摘要:

SUMMARYIn an acid sandy loam soil (pH 3.8), field production of N2O was two orders of magnitude higher at thaw in the spring than at any time during the rest of the year. Soil thaw in midwinter did not result in any increase in N2O flux. Soil water content remained at, or above field capacity during measurements; nitrate was added in excess. This effect could be reproduced in the laboratory: thawing soil cores at controlled temperature, nitrate and moisture yielded a large flush of N2O compared to an unfrozen control. The results indicate the importance of microbial N2O production during thaw for total annual N2O‐emissio

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00039.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYThe effects of an intercrop catch crop (Italian ryegrass) on (i) the amounts and concentrations of nitrate leached during the autumn and winter intercrop period, and (ii) the following crop, were examined in a lysimeter experiment and compared with that from a bare fallow treatment. The catch crop was grown in a winter wheat/maize rotation, after harvest of the wheat, and incorporated into the soil before sowing the maize. A calcium and potassium nitrate fertilizer labelled with15N (200 kg N ha−1; 9.35 atom per cent excess) was applied to the winter wheat in spring. Total N uptake by the winter wheat was 154 kg ha−1and the recovery of fertilizer‐derived N (labelled with15N) was 60%. The catch crop (grown without further addition of N) yielded 3.8t ha−1herbage dry matter, containing 43 kg N ha−1, of which 4.1 % was derived from the15N‐labelled fertilizer. Two‐hundred kg unlabelled N ha−1was applied to the maize crop. During the intercrop period the nitrate concentration in water draining from the bare fallow lysimeters reached 68 mg N1−1, with an average of 40 mg N1−1. With the catch crop, it declined rapidly, from 41 mg N I−1to 0.25 mg N I−1, at the end of ryegrass growth. Over this period, 110 kg N ha−1was leached under bare fallow, compared with 40 kg N ha−1under the catch crop.15N‐labelled nitrate was detected in the first drainage water collected in autumn, 5 months after the spring application. The quantity of fertilizer‐N that was leached during this winter period was greater under bare fallow (18.7% of applied N) than when a catch crop was grown (7.1%).In both treatments, labelled fertilizer‐N contributed about 34% of the total N lost during this period. With the ryegrass catch crop incorporated at the time of seedbed preparation in spring, the subsequent maize grain‐yield was lowered by an average of 13%. Total N‐uptake by the maize sown following bare fallow was 224 kg N ha−1, compared with 180 kg ha−1with prior incorporation of ryegrass; the corresponding values for uptake of residual labelled N were 3% (bare fallow) and 2% (ryegrass) of the initial application. Following the maize harvest, where ryegrass was incorporated, 22.7% of the previous year's labelled fertilizer addition was present in an organic form on the top 30 cm of lysimeter soil. This compares with 15.7% for the bare fallow intercropping treatment. Tracer analyses showed overall recoveries of labelled N of 91.7% for the winter wheat/ ryegrass/maize rotation and 97% for the winter wheat/bare fallow/maize rotation. The study clearly demonstrated the ecological importance of a catch crop in reducing N‐leaching as well as its efficient use of fertilizer in the plant‐soil system from this particular rotation. However, the fate of the organic N in the ploughed‐down catch crop is uncertain and problems were enc

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00040.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYThe chemical characteristics of the organic matter of five horizons from a Terra fusca Rendzina under beech were compared by fractionation. Two litter layers, sampled in autumn, were compared: fresh leaf litter and the remains of the previous years’ litter. Three mineral horizons, A, B and C were also sampled.While decomposition in the litter layer led only to slight redistribution of the fractions, much greater changes occurred when the organic matter became incorporated into the mineral soil. The proportion of C and N in amino sugars rose markedly down the profile. The percentage of more readily soluble substances, such as the hot water‐soluble fraction or the unidentified, hydrolysable component, in the mineral soil was twice the amount found in the leaf litter. The percentage of non‐hydrolysable compounds in the mineral horizons was nearly half that found in the leaf litter. The C/N ratio fell from 44 in fresh leaf litter to 10 in the C horizon. The amino acid C/amino sugar C ratio fell from 45 in fresh leaf litter to 3.0 in the B horizon. The glucosamine/galactosamine ratio fell from infinite in galactosamine‐free fresh leaf litter to 1.2 in residual litter and increased again to 2.5 in the C horizon. The hexose/pentose ratios were virtually the same in all horizons. The glucose/ mannose ratio showed a steady fall down the profile, from 7.7 to 1.6. The glucuronic/ galacturonic acid ratio increased from 0.1 and 0.2 in the two litter layers to 4.7 in the C

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00041.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYFour soils of different texture (6% to 46% clay and 6% to 32% silt) were incubated with14C‐labelled barley straw for periods ranging from 16 to 18 years. Incubations were either under laboratory (three soils, 20°C) or field conditions (one soil).Soil samples were separated into clay‐size (<2 μm), silt‐size (2–20 μm) and sand‐size (20–2000 μm) fractions using ultrasonic dispersion and gravity sedimentation. Whole soils and size fractions were hydrolysed with 12 M sulphuric acid, the released neutral sugars isolated and their amounts and specific activities determined.The proportion of C present as carbohydrate decreased in the order clay, silt and sand, but the sugar compositions of the various fractions were similar. No relationship could be discerned between the soil particle‐size distributions and the persistence of sugars.The specific activity of xylose remained higher than that of any major sugar, indicating the persistence of straw polysaccharides. This was true for all soils and size fractions. Galactose and mannose retained a greater proportion of their initial activity than did arabinose, xylose or glucose. This was ascribed to synthesis of microbial polysaccharides.Total activity present in whole soils decreased in the order glucose, xylose and arabinose, galactose and mannose. The total activity of individual sugars in size fractions decreased in the order clay, silt, sand. The hexose to pentose ratio (galactose+mannose to arabinose + xylose) tended to be higher for silt than for clay, but the trend was not consist

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00042.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYA sequential alkali extraction procedure followed by ultrafiltration and quantitative31P nuclear magnetic resonance (NMR) spectroscopy was used to examine organic phosphorus in a Brown Chernozem, and an adjacent Gleysol developed under native prairie vegetation, and a Grey Luvisol formed under aspen forest in Saskatchewan, Canada. Differences in the nature of organic P in the native soils were related to moisture status and vegetation. In the grassland soils, a greater proportion of orthophosphate diester P was found in the bottom‐slope Gleysol. This difference was partly attributed to less favourable conditions for mineralization in the bottom slope soil compared with the mid‐slope Brown Chernozem. Teichoic acid P occurred only in the native Grey Luvisol (NMR δ p.p.m. 0.36‐0.95) under boreal forest and not under native grassland. At all three sites, soils under long‐term cultivation were also examined and while orthophosphate monoester P (83.4‐94.6% total Po), orthophosphate diester P (3.9–8.7% total Po) and teichoic acid P (12.7% total Po in forested Grey Luvisol) were detected in native soils, only orthophosphate monoester P was found in the corresponding soils that had been cultivated for 70–80 years. These findings suggest that orthophosphate diester P and teichoic acid P are more readily mineralized in the soil environment than orthophosphate mon

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00043.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYArtificial polymers prepared by acid‐catalysed dehydration of glucose were examined by13C nuclear magnetic resonance (NMR), and subjected to degradation procedures frequently used in the study of humic substances: acid hydrolysis, persulphate degradation, alkaline permanganate oxidation, perborate depolymerization and cupric oxide oxidation. The ethyl acetate–soluble degradation products were identified by combined gas chromatography‐mass spectrometry. The compounds released in the perborate and in the cupric oxide treatments are as would be expected from a polymer derived from glucose dehydration products. Such degradation patterns contrasted with those obtained by alkaline permanganate oxidation, which resembled those of humic‐ or lignin‐like s

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00044.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYThe molecular weight distribution of humic substances in soil solutions from a dystric cambisol and an orthic podzol were determined by high‐performance gel filtration chromatography (Toyo Soda TSKgel G2000 SWXL column). The retention volumes of humic substances were compared to those of narrow molecular weight distributed poly‐styrenesulphonates with known molecular weight.A change in mobile phase pH or ionic strength usually resulted in an almost identical change in retention volume for the polystyrenesulphonates and the bulk of humic substances. The possibility of using the former to calibrate the column is discussed. The molecular weight at peak maximum was 1120–1190 and 1800 for the cambisol and podzol samples, respectively, using polystyrenesulphonates as molecular weight standards. No pretreatment of samples was used, but ion exchange is recommended for samples with high Al concentra

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00045.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYThe effects of soil organic matter content, soil water content and duration of wet‐sieving on aggregate stability of soils with contrasting cropping histories were investigated. Long‐term pasture samples had a greater aggregate stability than long‐term arable samples. However, air‐drying aggregates before wet‐sieving increased the aggregate stability of long‐term pasture samples, but decreased that of long‐term arable samples. With increasing duration of wet‐sieving, the proportion of water‐stable aggregates declined until a near‐constant value was reached for each sample. Thus, within a sample there are aggregates possessing a wide range of stabilities; with increasing time under arable cropping there is an increase in the proportion of unstable aggregates present, and the measured aggregate stability, therefore, declines. Unstable aggregates (defined as those dispersed after wet‐sieving for 1 min) generally had lower organic matter content than stable ones (those still intact after sieving for 15 min).The aggregate stability of a regrassed site (13 years of arable plus 2 years of pasture) was markedly higher than that of a corresponding site from 15 years of arable cropping. Nonetheless, levels of organic matter (organic C, total N and hydrolysable carbohydrate) were almost identical at the two sites. However, aggregates from the regrassed site did have a higher biomass C and water‐extractable carbohydrate content than those from the 15‐year arable site. For a group of soils with varying cropping histories, aggregate stability was significantly more closely correlated with hot water‐extractable carbohydrate content than with organic C or hydrolysable carbohydrate content. It is suggested that the hot water‐extractable carbohydrate fraction may represent a pool of carbohydrate involved in the f

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00046.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYReduction of aggregate size in the upper, tilled soil layer as a result of one wet/dry cycle was observed for sandy soil and clay soils. Bulk density of aggregates tended to increase as their size diminished for the clay after the wet/dry cycle, whereas similar changes in bulk density for the loam were observed only for aggregates smaller than 5.6 cm. Slaking and water‐drop impact seem to be the major factors in reducing the aggregate size of the sandy loam, while swelling and shrinkage affect clay aggregates of all size groups, but only aggregates smaller than 4.0 mm for the sandy loam.Tensile strength of the sandy loam aggregates was related to the axis along which the stresses were applied. A definite directional dependence of tensile strength was observed, e.g. the shorter the axis, the larger the tensile strength. The directional strength dependence was apparently not affected by one wet/dry cycl

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00047.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYThe examination of soil fracture surfaces that are created under tensile stress may reveal a great deal about the internal structural condition of the soil. A simple technique for quantifying the roughness of soil fracture surfaces from a measurement of their topography in cross‐section is described. The technique involves calculating the standard deviation of the differences between the measured elevations of soil fracture surfaces and their corresponding running‐mean values. The standard deviation, σR, is used as a measure of the fracture surface roughness. Advantages of this technique over others are discussed.Two methods for measuring the topography of soil fracture surfaces are presented: a bisection (single transect) method, and a laser scanning (multiple transects) method. The laser scanning method is to be preferred because it requires no sample preparation and enables greater and more rapid replication. Also, fracture surfaces created by applying direct tension in the hands produced values of σRthat were statistically indistinguishable from those created using indirect tension in a loading frame. This result makes the technique of fracture surface analysis usable for both laboratory as well as field investigations of soil structure.An example of the technique is presented to illustrate the role that air‐filled pores play in the brittle fracture of unsaturated soil (air‐filled pores are closer together in drier soils and further apart in wetter soils.) A strong positive linear correlation was found between the gravimetric water content,wand σRof natural soil clods, which supports the contention that brittle fracture of unsaturated soils under tensile stress occurs at least partly because of the propagation of air‐

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00048.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY