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1.
Microorganisms can clog pores in soils and decrease hydraulic conductivity and infiltration. We did three column experiments to clarify the effects. In all three columns, glucose solution of 50 μg cm−3 was percolated for 120 days, and both the saturated hydraulic conductivity, K s, and the volume ratio of the gas phase, a , were measured continuously. The K s decreased rapidly for the initial 10 days, and it slowly decreased for the following 110 days. By adding chloramphenicol to the second column as bactericide and cycloheximide to the third column as fungicide, we observed clogging by bacteria and fungi, respectively, bacterial clogging proceeding more rapidly than the fungal clogging. The volume of the gas phase increased and reached the maximum value of 30.6% after 103 days from the beginning of percolation. This large amount of gas was retained in the soil pores as bubbles and occluded the pathways of water, resulting in the decrease in K s. When the percolating solution was changed to sodium azide (a strong biocide), after 120 days the volume of the gas phase decreased rapidly, and K s increased simultaneously.  相似文献   

2.
Abstract. We studied the effects of red deer grazing and fence-line pacing on soil losses of contaminants (suspended sediment, Escherichia coli , phosphorus) and nitrogen species (ammonia, nitrate) via overland flow and soil physical properties (macroporosity, bulk density, saturated hydraulic conductivity, K sat) soon after (1 day) and 6 weeks after grazing on a Pallic pastoral soil in southern New Zealand. Fence-line pacing decreased the soil volume occupied by water, macroporosity and K sat, while increasing suspended sediment (to 0.226 g 100 mL−1), total P (to 2.0 mg L−1), mainly as particulate P (up to 90% of total P), and E. coli (to 3.52 log10 c.f.u. 100 mL−1) concentrations in overland flow at 1 day after grazing compared with soils from the rest of the paddock (0.148 g 100 mL−1, 0.86 mg L−1 and 2.86 log10 c.f.u. 100 mL−1, respectively). Although concentrations in overland flow were less at 6 weeks after grazing than at 1 day after grazing, losses of P, especially in fence-line soils, were still above recommended limits for surface water quality. Compared to P, losses of N species would be unlikely to have a significant impact on downstream water quality. Management strategies should be directed towards minimizing the occurrence of fence-line pacing to prevent contaminant loss and maintain water and soil quality.  相似文献   

3.
I. Celik   《Soil & Tillage Research》2005,83(2):270-277
Forest and grassland soils in highlands of southern Mediterranean Turkey are being seriously degraded and destructed due to extensive agricultural activities. This study investigated the effects of changes in land-use type on some soil properties in a Mediterranean plateau. Three adjacent land-use types included the cultivated lands, which have been converted from pastures for 12 years, fragmented forests, and unaltered pastures lands. Disturbed and undisturbed soil samples were collected from four sites at each of the three different land-use types from depths of 0–10 cm and 10–20 cm in Typic Haploxeroll soils with an elevation of about 1400 m. When the pasture was converted into cultivation, soil organic matter (SOM) pool of cultivated lands for a depth of 0–20 cm were significantly reduced by, on average 49% relative to SOM content of the pasture lands. There was no significant difference in SOM between the depths in each land-use type, and SOM values of the forest and pasture lands were almost similar. There was also a significant change in soil bulk density (BD) among cultivation (1.33 Mg m−3), pasture (1.19 Mg m−3), and forest (1.25 Mg m−3) soils at depth of 0–20 cm. Only for the pasture, BD of the depth of 0–10 cm was significantly different from that of 10–20 cm. Depending upon the increases in BD and disruption of pores by cultivation, total porosity decreased accordingly. Cultivation of the unaltered pasture obviously increased the soil erodibility measured by USLE-K factor for each soil depth, and USLE-K factor was approximately two times greater in the cultivated land than in the pasture indicating the vulnerability of the cultivated land to water erosion. The mean weight diameter (MWD) and water-stable aggregation (WSA) were greater in the pasture and forest soils compared to the cultivated soils, and didn’t change with the depth for each land-use type. Aggregates of >4.0 mm size were dominant in the pasture and forest soils, whereas the cultivated soils comprised aggregates of the size ≤0.5 mm. I found that samples collected from cultivated land gave the lowest saturated hydraulic conductivity values regardless of soil depths, whereas the highest values were measured on samples from forest soils. In conclusion, the results showed that the cultivation of the pastures degraded the soil physical properties, leaving soils more susceptible to the erosion. This suggests that land disturbances should be strictly avoided in the pastures with the limited soil depth in the southern Mediterranean highlands.  相似文献   

4.
Hydraulic properties of a Walla Walla silt loam were significantly changed by 50 years or more of cultivation under either a wheat-peas rotation (tillage depth 30 cm) or a wheat-summerfallow rotation (tillage depth 15 cm) as compared with no cultivation. Soil pH was reduced to depths as great as 60 cm in the cultivated sites; dry bulk density was increased to depths as great as 40 cm. Expressions of these changes were greater in the wheat-peas rotation because tillage was deeper than in the wheat-summerfallow rotation. Small reductions in soil organic matter were also noted in the cultivated sites. In the 60- to 90-cm depth, all three sites had similar bulk density, pH, cation exchange capacity, soil texture, desorption water characteristic, and hydraulic conductivity. In the upper 40-cm layer the desorption water characteristic showed that cultivation produced more smaller pores at the expense of large pores; in the upper 30-cm layer of the cultivated soils hydraulic conductivity was reduced at least 10-fold for water potentials > −100 cm of H2O. Steady-state drainage profiles and associated assumptions suggest that long-term cultivation increased the hydraulic gradient in the upper 35 cm, and that the low saturated conductivity of the 0- to 15-cm layer had an overall drying effect on the 15- to 35-cm layer. In the cultivated soils increased runoff and denitrification in the plow layer should both be expected and water relations in the 15- to 35-cm layer should favor microorganisms sensitive to high water potentials. Simulations suggested that long-term cultivation decreased evaporation rates an estimated 40% and in wet soil, increased the drying time needed to attain optimum moisture for tillage.  相似文献   

5.
Tillage management can affect crop growth by altering the pore size distribution, pore geometry and hydraulic properties of soil. In the present communication, the effect of different tillage management viz., conventional tillage (CT), minimum tillage (MT) and zero-tillage (ZT) and different crop rotations viz. [(soybean–wheat (S–W), soybean–lentil (S–L) and soybean–pea (S–P)] on pore size distribution and soil hydraulic conductivities [saturated hydraulic conductivity (Ksat) and unsaturated hydraulic conductivity {k(h)}] of a sandy clay loam soil was studied after 4 years prior to the experiment. Soil cores were collected after 4 year of the experiment at an interval of 75 mm up to 300 mm soil depth for measuring soil bulk density, soil water retention constant (b), pore size distribution, Ksat and k(h). Nine pressure levels (from 2 to 1500 kPa) were used to calculate pore size distribution and k(h). It was observed that b values at all the studied soil depths were higher under ZT than those observed under CT irrespective of the crop rotations. The values of soil bulk density observed under ZT were higher in 0–75 mm soil depth in all the crop rotations. But, among the crop rotations, soils under S–P and S–L rotations showed relatively lower bulk density values than S–W rotation. Average values of the volume fraction of total porosity with pores <7.5 μm in diameter (effective pores for retaining plant available water) were 0.557, 0.636 and 0.628 m3 m−3 under CT, MT and ZT; and 0.592, 0.610 and 0.626 m3 m−3 under S–W, S–L and S–P, respectively. In contrast, the average values of the volume fraction of total porosity with pores >150 μm in diameter (pores draining freely with gravity) were 0.124, 0.096 and 0.095 m3 m−3 under CT, MT and ZT; and 0.110, 0.104 and 0.101 m3 m−3 under S–W, S–L and S–P, respectively. Saturated hydraulic conductivity values in all the studied soil depths were significantly greater under ZT than those under CT (range from 300 to 344 mm day−1). The observed k(h) values at 0–75 mm soil depth under ZT were significantly higher than those computed under CT at all the suction levels, except at −10, −100 and −400 kPa suction. Among the crop rotations, S–P rotation recorded significantly higher k(h) values than those under S–W and S–L rotations up to −40 kPa suction. The interaction effects of tillage and crop rotations affecting the k(h) values were found significant at all the soil water suctions. Both S–L and S–P rotations resulted in better soil water retention and transmission properties under ZT.  相似文献   

6.
Abstract. The phosphorus (P) sorption and desorption dynamics of eleven major agricultural grassland soil types in Ireland were examined using laboratory techniques, so that soils vulnerable to P loss might be identified. Desorption of P from soil using the iron-oxide paper strip test (Pfeo), water extractable P (Pw) and calcium chloride extractable P (Pcacl2) depended on soil P status in all soils. However, soil types with high organic matter levels (OM), namely peat soils (%OM >30), had lower Pfeo and Pw but higher Pcacl2 values compared to mineral soils at similar soil test P levels. Phosphorus sorption capacity remaining (PSCr) was measured using a single addition of P to soils and used to calculate total P sorption capacities (PSCt) and degree of P saturation (DPS). Phosphorus sorption capacities correlated negatively with % OM in soils indicating that OM may inhibit P sorption from solution to soil. High organic matter soils exhibited low P sorption capacities and poor P reserves (total P, oxalate extractable P) compared to mineral soils. Low P sorption capacities (PSCt) in peat soils were attributed to OM, which blocked or eliminated sorption sites with organic acids, therefore, P remained in the soil solution phase (Pcacl2). In this work, peat and high organic matter soils exhibited P sorption and desorption characteristics which suggest that these soils may not be suitable for heavy applications of manure or fertilizer P owing to their low capacities for P sorption and storage.  相似文献   

7.
We assessed the effect of different tillage practices (i.e. conventional tillage and direct drilling) and pasture conditions on the infiltration and distribution of infiltrated rain water in an Australian Alfisol. Bromide was used as a tracer for the infiltrated rain under simulated rainfall conditions. The different infiltration patterns were then related to the macroporosity of the soils.

A 25-year-old permanent pasture was found to have the highest density (number per area) of macropores and percentages of transmitting macropores. A 9-year-old pasture phase in a pasture/crop rotation did not fully restore the macroporosity of the soil. Conventional cultivation by scarifying to 0.1 m for 4 years significantly reduced macropore density as well as continuity when compared with the pasture soil. The reduced macroporosity led to increased run-off by reducing preferential flow and altered the pathway of infiltrated water movement. As a consequence, the increase in water content below 0.1 m in the cultivated soil was predominantly from downward displacement of original soil solution, resulting in more leaching. The infiltrated rain water largely remained on the surface 0.1-m layer. In contrast, macroporosity found under direct drilling was similar to that of the pasture soil.  相似文献   


8.
Phosphorus (P) losses from intensively farmed dairy pastures can impair surface water quality. One of the first steps in mitigating this loss is to determine where in a field the potential for P loss is greatest. This study compared P export in overland flow from grazed pasture with areas that receive elevated P inputs and stock traffic (e.g. gateway, water trough, stream crossing and cattle lane). Intact soil blocks were removed, simulated rainfall applied and overland flow analysed for P fractions and suspended sediment (SS). Soil bulk density, hydraulic conductivity, porosity, Olsen P and water soluble P were also measured. P loss from the sites was in the order: trough > crossing > gateway > pasture. Total P losses from the trough averaged 4.20 mg P/m2 while the pasture exported 0.78 mg P/m2. In addition, runoff from lane soil was measured with total P averaging 5.98 mg P/m2, however the method used was different from the other soils. Using stepwise linear regression, Olsen P or H2O-P, % bare ground and % saturation were the most commonly occurring variables to predict P loss among the sites. This suggests that locating and minimizing the size of these areas in fields has the potential to significantly decrease P loss to surface waters.  相似文献   

9.
Abstract. The behaviour of potassium (K) in a range of arable soils was examined by plotting the change in exchangeable K of the topsoil (Δ Kex) at the end of a 3–5 year period against the K balance over the same period (fertilizer K applied minus offtake in crops, estimated from farmers' records of yield and straw removal). Based on the assumption that values for offtake per tonne of crop yield used for UK arable crops MAFF 2000) are valid averages, 10–50% of Δ Kex was explained by the balance, relationships being stronger on shallow/stony soils. Excess fertilizer tended to increase Kex and reduced fertilization decreased it, requiring between 1.2 and 5.4 kg K ha−1 for each mg L−1Δ Kex. However, merely to prevent Kex falling required an extra 20 kg K ha−1 yr−1 fertilizer on Chalk soils and soils formed in the overlying Tertiary and Quaternary deposits, despite clay contents >18%. Whereas, on older geological materials, medium soils needed no extra K and clays gained 17 kg K ha−1 yr−1. It is unlikely that the apparent losses on some soil types are anomalies due to greater crop K contents. Theory and the literature suggest leaching from the topsoil as a major factor; accumulation in the subsoil was not measured. Recommendations for K fertilization of UK soils might be improved by including loss or gain corrections for certain soil types.  相似文献   

10.
Abstract. Uptake in grass crops of ammonium acetate lactate extractable K (KAL) and reserve K (interlayer K + structural K) in soil was studied in 16 field experiments at different locations on a range of mineral soil types in Norway. The K uptake from soil, both from KAL and reserve K, was considerable, often even at the highest level of K fertilizer. During three years, only on the sandy soils with a low level of acid soluble K (KHNO3 minus KAL) was there a yield response to K fertilization. The KAL values declined rapidly and flattened off at a 'minimum level' which differed with soil type. This minimum level for KAL is a useful parameter in fertilizer planning, because the grass usually took up the K in excess of the minimum level over two years. The minimum value of KAL was significantly correlated with the content of clay + silt in soil. The decrease in KAL during the growing season was closely correlated to the KAL value in spring minus the minimum value and, therefore, the amount of K supplied to the grass from the KAL fraction can be calculated. Furthermore, the KAL value for the following spring may be estimated. The release from reserve K was partially related to acid soluble K.  相似文献   

11.
Changes to soil physical properties after grazing exclusion   总被引：3，自引：0，他引：3  
Abstract. The potential for degraded physical properties of soil to regenerate naturally after exclusion of grazing animals was examined at a long-term stocking rate trial near Armidale, New South Wales, Australia. Unsaturated hydraulic conductivity was measured before grazing was excluded, and after 7 months and 2.5 years' grazing exclusion. These data were compared with controls grazed at 10,15 and 20 sheep/ha. After 2.5 years, there were significant increases in unsaturated hydraulic conductivity at 5 and 15 mm tension in the ungrazed treatments compared with the grazed controls. The unsaturated hydraulic conductivities and bulk density of surface soils under pasture which had been ungrazed for 2.5 years were comparable to those where the pasture had been ungrazed for 27 years. We speculate that the natural amelioration of soil physical properties in these soils was due to biological activity and wetting and drying cycles, in the absence of the compactive effect of animal treading.  相似文献   

12.
Abstract. Soils in areas with high livestock density contribute to the eutrophication of aquatic ecosystems through loss of nutrients, especially phosphorus (P). In order to identify the potential for P loss from such soils we determined phosphorus extracted by water (H2O-P), by double lactate (DL-P), and P sorption capacity (PSC) and degree of P saturation (DPS) in soil samples from two counties, one with low (Harle-catchment) and the other with very high livestock density (Vechta). Both catchments are hydrologically connected with the tidal areas of the North Sea.
The mean concentrations of H2O-P (0.4mmol/kg) and DL-P (3.9 mmol/kg) were lower in the Harle-catchment than in the Vechta area (1.2 mmol/kg, 6.8mmol/kg). Although oxalate-extractable Al (Alox) and Fe (Feox) and the derived PSCs varied according to soil type and to land use, the livestock density and the resulting high concentrations of oxalate-extractable P (Pox) were shown to be the main reason for the very high DPS of up to 179% in the county of Vechta. These values exceeded DPS reported from other intensive pig feeding areas in western Europe and indicate the potential for significant P loss. Less than 40% of the variation in Pox could be explained by the routinely determined H2O-Por DL-P. Geostatistical analyses indicated that the spatial variability of Pox depended on manurial history of fields and Alox, showed still smaller-scale variability. These were the major constraints for regional assessments of P losses and eutrophication risk from agricultural soils using available soil P-test values, digital maps and geostatistical methods.  相似文献   

13.
Determination of the gas diffusion coefficient D s of peat soils is essential to understand the mechanisms of soil gas transport in peatlands, which have been one of major potential sources of gaseous carbons. In the present study, we aimed at determining the D s of peat soils for various values of the air-filled porosity a and we tested the validity of the Three-Porosity Model (Moldrup et al. 2004) and the Millington-Quirk model (1961) for predicting the relative gas diffusivity, the ratio of D s to D 0, the gas diffusion coefficient in free air. Undisturbed peat soil cores were sampled from aerobic layers in the Bibai mire, Hokkaido, Japan. The MQ model reproduced the measured D s/ D 0 curves better than the TPM. The TPM, a predictive model for undisturbed mineral soils, overestimated the D s/ D 0 values for peat soils, implying that in the peat soils the pore pathways were more tortuous than those in the mineral soils. Since the changes in the D s/ D 0 ratios with the a values of a well-decomposed black peat soil tended to be more remarkable than those of other high-moor peat soils, the existence of a positive feedback mechanism was assumed, such that peat soil decomposition itself would increase the soil gas diffusivity and promote soil respiration.  相似文献   

14.
Abstract. There is increasing evidence that phosphorus has been accumulating in the surface horizons of agricultural soils to the extent that some soils represent a potential diffuse source of pollution to surface waters. The relationships between equilibrium phosphorus concentration at zero sorption (EPC 0) of soil and a number of soil physicochemical variables were investigated in the surface layers of arable and grassland agricultural soils sampled from the Thame catchment, England. Soil EPC0 could be predicted from an equation including soil test (Olsen) P, soil phosphate sorption index (PSI) and organic matter content (OM) (R2=0.88; P <0.001) across a range of soil types and land use. The simple index Olsen P/PSI was found to be a good predictor of EPC0 (R2=0.77; P <0.001) and readily desorbable (0.02 m KCl extractable) P (R2=0.73; P <0.001) across a range of soil types under arable having soil organic matter contents of <10%.  相似文献   

15.
The effect of treatment with either gypsum or sodium chloride on the saturated hydraulic conductivity ( K s) of repacked soil columns and modulus of rupture (MOR) was studied on surface samples of two red-brown earth soils from SE wheat belt in Australia.
When the exchangeable sodium percentage (ESP) of the two soils was increased to >80, K S was substantially reduced and MOR increased relative to the untreated soil; the values of the parameters were nearly equal for these pairs of high ESP soils. However, after treatment with gypsum the Raywood soil had a K s twice, and a MOR less than half, the corresponding values for the Glenloth soil.
Micromorphological and scanning electron microscope (SEM) observations suggest that the increase in K s following gypsum treatment is associated with an increase in visible macropores and reduced clay dispersion; Na treatment increased dispersion at the soil surface, with the clay particles forming an impermeable surface seal and illuviation argillans.  相似文献   

16.
Abstract. A two year field study was conducted to evaluate the effects of straw management and tillage on the soil profile (1.5m) water storage, nature of the moisture profile, infiltration and sorptivity as influenced by rainfall, evaporativity (E0) and soil texture. The straw mulch treatment stored more moisture under low E0 rainy conditions in three coarse to medium textured soils. Straw incorporation treatment was better under low E0 rain free conditions, as well as under high E0 rainy conditions in the two coarser textured soils. In the coarsest textured soil, tillage and straw mulching were not effective in maintaining greater soil water storage under high E0 because of the very open nature of the soil. The soil moisture profiles showed a sharper increase in water content below the tilled layer in the tillage and straw- incorporation treatments than the untreated and straw mulch treatments. Tillage and straw incorporation treatments increased the sorptivity of the soil compared with untreated and straw mulch treatments respectively. The results of this study suggest that when selecting a suitable soil water conservation practice to increase water storage in the soil profile, information on soil texture and weather (rain and evaporativity) must be considered.  相似文献   

17.
Methane fluxes on agricultural and forested boreal organic soils   总被引：2，自引：0，他引：2  
Abstract. Annual methane fluxes from an organic soil in eastern Finland, originally drained and planted with birch ( Betula pendula ) and then later cultivated, were studied for two years using a chamber technique. The agricultural soils growing grass or barley or without vegetation, generally acted as sinks for CH4. Surprisingly, the agricultural soils emitted CH4 during a warm dry summer. The CH4 oxidation capacity and CH4 uptake rate of the forested site was three times that of agricultural soils. Also, the forest soil better retained its capacity to take up CH4 during a dry summer. Despite periods of CH4 emission, the agricultural soils were annual sinks for CH4, with uptake rate of CH4-C varying from 0.1 to 3.7 kg ha−1 yr−1. The forested soil had a methane uptake rate of 3.9 kg CH4-C ha−1 yr−1. All the soils acted as sinks for CH4 during winter, which contributed up to half of the annual CH4 uptake. The capacity of soils to transport gases did not explain the larger CH4 uptake rate in the forest soil. At the same gas filled porosity, the forest soil had a much larger CH4 uptake rate than the agricultural soil. Neither the soil acidity (pH 4.5 and 6.0) nor high ammonium content appeared to limit CH4 uptake. The results suggest that CH4 oxidation in agricultural organic soil is more sensitive to soil drying than CH4 oxidation in forested organic soil.  相似文献   

18.
Effects of organic matter and calcium on soil structural stability   总被引：2，自引：0，他引：2  
The cationic bridging effect of the calcium ion (Ca2+) and the flocculating ability of clay and organic matter are crucial in the formation and stability of soil aggregates. They are therefore likely to influence the soil's saturated hydraulic conductivity ( K s). We tested the individual effects of these factors on aggregate stability and related hydraulic properties, and studied the influence of clay mineralogy also. Samples from the surface (0–10 cm) of three contrasting soils in Trinidad were used. The soils were treated with three levels of Ca2+ and three levels of organic matter in a 3 × 3 × 3 factorial design and incubated for 14 days. Both aggregate stability and saturated hydraulic conductivity were influenced by all factor combinations. Interactions between soil type and Ca2+ revealed the importance of polyvalent cations in aggregate stability of soils with low activity minerals. The influence of organic matter varied with quantity; the more there was, the more stable the soil became, particularly in the soil containing little clay. Clay dispersion and slaking of expanding minerals occurred even with large additions of Ca2+ and organic matter, emphasizing the overall influence of mineralogy in determining the response of soils to stability treatments.  相似文献   

19.
Application of iron (Fe) -rich amendments to soils has been proposed as a means of decreasing phosphorus (P) losses from soils. However, anoxic conditions following soil saturation are known to increase Fe and P solubility in soils, thus cancelling out the potential benefits. Our aim was to evaluate the effects of continuous oxic, continuous anoxic and alternating anoxic/oxic conditions on P exchangeability and Fe forms in soil amended with Ca(OH)2 and FeSO4. We incubated amended and unamended soils under these conditions for 8 weeks and measured Fe forms and P exchangeability. Under oxic conditions, addition of Ca(OH)2 and FeSO4 resulted in a strong decrease in P exchangeability and an increase in oxalate-extractable Fe. Mössbauer analyses suggested that an unidentified Fe oxide (D1oxide) with a strong sorbing capacity for P was precipitated. Under continuously anoxic conditions, P exchangeability and oxalate-extractable Fe increased with or without the amendments. Mössbauer analyses suggested that there was a partial dissolution of the D1oxide phase, precipitation of another unidentified Fe oxide (S3) and a reduction of structural Fe3+ in phyllosilicate, thereby increasing soil negative charge. These transformations resulted in a strong increase in rapidly exchangeable P. Alternating anoxic and oxic periods induced the dissolution and precipitation of iron oxides and the increase and decrease in P exchangeability. Implications of the results for limiting P losses from grassland soils are discussed.  相似文献   

20.
This paper reports a systematic MRI study at 2 tesla of 23 soils, each separately saturated with a known amount of water. The percentage of that water which could be detected using various MR methods was determined by comparison with a liquid reference sample. A pulse-acquire sequence gave a bulk detection of between 47 and 94% of the known water content of saturated soil. Also for bulk measurements, the inversion-recovery sequence used for determining T1 values detected a range of 0.7–75% of the existing soil water. The CPMG sequence with an echo time (TE) of 1 ms used for determining the bulk T2 values gave lower values, in the range of 0.4–66% overall. A spin-echo MRI sequence with a TE of 2.9 ms gave an even lower bulk detection, ranging from 0.2 to 57%. These low values for the detectable water content of bulk saturated soil water reflect the loss of water magnetization which occurs even during short echo time MR sequence at 2 tesla field strength. The source of the above findings was investigated by measurements of the longitudinal (T1) and transverse (T2) relaxation times and spectral linewidths of the soil-water protons, and by conventional analysis of soil properties. The MR parameters of critical importance to water quantification are T2 and T2*, shorter values of which lead to a progressively greater loss of signal intensity for all MR protocols. Those parameters are affected by the following soil chemical and physical features: soil magnetic susceptibility, and the content of free iron oxides, clay, sand, exchangeable cations (K, Na and Ca), and organic matter. The implication of this work is that the only soil water which can be detected quantitatively at 2 tesla using a conventional spin-echo MRI protocol with echo times of 3 ms or longer is that located in the relatively large soil pores. Using the protocols investigated in this work, water in smaller pores will only be detected accurately for soils which have relatively low paramagnetic-metal impurities and/or have low clay content. Future MR studies of soil water should consider the use of other MRI protocols (e.g. for solid state), and measurement at low magnetic fields.  相似文献   

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