Tag Archives: soil

A walk in the woods – 17 years later

by Ian Yesilonis (Baltimore Ecosystem Study LTER)


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Figure 1. Ian Yesilonis by a large tulip poplar tree in Gwynns Falls/Leakin Park in Baltimore City.

Walking through the woods and observing the trees and animals is something I have always loved to do growing up in Baltimore.  Our temperate deciduous forests in the city are typically smaller patches; however, one park, the Gwynns Falls/Leakin Park (1,216 acres), is quite large and also has big trees (see Figure 1).  The trees and soils of these forest fragments, which make up 34 percent of the city’s tree canopy, provide important ecosystem services such as stormwater runoff reduction, water purification, food and habitat for animals, air temperature reduction, and carbon sequestration.  These patches are surrounded by an urban matrix comprised of roads, houses, and people.  The surrounding urban environment influences these patches many different ways, for instance, as sources of metals and nutrients, as well as invasive species.  Over time, these different influences may affect the general health of the forest.  In my project, I wanted to look at changes in soil properties over 17 years, specifically in carbon, nitrogen, pH, calcium (Ca), and magnesium (Mg) content in urban remnant forests and compare those to rural areas in Baltimore County.

Most temperate deciduous forest soil resampling studies investigate acid deposition before the 1970’s and consider the leaching of Ca and Mg through the soil profile.  Acid rain affected the entire forest ecosystem, including plants and wildlife, all over the east coast of North America.  Generally, these studies have found a decrease in Ca which is needed for a variety of living organisms such as arthropods and snails to create exoskeletons and shells. However, no retrospective studies have been done in an urban context to investigate the effects of the urban matrix.  In urban environments, the soils are not necessarily depleted in Ca. We are finding the opposite, i.e. there is excess Ca due to the abundance of cement and concrete which erodes and washes into nearby soils, or it becomes dust, deposited on the soil surface and by wind. This constant source of CaCO3 could affect soil pH which determines what plants can grow successfully.

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Figure 2. Big wheel tire in the middle of our Hillsdale plot, Baltimore City.

Carbon dynamics in our forest systems are complex.  A confounding issue found in the urban forests is the presence of earthworms that mix and integrate leaf material into the upper horizon of the soil profile therefore affecting where carbon is stored.  Another difference between urban sites and rural sites is the increased temperatures in the urban area.  As temperature rises, assuming the moisture is the same, decomposition of organic matter also is accelerated which decreases the amount of organic matter in the soil.

 

Our hypotheses are 1) soil organic carbon will decrease in urban areas due to increased temperatures and worm activity, and 2) conversely, pH will increase because of the amount of calcium prevalent in the urban system from cement.  In order to detect change, we resampled forest fragment plots from a previous study, conducted in 2001.  One challenge was to find the same plots.  Two things helped us achieve this goal: first, we had original hand-drawn field maps with detailed descriptions of the locations and second, we were able to talk with the investigator of the 2001 sampling. This guided our interpretation of these maps and ensured that collection methods were comparable.  Remember, this was a time before GPS was readily available and the signals from satellites were scrambled for civilians.  Although it was a challenge, I learned a great deal of patience, and ultimately enjoyed finding the exact same sites.  The maps were detailed, but things change over time in forests; trees fall over in wind storms, thus trees, originally standing on the 2001 field map, were on the ground in 2018.  On the other end of the spectrum, some things didn’t change at all; for example, at one site in Hillsdale, which is surrounded by neighborhoods, we found the exact same toy big wheel tire in the center of the plot that was drawn on the field sheets.  It hadn’t moved and, being plastic, had not decomposed in 17 years (Figure 2).  In the field, I was prepared for mosquitos, dog and black-legged ticks, and poison ivy but I was not prepared for stray dogs.  At one plot in Leakin Park, we saw two large dogs barking at us on the other side of the ravine.  They started running towards us and tried to find a way to cross the ravine.  I did not want to challenge their territory so we quickly made our way to the van and ended field work for the day.  Despite some unpleasant encounters such as this, I enjoy working in the forest, digging in dirt, enjoying the shade of the trees, hearing the birds, and watching the squirrels.

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Figure 4. Soils from different plots throughout Baltimore.

 

After our summer field work was completed, we worked in the lab to analyze the samples for nutrients, pH, and determined soil density (Figure 3).  Currently, we are examining data and look forward to exploring the results which could be used to predict future conditions and better plan for management of forest patches.

Author Biography: Ian Yesilonis is a Soil Scientist for the USDA Forest Service Northern Research Station and a PhD student at Johns Hopkins University in Dr. Szlavecz’s lab in the Department of Earth and Planetary Sciences.  He studies all aspects of urban soils including metal contamination (especially Pb), soil respiration, carbon storage, and decomposition.

Roadblocks and Rocks:  How to Measure Soils in Forest Ecosystems

by Karla Jarecke and Adrian Gallo (HJ Andrews Experimental Forest)


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Fig.1 Dave Frey scouts location for a quantitative soil pit in Watershed 2 by taking a tree core in one of the younger trees in the old growth stand to date the age of the most recent landslide. Photo credit: Adrian Gallo

Standing in a meter deep hole at the HJ Andrews Experimental Forest (HJA), Adrian arranged his soil tools on the mossy surface where I stood. Knife, check. Trowel, check. Meter tape, check. I stood above him, waiting patiently to record his observations. “Subangular blocky”, he announced. Clipboard in hand, I took note. For nearly two hours Adrian stood belowground, meticulously describing the soil profile. He noted how they clumped together, how they smeared in his hand, their color, the quantity of roots or rocks, and other details that soil scientists across the U.S. have used to standardize and compare soils of different regions. I looked up at the old growth canopy thinking about the water that moves from the roots at Adrian’s feet to the leaves 90 m above us.

When he finished, we switched places. I sat on the edge of the wall and lowered my feet to the bottom of the hole. The moist and cool temperatures of the deep soil was a relief from the hot July air. I used a small trowel to scoop soil from the horizons that Adrian had identified and marked with a golf tee. I placed the soil into quart-sized plastic bags labeled with our location, the date, and the soil depth. The samples would make their way to Oregon State University for lab analysis—information that would become publically available through the HJA database.

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Fig.2 Soil profiles from three different watersheds with increasing rock content from 15% to >35% from left to right.

HJA, located in the western Cascades of Oregon, is a Long-term Ecological Research (LTER) site. Elevation ranges from 410 to 1630 m creating diverse microclimates and steep slopes. HJA is unique from other LTER sites in that it has one of the highest rates of biomass production with ~40% of the 15,800 acre basin covered in huge old-growth that sometimes exceed 700 years of age. The need for a systematic soil-sampling program sprouted from hydrologists, stream ecologists, and other scientists asking “uphill” questions without many answers. The questions that developed over the decades (HJA was a charter member of the LTER program in 1980) of ecosystem monitoring began narrowing to terrestrial-hydrologic connections and nutrient inputs to streams. However, that’s where we lacked any representative data to inform these questions.

Jeff Hatten, a professor at Oregon State University, spearheaded this field intensive soil sampling project in 2015 to quantify soils across HJA. The analysis continues to this day because we’ve moved over 45,000 kg of material and acquired more than 600 samples that are currently being analyzed for their chemical (e.g., C:N and micronutrients) and physical characteristics (e.g., rock density and soil texture). One of the main objectives of the soil sampling program was to understand belowground carbon and nutrient stocks at HJA. The majority of terrestrial organic carbon that cycles on human timescales is stored belowground in soils (Stockman et. al, 2013). To know how much carbon is stored in soil and how this is changing over time requires precise estimates of belowground bulk density, or the mass of soil in a known volume. Bulk density is a crucial scaling factor when converting the elemental-percent of soil carbon and nutrients to its storage capacity across the landscape.

Forest ecosystems are often more difficult areas to quantify bulk density because large rocks and roots prohibit the standard sampling methodology often used in grasslands or agricultural fields. In areas with minimal rocks, soil scientists pound a 300 cm3 metal cylinder into the soil and carefully weigh the mass of soil inside. These tools are small, easily transportable in the field, and simple to operate. However, rocks or roots >5 cm diameter do not fit in the cylinders and larger diameter cylinders have too much friction against the soil that pounding them into the ground is nearly impossible. To accurately measure bulk density in forests (where rocks and roots are abundant) we need a larger sampling volume! More space allows us to remove any biases we may introduce by avoiding large rocks with the cylinder method. Hence we dig, and weigh, a perfectly square cubic-meter worth of material—a method known as quantitative soil-pit.

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Fig.3 Chris So separates large rocks from soil and root material. Photo credit:  Jeff Hatten

Digging these quantitative soil pits is no small task. A single pit could take up to 8 hours of physical labor depending on the size of the crew and the amount of rocks. The process involves filling 5-gallon buckets with excavated soil, weighing each bucket and sieving the bucket’s material into three categories:  rocks, roots, and soil. These categories were weighed and subsampled for future lab analysis. Once the cubic-meter is excavated, we take high-resolution photographs and spend hours describing the color, structure, and texture of the soil horizons. By sorting and weighing an entire cubic-meter of soil, by all its component parts—rocks, roots, and soil—we’re able to obtain far more precise and accurate estimates of bulk density, rock content, large roots, water storage, and nutrient storage.  While rocks sometimes occupied over half of the volume of the cubic meter pit, the soil bulk density at HJA is relatively low due to high organic matter input, large macropore structure, and volcanic mineralogy.

Since 2015, we’ve excavated and quantified materials from 18 cubic-meter soil pits starting in low elevation watersheds. Now that the soil sampling program has a plethora of samples, we’re beginning to piece together datasets that are more representative of the spatial variability of soil properties within and among watersheds. This all sounds great for researchers who focus on soils, but the benefits of these data and sample collection will help other scientists as well. Some data include clay content for individual horizons that can be used to predict nutrient holding capacity and soil-water retention and infiltration; the former can inform ecophysiologists and the latter can inform hydrologists (Rasmussen et al., 2018). How quickly streams respond to rainfall events could be related to the size and distribution of macropores in the soil. Tree productivity could be explained by rock content allocating more space for soil that holds more nutrients than rocks or by organic matter content, which holds far more water by weight compared to mineral soil.

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Fig.4 Karla takes a traditional bulk density sample at 100 cm. Photo credit: Adrian Gallo

Factors that influence soil bulk density, and by extension, soil carbon and water storage are complex. Our hope is that these soils data can be integrated not only at the hillslope scale, but across the watershed so other researchers can turn to soils as a way to explain ecological and physical processes. The Long-term Soil Measurement Program will continue to add knowledge of soils at HJA by routinely adding quantitative soil pits each summer. It will also allow for resampling on a multi-decadal time step to assess long-term changes in soil properties. Linking soil data to long-term monitoring of water, carbon, and nutrients in streams and hillslopes will further our understanding of ecological connectivity in forests and raise the alarm if we see imbalances.

Readers can learn more about HJA Long-term Soil Measurement Program and the Oregon State Forest Soil group at:  HJA Website and OSU Forest Soils Homepage.

Citations

Rasmussen, et al., 2018 – Beyond clay: towards an improved set of variables for predicting soil organic matter content. Biogeochemical Letters.

Stockman et al., 2013 – The knowns, known unknowns, and unknowns of sequestration of soil organic carbon. Agriculture, Ecosystems and Environment. 164:80-99.


Authors’ biography:

Karla Jarecke (below, left) is a PhD candidate at Oregon State University in the Forest Ecohydrology and Watershed Science Lab. She studies soil hydrology and plant water availability at HJA. Karla collected soil samples for lab analysis of soil hydraulic properties and installed a network of soil moisture sensors at HJA. She is currently planning a study to look at carbon isotopes in tree rings to identify signs of tree water stress.

Adrian Gallo (below, right) is a PhD student who initially focused on forest floor controls of biophysical factors in soil within the Oregon Cascades, but has since expanded to carbon cycling in soil across North American wildland ecosystems through the NEON project.

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