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1.
The aim of this study was to investigate how long‐term Mancozeb application to banana plantations affects the occurrence of pollutants in drainage‐channel sediment and water under tropical conditions. We estimated the possible accumulation of Mancozeb's principal metabolite ethylenethiourea (ETU), as well as manganese (Mn) and zinc (Zn) as components in channel sediment and water. We took samples during the tropical‐rainfall season and the low‐rainfall season. For sediment samples, we determined the contents of ETU, Mn, and Zn. For water samples, we determined the concentration of ETU. Additionally, we took water samples from a runnel that is the receiving body of hydraulic flow from the system. In both seasons, ETU in the sediment was near the detection limit (0.01 mg kg–1) and did not show any accumulation. However, Mn in sediment at all sampling sites exceeded the threshold values for aquatic life of 630 mg kg–1 with values between 635 and 7256 mg kg–1. The Zn concentrations in sediment varied from 87 to 190 mg kg–1 and exceeded the threshold values for aquatic life of 98 mg kg–1 at several sites. Furthermore, we determined an accumulation of these heavy metals in the sediments of the banana‐planted zone in comparison with sediments in pasture reference sites. In contrast to the low concentration of ETU in sediment, its concentration in drainage and runnel water (5.9–13.8 μg L–1) exceeded the EU threshold value for drinking water (0.1 μg L–1) by up to nearly 140 times. However, the threshold value for aquatic life was not exceeded. We conclude that long‐term Mancozeb application does lead to a severe accumulation of Mn in sediments and of ETU in surface water. New strategies should be used to control black Sigatoka, including integrated methods of pest control so that long‐term negative effects on the environment can be avoided.  相似文献   

2.
Repeated applications of bordeaux mixture （a blend of copper sulfate and calcium hydroxide） and pyrethroid insecticides （Pys） have led to elevated copper （Cu） and Pys concentrations in vineyard surface soils. To understand the potential influence of Cu on the fate of Pys in the soil environment, we selected two Pys, cypermethrin （CPM） and lambda-cyhalothrin （λ-CHT）, and two typical Chinese vineyard soils, Haplic Acrisol and Luvic Phaeozem, as experimental samples. The dissipation experiment was conducted at room temperature in the dark, and the transport of both Pys through the soils was investigated using soil thin-layer chromatography. The results showed that the transport of Pys in both soils increased as the Cu2+ concentration increased from 0 to 100 mg L-1 , and Pys were more transportable in Haplic Acrisol （HA） than in Luvic Phaeozem （LP） under the same experimental conditions. For CPM, only 100 mg L-1 of Cu2+ significantly （P<0.05） increased Pys transport through both soils relative to water. Lambda-CHT was significantly （P<0.05） transported through HA by all the Cu2+ concentrations compared to water, and all but the 1 mg L-1 of Cu2+ significantly （P<0.05） increased the transport of λ-CHT through LP relative to water. However, the dissipation rates of CPM and λ-CHT decreased with the addition of Cu to soils. Our findings suggest that the risk of groundwater contamination by Pys increases in the soils with elevated Cu concentrations.  相似文献   

3.
Paddy soil management is generally thought to promote the accumulation of soil organic matter (SOM) and specifically lignin. Lignin is considered particularly susceptible to accumulation under these circumstances because of the recalcitrance of its aromatic structure to biodegradation under anaerobic conditions (i.e ., during inundation of paddy fields). The present study investigates the effect of paddy soil management on SOM composition in comparison to nearby agricultural soils that are not used for rice production (non‐paddy soils). Soil types typically used for rice cultivation were selected, including Alisol, Andosol and Vertisol sites in Indonesia (humid tropical climate of Java) and an Alisol site in China (humid subtropical climate, Jiangxi province). These soil types represent a range of soil properties to be expected in Asian paddy fields. All upper‐most A horizons were analysed for their SOM composition by solid‐state 13C nuclear magnetic resonance (NMR) spectroscopy and for lignin‐derived phenols by the CuO oxidation method. The SOM composition was similar for all of the above named parent soil types (non‐paddy soils) and was also not affected by paddy soil management. A substantial proportion (up to 23%) of the total aryl‐carbon in some paddy and non‐paddy soils was found to originate from condensed aromatic‐carbon (e.g ., charcoal). This may be attributed to the burning of crop residues. On average, the proportion of lignin was low and made up 20% of the total SOM, and showed no differences between straw, particulate organic matter (POM), and the bulk soil material. The results from CuO oxidation are consistent with the data obtained from solid‐state 13C NMR spectroscopy. The extraction of lignin‐derived phenols revealed low VSC (vanillyl, syringyl, cinnamyl) values for all investigated soils in a range (4 to 12 g kg−1 OC) that was typical for agricultural soils. In comparison to adjacent non‐paddy soils, the data do not provide evidence for a substantial accumulation of phenolic lignin‐derived structures in the paddy soils, even for those characterized by higher organic carbon (OC) contents (e.g ., Andosol‐ and Alisol (China)‐derived paddy soils). We conclude that the properties of the parent soil types are more important for the lignin content of the soils than the effect of paddy management itself.  相似文献   

4.
In the humid tropics, soil erosion due to the impact of high‐intensity tropical rainfall is one of the important environmental problems. A quick assessment of slaking sensitivity of soils that are frequently subjected to the fast wetting of intense rainfall of the humid tropics is necessary for the selection of appropriate soil management practices to avoid soil structure deterioration that results in runoff, seal formation, erosion and eventual degradation. Unfortunately, field and laboratory measurements of slaking sensitivity are tedious, time consuming and expensive. Therefore, a slaking sensitivity ranking framework using readily available soil data, namely, clay content, organic matter content, exchangeable sodium percentage (ESP) and cation exchange capacity (CEC) determined to be important in slaking sensitivity and structural degradation under intense rainfall was developed. The ranking framework was subsequently used to classify 23 agriculturally important Trinidadian soils into slaking sensitivity classes for management recommendation. A simple mathematical model that provides a rapid assessment of slaking sensitivity was also developed using the soil data of 14 out of the 23 soils and subsequently tested on the remaining nine soils. Our results suggest that about 80 per cent or more of the soils are highly sensitive to slaking pressures, highly vulnerable to degradation and require management practices that reduce the rate of wetting and thus degradation of aggregates under intense rainfall. The developed model performed with a high degree of accuracy as the predicted values were in close agreement with measured values (r = 0·93). This suggests that the model gives a good indication of the structural degradation vulnerability of the soils studied under the conditions applied and criteria used. The model is, therefore, recommended for use in the tested humid tropical soils. However, more comprehensive testing is required on a broader range of soils prior to its more widespread application in other climatic conditions. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

5.
The mineralogy of five soils situated on a south-west to north-east transect of Guanxi Province, south-east China was determined by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA) and chemical composition. One soil had formed on granite under tropical conditions, the others on sedimentary rocks under subtropical conditions. In all soils, kaolinite dominates the clay fractions, and is accompanied by vermiculite or chloritized vermiculite. In the saprolites of the granite-derived and sandstone-derived soils, the kaolinite has a morphology close to that of hydrated halloysite. The formamide and hydrazine/water tests suggested the presence of both poorly-crystalline (dehydrated halloysite) and well-ordered kaolinite. Gibbsite was identified in the saprolites of the granite-derived and sandstone-derived soils but not in the soils themselves. Gibbsite was also identified in the lowermost horizon of a soil derived from Pleistocene sediments. The upper horizon clay of a Rendzina soil formed on Permian limestone contains much more gibbsite and is much more weathered than the corresponding lower horizon clay. Only in the more humid, southernmost soil can the mineral composition be explained by present-day climatic conditions. In the soils from drier areas, the clay mineral composition reflects weathering that had taken place under moister, paleoclimatic conditions. Some of these paleoclimatic conditions had been conducive to lateritic weathering, as is indicated by the presence of goethite-containing pisolitic nodules in one of the soils and in its parent material. Two of the soils appear to be polymorphic, with one part of the soil having weathered more strongly than the other part.  相似文献   

6.
The effects of wheat, potato, sunflower, and rape residues and calcite were evaluated in soil that received sodic water. These materials were added to a sandy‐loam soil at a rate of 5%, after which the treated soils were incubated for 1 month at field‐capacity moisture and a temperature of 25°C–30°C. Column leaching experiments using treated soils were then conducted under saturated conditions using water with three sodium‐adsorption ratios (SAR) (0, 10, 40) with a constant ionic strength (50 mmol L–1). The results indicated that the application of plant residues to soils caused an increase in cation‐exchange capacity and exchangeable cations. Leaching experiments indicated that the addition of plant amendments led to increased Na+ leaching and decrease in exchangeable‐sodium percentage (ESP). The ESP of the control soil, after leaching with solutions with an SAR of 10 and 40, increased significantly, but the level of sodification in soils treated with plant residue was lower. Such decreases of soil ESP were greatly affected by the type of plant residues, with the order of: potato‐treated soil > sunflower‐treated soil > rape‐treated soil > wheat‐treated soil > calcite‐treated soil > control soil.  相似文献   

7.
Abstract

Irrigation is becoming a more commonly used practice on glacially derived soils of the Northern Great Plains. Threshold salinity and sodicity water quality criteria for soil‐water compatibility in these sulfatic soils are not well defined. This study was conducted to relate soil salinity and sodicity to clay dispersion and saturated hydraulic conductivity (Ksat) in four representative soils. Soil salinity (EC treatment levels of 0.1 and 0.4 S m‐1) and sodicity (SAR treatment levels of 3, 9, and 15) levels were established to produce a range of conditions similar to those that might be found under irrigation. The response of each soil to changes in salinity and sodicity was unique. In general, as sodicity increased clay dispersion also increase, but the magnitude of the increase varied among the soils. In two of the soils, clay dispersion across a range of sodicity levels was lower under the 0.4 S m‐1 treatment than under the 0.1 S m‐1 treatment and in the other two soils, clay dispersion across a range of sodicity levels was similar between the two salinity treatments. Changes in Ksat were greatest in the finer textured soil (decreasing an order of magnitude across the range of sodicity levels), but was unchanged in the coarse textured soils. Results suggest that these sulfatic soils are more susceptible to sodicity induced deterioration than chloridic soils. These results and earlier field observations suggest that sustainable irrigation may be limited to sites with a water source having a SAR <5 and an EC not exceeding 0.3 S m‐1 for these sulfatic glacially derived soils.  相似文献   

8.
A key issue when researching land degradation is the pollution of soils. For bioremediation of contaminated soil, Burkholderia sp. XTB‐5 cells were obtained from soil and grown on mineral salt medium with initial phenol concentrations of 650 mg L−1 and 850 mg L−1, which were found to degrade more than 98% of phenol content in less than 4 days. About 90% of phenol content (with initial concentration of 250 mg kg−1 soil) was removed from soil inoculated with XTB‐5 cells in 6 days. More than 90% of phenol content was removed within 20 days after co‐introduction of XTB‐5 cells and plants to sterilized soil in a greenhouse or to natural soil in field trials. But under the same conditions, individual introduction of plants to sterilized soil in the greenhouse reduced phenol content by about 50% and introduction to natural soil in field trials reduced phenol content by about 38%, suggesting that phytoremediation of phenol is often inefficient and microorganisms can efficiently degrade this pollutant. In addition, strain XTB‐5 was found to solubilize phosphate and produce 1‐Aminocyclopropane‐1‐Carboxylate (ACC) deaminase and siderophore. Strain XTB‐5 promoted plant growth in both phenol‐absent and phenol‐spiked soil under greenhouse and field conditions. Considering that ACC deaminase is beneficial to plant growth under adverse environmental conditions, plant growth promotion by XTB‐5 in phenol‐contaminated soil is not only due to XTB‐5 cell‐degradation of phenol and reduced phytotoxicity but also to production of ACC deaminase. Hence, Burkholderia sp. XTB‐5 presents an attractive microorganism for phytoremediation of contaminated soil and agronomic application. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

9.
Dissolved organic matter (DOM) is important for the cycling and transport of carbon (C) and nitrogen (N) in soil. In temperate forest soils, dissolved organic N (DON) partly escapes mineralization and is mobile, promoting loss of N via leaching. Little information is available comparing DOC and DON dynamics under tropical conditions. Here, mineralization is more rapid, and the demand of the vegetation for nutrients is larger, thus, leaching of DON could be small. We studied concentrations of DOC and DON during the rainy seasons 1998–2001 in precipitation, canopy throughfall, pore water in the mineral soil at 5, 15, 30, and 80 cm depth, and stream water under different land‐use systems representative of the highlands of northern Thailand. In addition, we determined the distribution of organic C (OC) and N (ON) between two operationally defined fractions of DOM. Samples were collected in small water catchments including a cultivated cabbage field, a pine plantation, a secondary forest, and a primary forest. The mean concentrations of DOC and DON in bulk precipitation were 1.7 ± 0.2 and 0.2 ± 0.1 mg L–1, respectively, dominated by the hydrophilic fraction. The throughfall of the three forest sites became enriched up to three times in DOC in the hydrophobic fraction, but not in DON. Maximum concentrations of DOC and DON (7.9–13.9 mg C L–1 and 0.9–1.2 mg N L–1, respectively) were found in samples from lysimeters at 5 cm soil depth. Hydrophobic OC and hydrophilic ON compounds were released from the O layer and the upper mineral soil. Concentrations of OC and ON in mineral‐soil solutions under the cabbage cultivation were elevated when compared with those under the forests. Similar to most temperate soils, the concentrations in the soil solution decreased with soil depth. The reduction of OC with depth was mainly due to the decrease of hydrophobic compounds. The changes in OC indicated the release of hydrophobic compounds poor in N in the forest canopy and the organic layers. These substances were removed from solution during passage through the mineral soil. In contrast, organic N related more to labile microbial‐derived hydrophilic compounds. At least at the cabbage‐cultivation site, mineralization seemed to contribute largely to the decrease of DOC and DON with depth, possibly because of increased microbial activity stimulated by the inorganic‐N fertilization. Similar concentrations and compositions of OC and ON in subsoils and streams draining the forested catchments suggest soil control on stream DOM. The contribution of DON to total dissolved N in those streams ranged between 50% and 73%, underscoring the importance of DOM for the leaching of nutrients from forested areas. In summary, OC and ON showed differences in their dynamics in forest as well as in agricultural ecosystems. This was mainly due to the differing distribution of OC and ON between the more immobile hydrophobic and the more easily degradable hydrophilic fraction.  相似文献   

10.
Conversion of natural forest to intensive cultivation makes to soil susceptible to flooding, declining fertility and loss of organic matter (OM) and reduced water movement into and within the soil. We studied infiltration rates and related soil penetrating indicators of forested and cultivated soils in humid tropical coastal plain sands in Southern Nigeria. Results showed that mean-weight diameter (MWD) and water stability of aggregates were higher in forested than cultivated soils. Stable aggregates > 1.00 mm were 16.5% and 31.1% respectively, for cultivated and forested soils at 0–15 cm depth, indicating formation of more macro-aggregates in forested soil. Soil disturbance through cultivation decreased hydraulic conductivity and increased bulk density of the soil. Infiltration rate attained after 2 hours was higher in forested soil. Temporary infiltration rate of 178 mm hr?1 at initial time in cultivated soil was followed by very low infiltration rate of 7 mm hr?1 after 2 hours. Soil organic matter (SOM), saturated hydraulic conductivity, MWD and total sand correlated positively with infiltration rates are r = 0.76, 0.61, 0.57 and 0.51 respectively. Changes in these parameters are dependent on surface soil disturbance by cultivation. Cultivation of forest decreased infiltration rates and water transmission properties of the soil.  相似文献   

11.
Abstract

Loss of soil‐water saturation may impair growth of rainfed lowland rice by restricting nutrient uptake, including the uptake of added phosphorus (P). For acidic soils, reappearance of soluble aluminum (Al) following loss of soil‐water saturation may also restrict P uptake. The aim of this study was to determine whether liming, flooding, and P additions could ameliorate the effects of loss of soil‐water saturation on P uptake and growth of rice. In the first pot experiment, two acid lowland soils from Cambodia [Kandic Plinthaqult (black clay soil) and Plinthustalf (sandy soil)] were treated with P (45 mg P kg?1 soil) either before or after flooding for 4 weeks to investigate the effect of flooding on effectiveness of P fertilizer for rice growth. After 4 weeks, soils were air dried and crushed and then wet to field capacity and upland rice was grown in them for an additional 6 weeks. Addition of P fertilizer before rather than after flooding depressed the growth of the subsequently planted upland rice. During flooding, there was an increase in both acetate‐extractable Fe and the phosphate sorption capacity of soils, and a close relationship between them (r2=0.96–0.98). When P was added before flooding, Olsen and Bray 1‐extractable P, shoot dry matter, and shoot P concentrations were depressed, indicating that flooding decreased availability of fertilizer P. A second pot experiment was conducted with three levels of lime as CaCO3 [to establish pH (CaCl2) in the oxidized soils at 4, 5, and 6] and four levels of P (0, 13, 26, and 52 mg P kg?1 soil) added to the same two acid lowland rice soils under flooded and nonflooded conditions. Under continuously flooded conditions, pH increased to over 5.6 regardless of lime treatment, and there was no response of rice dry matter to liming after 6 weeks' growth, but the addition of P increased rice dry matter substantially in both soils. In nonflooded soils, when P was not applied, shoot dry matter was depressed by up to one‐half of that in plants grown under continuously flooded conditions. Under the nonflooded conditions, rice dry matter and leaf P increased with the addition of P, but less so than in flooded soils. Leaf P concentrations and shoot dry matter responded strongly to the addition of lime. The increase in shoot dry matter of rice with lime and P application in nonflooded soil was associated with a significant decline in soluble Al in the soil and an increase in plant P uptake. The current experiments show that the loss of soil‐water saturation may be associated with the inhibition of P absorption by excess soluble Al. By contrast, flooding decreased exchangeable Al to levels below the threshold for toxicity in rice. In addition, the decreased P availability with loss of soil‐water saturation may have been associated with a greater phosphate sorption capacity of the soils during flooding and after reoxidation due to occlusion of P within ferric oxyhydroxides formed.  相似文献   

12.
Abstract

On air‐drying, the ATP contents of two moist soils fell to about one quarter of their original values. When a freshly‐sampled soil (field temperature 5.5°C) was stored moist (43% water holding capacity) for 7 days at 25°C the ATP content increased from 4.54 to 7.84 μg ATP g‐1 soil. Storage at 10°C caused a smaller increase; to 5.39 μg g‐1 soil. Microbial biomass C also increased on storage but the relative increase was less than that of ATP. Thus the biomass C/ATP ratio fell from 234 in the freshly sampled soil to 168 in the soil stored moist for 7 days at 25°C. The ATP content declined to less than half its starting value if storage was under waterlogged conditions.

The ATP method for determining microbial biomass in soil depends on the use of a constant factor (5.85 mg ATP g‐1 biomass C) for converting ATP content to biomass C. This factor came from work on soils that had been stored moist at 25°C for several days before biomass C and ATP measurements were made: it is only applicable to soils that have been stored in this way.  相似文献   

13.
Calcium hypochlorite (Ca(OCl)(2)) and chlorine dioxide (ClO(2)), common disinfecting and bleaching chemicals used in the food industry, are potent oxidizing agents. In this paper, the degradation effects of chlorine dioxide on mancozeb and ethylenethiourea (ETU) residues were investigated in a model system and compared with those of liquid chlorine, under various conditions such as differing concentration, pH, reaction time, and temperature. All samples were analyzed for residues by GLC and HPLC. Rate of mancozeb degradation was dependent on pH, with pH 4.6 being the most effective. Mancozeb residues decreased 40-100% with chlorine and chlorine dioxide treatments. ETU residue concentrations in mancozeb solutions were monitored over 60 min. Under controlled conditions, the ETU residue concentrations increased up to 15 min reaction time and then decreased in all three pH ranges. Treatment with both chlorine and chlorine dioxide at pH 4.6, yielded no ETU residues at both 10 and 21 degrees C. The results show that chlorine dioxide gives excellent degradation effects at lower concentrations than liquid chlorine.  相似文献   

14.
Degradation of atrazine and alachlor in saturated aquifer materials and soil was studied in the laboratory. A static aquifer was represented by a set of stagnant flasks and a well-mixed aquifer was simulated by recirculating columns. Water was tested at selected time intervals over six months and analyzed for herbicides and metabolites. Under all conditions, atrazine was more persistent than alachlor. Increased temperature had little effect on atrazine dissipation but did increase alachlor degradation rates, especially in the sterilized treatments. The addition of carbon and nitrogen prolonged the initial period before the onset of degradation in some of the columns. Enhanced mass transfer of the herbicides, nutrients, and oxygen in the recirculating columns dramatically increased dissipation of atrazine and alachlor. The degradation rates of atrazine and alachlor were 2 to 5 times faster in the recirculating columns than in the stagnant flasks. Atrazine was more persistent in the aquifer materials than in the soils, while alachlor dissipation was similar in the soils and recirculating aquifer columns, but was slower in the stagnant flasks. The prolonged persistence of atrazine under static, aquifer conditions (t 1/2 = 206 to 710 days) indicates that natural mechanisms are not sufficient to alleviate the risk of atrazine buildup over time; however, in a well mixed aquifer, atrazine degradation rates should be higher (t 1/2 = 66 to 106 days) and the threat of atrazine accumulation is diminished. Alachlor persistence at low concentrations (< 10 μg L?1) in aquifers should not pose a long-term threat to ground water supplies.  相似文献   

15.
Knowledge of soil water repellency distribution, of factors affecting its occurrence and of its hydrological effects stems primarily from regions with a distinct dry season, whereas comparatively little is known about its occurrence in humid temperate regions such as typified by the UK. To address this research gap, we have examined: (i) water repellency persistence (determined by the water drop penetration time method, WDPT) and degree (determined by the critical surface tension method, CST) for soil samples (0–5, 10–15 and 20–25 cm depth) taken from 41 common soil and land‐use types in the humid temperate climate of the UK; (ii) the supposed relationship of soil moisture, textural composition and organic matter content with sample repellency; and (iii) the bulk wetting behaviour of undisturbed surface core samples (0–5 cm depth) over a period of up to 1 week. Repellency was found in surface samples of all major soil textural types amongst most permanently vegetated sites, whereas tilled sites were virtually unaffected. Repellency levels reached those of the most severely affected areas elsewhere in the world, decreased in persistence and degree with depth and showed no consistent relationship with soil textural characteristics, organic matter or soil moisture contents, except that above a water content of c. 28% by volume, repellency was absent. Wetting rate assessments of 100 cm3 intact soil cores using continuous water contact (–20 mm pressure head) over a period of up to 7 days showed that across the whole sample range and irrespective of texture, severe to extreme repellency persistence consistently reduced the maximum water content at any given time to well below that of wettable soils. For slightly to moderately repellent soils the results were more variable and thus hydrological effects of such repellency levels are more difficult to predict. The results imply that: (i) repellency is common for many land‐use types with permanent vegetation cover in humid temperate climates irrespective of soil texture; (ii) supposedly influential parameters (texture, organic matter, specific water content) are poor general predictors of water repellency, whereas land use and the moisture content below which repellency can occur seem more reliable; and (iii) infiltration and water storage capacity of very repellent soils are considerably less than for comparable wettable soils.  相似文献   

16.
Ryegrass uniformly labelled with 1 4C was allowed to decompose for 10 years under field conditions in a range of contrasting soils. The amount of organic matter already in a soil had no effect on the retention of labelled C by that soil, nor had a variation in soil pH of from 4.9 to 8.1. Decomposition was initially slower in a strongly acid soil (pH 3.7) but by the end of 5 years the difference between this soil and the others had almost disappeared. The more clay in a soil, the greater the retention of labelled C over the whole 10 year period; this was true of both strongly acid and near-neutral soils. More labelled organic matter was leached from a soil containing 7.6% clay than from one with 17.5% clay, but the amount thus lost was insufficient to account for the difference in retention of C by the two soils. The decomposition of labelled plant material was faster in bare soil than in soil growing grass but the ‘protection’ thus given to the labelled C by the growing grass ended when the grass was removed. In bare soil about one third of the labelled ryegrass C was left after one year but thereafter decomposition became very much slower and about one eighth of the labelled C still remained in the soil after 10 years. The decay curve can be represented by a two compartment model, in which about 70% of the ryegrass C decomposed by a first order process of half life 0.25 years and the remainder by a similar process of half-life 8 years.  相似文献   

17.
Ferralsols have high structural stability, although structural degradation has been observed to result from forest to tillage or pasture conversion. An experimental series of forest skidder passes in an east Amazonian natural forest was performed for testing the effects of mechanical stress during selective logging operations on a clay‐rich Ferralsol under both dry and wet soil conditions. Distinct ruts formed up to 25 cm depth only under wet conditions. After nine passes the initially very low surface bulk density of between 0.69 and 0.80 g cm?3 increased to 1.05 g cm?3 in the wet soil and 0.92 g cm?3 in the dry soil. Saturated hydraulic conductivities, initially >250 mm h?1, declined to a minimum of around 10 mm h?1 in the wet soil after the first pass, and in the dry soil more gradually after nine passes. The contrasting response of bulk density and saturated hydraulic conductivity is explained by exposure of subsoil material at the base of the ruts where macrostructure rapidly deteriorated under wet conditions. We attribute the resultant moderately high hydraulic conductivities to the formation of stable microaggregates with fine sand to coarse silt textures. We conclude that the topsoil macrostructure of Ferralsols is subject to similar deterioration to that of Luvisols in temperate zones. The stable microstructure prevents marked compaction and decrease in hydraulic conductivity under wetter and more plastic soil conditions. However, typical tropical storms may regularly exceed the infiltration capacity of the deformed soils. In the deeper ruts water may concentrate and cause surface run‐off, even in gently sloping areas. To avoid soil erosion, logging operations in sloping areas should therefore be restricted to dry soil conditions when rut formation is minimal.  相似文献   

18.
The proportional differences in soil organic carbon (SOC) and its fractions under different land uses are of significance for understanding the process of aggregation and soil carbon sequestration mechanisms. A study was conducted in a mixed vegetation cover watershed with forest, grass, cultivated and eroded lands in the degraded Shiwaliks of the lower Himalayas to assess land‐use effects on profile SOC distribution and storage and to quantify the SOC fractions in water‐stable aggregates (WSA) and bulk soils. The soil samples were collected from eroded, cultivated, forest and grassland soils for the analysis of SOC fractions and aggregate stability. The SOC in eroded surface soils was lower than in less disturbed grassland, cultivated and forest soils. The surface and subsurface soils of grassland and forest lands differentially contributed to the total profile carbon stock. The SOC stock in the 1.05‐m soil profile was highest (83.5 Mg ha−1) under forest and lowest (55.6 Mg ha−1) in eroded lands. The SOC stock in the surface (0–15 cm) soil constituted 6.95, 27.6, 27 and 42.4 per cent of the total stock in the 1.05‐m profile of eroded, cultivated, forest and grassland soils, respectively. The forest soils were found to sequester 22.4 Mg ha−1 more SOC than the cultivated soils as measured in the 1.05‐m soil profiles. The differences in aggregate SOC content among the land uses were more conspicuous in bigger water‐stable macro‐aggregates (WSA > 2 mm) than in water‐stable micro‐aggregates (WSA < 0.25 mm). The SOC in micro‐aggregates (WSA < 0.25 mm) was found to be less vulnerable to changes in land use. The hot water soluble and labile carbon fractions were higher in the bulk soils of grasslands than in the individual aggregates, whereas particulate organic carbon was higher in the aggregates than in bulk soils. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

19.

Purpose

Denitrification has been extensively studied in soils from temperate zones in industrialized countries. However, few studies quantifying denitrification rates in soils from tropical and subtropical zones have been reported. Denitrification mechanisms in tropical/subtropical soils may be different from other soils due to their unique soil characteristics. The identification of denitrification in the area is crucial to understand the role of denitrification in the global nitrogen (N) cycle in terrestrial ecosystems and in the interaction between global environmental changes and ecosystem responses.

Materials and methods

We review the existing literature on microbially mediated denitrification in tropical/subtropical soils, attempting to provide a better understanding about and new research directions for denitrification in these regions.

Results and discussion

Tropical and subtropical soils might be characterized by generally lower denitrification capacity than temperate soils, with greater variability due to land use and management practices varying temporally and spatially. Factors that influence soil water content and the nature and rate of carbon (C) and N turnover are the landscape-scale and field-scale controls of denitrification. High redox potential in the field, which is mainly attributed to soil oxide enrichment, may be at least one critical edaphic variable responsible for slow denitrification rates in the humid tropical and subtropical soils. However, soil pH is not responsible for these slow denitrification rates. Organic C mineralization is more important than total N content and C/N in determining denitrification capacity in humid subtropical soils. There is increasing evidence that the ecological consequence of denitrification in tropical and subtropical soils may be different from that of temperate zones. Contribution of denitrification in tropical and subtropical regions to the global climate warming should be considered comprehensively since it could affect other greenhouse gases, such as methane (CH4) and carbon dioxide (CO2), and N deposition.

Conclusions

Tropical/subtropical soils have developed several N conservation strategies to prevent N losses via denitrification from the ecosystems. However, the mechanisms involved in the biogeochemical regulation of tropical and subtropical ecosystem responses to environmental changes are largely unknown. These works are important for accurately modeling denitrification and all other simultaneously operating N transformations.  相似文献   

20.
Development of alternative sources through wastewater reuse is important to meet water demands in arid regions. However, effects of wastewater irrigation on soil properties and crop performance must be evaluated before advocating its widespread use. Objectives of this study were to evaluate: (i) effects of prior evaporative disposal of saline‐sodic blowdown water (BW) on soil (fine‐loamy, mixed, and thermic Typic Calciorthods) properties in the disposal area, and (ii) effects of flood irrigation with three water qualities (control, BW 1X, and BW 2X) on soil salinity and alfalfa performance using a greenhouse soil column study (soil collected from same study area as objective (i)). Results indicated that although prior land disposal of BW had increased salinity and sodicity of soil, they were within the tolerance limits of the intended crop, alfalfa. Mass balance calculations indicated measured (15·6 Mg ha−1) and calculated (13·2 Mg ha−1) salt accumulation at the test site used for evaporative disposal were similar. Alfalfa grown using BW under greenhouse conditions produced prime quality hay and biomass yield similar to the control treatment (8·3 g column−1 vs. 10·5 g column−1 in control). Although 3·6 years equivalent of flood irrigation with BW 1X did not result in saline soil (BW 1X irrigated soils EC ranged from 2·2 to 3·5 dS m−1), BW 2X irrigation resulted in saline soils. Sodicities of irrigated soils were greater in fine textured deep soils than coarse textured surface soils (e.g., SAR of 6·1 at 0–5 cm vs. 19·5 mmol1/2 L−1/2 at 30–60 cm in BW 1X), indicating the need for high solubility Ca amendments for long‐term irrigation with BW on fine texture soils within the soil profile. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

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