Carbon Flux and Spectral Response of Biological Soil Crust to Soil Moisture Status.

Soil water content is known to be vital for biological soil crust (BSC) activity and carbon flux.  However, few studies have simultaneously examined the inter-relationships among soil water, carbon flux and reflectance spectra of BSC organisms.  We report on such a study, which was conducted during water addition and dry-down cycles in BSC samples collected near Moab, UT and Indian Springs, NV.  Experiments were conducted using a custom-designed chamber, into which BSC samples were placed. Dual-probe heat pulse sensors were then inserted into the BSC sample to measure water content. Carbon flux was determined using a LI-COR 6400 gas exchange instrument.  Measurements were made before water addition, immediately after water addition, 5 minutes after water addition and at 30-minute increments thereafter until steady state conditions were reached.  The spectral response measurements were taken contemporaneously using an ASD FieldSpec pro (350-2500 nm). The spectral response to water addition was an overall decrease in reflectance across the spectrum, regardless of crust presence and crust type (e.g., lichen, moss or cyanobacteria).  While moss spectra revealed a significant increase in red wavelength absorption, the lichen and cyanobacteria spectra did not have a significantly increased red absorption.  However all crust types did have significant spectral index increases (predominantly red-to-near infrared indices) and gradual declines in these indices over time as the soil dried.  Carbon flux patterns, as anticipated, were highly correlated to soil water content under simulated Spring temperature and light intensity conditions. The results highlight the importance of very-near surface water contents on biological activity of soils populated with crusts and the potential for assessing BSC carbon flux with remote sensing.