Soil CO2 fluxes beneath barley on the central Spanish plateau

Soil CO2 fluxes were measured, from November 1998 to October 2000, over non-irrigated barley in Central Spain using a gas-exchange soil chamber, and covering a 10 cm soil temperature range of 1–29 °C. Soil CO2 fluxes exhibited seasonal variation with an increase from March to October, peaking in May, and a decrease of around a factor of 2 during the winter. The mean of the data recorded during 53 days and covering all the seasons of the year was 2.0 μmol m−2 s−1, and the extreme values ranged from 0.2 to 8.0 μmol m−2 s−1. During the sampling campaign, 10 cm soil temperature measurements were also conducted and NDVI data were calculated by means of NOAA satellite images as 10-day maximum composite values. With these data, two different empirical equations were fitted for soil CO2 predictions. Both were dependent on a second order 10 cm soil temperature but differed in the way they considered a surrogate of soil water content and plant vitality as second input data. The first equation roughly took into account the influence of soil water content by sorting out the whole days in two clusters, those corresponding to rainfall occurrences on the measuring day or on the day before measurement and the remaining ones. The second was on NDVI. The squared correlation coefficient of each empirical equation was 71% and 74%, respectively. The maximum fluxes were reached at a 10 cm soil temperature of 17 °C. The relationship with soil temperature indicated that the flux should be negligible below 3 °C and above 31 °C.