摘要:

SUMMARYTriaxial tests were conducted on 436 reconstituted samples of two agricultural soils (a sandy loam and a loam) at moisture contents from air dryness to close to the respective liquid limits.Volume‐change behaviour was largely consistent with the critical‐state concept, the role of soil‐moisture content being expressed in the values of the measured critical‐state parameters. The most important parameters, and their inherent variability, have been specified. Most of the parameters varied systematically with soil‐moisture content, but differently for the two soils. Some of the parameters were linearly correlated. The specific volume (v) was a linear function of the logarithm of the mean normal stress (Inp) during normal compression and along the critical‐state line (CSL) with significantly different gradients (λ and λ*, respectively) at most moisture contents except close to the liquid limits. The standard errors of λ* were typically of the same magnitude as the gradients found during hydrostatic unloading from two differentv‐levels whereas the standard errors of λ were up to a factor of ten smaller. The linear projections of the CSLs on the deviatoric stress (q)–mean normal stress plane showed intercepts on theq‐axis. Generally, precompaction leading to brittle failure had very little influence on the shear strength as compared with the stren

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00462.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYSorptivity for tension wetting in the field was determined and weighted‐mean diffusivity calculated. Both tension sorptivity and weighted‐mean diffusivity could be described by an exponential function of the formaexp(bθ) where θ is either mean water content or dimensionless water content andaandbare parameters. This relation was valid for an initial water content between 0.016 and 0.047 m3m−3and a final water content between 0.154 and 0.316 m3m−3. Tension sorptivity was more strongly correlated with average water content than dimensionless water content. However, in the case of weighted‐mean diffusivity, the correlations with both dimensionless water content and average water content were almo

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00463.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYSoil and foliage samples were collected from 136 sample sites in forested areas of the Dhulikhel Watershed, Nepal. Analyses showed that the soils have small values for pH, base saturation, total nitrogen, carbon and available phosphorus. Sample sites were stratified on the basis of aspect and elevation, soil type, forest type and management and the strata were compared to determine the influence of site factors on forest soil fertility.Soils on south‐facing slopes at low elevation contained significantly less total nitrogen and organic carbon compared with soils from north‐facing slopes at high elevation. Foliage of both sal (Shorea robustaGaertn.) and chir pine (Pinus roxburghiiSargent) growing on red soils (Rhodustults and Haplustults) contained significantly less phosphorus compared with non‐red soils (Ustochrepts and Dystrochrepts). Soils under stands of sal have the poorest soil fertility levels in comparison with chir pine and hardwood sites. The continuous removal of base‐rich litter from sal sites may account for the poor fertility conditions. Soil fertility levels are greater on sites which have been protected from biomass removal for at least 15 years relative to sites which have come under protection in the past

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00464.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYThe mound of the fungus‐growing termiteMacrotermes carbonarius(Hagen) contained significantly larger proportions of clay (P<0.05) than the adjacent soil. The largest clay content was found in the brood chamber. Concentrations of organic carbon, nitrogen, potassium and calcium in various parts of the mound were enriched compared to the adjacent soil at 10–40 cm depth from which they might have been derived. The activities ofM. carbonariusdid not significantly alter the pH or the bulk density of the mound soil compared to the adjacent tops

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00465.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYPorous ceramic cups are widely used for measuring nitrate leaching from agricultural land, but it is not fully clear what procedures give the most reliable results, or what factors could limit the validity of their use. This paper reports improvements in methodology made during tests of the technique when it was first used on a large scale, on light soils.Porous cup assemblies must be installed carefully to avoid preferential flow through the disturbed soil around them, and to maximize contact between cup and soil. Newly‐installed and well‐established porous cups could give differing results. Sampling must start as soon after return to field capacity as possible, when concentrations are often highest. The estimate of nitrate loss can be greatly affected by the calculated date of start of drainage, and some independent check on this date is advisable. Sampling frequency must be sufficient to define the shape of the curve of concentration against drainage, but this curve is normally fairly smooth. Methods of calculating nitrate loss are discussed. The spatial variability between cups in arable soils is typically 30–60% of the mean, less than between individual soil samples. Because porous cups can be leftin situ, changes over time are not confounded with changes due to sampling from differing locations, as they are with successive soil sa

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00466.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYThe brigalow clay soils of central Queensland in eastern Australia contain large quantities of nitrate‐N in the subsoil beneath shallow rooting cultivated crops. A laboratory incubation study was conducted to determine whether nitrate accumulation at depth beneath these crops was due toin situnitrogen mineralization.Intact soil cores, 5 cm long and 5 cm diameter, were obtained at four depths to 120 cm beneath cultivated black gram (Vigna mungo) and green panic (Panicum maximumvartrichoglume) permanent pasture and incubated for 12 weeks at 60% water‐filled pore space and 25°C.Net mineralization of organic N occurred in all soil cores obtained from under black gram with values ranging from 4.3 to 9 mg N kg−1soil at 12 weeks. Beneath the pasture, net mineralization had not commenced by the end of 12 weeks.Potentially available nitrogen (Na) ranged from 1.2 to 62.7 kg N ha−1under black gram, and from 10.2 to 136.9 kg N ha−1under pasture. A significant relationship was observed betweenNaand total N beneath both crops, and betweenNaand total C under the pasture.Leaching of N mineralized in the surface layers of soil appears to be the main avenue of nitrate build‐up in the subsoil beneath black gram, with subsoil mineralization making only a partial contribution to the accumulated

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00467.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYSulphate release from unsterilized and γ‐irradiation sterilized soil samples was studied in leaching funnels over a 22‐week incubation period. In unsterilized soil samples, lime addition to the acid forest topsoil caused an immediate increase in extractable sulphate, but enhanced sulphate release rates were detectable only during the first 4 weeks of incubation. Soil samples from an adjacent five‐year‐old limed plot displayed significantly higher sulphate release rates than the control throughout the experiment. The cumulative amount of sulphate extracted from the unsterilized treatments was modelled with parabolic functions and reached 4.6%, 6.8% and 8.2% of the initial total S‐contents of the control, the lime treatment and limed plot respectively. Mineral‐N release was also greatest in the samples from the old limed plot while fresh lime additions reduced net N‐mineralization due to microbial immobilization. In the sterilized samples, γ‐irradiation initially caused an increase in extractable sulphate and ammonium which is mainly attributed to the destruction of microbial biomass. After the second week, no further differences in extractable sulphate were detectable between the treatments. After 22 weeks, between 7 and 9 mg SO42−‐S kg−1soil was released from the sterilized samples which was 25–50% of the respective amounts in the unsterilized samples. A similar relationship was found for N. The data indicate that even 5 years after lime addition the observed increased sulphate concentrations in the soil solution can be attributed to en

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00468.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYIn order to develop a method for estimating labile, soil organic phosphorus (Po), macroporous anion‐exchange and adsorbent (XAD) resins were tested for their ability to extract Pofrom a permanent pasture soil. Experimental variables included: ionic form, soil‐water resin ratio, addition of cation‐exchange resin, extraction time, extraction temperature, continuous and stepwise extraction and soil pretreatment (chloroform and microwaves).The amounts of extracted Poand Pi(inorganic phosphorus) showed little variation between the anion‐exchange resins, when used in the bicarbonate form. In the chloride form, the amounts extracted were less and more variable. XAD resins extracted much less Poand very little Pi. The macroporous anion‐exchange resin, Lewatit MP500A in the bicarbonate form, was chosen for further studies. It extracted 21 mg Pokg−1from air‐dried soil and 8 mg Pokg−1from moist, incubated soil.The specific amounts of Poand Piextracted were little affected by variations in the ratio between soil, water and resin, but increased with extraction time and temperature. Chloroform pretreatment of the soil mainly increased extracted Pi, whereas microwave pretreatment only increased extracted Po. The magnitude of these increases was approximately constant irrespective of extraction temperature, indicating that the increases came from killed

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00469.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYThe Organisation for Economic Cooperation and Development (OECD)‐recommended biometer flask (OECD Guideline 304A) was modified to allow rapid and accurate sampling of large numbers of flasks during long‐term soil biodegradation experiments. Use of the OECD flask involves tedious manipulation each time the flasks are sampled. The transfer of alkali to and from the side‐arm reservoir, followed by dilution, is open to operator error and is time consuming. The new flask was found to give comparable or higher recoveries of CO2compared to the OECD‐recommended apparatus whilst minimizing the manipulation of14C‐loaded alkali. By avoiding dilution of the alkali trap, the sensitivity of the test is increased by an order of magnitude and the sampling time is reduced by t

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00470.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYA novel approach to the study of organic‐matter distribution in soil microaggregates (<20 μm) using high‐energy ultraviolet (UV) radiation in the presence of oxygen (photo‐oxidation) is reported. The method quantitatively destroyed complex organic materials through oxidation, even in the presence of clay, provided the organic materials were directly exposed to the UV radiation.Photo‐oxidation of clay and silt fractions for periods up to 8 h demonstrated that a considerable proportion of the organic matter was physically protected within clay‐ and silt‐sized aggregates. In some clay fractions, up to 23% of the organic carbon could be considered as physically protected whereas in silt fractions this was as high as 36%.Infrared spectroscopy demonstrated that the materials external to both clay‐ and silt‐sized aggregates were largely proteinaecous in nature, while the materials in the interior of the aggregates resembled humic acids. These humic materials appeared to be physically shielded against photo‐oxidation, rather than being chemically recalcitrant. Using the clay‐ and silt‐sized fractions from one soil,14C accelerator mass spectrometry demonstrated that, although both clay and silt fractions contained essentially modern carbon, after 4 h of photo‐oxidation much older organic carbon with a mean resonance time (MRT) of between 200±80 and 320±80 years before the present (BP) remained. This protection from photo‐oxidation, therefore, appears to mirror the process which physically protects organic substances in soils against microbial degradation. Photo‐oxidation of the clay‐plus‐silt fractions also resulted in a considerable reduction in particle size as the organic‐cementing agents, consisting of proteinaecous and humic materials, were oxidized.Using data from the photo‐oxidation method along with infrared spectroscopy, radiocarbon dating and scanning electron microscopy, a simple model is proposed that spatially relates the various organic structures present to the

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00471.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY