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Currently, drylands occupy more than one-third of the global terrestrial\udsurface and are recognized as areas vulnerable to land degradation. The\udconcept of land degradation stems from the loss of an ecosystem's biological\udproductivity due to long-term loss of natural vegetation or depletion of\udsoil nutrients. Drylands' key role in the global carbon (C)\udbalance has been recently demonstrated, but the effects of land\uddegradation on C sequestration by these ecosystems still need to be\udinvestigated. In the present study, we compared net C and water vapor\udfluxes, together with satellite, meteorological and vadose zone (CO2,\udwater content and temperature) measurements, between two nearby\ud(∼ 23 km) experimental sites representing <q>natural</q>\ud(i.e., site of reference) and <q>degraded</q> grazed semiarid grasslands. We utilized data acquired over 6 years from two eddy covariance stations located in southeastern Spain with highly variable precipitation magnitude and distribution. Results show a striking difference in the annual C balances with an average net CO2 exchange of 196 ± 40 (C release) and −23 ± 2 g C m−2 yr−1\ud(C fixation) for the degraded and natural sites,\udrespectively. At the seasonal scale, differing patterns in net CO2\udfluxes were detected over both growing and dry seasons. As expected, during\udthe growing seasons, greater net C uptake over longer periods was observed at\udthe natural site. However, a much greater net C release, probably derived\udfrom subterranean ventilation, was measured at the degraded site during\uddrought periods. After subtracting the nonbiological CO2 flux from net CO2 exchange, flux partitioning results point out that, during the 6 years of study, gross primary production, ecosystem respiration and water use efficiency were, on average, 9, 2 and 10 times higher, respectively,\udat the natural site versus the degraded site. We also tested\uddifferences in all monitored meteorological and soil variables and CO2 at 1.50 m belowground was the variable showing the greatest intersite difference, with  ∼ 1000 ppm higher at the degraded site.\udThus, we believe that subterranean ventilation of this vadose zone CO2, previously observed at both sites, partly drives the differences in C dynamics between them, especially during the dry season. It may be due to enhanced subsoil–atmosphere interconnectivity at the degraded site

Can land degradation drive differences in the C exchange of two similar semiarid ecosystems?