Transect Particle Size Analysis

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Variation in the soil from place to place is a fact of nature and is substantial. It is a major source of uncertainty in soil survey and causes many of the difficulties of applying mathematics to the study of soil. Soil variability within small areas can be studied by using a straight line (transect) laid over a tract of known distance and can be sampled and observed equidistantly. Line transects give an indication of both the local and the general variation. The technique, however, may depend strongly on direction of the transect. Soil along the LTER I Control transect was sampled at 30 meter intervals from station C01 through C89. Soil characteristics were measured at 4 depths (30, 60, 90, and 120 cm) for the percent composition of clay, silt, sand, very coarse sand, coarse sand, medium sand, fine sand, very fine sand, calcium carbonate, organic carbon and coarse fragments greater than 2mm.

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Field data sheets

*A transect of 2670 meters in length served as a reference in the study area. Soil texture was determined on soil samples taken at 30 meter intervals along the transect at 30, 60, 90, and 120 cm depth on a parallel transect 7 m to the northwest side of the main transect (LTER I Control transect). Particle size distribution was determined with the pipette method, and sand fractions were separated by sieving (Day 1965). Those samples were taken in layers rather than genetic horizon. Individual mixed bulk samples were taken of each depth. The number of samples were 356 (89 locations X 4 depths). These samples were taken by an auger of 7 cm diameter and stored in water resistant bags to preserve the samples. **Calcium carbonate. Carbonates were determined by a titrimetric procedure, using a 0.5 N HCl soil treatment with a 0.5 N NaOH back titration with a Fisher Titralizer II titration system. **Particle size analysis. The pipette method was used to determine the percent of sand, silt and clay after pretreatment to remove organic matter and carbonate. The sand fractions (USDA sizes) were determined by dry sieving. **Organic carbon. The percentage or organic carbon by weight was determined by reduction of potassium dichromate by organic matter (Peech, 1965).

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In the spring of 1982, as part of the establishment of the Jornada Long-Term Ecological Research site in southern New Mexico, a 135 ha portion of a 1500 ha, internally drained, watershed was exclosed from grazing by domestic livestock. Prior to exclosure the watershed, as well as the rest of the Jornada basin, had been moderately to heavily grazed for the past 100 years. Concurrent with grazing, the vegetation had undergone a dramatic change from desert grassland, with an almost continuous cover of C4 perennial grasses, to isolated patches of the original grassland in a mosaic with desert shrub dominated plant communities (Buffington and Herbel, 1965). The exclosure lies along a northeast facing piedmont slope at the base of a steep isolated mountain peak, and covers a variety of component landforms from the foot of the mountain to the basin floor. The northeast side of the exclosure is immediately upslope of the College Playa located near the NMSU College Ranch. Three parallel transects (2.7 km in length) run from the middle of the College Playa up into the foot of Mt. Summerford. The Control transect is to the west, the Treatment transect on the east side of the Control transect, and the Alternate Control to the east of the Treatment transect. Each transect is 30 meters wide with a 45 meter buffer zone between each transect. The Treatment transect was treated annually until 1987 with NHNO3 in a concentration equal to 10g N/m2. The station markers at 30 meter intervals along each transect. Only the control transect was used in this study.


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