even if gains and losses are in balance, then N still continues to cycle in the soil. This chapter describes the current understanding of the conceptual basis of the processes involved in mineralization, relationships among the processes and other factors, and how their effects can be determined practically. The aim is to present this in a way that is relevant to current and future agricultural development and to environmental issues.

Nitrogen Mineralization in Temperate Agricultural Soils: Processes and Measurement

This article is concerned with the predominantly vertical movement of water through soils that to some degree have a network of large channels (macropores) and the consequences of this movement for the convective transport of solutes and suspended matter. Beven and Germann (1982) have reviewed the experimental evidence indicating that infiltration and redistribution of water in soils containing macropores are not adequately described by theories that treat the soil as a homogeneous medium conforming to Darcian principles of water flow. Such theories, developed for the mixing of solutions in hallow tubes (Taylor, 1953) and porous rock strata (Brigham et al., 1961), have been applied to miscible displacement experiments with columns of sand, resins, glass beads, or finely sieved and repacked soil. The underlying assumptions are that an unreactive solute moves through the medium at the same velocity as the water and all the analysis and interpretation of these experiments have been reviewed several times (Gardner, 1965; Biggar and Nielsen, 1967; Biggar and Nielsen, 1980; Wagenet, 1983) and will not be repeated here.

The Influence of Macropores on the Transport of Dissolved and Suspended Matter Through Soil

Increased awareness of the economic importance of losses of nitrogen (N) from agricultural land, together with the recognition that nitrate in drinking water is potentially detrimental to human health, have stimulated research on nitrate leaching1–4. Various reports5–7 have suggested that the leaching of nitrate from arable land is the major source of nitrate to water supplies in east and south-east England. In contrast, leaching from grassland is thought to he small5,8–10. However, this conclusion has been based largely on studies of leaching below cut swards, which exclude the effect of ruminants (cattle and sheep) on nitrate loss. This effect is likely to be substantial as <20% of the input of N to grassland systems is recovered in products of the ruminant animal11. Our study demonstrates that the amount of nitrate leached below a grass sward grazed by cattle was 5.6 times greater than that leached below a comparable cut sward and exceeded the losses normally observed from arable land.

Nitrate leaching from grassland

Net mineralization of N and net nitrification in field-moist clay soils (Evesham-Kingston series) from arable and grassland sites were measured in laboratory incubation experiments at 4, 10 and 20°C. Three depth fractions to 30 cm were used. Nitrate accumulated at all temperatures except when the soil was very dry (θ=0.13 cm3 cm−3). Exchangeable NH4-ions declined during the first 24 h and thereafter remained low. Net mineralization and net nitrification approximated to zero-order reactions after 24 h, with Q10 values generally <1.6. The effect of temperature on both processes was linear although some results conformed to an Arrhenius-type relationship. The dependence of net mineralization and net nitrification in the field soil on soil temperature (10 cm depth) and moisture (0–15, 15–25, 25–35 cm depths) was modelled using the laboratory incubation data. An annual net mineralization of 350 kg N ha−1 and net nitrification of 346 kg N ha−1 were predicted between September 1980 and August 1981. The model probably overstressed the effect of soil moisture relative to soil temperature.

Net mineralization and nitrification rates in a clay soil measured and predicted in permanent grassland from soil temperature and moisture content

Inputs and outputs to the N balance of a clay soil catchment (Evesham and Kingston series) under grassland and cereals at Wytham near Oxford were measured over 2 years. Soil mineral N (NH4 + NO3) was measured to 1 m depth at intervals of 2 to 8 weeks. The frequency distribution of these values was approximately log-normal and the geometric mean was used as an estimate of central tendency. Overall, soil mineral N tended to decrease during the study period, but marked fluctuations were observed in autumn (October–November) and early spring (February–March) in the grassland due to mineralization of soil organic N, and in the arable soil in April–May following the application of N fertilizer to the spring barley and winter wheat.

J. H. Macduff

Papers

Net mineralization and nitrification rates in a clay soil measured and predicted in permanent grassland from soil temperature and moisture content

Components of the nitrogen cycle measured for cropped and grassland soil-plant systems