摘要:

Short-term (i.e., seasonal) variations are expected with some soil physical properties, whereas other properties, such as particle size, are considered static. However, less is known about long-term (i.e., decadal) variations in properties, such as particle-size fractions and water-retention characteristics, caused by continuous annual cultivation. The objective of this study was to determine the effect of land use (i.e., undisturbed virgin prairie vs. cultivated agriculture) and years of continuous annual cultivation on soil particle-size fractions and water-retention characteristics determined using soil wetting curves. Particle-size fractions (i.e., 0.05–2 mm, 0.002–0.05 mm, and <0.002 mm representing sand-, silt-, and clay-sized fractions, respectively) were determined on soil samples collected in 1987 and 2001 from the 0- to 10-cm depth along a transect across four adjacent fields representing a range from 0 to 44 years under continuous annual cultivation. Soil wetting curve data, obtained from a dewpoint potentiameter by re-wetting air-dried, crushed, and sieved soil, were fit by nonlinear regression to determine modeled water-retention characteristics for the native prairie and cultivated agricultural soil from samples collected in 2001. Particle-size fractions and water-retention characteristics were significantly affected by land use and years under continuous cultivation. The sand-sized fraction decreased and the clay-sized fraction increased significantly for the agricultural soil that had been annually cultivated the longest. Results indicate that long-term changes in particle-size fractions can occur following decades of continuous annual cultivation and that the effects of tillage on soil-water-retention characteristics can be ascertained after removing the confounding effects of initial differences in soil and pore structure by using soil wetting curves.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Inositol phosphates are the dominant class of organic phosphorus (P) compounds in most soils, but are poorly understood because they are not easily identified in soil extracts. This study reports a relatively simple technique using solution31P NMR spectroscopy and spectral deconvolution for the quantification ofmyo-inositol hexakisphosphate (phytic acid), the most abundant soil inositol phosphate, in alkaline soil extracts. An authenticmyo-inositol hexakisphosphate standard added to a re-dissolved soil extract gave signals at 5.85, 4.92, 4.55, and 4.43 ppm in the ratio 1:2:2:1. Spectral deconvolution quantified these signals accurately (102 ± 4%) in solutions containing a mixture of model P compounds by resolving the envelope of signals in the orthophosphate monoester region. In NaOH-EDTA extracts from a range of lowland permanent pasture soils in England and Wales, concentrations ofmyo-inositol hexakisphosphate determined by spectral deconvolution ranged between 26 and 189 mg P kg−1soil, equivalent to between 11 and 35% of the extracted organic P. Concentrations were positively correlated with oxalate-extractable aluminum and iron but were not correlated with total carbon, total nitrogen, clay, or the microbial biomass. This suggests thatmyo-inositol hexakisphosphate accumulates in soils by mechanisms at least partially independent of those controlling organic matter stabilization and dynamics. Furthermore,myo-inositol hexakisphosphate concentrations were positively correlated with plant-available inorganic P and negatively correlated with the carbon-to-organic P ratio, suggesting that biological P availability may, in part, regulatemyo-inositol hexakisphosphate concentrations in soils, perhaps because organisms capable of degrading this compound are favored in more P-limited environments. Solution31P NMR spectroscopy and spectral deconvolution offers a relatively simple method of quantifyingmyo-inositol hexakisphosphate in soil extracts.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

The solubility of iron oxides in soils is governed by crystal size, crystal order, isomorphous substitutions, and associations with other minerals. Their dissolution occurs by protonation, reduction, or complexation with ligands such as phosphate and inositol phosphates. In this work, the interaction of inositol hexaphosphate (IHP) and phosphate (Pi) with ferrihydrite (Fh) or ferrihydrite-kaolinite systems (Fh-KGa2) was studied to assess the effects on iron oxide dissolution. Adsorption of IHP and Pi on a two-line Fh and Fh-KGa2 was performed in the range 0 to 0.004 mol P L−1, and the release of P and Fe from samples of Fh or Fh-KGa2 saturated with IHP or Pi was evaluated at different pHs.The amount of P adsorbed on Fh increased, reaching a plateau at 2.12 μmol m−2for IHP and 4.57 μmol m−2for Pi. Sorption on KGa2 was lower. It increased to 2.24 μmol m−2for IHP and to 2.96 μmol m−2for Pi on Fh-KGa2. On the basis of the IHP/Pi ratios, it was hypothesised that IHP interacted with Fh through two of its six phosphate groups, whereas it interacted with Fh-KGa2 through one P group. Phosphate desorption from these complexes occurred only at pH ≥ 4.5 and was higher for Pi than for IHP and from the Fh-KGa2 system than from Fh. The desorption of IHP was followed by a consistent Fe release, higher at basic pHs. By contrast, Pi adsorption inhibited dissolution of both minerals, although the anion was desorbed in higher amounts compared with the P organic form.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Rice plants (Oryza sativa L.) grown on soils containing low soil testextractable P frequently do not respond to fertilizer P application under reduced conditions. The lack of rice response to fertilizer P in soils with low extractable P has been attributed to increased solubility of Fe-associated P upon flooding. The increased solubility of Mn oxides and release of Mn-associated P in flooded soils may also increase P availability to rice plants. The effect of Mn oxides on P availability has not been adequately described or quantified. In this experiment we used rice soils with low soil test P and high or low Mn or Fe oxides to measure the effects of Mn oxides on P availability under field, greenhouse, and laboratory conditions. Soils were incubated under oxidized or reduced conditions for 35 days. At the end of 0, 2, 5, 8, 11, 15, 20, 25, 30, and 35 days, soil solution pH and Eh and soluble and extractable P, Fe, and Mn were determined. Greenhouse and field experiments showed that rice plants did not respond to P application in three of four soils despite low extractable P. A large increase in P concentration following reduction was observed and was correlated positively with reactive Fe and Mn oxides in soils. During incubation water-soluble P in soils increased from 0.06 to 0.37 mg P kg−1, Fe from 0.2 to 22.0 mg Fe kg−1, and Mn from 0.08 to 6.43 mg Mn kg−1. Linear stepwise regression analysis indicated that 65 to 91% of solution P variation was attributable to Fe oxides and about 14 to 28% was attributable to Mn oxides. The contribution of Fe reduction to increased P availability was highly significant and related to large amounts of poorly crystalline Fe oxides in soils extracted by oxalate. The manganese oxide contribution to P availability was important, especially in the early stages of soil reduction in soils with large amounts of reactive Mn. In soils dominated by Mn oxides, the reduction of Mn-associated P can be a significant source of P for plants.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Particle-size fractions (psf) of mineral soils and, hence, soil texture, are the most important attributes affecting physical and chemical processes in the soil. More often, psf data are available only at a few locations for a given area and, therefore, require some form of interpolation or spatial prediction. However, psf data are compositional and, therefore, require special treatment before spatial prediction. This includes ensuring positive definiteness and a constant sum of interpolated values at a given location, error minimization, and lack of bias. In order to meet these requirements, this study applied two methods of data transformation prior to kriging of the psf of soils in two regions of eastern Australia. The two methods are additive log-ratio transformation of the psf (ALROK) and modified log-ratio transformation (mALROK). The performance of the transformed values by ordinary kriging was compared with the spatial prediction of the untransformed psf data using ordinary kriging, compositional kriging (CK) (UTOK), and cokriging, based on the criteriaprediction bias or mean error (ME) and precision (root mean square error (RMSE)), and validity of textural classification. ALROKandmALROKoutperformed UTOKand CK in terms of prediction ME and RMSE. Because of the closure effect on the psf data, UTOK, and, to a lesser extent, CK, did not meet all of the requirements for spatially predicting compositional data and, therefore, performed poorly.mALROKoutperformed all of the interpolation methods in terms of misclassification of soils into textural classes. The results show that without considering the special requirements of compositional data, spatial interpolation of psf data will necessarily produce uncertain and unreliable interpolated psf values.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Narli Plain in eastern Turkey is a 26,500-ha irrigation area brought into a single irrigation scheme between 1975 and 1978. Soils are clay loam and clays. Field observations have shown that compaction limits root development in some areas. To evaluate this, 15 soil profiles were sampled in fallow land after wheat in a rotation that involved cotton, wheat, sugar beet, and maize. Soil structure was measured using bulk density, porosity, and penetrometer resistance. Structure was described using soil thin sections with image analysis and scanning electron microscopy. Soil profiles were ranked on quality. This identified three structural groups; well structured, moderately poorly structured, and very poorly structured.Well structured soils at all depths had densities that were usually less than 1.25 Mg m−3and were always less than 1.4 Mg m−3, which equated with macroporosity greater than 18% and 10%, respectively. Poorly structured soils had densities always greater than 1.44 Mg m−3, and commonly greater than 1.5 Mg m−3, equating with a macroporosity of 9% and 5%, respectively. These values, together with data on pore sizes and morphology, correspond with acceptable versus limiting porosity. An intergrade between these limits described moderately poorly structured soils. The degree of compaction was best explained by considering its spatial distribution. This showed that the most severe compaction occurred within 2 km of the river, near the principal town, and between an archaeological site and the river. This land has been used for 3000 to 4000 years. In contrast, well structured soils occurred in the southwest away from the main settlement and on land reclaimed from marshland in 1958 and cultivated for about 40 years. Land not belonging to these groups has been cultivated for about 400 years and is located further from the river and the settlement than the severely compacted soils and has a moderately poor structure. Poor correlations were found between soil structure and clay mineralogy and calcium carbonate. Clay content was significant but accounted for only 21% of the variation. The role of organic matter was clearer despite small but statistically significant numerical differences between moderately poorly structured soils together with very poorly structured soils (0–40 cm), compared with well structured soils.The more poorly structured soils had a mean organic matter content of 1.45% compared with 2.1% for well structured soils (0 to 40 cm). Thin sections suggested that the soil structure in compact soils had been produced by tillage, partial or total aggregate collapse, and from shrink-swell activity rather than nonplant biota. This is interpreted by the presence of vughs, planar voids, and curvo-planar voids that partially isolate former peds. Biopores and microaggregates comprising coprogenic fragments are rare. A sequence of steps for the formation of severe compaction is suggested with possible remedial actions identified. This work demonstrates that an holistic approach to soil management is required to remediate compaction and that this should take into account the historical aspects of agricultural development and good practice in machinery management.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID