Observations of the soil surface on the Sevilleta National Wildlife Refuge indicate that the surface is very dynamic. During collections of decomposition bags it has been noted that the bags are frequently resting on pedestals of soil, often 2-3cm above the surrounding surface. This study will attempt to measure the dynamics of the soil surface using a specially designed soil bridge. The soil bridge will allow for repeated measurements of the relative height of the soil surface at several sites on the Sevilleta. These measurements will be used to assess the impact of the changing soil surface on nutrient cycling.
The purpose of this experiment was to determine the magnitude of change in the soil surface at certain locations around the Sevilleta. The locations chosen were thought to be characteristic of the vegetation types in which they occurred, but this experiment was NOT designed to test whether or not certain vegetation types had more dynamic soil surfaces. To address that question, replication of the vegetation types would be necessary. The locations are not standardized for parent material (underlying geology), slope, aspect, or geomorphic position, although the sites were selected to minimize differences in these characteristics.
Each site has a total of 5 bridges that are about 75 m apart and covers a total of about 300 m. along an approximate contour at site 1 and 4 and perpendicular to the approximate contours over an elevational range at sites 2 and 3.
Installation of the bridge began by randomly selecting the spot for the first rebar after the approximate location was determined. The first rebar (24 in.) was driven in until about 15 to 20 mm extended above the soil surface. The bridge was then placed over the rebar and the location for the second rebar was determined by the position of the hole in the other end of the bridge. The second rebar was driven to the proper depth to level the bridge (determined by the bubble level permanently mounted on the bridge). When the bridge was level, wood shims were inserted into the end of the bridge to secure the bridge around the rebar and to make the bridge more rigid. A pointed aluminum pin (60 cm) was inserted through hole #16 (the middle hole) and lowered to the soil surface to indicate placement of the spike (with depression in head). The bridge was removed and the spike driven until the head was flush with the soil surface. The bridge was replaced, the middle pin inserted through the bridge into the depression in the head of the spike and shims inserted into the end of the bridge. Pins were inserted through the holes in the bridge and lowered to the soil surface. If the pin struck vegetation while being inserted, canopy coverage was recorded for that pin (indicated by a '1' in cover column; '0' indicates no cover). If the pin struck plant basal coverage at the soil surface, basal cover was recorded (indicated by a '1' in basal cover column; '0' indicates no basal cover). If the soil surface was not hard enough to support the pin, the pin was secured by a clothespin at the top of the bridge. The height of each pin above the bridge was recorded only to the nearest mm (or last mm mark to be seen above the pin). After heights of all 31 pins (including #16, the middle reference pin above the spike) and their canopy or basal intercept were recorded, the bridge was removed, leaving the two end rebar pieces and the middle spike for permanent reference.
Bridges 1.1, 1.2, 1.3, 1.4, and 1.5 are located east of the original fertilizer plots south of deep well. Bridges 2.1, 2.2, 2.3, 2.4, and 2.5 are located at the top of Goat Draw on the east side of the Los Pinos Mountains. Bridge 2.6 is a special bridge that was set at the top of an arroyo head-cut to measure dynamics in arroyo formations. Bridge 2.7 was set at an apparent nick on the slope where a rill formation may occur (the beginning of a potential arroyo).
Bridges 1.1 through 1.5 were identified within an area to be burned in the summer of 2003. In June of 2003, 5 new bridges were established at about 200 m to the west of the original bridges to serve as controls to maintain the long-term untreated measurements at that site. The original bridges (1.1 through 1.5) were measured on 11 June 2003 and the new bridges (identified as 7196, 7197, 7198, 7199, and 7200) were established on 13 June 2003. The new bridges were essentially the same with respect to plant cover, basal cover, and variation in microtopography as the original bridges. The original bridge sites were burned by prescribed fire on 19 June 2003, with the initial post-fire sampling occurring on 23 June 2003.
The soil bridge consists of a 6 ft. length of 1.5 in. (OD) aluminum square bar or tubing. Beginning at ---, brass bushings were set in the tubing at 5 cm intervals (31 each). A bubble level was secured to the top of the bridge (the bubble level was always on the left side of the bridge, hole #1 the closest to the level). A hole approximately --- from the end of the bridge was made in the bottom of the bar to fit over the rebar that is driven into the ground. The bridge is removed after each measurement and stored at the Biology Annex.
Data Collection Methods:
Data collected are date of sampling, location, height of each pin above the bar, and an indication of canopy and basal intercept for each pin. Data are recorded in a surveyor's book and then transcribed to computer spreadsheets for data management.
NOTE: Beginning on 19 April 2004, two additional data columns were initiated. These columns were cover and basal cover by annual herbaceous plants. These new data were collected in response to the large increase in annual herbaceous plants on the burned plots (1.1 through 1.5) This information has been collected an all sites following 19 April 2005.
9/12/94 Project approved by Sevilleta LTER PCP
9/20/94 File initiated by John A. Craig
10/21/94 Wrote initial abstract - JAC
12/18/95 appended initial data (analyzed for Sevilleta Symposium Jan. 1996)
9/26/96 Added description of variables
2/14/00 Appended data through 1998
2/25/05 Appended data through 2005
4/21/09 Appended data through 2008 -- Carl White
10/29/09 Appended data through 2009 -- Carl White
This research project measures soil microtopography of 5 bridges at 4 sites: Deep Well, Cerro Montosa, Red Tank, and Rio Salado. Deep Well site includes soil collections from under grass and open patches for N mineralization. Five additional sites installed on Deb Coffin's experimental plots: 6 bridges at 3 sites, 9 bridges at 2 sites
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