Temporal Coupling of Subsurface and Surface Soil CO2 Fluxes: Insights From a Nonsteady State Model and Cross-Wavelet Coherence Analysis

Inferences about subsurface CO2 fluxes often rely on surface soil respiration (R-soil) estimates because directly measuring subsurface microbial and root respiration (collectively, CO2 production, S-Total) is difficult. To evaluate how well R-soil serves as a proxy for S-Total, we applied the nonsteady state DEconvolution of Temporally varying Ecosystem Carbon componenTs model (0.01-m vertical resolution), using 6-hourly data from a Wyoming grassland, in six simulations that cross three soil types (clay, sandy loam, and sandy) with two depth distributions of subsurface biota. We used cross-wavelet coherence analysis to examine temporal coherence (localized linear correlation) and offsets (lags) between S-Total and R-soil and fluxes and drivers (e.g., soil temperature and moisture). Cross-wavelet coherence revealed higher coherence between fluxes and drivers than linear regressions between concurrent variables. Soil texture and moisture exerted the strongest controls over coherence between CO2 fluxes. Coherence between CO2 fluxes in all soil types was strong at short (similar to 1 day) and long periods (>8 days), but soil type controlled lags, and rainfall events decoupled the fluxes at periods of 1-8days for several days in sandy soil, up to 1week in sandy loam, and for a month or more in clay soil. Concentrating root and microbial biomass nearer the surface decreased lags in all soil types and increased coherence up to 10% in clay soil. The assumption of high temporal coherence between R-soil and S-Total is likely valid in dry, sandy soil, but may lead to underestimates of short-term S-Total in semiarid grasslands with fine-grained and/or wet soil. Plain Language Summary Soil CO2, which is produced underground by roots and microbes, is a major part of the global carbon cycle. There are large uncertainties over how soil CO2 will change as global temperatures and atmospheric CO2 rise. One source of uncertainty is how quickly soil CO2 moves from the sites where it is produced underground to the surface where it is released to the atmosphere. In this paper, we use a numerical model to test the common assumption that CO2 produced underground is released immediately to the atmosphere. We found that this assumption is valid when soil is coarse and dry, but there are delays between subsurface CO2 production and release to the atmosphere when the soil has a fine texture and/or is wet.