DOI: 10.48443/bdgb-5v03
title: Soil CO2 concentration (DP1.00095.001)
title: RELEASE-2023

description: CO2 concentrations are measured at different depths in the soil to allow the gradient method to be used to estimate soil CO2 efflux rates when combined with other NEON data products. Soil CO2 efflux is an important component of the carbon cycle because it is one of the largest exchanges of carbon between terrestrial ecosystems and the atmosphere. In addition, since the vast majority of soil CO2 is produced by microbial, root and soil faunal respiration, soil CO2 efflux is an indicator of total soil biological activity. CO2 concentrations are measured in all five Instrumented soil plots per terrestrial site and at various depths below the soil surface starting at approximately 2 cm and data are presented as 1-minute and 30-minute averages. CO2 sensors at different depths within a soil plot are typically located within 1 m horizontally of one another. The CO2 concentration of soil air is measured at three depths within each plot, starting at approximately 2 cm.

Latency:
Data collected in any given month are published during the second full week of the following month.

Soil CO2 concentration data from the NIWO site (mean elevation: 3490m) are currently flagged because the typical atmospheric pressure (~66 kPa) is below the manufacturer-specified lower range for pressure compensation (70 kPa). This data product is not currently available at the PUUM site (Hawaii) due to the initial soil characterization indicating the soil was too shallow to allow the standard soil temperature sensor to be installed. An alternative sensor was installed, but it is not currently compatible with the data processing pipeline (soil temperature is an input for the soil CO2 concentration data product).  Future upgrades to the data processing pipeline will allow the data product to be produced, including retroactively, but this is not expected to occur soon.

At four Alaskan sites (HEAL, BONA, TOOL, and BARR) the sensors experience frost heaving which regularly changes the measurement depths. New measurement depths are recorded when technician time and other priorities allow and depths are updated, but sensor depths may change between remeasuring events. In addition, the soil plots at HEAL have been subsiding faster than the surrounding area which may alter their representativeness of the overall site.
description: When possible the plots were arranged in a transect with the first plot approximately 15-40 m from the tower in the expected dominant airshed. The middle of airshed was used as the transect vector and plot spacing was based on the distance required for surface soil temperature and moisture measurements to be spatially independent at the 1 hectare scale during site characterization (capped at approximately 40 m due to logistical constraints). Soil plots were microsited as necessary to avoid obstacles (e.g., boulders, streams, and paths) and more compact plot layouts were used at small sites. The CO2 concentration of soil air is measured at three depths within each plot, starting at approximately 2 cm. The depths of the two deeper sensors vary among sites and were chosen based on megapit soil horizon data (NEON.DP1.00097), expected soil CO2 concentration, and ecosystem type (see NEON.DOC.003146). Instrumentation: Vaisala - GMP343