A Continental-Scale Investigation of Factors Affecting Soil Organic Matter Vulnerability to Decomposition.

Soil organic matter (SOM) is the largest actively-cycled terrestrial carbon (C) pool and is linked to atmospheric C and climate through soil respiration, one of the largest terrestrial C fluxes. Consequently, due to potential carbon-climate feedbacks, understanding SOM dynamics is critical to predicting the effects of global change drivers, such as climate and land use change. However, there has been a recent paradigm shift in regards to controls on SOM dynamics with SOM molecular recalcitrance playing a lesser role than soil and other ecosystem properties, which has led to a knowledge gap in being able to predict SOM vulnerability to environmental change. In an effort to address this knowledge gap, a year-long two-way factorial laboratory soil incubation is currently underway. The experiment involves incubating samples from two horizons (A and uppermost B) from 30 National Ecological Observatory Network (NEON) sites spanning a wide range of climates and ecosystems under two temperature levels (mean summer temperature (MST) and MST + 5 ºC) and three moisture levels (-33 kPa, -150 kPa, and -400 kPa). Over the course of the incubation, headspace CO₂ concentrations are being periodically measured to calculate cumulative C respired. Additionally, headspace δ¹³CO₂ measurements are being taken at t = 0, 3, 6, and 12 months to be used in combination with solid phase bulk and density fraction δ¹³C measurements to detect shifts in substrate usage. Cumulative C respired normalized to total sample organic C will be regressed against a number of variables, including temperature, moisture, and various soil properties, to determine which factors are most important in controlling SOM vulnerability to decomposition in response to environmental change.