摘要:

Degraded Sahelian Alfisols are prone to sealing because of low soil organic matter content and exposure of finer textured subsoil attributable to erosion. We characterized the occurrence of surface seals on a watershed in Niger and related spatial distribution and structure of surface seals to selected soil properties. Study sites were located on six soil series of the Hamdallaye watershed (500 ha), with soil textures ranging from sandy loam to sand, classified as Psammentic Kandiustalf and Petroferric Kanhaplustult. All seals sampled in the watershed were structural seals and were morphologically similar, with a 0.1 to 1.0 mm thick continuous plasmic clay layer within 4 mm of the surface. Clay content averaged 12.1% in the crust samples (0-3 mm depth), which was five times greater than the clay content of unsealed sites at 0-50 mm depth (2.5%). Sand content was significantly lower in the sealed than in the unsealed 0 to 50-mm soil layer. Of the 12 sites sampled in the Hamdallaye watershed, all those with more than 5% clay at 0-50 mm depth were sealed, whereas all sites with less than 5% clay were unsealed. pH values of sealed sites were 1.3 units lower than those of unsealed sites and were similar to those of the subsoil. Organic C contents of sealed sites and were low (1-2 g kg−1) at 0-50 mm depth and slightly higher at unsealed sites. Sealed sites had greater dry-stable and greater wet-stable aggregation than unsealed sites because of higher clay content; however, aggregation was too weak to withstand raindrop impact. Extractable Fe and Al contents of the six soil series were related to clay content, which was likely the controlling factor of seal formation. With simulated rainfall of 90 mm h−1, the same six soils formed a seal during the initial 30-minute rainfall event in most cases, with no change in layer thickness thereafter. Soil with more than 15% (silt+clay) content formed a 2-layer structural seal, whereas coarser textured soils developed 4-layer structural seals. Seal structure of intact field samples and rainfall simulator-prepared samples differed in that many field samples did not have characteristic layers above the clay accumulation layer, which was attributed to erosion.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

Field studies on the effects of heating soil are rare. Studies in which the temperature was varied and measured are even rarer. To cover this lack of knowledge, we conducted some experimental fires in which the fuel was controlled to obtain different fire intensities. Induced temperatures were measured by k-type thermocouples placed at various soil depths. These induced temperatures were highly variable at the soil surface, whereas below 2.5-cm depth, the temperature rise was negligible. Modifications of various soil physico-chemical parameters caused by fire were related to increasing temperatures at the soil surface. The passage of fire promoted the aggregation of clay particles into sand-sized particles, which become even more resistant to the disrupting action of water. The soil pH decreased at temperatures up to 395°C, followed by a clear increase at higher temperatures; however, both the cation exchange capacity and the organic matter content decreased continuously with increasing soil temperature. Total nitrogen content remained surprisingly constant as the soil surface temperature increased, whereas ammonium nitrogen peaked at 184°C. The organic phosphorus decreased continuously, and the available fractions increased.These results are in agreement with the results of our previous research on the effects of the artificial heating on soil properties and confirm that the processes occurring in soil are related to the temperature reached during the fire. The ashes, at least immediately after fire, do not affect these processes.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

The increased use of sewage sludge as a fertilizer, combined with reports that large fractions of sludge-borne heavy metals cannot be accounted for several years after land application, indicates that more detailed study of potential mobility of these metals in soils is needed. A field site that had received a single heavy application of municipal sewage sludge was re-investigated 15 years later to measure the degree to which toxic heavy metals as well as nutrient elements remained in the topsoil. Total elemental composition of soil samples collected recently at this site was measured by inductively coupled plasma (ICP) spectrometry analysis of acid digests as well as by neutron activation analysis (NAA). These two methods gave comparable results for most sludge-applied heavy metals, but not for those elements that reside largely in mineral lattice structures resistant to acid digestion.Based on elemental deficits calculated using soil chromium concentration as an indicator of the original sludge concentration in the soil, it is estimated that most of the sludge-applied Na, S, Ca, and Sr, about 40% of the Zn and Cu, and less than 30% of the Cd and P have been lost from the topsoil. There is limited evidence that about half of other heavy metals such as Hg, Ag, and Au has also been lost. The ICP analysis of water extracts of the soils shows present concentrations of water-soluble Cu, Zn, and Ni in the sludge-amended topsoil are more than 10 times higher than those in the control soil. Ion-selective electrode measurements confirm that a large fraction of soluble Cu appears to be in an organically complexed and mobile form, which leaches to shallow groundwater. To varying degrees, numerous other elements, including Cd, P, and S, have solubilities substantially higher than in the control soil after the 15-year period. Nevertheless, despite the fact that the elemental deficits are correlated to the distribution coefficients of the elements, the deficits can only be accounted for by leaching if those elements were more soluble in the soil immediately after sludge application than they are now. The Zn and Cd remaining in the topsoil (pH 6.5-7.0) after 15 years is plant-available, as indicated by excess uptake and severe phytotoxicity symptoms in vegetable crops.In summary, although the vertical metal concentration profiles failed to manifest substantial movement of most trace metals, it appears that large fractions of certain metals applied in the sludge amendment have redistributed and moved out of the soil surface by physical-chemical or biological processes and that there is potential for groundwater and surface water contamination.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

Accumulation of partially decomposed plant residues under no-tillage (NT) and cover crop management systems can affect herbicide fate in the soil. This study evaluated adsorption and desorption of cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile} in soils and herbicide-killed Italian ryegrass (Lolium multiflorumLam.) residues collected from a long-term conventional tillage (CT) and NT cotton field. The four cotton production systems included were CT and NT, each with and without ryegrass as a cover crop. Adsorption was determined by reacting 0.5 g of soil or ryegrass residue with 8 mL of14C-cyanazine solution (five concentrations: 0.13 to 15.68 μmol L−1) for 48 h. The Freundlich Kfvalues were higher in NT than in CT soils and higher in soils from ryegrass cover crop than in soils from no cover crop. The Kfwas higher in ryegrass residue (13.33) than in soils (1.77 to 2.94). The N values for soils (>0.90) and ryegrass residue (>0.95) indicated nearly linear adsorption. Time-course adsorption data analyzed by an equilibrium/kinetic model indicated that adsorption was rapid initially (within 1 h), followed by a slow increase in CT and NT soils from ryegrass plots. In contrast, adsorption achieved equilibrium within 48 h of reaction time in ryegrass residue. Cyanazine adsorption increased with increased decomposition of plant residues. The Kffor ryegrass residues sampled at 5 weeks after cotton planting was 17% higher than the residues sampled at 3 weeks before planting. The CaCl2-desorbable cyanazine in two consecutive 24-h cycles ranged from 77 to 88% in soils and from 46 to 47% of that adsorbed in ryegrass residues. Two additional 24-h desorptions with methanol removed most of the remaining cyanazine. Under field conditions, the plant residues on the soil surface in NT and cover crop systems can apparently intercept and temporarily retain cyanazine.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

A soil core method is described for monitoring rates and extent of N-mineralization and denitrification from intact leguminous cover crops (e.g., hairy vetch) as a function of soil water content. The method also allows for estimates of N-fixation in order to perform N-mass balances. Field conditions were simulated by growing cover crops in soil cores from seed to biomass levels comparable to the field, followed by harvest/kill. Soil cores were wetted periodically using a rain simulator. After simulated rain events, samples of leachate were obtained and soil water content adjusted by application of a vacuum (15 kPa) to the bottom of cores. The use of a PVC/silica filter (bubble point = 30 kPa) allowed for cores to be drained to field capacity without pulling ambient air through the soil. N-fixation (before harvest/kill) and N-mineralization (after harvest/kill) were determined by comparing NO − [over] 3 leached from vetch with fallow cores. Denitrification was determined by periodically sealing cores, injecting and recirculating acetylene throughout cores, and quantifying N2O production after 48 h. Preliminary experiments with hairy vetch (Vicia villosa) indicate that plants are reasonably efficient at taking up soil NO − [over] 3, intact roots decompose fairly rapidly in soil (<6 weeks), and there is potential for substantial losses of soil NO − [over] 3 as a result of denitrification at soil water contents ≥70%.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

In Antarctica, ornithogenic soils from penguin guano play an important role in nutrient cycles in the ecosystem. Soil organic matter (SOM) degradation and translocation are the main determinants of these dynamics, of which podzolization is probably one, in ornithogenic soil. The purpose of this study is to describe the SOM in the older ornithogenic soils of Coastal Continental Antarctica in order to learn more about the organic matter of these soils. The SOM investigations were carried out by means of wet chemical SOM analyses and carbon-13 and nitrogen-15 nuclear magnetic resonance (NMR) spectroscopy. The purpose is to compare these data with those obtained from spodic horizons formed under temperate climate conditions. This comparison should provide the first information concerning the possibility of podzolization in Antarctica. In contrast to the spodic horizons in Germany, the SOM of the antarctic soils is characterized by a high percentage of amino derivates from proteins, polysaccharides, urates, and chitin, resulting in a mean C-to-N ratio of 10 and a high content of carboxyl carbon units, which probably derived from amino and other organic acids. The15N-NMR spectrum of penguin guano suggests the presence of uric acid (2,6,8-trioxypurine). Our data suggest that migration of organic acids, not-humified carbohydrates, and N-containing moieties from the topsoil moving into the spodic horizons of the ornithogenic soils affect the podzolization process. In the SOM of the German Podzols, N-compounds and not-humified carbohydrates were of minor importance within the SOM translocation processes.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID