Methods of Soil Resampling to Monitor Changes in the Chemical Concentrations of Forest Soils

Repeated soil sampling has recently been shown to be an effective way to monitor forest soil change over years and decades. To support its use, a protocol is presented that synthesizes the latest information on soil resampling methods to aid in the design and implementation of successful soil monitoring programs.

The overall goal of this methodology is to design and implement a forest soil monitoring program to determine if key soil properties are being changed by large-scale environmental drivers, such as acidic deposition or climate change. This method can help answer how reductions in air pollution have affected soil-nutrient relationships tied to forest health and how changing climate influences the amount of carbon in the soil. The main advantage of this technique is that it allows direct quantification of changes in soil chemistry, greatly reducing the uncertainties of inferential methods such as modeling and watershed mass balance.

Locate a forested area with the characteristics desired for monitoring as described in the text protocol. Select locations for pits, avoiding land surfaces that are not representative of the study unit. Also avoid land surfaces where sampling methods are not possible because of perennial wetness, excessive rocks at or near the surface, or excessive density of trees.

Cover one side of the planned pit with a plastic garbage bag or something similar to protect from trampling and contamination during pit digging. This side will then be used for the profile description and sampling. Lay out a tarp adjacent to the location where our pit is to be excavated.

Begin excavating the pit by removing the forest floor with a shovel. If possible, keep the forest floor intact and place where it will not be mixed with mineral soil being removed from the pit. Excavate the pit with the smallest footprint possible until reaching the desired depth determined by the monitoring design.

Prepare a vertical pit face for description and sampling by lightly scraping downward with a hand trowel to remove any loose soil resulting from the excavation. Prune roots with hand snippers where necessary. Record any observations of water seeping into the pit from a pit face or from the bottom of the pit.

Visually evaluate the pit face from top to bottom for differences in color, texture, and structure. Remove small amounts of the differing soil and place it side by side on a white piece of paper to assist in identifying horizon boundaries. Mark horizon boundaries with T shaped pins or similar objects.

Take a digital photo of the profile with horizon markers and a tape in place to show the scale. Measure and record the depth of the top and bottom of each horizon with a metric tape relative to the interface between the air and soil surface. Select the horizons and/or depths to be sampled based on the study design and requirement.

Collect the soil from the selected horizons or depth intervals, starting with the deepest sample and working upward. To remove the sample from the pit face, insert the gardening trowel near the bottom of the layer that is being sampled. Then insert a flat trowel above the gardening trowel to loosen the soil so that it can be removed with the bottom trowel.

For both horizon and depth sampling, collect soil across the breadth of the pit face where the horizon can be sampled. Label the sample bag with the study unit, date, pit identification, horizon or depth interval, and sampler name. Place the samples in sealable plastic bags and double bag samples if soils are stony.

Once sampling is completed, backfill the pit with the mineral soil and coarse fragments. Place the forest floor on top of the mineral soil, keeping the organic material as intact as possible. Record the location of the pit with respect to the study unit monument.

Within 24 hours of collection, pour the samples out of the plastic bags into pans that will facilitate air-drying of the samples. Air-dry at approximately room temperature in a secure location that is protected from airborne contaminants, such as dust. Mix the samples in the pans every few days, depending on wetness.

Inspect each sample for visual and tactile evidence of dryness to determine if air-drying is near completion. Verify the completion of air-drying by weighing subsamples from several samples. Then oven-dry these subsamples for 24 hours.

After reweighing the subsamples, calculate the mass of moisture lost through drying as a percent of the total mass before drying. After two days, repeat the weighing/drying process and compare the moisture lost from the first oven-drying to that lost in the second oven-drying. If the moisture lost in each oven-drying is within 2%the soil can be considered air-dried.

Once air-drying is complete, place samples in plastic bags that can be sealed after expelling as much air as possible. To remove coarse fragments and roots, sieve all collected soil. Pass the organic samples through a sieve with openings of approximately four to six millimeters.

Pass mineral soil samples through a sieve with openings of two millimeters. Additional sieving through smaller openings may be required for specific chemical analyses. For resampling, make sure that the sieving procedure matches that of the previous sampling.

Archive the soil that remains after chemical analyses for future use. To do so, select the mass of soil to be saved based on how much soil was used for the full suite of measurements, the anticipated number of times samples will be reanalyzed in the future, and the available long-term storage space. With a permanent marker, write on an appropriately sized storage container.

Note the sample identification information, including horizon or depth increment, the sieve size, the date collected, and any necessary laboratory information such as sample identification code. Weigh and record the mass of the soil that is being archived for each sample. Store the cups in plastic storage containers configured to the available shelving.

Label the container with information on the samples to enable them to be located efficiently. Keep the archive room at a stable temperature. Store the information on each archived sample in a digital database that is routinely backed up.

The ability to detect changes depends on the sampling size. The number of detected changes went from only two with a sample size of four to five with a sample size of 12. The measure of uncertainty, shown by the p value, also decreased as sample size increased.

Reanalysis of a subset of archived soil samples for exchangeable calcium concentrations showed no statistical difference, little bias, and little unexplained variation when compared with original analysis results, making reanalysis of all archived samples unnecessary. For exchangeable aluminum concentrations in the Oa horizon, a statistical difference was detected between the original analysis and reanalysis for a subset of samples. However, a strong linear relationship between the two sets of data enabled the original measurements to be mathematically adjusted without reanalysis of all samples.

Original and reanalyzed data for a subset of archive samples were also statistically different for exchangeable aluminum in the B horizon. However, there was no relationship between the data sets to enable a mathematical correction, so all archive samples must be reanalyzed. Results of a study with sites throughout the northeastern U.S.and eastern Canada demonstrate that soil resampling studies with differing designs can be aggregated to address broad regional questions, such as how soils are recovering from acidic deposition.

Repeated soil sampling enables development of soil monitoring programs to build a better understanding of the effects of large-scale factors such as atmospheric nitrogen deposition, climate change, and invasive species. By following the proper procedures for designing and implementing your soil monitoring program, potential pitfalls that could lead to bias or excessively noisy data will be avoided.