摘要:

SUMMARYTime‐domain reflectometry (TDR) is becoming an increasingly popular technique for measuring soil water content. It is based on measuring the dielectric constant of soil from the propagation velocity of a pulse travelling along an electromagnetic transmission line embedded in the soil. Although various designs of transmission line have been proposed, there have been no attempts to contrast these designs to enable the experimenter to select an appropriate system. A second area which needs consideration is the choice of calibration function, as the use of an empirical calibration with no physical basis persists. In this paper, transmission line design (balanced and unbalanced) is discussed and a simple linear calibration function based on refractive index is develope

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00429.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYSteady‐state infiltration rates from tension infiltrometers were measured on ploughed and unploughed plots in a clay soil during the period June to October. Measurements were made both at the soil surface and at depths of 15 and 25 cm. Hydraulic conductivity in the water potential range zero to −11 cm was obtained using a piece‐wise exponentialK(Ψ) function and Wooding's solution for infiltration from a circular source. A two‐line regression model showed excellent fits to paired (InKΨ) values on all measurement occasions. This may indicate the existence of a bimodal pore system, reflecting the contributions of macro‐ and mesopores to the measuredK(Ψ) function. The break‐point potential dividing the two pore systems varied between c.−4 and −6cm. Significant variations in theK(Ψ) function between sampling occasions were found at the soil surface, but not at depths of 15 and 25 cm. MeasuredK(Ψ) values decreased during the growing season, particularly at potentials between −4 and −6 cm where reductions were up to one order of magnitude. This was attributed to soil structural breakdown by rain impact and surface capping or sealing. Hydraulic conductivity near the soil surface was significantly increased by disc harrowing in autumn. In contrast, no pronounced difference in theK(Ψ) function between ploughed and unploughed treatments could be discerned at 15 and

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00430.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYThe simplicity and utility of Burns' leaching equation make it worthy of study. The equation may be written aswhereXis the fraction of initially surface‐resident fertilizer leached below depthzby net rainfallI, in soil with a volumetric water content at ‘field capacity’ of θ. The equation is analysed using transfer functions. The analysis shows that Burns' equation is consistent with an ‘independent flow tube’ soil leaching model, rather than the soil solution being well‐mixed at each soil depth as Burns suggested. The flux and resident soil solution soil concentration profiles are shown to be quite different. An alternative definition of θ is suggested. The behaviour of ‘a Burns soil’ for different initial and boundary condit

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00431.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYThe leaching of nitrate‐N under autumn‐sown arable crops was measured using hydro‐logically isolated plots, about 0.24 ha in area, from 1984–1988. Fluxes of water and nitrate moving over the soil surface (surface runoff), at the interface between topsoil and subsoil (interflow), and in the subsoil (drainflow) were monitored in plots with mole‐and‐pipe drain systems (drained plots); surface runoff and interflow only were monitored in ‘undrained’ plots. Half the drained and undrained plots were direct‐drilled, and on the other half seedbeds were prepared by tillage to 200 mm. Tillage increased the total leaching loss of nitrate by 21 % compared with direct drilling in drained plots. About 95% or the nitrate moving from the soil was present in the water intercepted by the subsoil drains in these plots. In undrained plots less water and nitrate were collected in total; more of the nitrate was present in interflow on ploughed plots and in surface runoff in direct‐drilled land.Losses of nitrate for the whole experiment from 1978‐1988 were analysed. This showed that, between the harvest of one crop and the spring application of fertilizer to the next, loss of nitrate‐N from ploughed land (Lp) was approximated byLp=22+Fkg N ha−1, whereFwas the autumn fertilizer‐N applied. After fertilizer was applied in spring, loss of nitrate‐N depended on rainfall such that for 100 mm rainfall about 30% of the fertilizer‐N was lost by leaching. About 18% more nitrate‐N was lost from direct‐drilled land than from ploughed land in spring, but the total loss was generally small compared to that over winter.The apparent net mineralization of organic‐N was measured in 1988. In autumn and winter there was little effect of tillage treatment (26 and 31 kg N ha−1on direct drilled and tilled plots respectively). However, over the year 83 kg N ha−1were mineralized in tilled plots, and 67 kg N ha−1in direct‐drilled plots.Five factors governing the leaching of nitrate are assessed and this identified that fertilizer nitrogen application to the seedbed of winter sown crops and the mineralization of nitrogen from the residues of the previous crop are the most signif

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00432.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYComparisons were made between 1988 and 1991 to evaluate three methods of estimating the leaching of mineral nitrogen (N) from unstructured freely draining sandy loam and loamy sand soils. The studies compared the drainage patterns and quantities of N (almost exclusively nitrate) leached from monolith lysimeters with those estimated from ceramic suction cups and soil core extracts. The latter two methods gave direct measurements of the mineral N concentrations in drainage, but required an estimate of the drainage volume calculated from meteorological observations and evapotranspiration equations to give total N leached. A bromide tracer was also used to confirm conclusions from nitrate leaching studies. There was a delay in the onset of drainage from free draining lysimeters because they lack the subsoil matric potential of field soils. However, total annual drainage measured by lysimeters or calculated from meteorological observations was similar, providing that return to field capacity was correctly identified in the field soil. During the first year there were discrepancies between methods which were attributed to soil disturbance during lysimeter and/or ceramic cup installation. In the second and third years of the experiment, estimates of N leaching losses using the lysimeters and ceramic cups were in good agreement. Nitrate concentrations in soil solution at a depth of 130 cm measured from soil core extracts were smaller than found by the other methods during the second year and the peak concentrations were significantly different (P<0.05). However, total overwinter N leached was not significantly different. Thus, while lysimeters and cups can be used to quantify leaching losses on unstructured, free draining soils if used correctly, the use of soil core extracts is questionable.

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00433.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYWe studied the fate of 222 kg N ha−1applied in spring as K15NO3to winter wheat test crops which followed either continuous arable cropping (Arable) or a rotation in which a 3‐year grass/clover ley preceded the wheat (Ley). Denitrification losses (measured by an acetylene‐inhibition method) of over 1 kg N ha−1d−1were measured for short periods following heavy rain in mid‐May. However the generally dry and cool weather resulted in accumulated losses by denitrification between fertilizer application and anthesis equivalent to only 5.3% and 3.6% (±2%) of the applied N for the arable and ley treatments respectively. The smaller loss from the ley was despite this treatment containing more inorganic N and available carbon.15N balance indicated that, at anthesis, 1.5% and 11.5% (± 7%) of the labelled N was lost from the arable and ley treatments respectively. Given the precision of the 15N and the acetylene‐inhibition methods, the results are not significantly different. However, the larger difference between methods for losses from the ley treatment may be an underestimate because15N balance does not measure losses of unlabelled N. These were probably very small on the arable treatment but could have increased total N loss by 25% toc. 32 kg ha−1on the ley treatment compared with the 8 kg ha‐1 measured as denitrified. Such a large difference is unlikely to be an error but was probably due to ammonia volatilization from this crop which was severely infected by mildew. The results were thus a poor test of the acetylene‐inhibition method, but revealed another loss process which could be significa

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00434.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYLarge leaching losses of sulphate contribute to the inefficient use of sulphur (S) fertilizer in some grazed hill country pastures in New Zealand. Laboratory mineralization studies were conducted to measure S, nitrogen (N) and carbon (C) mineralization rates and the potential for S leaching from surface soils of low (LS, 0–12°) and medium (MS, 13–25°) slope positions on hill country pastures. The soils differed in C, N and S contents as a result of different fertilizer histories and the uneven redistribution of nutrients by the grazing animal. Soils were incubated in columns for 140 d and leached periodically with 0.01 M KCl to remove mineralized sulphate, nitrate and ammonium. The net amounts of N and S mineralized were greater in soils on low slopes than medium slopes and in soils from pastures which had received fertilizer continuously, compared with those hill pastures that had not received single superphosphate fertilizer in the previous 7 years. In both cases more mineralization was associated with smaller soil C:nutrient ratios. However, the amounts of C mineralized were greater in soils from medium slopes where C:nutrient ratios were larger. In general, the ratio of N:S mineralized was smaller than the N:S ratio of the soil organic matter, suggesting that N is conserved while S remains in the soil solution susceptible to leaching. It is suggested that the extent of S leaching loss under field conditions will reflect S mineralization pote

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00435.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYExtraction of soil with CaCl2, has been recommended as a measure of bioavailability of heavy metals. Interpretation of soil extraction data in terms of plant uptake potential may improve when the chemical behaviour of heavy metals in these extracts is ascertained. The effect of pH, Cd complexation by Cl, and competition between Cd and Ca on Cd sorption was studied at an ionic strength of 0.03m. Sorption of cadmium was measured in 0.01mCaCl2, in 0.01mCa(NO3)2, in a mixture of 0.02mNaCl and 0.01mNaNO3, and in 0.03mNaNO3, at different values of pH ranging from 3.8 to 4.9. Adsorption isotherms were all linear, with a negative intercept on the y‐axis. This intercept indicated (linear) desorption of only part of the initial soil Cd content. About 50% of the Cd in solution was complexed in the presence of 0.02mCl at ionic strength of 0.03. Due to competition between Cd and Ca, sorption of Cd was reduced by 80% in the Ca‐electrolytes as compared with the Na‐electrolytes. Sorption was highly sensitive to pH as each 0.5 unit increase in pH resulted in twice as much sorption of Cd. An empirical factor in the sorption equation that accounts for this effect of pH showed a similar response to changes in pH as a mechanistic factor. This mechanistic factor was developed by assuming that Cd and protons sorb onto the same sites and that a two‐site Langmuir sorption isotherm for protons was able to describe the titration curve of the soil. This similarity may explain the successful application of the empirical factor in this and previous

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00436.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYA pot experiment was conducted to compare the behaviour and bioavailability of Cd and Pb from two soils mixed with sewage sludge at three rates (0, 50 and 150 t ha−1) and maintained at two contrasting ambient temperatures (15°C and 25°C) over a period of one year following the treatments. Ryegrass (Lolium perenne) accumulated Cd and Pb in the sewage sludge treated soils, although accumulation was significantly lower in the soils treated at the high rate (150 t ha−1) compared to the low rate (50 t ha−1). Ryegrass grown in the warm environment (25°C) accumulated significantly higher levels of Cd and Pb than that grown in cooler conditions (15°C). Samples of the soils spiked with nitrate salts of Cd and Pb at equivalent rates of metal loading resulted in the ryegrass accumulating much higher levels of both the metals than on the sludge treated soils.Metal uptake by the ryegrass from the sludge treatments increased over successive harvests while that from metal salt treatments decreased. The observed trend of increasing plant metal uptake over time coincided with a trend of decreasing pH in the sludge treatments. However, the concentrations of Cd and Pb extracted by DTPA failed to predict the changes in plant metal uptake. The importance of sewage sludge as both a source and a sink of pollutant metals and the trend of increasing bioavailability over time shown by this experiment are

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00437.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

SUMMARYSoils with and without organic manuring from 10 long‐term manurial experiments in East Germany were fractionated into organo‐mineral particle‐size separates by ultrasonic disaggregation and sedimentation/decantation. The cation exchange capacities (CECs) buffered at pH 8.1 were determined for the size fractions fine+medium clay, coarse clay, fine, medium and coarse silt, sand, and for the total soil samples.In the samples from nine field experiments the CECs decreased with increased equivalent diameters (fine+medium clay: 489–8 13 mmolckg−1, coarse clay: 367–749 mmolckg−1, fine silt: 202–587 mmolckg−1. medium silt: 63–345 mmolckg−1, coarse silt: 12–128 mmolckg−1and sand: 10–156 mmolckg−1. The CECs varied with genetic soil type, mineralogical composition of thefresh farmyard manure

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1993.tb00438.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY