Pulse-Press Project (P3): Continuous soil temperature and volumetric water content (VWC) measurements, McMurdo Dry Valleys, Antarctica (2012-2021, ongoing)

Abstract:

Climate warming in polar regions is associated with thawing of permafrost, resulting in significant changes in soil hydrology, biogeochemical cycling, and in the activity and composition of soil communities. While ongoing directional climate warming presses can elicit such responses over decadal time scales, their manifestation typically occurs as discrete thawing pulses. Indeed, in the McMurdo Dry Valleys of Antarctica, abrupt changes in community structure and biogeochemical cycling in terrestrial and aquatic ecosystems following a summer warming event (Jan. 2002) exceeded the influences of a decadal cooling trend in both magnitude and rate of response. Thus, we anticipate that climate-mediated permafrost changes and their associated impacts on soil communities and biogeochemical cycles may occur over seasonal time scales. The Pulse-Press Project (P3) experiment was established in 2012 as part of the McMurdo Dry Valleys Long Term Ecological Research (LTER) program to investigate impacts of seasonal wetting on ecosystem structure and functioning by simulating different frequencies of permafrost thawing events in Antarctic permafrost soils. Since the top horizons of most Antarctic soils are dry permafrost (i.e., there is insufficient water content to generate ice-cement), with ice-cement or massive ice typically below 30 cm, permafrost thawing events are likely to result in subsurface movements of water that may manifest as groundwater seeps down gradient. The P3 experiment consists of three permanent plots situated on the south-facing hillslope above Many Glaciers Pond in Taylor Valley. Each plot is 15 m by 7.5 m with a trench on the upslope end that is used for experimental wetting events. The Press plot receives water every austral summer, the Pulse plot receives water every other austral summer, and the Control plot never receives water, serving as the ambient treament. Each plot is instrumented with a network of soil moisture and temperature sensors, positioned at 13 locations along the hillslope in vertical arrays at varying depths. This data package consists of these continuously recorded soil temperature and soil moisture data, complementing other data packages associated with the P3 experiment.

LTER Core Areas:

disturbance

Dataset ID:

4017

Associated Personnel:

1171

1173

1172

Short name:

P3_SENSOR

Data sources:

P3_CONTROL_TEMP

P3_CONTROL_VWC

P3_PRESS_TEMP

P3_PRESS_VWC

P3_PULSE_TEMP

P3_PULSE_VWC

Methods:

The P3 experiment consists of three permanent treatment plots situated on the south-facing hillslope above Many Glaciers Pond in Taylor Valley. Each plot is 15 m by 7.5 m. A trench (20 cm deep), located at the top of each plot, is used for experimental wetting events.

Press (H): Receives yearly water additions sufficient to raise soil water to 10% water content (as observed during the melt event in Jan. 2002). This is achieved by pumping water from Many Glaciers Pond to a holding tank above the experimental plots, from which water is applied by gravity feed to a trench at the top of the hillslope during the warmest week of the year (typically the 1st or 2nd week of Jan.). The water from the trench then seeps down through the soil.
Pulse (L): Same as above, but water additions occur in alternate years to simulate a lower frequency of thawing permafrost.
Control (C): Trenched and instrumented, but receives no water addition treatments.

In January 2012, each treatment plot was instrumented with a network of soil moisture and temperature sensors, which are located down the center of each plot at 13 locations (blocks) at varying distances (m) from the trench and at varying depths in the soil (cm). There are 2-3 sensors deployed in each block. One is shallow, at 5 cm deep in the soil, whereas the other 1-2 sensors are deeper, >10 cm deep in the soil. The depth of the deeper sensors was dependent on how frozen the ground was at the time of deployment. Data from each plot are collected continuously on a 5 to 60-minute interval by an associated Campbell Scientific CR1000 datalogger paired with an AM1632 and AM25T multiplexer.

Decagon 5TM soil moisture and temperature sensors are used to measure soil volumetric water content (VWC) and soil temperature in blocks 1, 2, 3, 5, 7, 9, 11 (except at 26 cm in the control plot), and 15 (except at 35, 26, and 21 cm in the control, press, and pulse plots respectively), while thermocouples measure temperature in the other blocks (4, 6, 8, 10, 11 (at 26 cm in the control plot only), 13, and 15 (at 35, 26, and 21 cm in the control, press, and pulse plots respectively). The data collection frequency has changed several times over the lifespan of the project, ranging 5-min to 1-hour collection intervals. Generally, the frequency of data collection occurs every 5 minutes during the period of wetting, then is decreased to either every 30 or 60 minutes depending on the time of year.

Note that datalogger memory problems and sensor malfunctions over the years have resulted in some missing data. Also, due to the difficulty in calibrating the EC-TM Soil Moisture sensors to polar desert soils, the VWC data must be looked at as relative to the background for each probe.

Additional information:

Funding for this project was provided by several grants from the US National Science Foundation for Long Term Ecological Research, most recently under award #OPP-1637708.