Abstract
Soil respiration represents one of the dominant fluxes of CO2 from
terrestrial ecosystems to the atmosphere, therefore, it is important to
understand how it is controlled across a wide variety of sites. As part
of developing a soil respiration data product based on freely available
National Ecological Observatory Network (NEON) data, soil respiration
rates were calculated at 30 sites throughout the USA (from Puerto Rico
to North Dakota and Virginia to California) to investigate controls on
soil respiration at a continental scale. The sites spanned a wide range
of ecosystems, including deserts, grasslands, and forests, as well as
managed and wildland sites. Soil respiration was calculated in 30-minute
intervals using the gradient method based on soil CO2 concentrations
measured at three different depths in conjunction with estimates of soil
CO2 diffusivity based on soil physical properties, soil moisture and
temperature profiles, and barometric pressure. Inevitably with such a
large number of data inputs, the temporal coverage of good quality
(i.e., unflagged) soil respiration values was relatively low (8%)
because one or more of the input data were flagged, but this was
significantly higher for some sites and soil plots (maximum: 58%).
Ongoing efforts to increase the quality of input data are expected to
substantially improve temporal coverage. Despite these gaps, over 54,000
unflagged half-hourly soil respiration data points were generated for
the period of Apr-Jun 2019 (the temporal range and the number of sites
will be increased further over coming months). Across all sites and
times, soil temperature and soil moisture explained only a moderate
amount of variation in soil respiration (20%). However, including site
in the model increased the proportions of variation explained to 85%,
indicating the importance of site-specific properties, such as
vegetation and microbial community composition and the accessibility of
soil carbon, in controlling soil respiration rates. Within each site,
soil temperature was typically positively correlated with respiration
and in the cases where it was not, this was often due to large fluxes of
CO2 leaving the soil during snowmelt. The relationship between soil
moisture and respiration within each site was more variable, with both
positive and negative relationships observed.