Continental representativeness of NEON's design for sensor-based measurements of soil properties

Background/Question/Methods

Soils are one of the most complex components of terrestrial ecosystems and play a key role in a range of ecological properties and processes, including carbon storage, ecohydrology, nutrient cycling, and supporting hyper-diverse biological communities. As a result soil measurements represent a major focus of the National Ecological Observatory Network (NEON) and soil-based sensors will monitor profiles of soil temperature, moisture, CO₂, and root activity, as well as soil heat flux. Here we describe results of NEON’s strategy to ensure the representativeness of sensor-based soil measurements at scales of the continent (>1000s km), site (100s m), and plot (≤5 m horizontally; ≤3 m deep) by considering each scale in turn.

Results/Conclusions

Continental scale: The initial 60 NEON sites include representatives of every major soil order found in the US. Some common soil orders are currently underrepresented (e.g. Alfisols and Entisols), while others are overrepresented (e.g. Inceptisols and Spodosols). However, NEON’s 40 Relocatable sites, which will move approximately every 5 years, are expected to reduce under- and over-representation over NEON’s 30-year lifespan. Site scale: There will be five replicate soil plots at each site. A geostatistical site characterization was conducted to determine the spatial scale of independence for soil temperature and moisture at each site, i.e. the distance at which these variables were no longer autocorrelated. This distance determined the spacing between neighboring soil plots at a site, although the maximum distance between plots was limited to 40 m due to budget constraints. Based on our geostatistical analysis, the soil plots will be fully independent at the site scale at 45% of sites, and the two most distant soil plots will be fully independent at >85% of sites. Plot scale: Soil plots will generally be 5 x 5 m in size and sensors will be placed up to 3 m deep. A typical soil plot will include 8 temperature sensors, 8 moisture sensors, 3 CO₂ sensors, 3 minirhizotron tubes, and 1 heat flux sensor. The protocol for sensor placement is under development at the time of abstract submission; however, factors that will be considered include: 1) soil horizons; 2) placing sensors at identical depths when possible; 3) maximizing profile depth for temperature and moisture sensors; and 4) placing the uppermost CO₂ sensor near the soil surface to ensure total soil CO₂ efflux can be estimated. Additional details will be presented on the poster.