摘要:

The oxygen diffusion rate (ODR) technique measures the flux of oxygen to a platinum microelectrode, similar to oxygen movement to roots. Instead of merely determining mean ODR, the ODR can be measured along a transect. The spatial pattern can then be examined and related to patterns of aerated and anoxic zones in the soil. The purpose of this study was to examine the within-sample variability of ODR as a tool to characterize soil variability at the small scale, autocorrelation, and binary analysis of the two-dimensional plane. Undisturbed samples (640 cm3) were collected from pots in a greenhouse study, in which the soil had been pulverized, and from plots in a field trial under minimum tillage. After equilibrating the samples at 100 mm negative soil water head, horizontal ODR transects were measured in the center plane of each sample. The ODR values varied considerably within the samples, and mean ODR values were larger for the field samples (55 μg m−2s−1) than for the pulverized greenhouse samples (26 μg m−2s−1). The autocorrelation lengths were much greater for the disturbed greenhouse samples (53 mm) than for the field samples (5 mm). Variances may not have been stationary for the homogeneous greenhouse samples. The fraction of ODR reading >33 μg m−2s−1was 0.19 for the greenhouse samples and 0.61 for the field samples. For the same ODR cutoff, the ratio of cluster size to sample area was 0.03 for the greenhouse samples and 0.06 for the field samples. In summary, the two-dimensional analysis provided more insight than the mean ODR values alone. The ODR transect technique may provide useful information on aeration patterns in soil that will complement more detailed image analysis approaches that have a finer pixel scale.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Improved symbiosis establishment between effective rhizobia and forage legumes is often important for maximizing sward yields, particularly when seeding is done on acidic, low-fertility soils. New information regarding the earliest events in this process could provide insights into the development of germplasm, both plant and bacterial, better adapted to such soils. A recently developed plate model system containing a narrow range of limed (pH 4.71–4.99), nonsterile soils (see article by Staley in Soil Science 167, pp. 211–221) was employed to investigate initial nodulation of white clover (Trifolium repensL.) seedlings. Our objectives were to further evaluate nodulation assessment methodology (staining and nodule stage), distinguish between soil pH-induced plant and rhizobia limitations on nodulation by employing (daily) saturation inoculation, and determine if previously observed nodulation reduction by low pH soil was a consequence of inhibition of symbiosis establishment or nodule maturation by utilizing agusA-marked mutant ofRhizobium leguminosarumbv.trifolii.Over the course of all experiments in all soils, rhizobial MPN populations never declined below 1.3 × 104CFU g−1dry soil. Significant (P≤ 0.05) nodulation responses to soil pH increase were found for both the wildtype (gus−) and the mutant (gus+) stains, whether assessed using unstained (fresh) or stained (methylene blue or GUS) roots. The pattern ofgusAnodule ontogeny under saturation levels of rhizobia, relative to soil pH, suggests that inhibition of symbiosis establishment (cellular effects), rather than differences in nodule maturation (tissue effects), is the explanation for our observed soil pH effects on nodulation. Re-investigation of root responses revealed small but significant (P≤ 0.05) differences, due to soil pH, primarily for lateral roots. Taken together, these results corroborate our previous, wildtype rhizobia findings of a positive effect of soil low-level liming (only 0.25 pH increase) on nodulation and extend them to earlier times (4–6 DAP) in symbiosis establishment, but they refute our previous finding of no root growth and developmental effects. More importantly, they suggest that nodulation reduction in the lower pH soils of our model system was caused by disruption of rhizobia function (but not viable populations) or root hair growth and/or function.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Aggregate slaking is one of the main mechanisms responsible for the breakdown of aggregates. Exchangeable sodium percentage (ESP) and electrolyte concentration (C) of the soil solution play a significant role in determining soil physical properties and the response of soil clays to dispersion and swelling. However, studies of sodicity and C effects on aggregate stability have yielded inconsistent results. Our objective was to evaluate the effects of sodicity and C on aggregate slaking in soils with different clay contents. Using the high-energy-moisture-characteristics (HEMC) method, we studied aggregate slaking in 56 samples of Israeli topsoils varying in clay content (80–675 g kg−1) and ESP levels (0–30). In this method, accurately controlled wetting of the aggregates (i.e., the driving force for slaking) was the only force exerted on the aggregates. Aggregates (0.5–1.0 mm) were placed in a funnel equipped with a fritted disk and, using a peristaltic pump, were wetted either fast (100 mm h−1) or slowly (2 mm h−1). Two salinity levels were studied: dionized water (DW, C = 0.04 mmolcL−1) and saline water (SW, C = 20 mmolcL−1). Thereafter, the aggregates were subjected to stepwise increases in matric potential up to 5.0 J kg−1to obtain a moisture retention curve that served as the base for calculation of aggregate susceptibility to slaking. The latter was expressed in terms of stability ratio (SR). The SR of low ESP soils increased from 0.298 to 0.751 and from 0.508 to 0.799 for DW and SW, respectively, with the increase in soil clay content from 11.3 to 67.4%. Increase in ESP decreased SR (i.e., enhanced aggregate slaking), whereas use of SW increased SR compared with DW. A triple interaction among ESP, C, and clay content (P = 0.001) in their effect on aggregate slaking suggested that the combined effects of these variables on slaking were complex. Our results suggested that: (i) in soils having inherently low aggregate stability (clay < 25%) use of SW was effective in decreasing aggregate slaking at ESP ≤ 5, and (ii) for inherently stable soils (clay > 35%), aggregate slaking decreased upon use of SW only when ESP > 15.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Hydric soil field identification is a common activity for natural resource professionals and planners, but it can be time consuming and labor intensive. This study used Soil Survey Geographic Database (SSURGO), National Wetlands Inventory (NWI), National Land Cover Data (NLCD), and other public domain data to make digital hydric soil predictive maps of two study areas in western Virginia. Soil scientists used the predictive maps as guides to conduct hydric soil field surveys and compared the results to delineations of SSURGO map units dominated by hydric soils and NWI and NLCD wetlands. At Stuarts Draft, 15% of the 1296-ha study area was composed of hydric soils compared with 14% estimated by SSURGO. At Blacksburg, 3% of the 828-ha study area was composed of hydric soils compared with 1% estimated by SSURGO. Both NWI and NLCD estimated 1% wetlands at each area. Locational correspondence was higher between the field survey and SSURGO than between the field survey and the NWI and NLCD wetlands at both study areas. The predictive maps were useful because the SSURGO delineations were closely aligned with field survey delineations, had <2% false negative identifications compared with >13% for NWI and NLCD at Stuarts Draft, and had ≤ 2% false positive identifications. Overlaying NWI and NLCD onto SSURGO polygons resulted in ≤ 1% improvement of predictive map utility, but all indicators of hydric soils were useful in narrowing the specific location of hydric soils within large SSURGO delineations.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Laboratory simulations were used to study the potential influence of bases (K+, Mg++, Ca++) reaching soil via the burning of natural vegation—a common practice of shifting agriculture—on the genesis of argillic horizons and the evolution of Oxisols to Ultisols on the plateaus of Cabo Delgado (NE Mozambique). The effect of such a practice on the gradual enrichment with sand of the soils on slopes and in the bottom of valleys and dambos imbedded in the plateaus was also examined. The results suggested that the burning of natural vegetation before the establishment of shifting agriculture on plateaus in the province of Cabo Delgado causes the pH of the topmost soil layer to rise and facilitates its disaggregation. This, in turn, results in clay illuviation to subsurface horizons, as well as the erosion of fine sand by runoff water and its accumulation on slopes and in valley bottoms, all of which will have major implications on soil morphology and evolution.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Slumping of hardsetting seedbeds upon wetting is likely to determine the shrinking and development of strength on drying. Different processes have been invoked, including aggregate disruption, material relocation, and compaction. To gain a better understanding of the role played by compaction compared with aggregate disruption in seedbed slumping and shrinking, mechanical analysis was combined with previous morphogenetical description. The global structural behavior of repacked seedbeds of a hardsetting sandy loam soil was studied after wetting and again after subsequent drying. Bulk density was measured in 5-mm-depth increments using gamma attenuation, and water content was determined at 10-mm-depth increments. Various wetting conditions were used to simulate a range of climatic and management conditions, including flood irrigation, furrow irrigation of a formed seedbed, drip irrigation, and rainfall. Aggregate coalescence under overburden pressure played the main role in slumping, even though microcracking enhanced coalescence. Most of the slumping occurred at calculated effective stress > 1.1 kPa. Intense aggregate breakdown at the top of seedbeds under fast wetting led to slight slumping because the resulting clogging of the initial interaggregate packing voids was balanced, in part, by the increase in microporosity resulting from aggregate disruption. However, aggregate coalescence induced by overburden pressure developing at the seedbed bottom often resulted in a strong decrease in total porosity. The effect of rainfall kinetic energy on crust bulk density was strong compared with the effect of fast wetting (bulk density increase of about 0.07 Mg m−3and 0.03 Mg m−3, respectively) and could be ascribed to compaction rather than to aggregate breakdown. Shrinking on drying was related to the continuity of the microstructure resulting from wetting rather than to the intensity of slumping. Aggregate breakdown led to more shrinking than did aggregate coalescence.

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