Deep Soil Cores Research Articles

Field scale transport of bromide in an unsaturated soil: 1. Experimental methodology and results

Modeling the field scale movement of chemicals in unsaturated soil is of intense interest to both the public and private sectors and has become an area of active theoretical research in a number of environmentally based disciplines. However, the experimental data needed to validate existing solute transport models and to inspire the development of more refined approaches is very limited. In this research study, the movement of a mobile tracer (Br−) was monitored as it moved through the unsaturated zone beneath the soil surface of a 0.64 ha loamy sand field. Under flux‐controlled, steady state water flow achieved by bidaily sprinkler irrigation, a narrow pulse of 58.9 mol/m−3 NaBr(aq) was applied uniformly to the field and subsequently leached downward while monitored by vacuum solution samplers replicated 16 times at each of 6 depths between 0.3 and 3.0 m and 6 times at the 4.5‐m depth. Six deep soil cores to a maximum of 25 m were taken to characterize the final field average bromide depth profile after the pulse had passed the 4.5‐m depth. Although the mass recovery of the area‐averaged pulse was near 100% at all depths, the coefficient of variation (CV) of mass recovery between samplers at a given depth was near 50%. Lateral variations in apparent vertical solute velocity or in solute transport volume were considerable, with CVs near 50% in the shallow monitoring depths. However, variations in transport volume with depth at a given site were also large, even though the solution samplers for different depths were displaced laterally by only 0.3–0.6 m at different sites. The mean vertical velocity of the area‐averaged solute pulse was significantly less than the ratio of the average net water flux to the average volumetric water content, until approximately 1.8 m. The difference between the two average velocities near the surface was large enough (nearly a factor of 2) to suggest that transient effects from the bidaily irrigations were influencing solute transport.

Nonbiological Nitrous Oxide Production from Vadose Soil Catalyzed by Galvanized Steel Tubing

AbstractStudies of microbial processes in the vadose zone are hampered by difficulties in obtaining uncontaminated samples. We initiated a study in 1985 to quantify microbial denitrification in the vadose zone of agricultural land located in the Atlantic Coastal Plain. In the initial stages of this study, deep soil cores (360 cm) were collected and subcored with sterile galvanized steel tubes to eliminate contamination by surface soil. We observed N2O production from subsoil incubated with C2H2 in galvanized tubes, but in the same soil incubated anaerobically with added glucose and NO−3, denitrification activity was not detected. Subsequent comparisons of N2O production in surface and subsurface soil incubated in galvanized and plastic tubes indicated that chemodenitrification was the likely mechanism responsible for the N2O production observed in soil contained in galvanized steel tubes. These results suggest that, in field or laboratory experiments designed to investigate soil‐N transformations, galvanized steel enclosures should be used with caution.

Effect of irrigation method on the recovery of 15N fertilizer in a slowly permeable clay soil cropped with maize

A study using 15N~labelled fertilizer was initiated in a lysimeter facility to quantify the amount of N assimilated by maize plants and that which remained in the soil at the end of a cropping season. Maize was planted in 0.43 m2 by 1.35 m deep intact Marah clay loam soil cores removed from an improved pasture in mid-October 1983. Two irrigation treatments, flood-impounding water on the soil for up to 72 h, and control-applying enough water to prevent plant stress without ponding, were employed. The crop was harvested in early April 1984 and the amount of fertilizer- and soil-derived N in the plant and remaining in the soil was determined. Grain yields were reduced about 33% by flood irrigation. Although about 30 kg N ha-1 more fertilizer N was lost from the flood-irrigated system, the difference in N recovery between the flood- and control-irrigated soils was not sufficient to account for the reduced grain yield. Flood-irrigated plants were less efficient in transporting fertilizer N to the seed than were control irrigation plants. The data suggest that the reduced seed yield and total N content of maize plants grown under flood irrigation was metabolically controlled rather than being derived from a difference in soil mineral N content compared with control-irrigated soils.

Abundance, Distribution, and Effects of Clearcutting on Cryptostigmata in the Southern Appalachians

Oribatid mites were sampled from deep soil, soil cores, litter bags, and woody litter on a clearcut and adjacent control hardwood watershed at Coweeta Hydrologic Laboratory, North Carolina. The inclusion of woody litter and deep soil samples caused the total number of genera found to reach 72, as opposed to the 37–42 genera range reported in other studies. The more common genera were assigned to three habitat types based on stratification data. The fauna was similar to those of other holarctic study sites. Sampling a greater variety of habitat types yielded a richer fauna than intensive sampling of a few habitat types. Clearcutting caused a reduction in numbers and a shift in faunal dominance. This effect is attributed to temperature-humidity phenomena rather than to food availability.

Ground-Water Contamination by Fertilizer Nitrogen

Nitrogen percolation beneath the crop root zone was significantly reduced by careful irrigation management using the USDA irrigation scheduling program. Deep percolation losses were measured directly using vacuum extractors. Analyses of both deep soil cores and water samples from piezometers suggest that significant amounts of percolating nitrate are removed by denitrification, either before or soon after reaching the water table, thus reducing the potential for ground-water contamination.

Minimum vs. Conventional Tillage in Commercial Sugarbeet Production1

AbstractIn recent years, minimum tillage practices have been applied to sugarbeet (Beta vulgaris L.) production in northeastern Colorado to reduce soil erosion and fuel demands. The objective of this study was to compare the effectiveness of rotary‐strip, no‐plow minimum tillage to conventional plow‐based tillage on commercial sugarbeet fields under center‐pivot sprinkler irrigation relative to soil nitrate movement, weed control, stand establishment, soil compaction, field operations, yield and quality. Minimum tillage was compared to conventional plow‐based tillage in 16 center‐plot‐irrigated fields in eastern Colorado. A 0.10‐ha study site was located in a representative portion of each field. Soils were aridic Argiustoll members of the fine montmorillinitic mesic family. In the spring and midsummer, stand counts and weed counts were conducted to determine stand establishment and the effects of tillage systems on weed populations. Five‐foot deep soil cores were collected in June, July, and November to monitor nitrate movement in each field. After all field operations were completed prior to harvest, water infiltration, soil compaction, soil aggregate distribution, and aggregate stability studies were conducted to determine any differences due to tillage system. Minimum‐tilled sites had significantly lower proportions of furrow soil aggregates 6.4 mm and less in diam but had a higher percentage of aggregates 12.7 to 38.0 mm in diam. Minimum tillage significantly reduced the percentage and stability of row soil aggregates 0.84 to 6.4 mm in diameter. The stability of row and furrow aggregates decreased with increasing size. Nitrate movement was greater and larger concentrations of nitrate were moved to a depth of 75 cm under minimum tillage. The method of tillage had no effect on row and furrow bulk density, water infiltration rate, weed populations, emergence percentage, final stand, yield, quality, or recoverable sucrose. The results demonstrate that minimum tillage produced a sugarbeet crop of comparable tonnage and quality to that of conventional tillage while significantly reducing the number of field operations by 40%.

Evaluation of the acetylene-reduction assay of nitrogen fixation in pastures using small soil-core samples

Abstract Acetylene reduction (AR) assays of N2 fixation in clover-ryegrass pasture were made on bulked samples of 2.54 cm diameter by 7.5 cm deep soil cores. The assays gave a reasonable reflection of N2 fixation as indicated by their relationship to assays on pasture turfs and by their responses to seasonal conditions and imposed treatments. However, variability in the assay was large (25% coefficient of variation on a 12-core sample) apparently because of irregular spatial distribution of N2 fixation in pasture, and AR activity was subject to sharp short-term fluctuations presumably reflecting climatic conditions before the assay. Consequently, quantitative interpretation of the assay in terms of N2 fixed was considered liable to substantial error. There was also some indication that the AR potential of pastures was not fully expressed by the assay method used. The experiments suggest that the AR assay using small soil cores could be a useful index of N2 fixation in making between-treatment comparisons ...

Nitrate Concentrations in Deep Soil Cores as Related to Soil Profile Characteristics

AbstractFifteen sites within a 30‐ha study area were studied to determine the effect of soil profile characteristics on nitrate concentrations (NO3−‐N) below the root zone. The N input, crop removal of N, and water management were similar for all sites during the previous 6 years.The soils of the study area were primarily Alfisols and Entisols of fine‐loamy, coarse‐loamy, sandy and sandy over loamy families. Nitrate concentrations varied between sites and with depth. The average nitrate concentrations below the root zone ranged from 4.9 to 15.3 µg/g when expressed on an oven‐dry soil basis.Profile characteristics were significantly correlated with the average nitrate concentrations below the root zone. A regression equation relating average nitrate concentration in the 1.8–8 m depth to control section characteristics explained 86% of the variability of the NO3−‐N concentration and was highly significant. Hence, to minimize ground‐water pollution, soil profile characteristics should be considered in selecting land for high N input whether by fertilization or waste disposal.