摘要:

It is extremely challenging, if not impossible, to measure distribution of root-water-uptake or root density directly, either in the laboratory or in the field. To estimate root-water-uptake distribution in soils, the authors of this paper developed an inverse method, whereby measured values of water content in soil profiles with root-water-uptake were used as input information, which can be acquired relatively easily and accurately. Then a soil water flow equation incorporating an unknown sink term was solved inversely for the sink term, which characterizes the root-water-uptake distribution in the root zone. The estimated distribution of root-water-uptake rate was used to calculate the normalized distribution of relative root density, from which the root-water-uptake model was established. Based on the model, soil water flow with root-water-uptake was simulated successfully. Examples of numerical experiments were designed to examine the accuracy and stability of the proposed approach for estimating the average root-water-uptake rate, considering different time intervals between successive measurements of the input distributions of soil water content, spatial intervals of the measurement points along the soil profile, simulating depth, different soils, variation of the saturated hydraulic conductivity, and the stochastic nature of the measured soil water content. Examples were also used to demonstrate applications of the method. The results showed that the inverse method is reliable for estimation of distribution of the root-water-uptake rate, compared with the theoretical values. The proposed method is useful for estimation of the normalized distribution of the relative root density, from which the root-water-uptake model can be determined and the soil water flow with root-water-uptake can be simulated continuously.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Preferential flow in golf greens causes turfgrass quality deterioration and shortens solute travel time. Little is known how surfactants affect the flow pattern in golf greens. This study visualizes the changes in flow patterns and compares leaching of a nonadsorbed and an adsorbed chemical as a result of the addition of a nonionic surfactant in a simulated United States Golf Association (USGA) putting green profile in a twodimensional sand slab chamber. The USGA putting greens have three distinct layers that become coarser with depth. The addition of the surfactant changed the unstable preferential flow pattern to a uniform flat wetting front in the first 11 cm. Below this, small perturbations occurred, and at a depth of 17 cm, the wetting front became unstable and formed six fingers. After the first 17 cm, the infiltration process was not affected by the addition of the surfactant. Water infiltrated into the second layer as a wavy front and developed a single finger at the third layer. After the irrigation was stopped and water reapplied, the water followed the same pathways established during the first infiltration cycle. Chloride moved two times more slowly in the surfactant-treated profile compared with the nontreated profile. A second application of the surfactant at the same rate did not alter the flow patterns. The study showed that surfactants can provide an inexpensive way to decrease the travel velocity of the pollutants and increase the efficacy of water and applied chemicals through coarse-textured turfgrass profiles.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Information regarding the mobility of applied chemicals to soils is essential for environmental assessment of potential leaching to groundwater supplies. In this study, the mobility of deltamethrin applied to soil columns with different properties was investigated. The approach used was that of a modified miscible displacement, or transport approach, where soil columns under soil-water-saturated conditions and constant flow velocity were maintained. The extent of deltamethrin leaching losses was measured by quantifying deltamethrin concentration in the effluent solution versus time. After the transport experiments were conducted, each soil column was sectioned into 2-cm increments, and the amount of deltamethrin retained by the soil was determined from sequential methanol extractions. All measured breakthrough curves (BTCs) exhibited extensive asymmetry with sharp adsorption fronts. Extremely low mobility, and thus strong retention for deltamethrin in the soil columns, was observed. With continuous leaching of up to 80 pore volumes, effluent results showed that cumulative leaching of deltamethrin was small and ranged from only 3 to 8% of that applied. Moreover, 92 to 93% of that applied remained within the 0–2-cm layer, with only 0.5–1% extracted from the subsequent lower depths (2–4-cm and 4–6-cm layers). Deltamethrin mobility was relatively higher in a reference sand material. The use of a multireaction and transport model (MRTM) proved successful in describing the BTC from the sand column and was in close agreement with measured deltamethrin results. Based on its extremely low mobility in soils, deltamethrin is considered to be not susceptible to leaching from the soil profile.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

In tropical Asia, multiple annual cropping of lowland rice and the associated submerged soil conditions have been linked to long-term changes in nitrogen (N) cycling and the chemical nature of soil organic matter. To identify changes in organic matter properties, two humic acid fractions and whole soil samples were obtained from field treatments of lowland rice that varied in cropping intensity, fertilizer management, and location. These samples were methylated and analyzed by pyrolysis-gas chromatography-mass spectrometry. With compounds expressed in relative abundance, whole soil was enriched in nonmethoxybenzene compounds and heterocyclic N compounds compared with extracted humic acids. The young mobile humic acid (MHA) fraction had a wide diversity of methoxybenzenes that are associated with lignin residues. The more recalcitrant calcium humate (CaHA) fraction had characteristics intermediate between whole soil and MHA. Under intensified cropping and increased soil submergence, lignin residues increased in relative abundance in all three fractions. Heterocyclic N compounds decreased with intensified cropping, consistent with previous analysis by15N nuclear magnetic resonance spectroscopy. Their parent compounds may be primarily naturally occurring heterocyclic N compounds. For whole soil, and especially the MHA, submergence effects were accentuated in treatments with high fertilizer rates, trends that may be related to the balance between input rates and degradation rates of crop residues. The ratio of myristic acid: stearic acid varied with soil submergence, fertilizer rate, and type of fraction in patterns following the abundance of methoxybenzenes. In general, responses of the MHA and CaHA to field treatments were representative of whole soil.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

The phosphorus (P) retention characteristics of soil layers are critical for predicting the effectiveness of a constructed wetland, built in high water table soils, in reducing P transport to the receiving water body. Soil samples were collected from surface and subsurface horizons for P sorption and mineralogical studies directed to the assessment of a proposed site for stormwater treatment area (STA) construction in the Okeechobee Basin, Florida. The P sorption maxima were positively correlated with oxalate-extractable aluminum (ox-Al) and citrate dithionite bicarbonate-extractable aluminum (CDB-Al) under anaerobic conditions, but there was no significant correlation with either ox- or CDB-extractable iron (Fe). The ox/CDB ratio for extractable Fe was relatively high (>0.5), indicating that most of the Fe was in noncrystalline form, i.e., highly reactive and prone to rapid dissolution under reducing conditions. The X-ray Diffraction (XRD) patterns indicated that smectite was the dominant mineral in the clay-size fraction. The permanent negative charge of smectite limits contribution to P sorption. The equilibrium P concentrations (EPC0) of the soils were significantly correlated toCDB-Albut not toCDB-Fe, suggesting that Al-associated P is involved primarily in elevating EPC0in these soils. Both P sorption capacity and EPC0were fairly low for soils from most locations, indicating that the proposed STA cannot remove P from the rerouted water to a significant extent under ambient conditions and would not also cause significant P loading to the water table aquifer. Thus, alternatives for effective P sequestration, such as establishment of vegetative communities or chemical amendment, would be required.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Reduction in the use of fallow in traditional wheat-fallow cropping systems, along with other management changes such as reduced tillage, raise questions about nutrient management, particularly phosphorus (P) management, in such soils. Phosphorus is focused on because of its low soil mobility and its common application in wheat cropping systems. Consequently, we initiated a study with the following objectives: (i) Measure changes in soil available P status during the fallow year under two different cropping intensities; (ii) Determine differences in P status with soil depth and topographic position; (iii) Relate changes in P fractions (organic, inorganic) to P availability; and (iv) Relate P availability (as measured by Bray and Kurtz [B&K] extraction) to soil phosphatase activity. We sampled soils by depth, topographic position, and season during the fallow year in farmers’ fields used for wheat-fallow (w-f) or wheat-proso millet-fallow (w-m-f) cropping systems located near Sidney, Nebraska. Soils were analyzed for B&K-P, total P (TP), inorganic P (IP), organic P (OP), organic matter (OM), and phosphatase activity. Soils under w-m-f were up to 5 mg P kg−1soil higher in B&K-P than those under w-f, but differences were not significant. Soils at footslope positions were significantly higher in OP, IP, TP, OM, and phosphatase activity, probably because of a combination of deposition of eroded soil and higher average soil moisture content. Differences in P-related properties across seasons were inconsistent. B&K-P was strongly correlated to phosphatase activity, OM, IP, and TP (allP< 0.0001). The general absence of differences in any measured quantity between cropping systems suggests that cropping intensity can be increased in semiarid climates such as the study site, at least over the short-term, without concerns of inducing P deficiencies. Phosphatase activity, although related to P availability, does not offer greater benefits in predicting soil P supply than established soil tests.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID