摘要:

We present a simple method for measuring unsaturated hydraulic conductivities at low matric pressure head under a drying process. This method determines hydraulic conductivities from a water content profile under a steady-state upward flow. A steady state was induced by setting both constant meteorological conditions and a constant inflow rate from the bottom. Under our experimental conditions, steady states were attained within a week for two disturbed soils (a loamy sand and a loam). Isothermal water vapor flux was taken into account in the data analysis. Measured hydraulic conductivities agreed well with those obtained from an instantaneous profile method. The simulated time evolution of the evaporation amounts and final water content profiles using the hydraulic conductivity function determined with the present method were also in good agreement with measured values. These results confirm the reliability and applicability of the present method

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Agricultural producers who choose to supplement their crops’ water requirement are able to determine irrigation scheduling practices better when the soil water content of their fields is known. The objective of this study was to statistically evaluate numerous sensors for their ability to accurately estimate water content in a 90-cm soil profile, based on calibrated neutron probe measurements. The sensors tested were Irrometers, Watermarks, EnviroScan, Troxler Sentry, AquaTel, AquaFlex, Trime, AquaPro, and GroPoint. The sensors were field tested at different water content levels and a variety of irrigation frequencies over a 3-year period in a Warden silt loam soil (Coarse-silty, mixed, mesic, Xerollic Camborthids) planted to alfalfa. The default factor calibration was used to estimate the soil water content from all sensors except the neutron probe, which was calibrated for the soil using the gravimetric moisture content method. The Irrometer and Watermark sensors utilized a local soil water retention relationship in order to convert soil water potential into volumetric water content. The results suggest that most sensors were able to follow the general trends successfully as soil water content changed during the growing season, and there was significant correlation between the sensors and the neutron probe readings. Although sensor trends were similar, visual and statistical analyses indicated that the actual measured values varied significantly between the sensors and the calibrated neutron probe measurements. Therefore, a soil specific calibration of each sensor would have been necessary to obtain a high degree of absolute accuracy in soil water content measurements. The results suggest that irrigators can still use uncalibrated sensors to improve their watering schedules by setting irrigation trigger points that may relate only to a specific sensor in a specific soil. These trigger points cannot easily be related to different soils, different sensors, and other sources of information such as extension fact sheets and research publications, however, because the actual water content measurements may not be correct.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

This objective of this study was to determine if mineralization of C, N, and P was pH stressed and/or nutrient limited in a bog and fen in northeastern Minnesota. Although soil activity in northern peatlands can be limited by low pH, low temperatures, high C:N and C:P ratios of soil organic matter, and/or anaerobic conditions, different classes of peatlands (bog and fen) vary in the type and degree of stress affecting soil processes. We employedin situstudies, laboratory incubations, and a fertilization and liming experiment to understand peatland mineralization dynamics further. Nitrogen mineralization in the field was higher in the fen than in the bog, but net P mineralization was close to zero in both sites. Soil cores were removed from plots that had been treated for 2 years with NH4+, PO4−2, and/or CaCO3, followed by a 30-week lab incubation. Nutrient additions in the bog increased the labile N pool (N0), and cumulative N and P mineralization over 30 weeks, but lime addition had no effect. Nutrient additions to the fen did not significantly alter nutrient pool sizes (N0, P0), cumulative mineralization totals, or mineralization rates. However, lime additions decreased potentially mineralizable N and cumulative N mineralization in the fen aerobic incubations, but increased cumulative P mineralization in the anaerobic incubations. Although both are peatlands, bog and fen nutrient cycles are controlled by different factors that may explain the differences in mineralization, total soil N and P, and respective labile pools. This information may be especially useful in anticipating changes brought about by anthropogenic nutrient/cation inputs and hydrologic alterations.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

It has become common practice to use feed additives (such as phytase) to increase the digestibility of phytate-P to monogastric animals, decreasing the need for P supplements to diets and decreasing the total P content in manures. In this study, the effect of incorporating phytase/and 25-hydroxycholecalciferol (25OH-D3) in turkey diets on the solubility of P in turkey manures (TMs) and TM-amended soils was investigated. Turkey manures were collected from turkeys fed (i) a normal diet, and three diets containing reduced nonphytate P (nPP) called (ii) P deficient, (iii) phytase, and (iv) phytase+25OH-D3. The TMs were added to five soils at the same total P rate (150 kg P ha−1) and incubated for 42 days. The dried TMs were analyzed for water soluble-P (WS-P) and total P, whereas the incubated soils were analyzed for pH, organic matter (OM), Mehlich-3 P, Al, and Fe (M3-P, M3-Al and M3-Fe), water soluble-molybdate reactive P (WS-MRP), and organic-P (WS-OP). The normal diet produced TMs that had six times higher WS-P and more than 40% greater total P than any of the reduced nPP diets. Adding phytase to the reduced nPP diets did not increase the percentage of total P that was water soluble. As a result of its greater P content, TM from the normal diet increased soil WS-MRP, but not M3-P or WS-OP, to a greater extent than the reduced nPP diets. All TMs increased WS-MRP relative to the unamended soil, and these increases were inversely related to the M3 [Al+Fe] content of each soil. Converting from a normal turkey diet to one that contains reduced nPP and phytase/and 25OH-D3shows potential for reducing total and soluble P in manures and soluble P in manure-amended soils. As reduced nPP, phytase/and 25OH-D3in diets lead to reduced TM total P, long-term applications of these reduced P TMs using nitrogen-based management would be expected to reduce total P and Mehlich-3 P in soils, compared with TMs from normal diets.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Management practices that influence the quantity of C inputs returned to the soil from cropping systems and compost applications alter subsequent biotic activity broadly, contribute to seasonal fluctuations in nutrient dynamics, and may increase C sequestration. The effects of crop rotations and compost applications on soil-C sequestration and decompostion, and the turnover time of C4-derived corn C were assessed via changes in the C content and13C values of particulate organic matter (POM) and total soil organic C (SOC). The majority of organic inputs entered the POM fraction, defined as the sand-sized soil separates remaining on a 53-μm sieve after removal of residues (>2 mm), dispersion in 5% sodium polyphosphate, and 12 h of shaking. Before the application of compost to soil, 85% of the C in the compost material was classified as POM. Measurements of POM-C in the soil were 45% higher and SOC was 16% greater where compost was applied in place of N fertilizer. Addition of compost to POM-C diminished the value of POM as an indicator of short-term changes in nutrient dynamics. However, POM-C remaining from compost applications made during the period 1993 to 1997 may be an indicator of enhanced macroaggregate stability: improved soil tilth and the retention of soil C and N. The turnover time of C4-derived C in the POM fraction was 11 years compared with 22 years in SOC. The presence of compost C did not affect the turnover time of corn-derived C. High cropping intensity and chisel plow management increased C sequestration relative to the preceding alfalfa management.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Quantitative soil-landscape models, based on topographic attributes, make possible the characterization of large areas because of the widespread availability of digital elevation models (DEMs). However, these soil-landscapes models, which are usually generated and validated on the same detailed, single research site, such as a hillslope or an elementary catchment, may show high prediction errors when applied to other areas of a region. The effect of the regional variations of topography, climate, parent material, land-use, and/or soil has seldom been analyzed. The objective of this study was to test multiple-regression relationships between the soil hydromorphic index (HI) and topographic attributes on different catchments of the Armorican Massif (30,000 km2) in western France. Regression models were validated using 565 data points collected from four sites along hillslopes. These four sites, located throughout this region, were included in three catchments with surface areas of 78, 82, and 120 ha, respectively, and differing in topography (mean elevation from 39 to 202 m and slope gradient from 3.4 to 7.9%), parent material (granite and schist), and precipitation (700 to 900 mm y−1). The existing models were multiple regression equations between the HI and the elevation above the stream bank, the compound topographic index (regression 1,r2= 0.84), or the upslope drained area (regression 2,r2= 0.86). At each validation point, systematic soil observation for HI estimation was compared with estimations from terrain attributes derived from DEMs at a 30-m resolution. Results showed small prediction errors for all study sites, with mean absolute errors between 5 and 15% of the HI range. Errors were not spatially correlated. Minimum prediction errors were encountered in the catchment for which the models were generated and also in one other that differed only in the parent material. In the other validation site, the models systematically overestimated HI. At the site of model generation, both regressions were accurate. However on the other sites, prediction errors using regression 2 were systematically higher than for regression 1, which uses a topographic index with a physical basis. These results revealed that soil-landscape models may be useful for predicting soil hydromorphy over a region but only when validated under several environmental conditions.

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

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID