The overall goals of this study are first to provide an efficient method for quantifying and describing spatial patterns of soil erosion and deposition at field scales and second, to test a conceptual model relating these patterns to near surface soil carbonates. These methods give us a better understanding of the inter relationships between surface soil carbonates and the land surface processes of erosion and deposition. The main advantages of these techniques are the accurate detection of changes in soil surface elevation over time and the rapid analysis of soil carbonates over many soil samples.
We first had the idea for this research after updating the digital elevation maps. A map of the elevation differences showed clear patterns of erosion and deposition and we wanted to look at how that related to carbonate concentrations. For this protocol, use field software to record the RTK GPS position data at every five minutes throughout the survey area.
Begin by performing a site calibration to local benchmarks to ensure the highest level of accuracy. A local site calibration for the area to be surveyed, provides necessary horizontal and vertical control for the GPS method. This ensures a centimeter level accuracy needed to detect small elevation changes.
For efficient data collection, mount a rover GPS antenna at a fixed height above the ground on a vehicle. Then import the transect in points into the GPS data collector, and navigate between them while driving and collecting data at a constant speed to map five meter intervals. Additionally, have a vehicle mounted hydraulic soil coring machine and use a sampling tube of desired soil core diameter.
To extract cores at locations identified with RTK GPS position data. Soil sampling is performed by pulling a soil core with the soil coring machine. Push the soil core out of the sampling tube and cut into desired depth increments.
Transfer the soil samples into pre-labeled sealable plastic bags. Then store the samples in coolers to transport them to the laboratory. When repeating these analysis after significant soil erosion and deposition events have occurred, return to the same sample locations using the GPS and collect new soil cores.
In the lab, prepare the soil samples for analysis. To begin, dry them in a 60 degree celsius oven overnight. The next day, grind the dry soil samples through a two millimeter sieve, using either a motorized grinder or grind manually with a mortar and pestle.
The next step, requires setting up a modified pressure calcimeter apparatus. First, connect a pressure transducer to a power supply with 14 gauge wire. Include an attached digital volt meter to monitor output from the transducer.
Include a 0.6 micron particle filter to collect an even flux and thus, protect the pressure transducer. Next, attach tubing to the base of the particle filter and connect the tubing to an 18 gauge luer lock hypodermic needle. Next, determine if the reaction vessels should be at 20 or 100 milliliter serum bottle.
Wet a metal tablespoon with water and scoop up about a teaspoon of soil that may have a high carbonate concentration. Pipette one milliliter of 0.5 anhydrous sulfuric acid onto the soil and observe effervescence. If effervescence is high, then assume calcium carbonate is greater than 15%and use a 100 milliliter vessel.
Otherwise, use a 20 milliliters serum bottle. To determine the carbonate concentration, weigh one gram of prepared soil sample into a labeled reaction vessel. Next, pipette two milliliters of acid reagent into a vial.
Gently place the vial into the reaction vessel, so it doesn't spill, then carefully seal the vessel using a gray butyl rubber stopper and clip it with an aluminum sealing ring. Now shake the vessel to spill the acid onto the soil sample and let the reaction proceed for at least two hours. While waiting, make a standard gar using standards in a parallel setup with the same parameters.
Make standards by mixing 100%calcium carbonate with glass beads or sand based on weight, not volume. When determining a standard gar, it's important to accurately know the voltage of the zero calcium carbonate since this defines the detection limit of the method. I typically measure the blank three times along with a known standard concentrations to derive a gar.
After the soil sample reaction is complete, pierce the rubber septum of the reaction vessel and record voltage output by the pressure transducer. Later, solve for the calcium carbonate percentage in the sample using the standard curve. Mapping DEM differences from 2001 and 2009, revealed erosion and deposition over that eight year period.
With decimeter level changes in elevation over most areas. Find scale erosional and depositional patterns that resemble water flow paths occurred in areas of topographic convergence. Calcium carbonate samples were taken in 2001 and 2012, to compare with the erosion and deposition that occurred in the study area and to test the conceptual model.
When comparing soil samples of the east and west block of management strips, an inverse relationship was found between the change in surface soil calcium carbonate and the change in land surface elevation. An analysis of variants show that change in elevation is significantly affected by all side variables. Whereas, changes in calcium carbonate were most significantly affected by erosional class or EDU or mapped soil unit.
Five soil units were determined previously by a soil survey of the site. Interpolations of the point sample results show that the relatively small concentrated areas of high and low calcium carbonate in 2001 were not evident in 2012. The spatial pattern had become less complex.
The sensitivity of elevation change detection provided by repeated RTK GPS ground surveys appears optimal in quantifying and describing the erosion and deposition processes of the field scale. The modified pressure proximity method is a great technique for measuring carbonates and we can process many samples in a short amount of time. Soil surface elevation were more readily detected than changes in soil carbonates, probably due to the core's increment of sampling which was 30 centimeters in 2001.
Future sampling increments of 15 centimeters should improve our detection of soil carbonates. The conceptual model and methods described here can be applied to other sides where calcareous layers are present near the soil surface.
