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

Abstract. The concept of land degradation stems from the loss of an ecosystem's biological productivity, which in turn relies on several degradation processes, such as long-term loss of natural vegetation, depletion of soil nutrients, soil compaction or water and wind erosion, to which drylands are especially vulnerable. Currently, drylands occupy more than one third of the global terrestrial surface and will probably expand under future climate change scenarios. Drylands' key role in the global C balance has been recently demonstrated, but the effects of land degradation on C sequestration by these ecosystems needs further research. In the present study, we compare net carbon exchange, together with satellite data and meteorological, ambient and vadose zone (CO2, water content and temperature) variables, between two nearby (~23 km) experimental sites representing natural (i.e. site of reference) and degraded grazed semiarid grasslands located in SE Spain, via eddy covariance measurements over 6 years, with highly variable precipitation magnitude and distribution. Results show a striking difference in the annual C balances with an average release of 196 ± 40 and −23 ± 20 g C m−2 yr−1 for the degraded and natural sites, respectively. At the seasonal scale, differing patterns in net CO2 fluxes were detected over both growing and dry seasons. As expected, during the growing seasons, greater net C uptake over longer periods was observed in the natural site, however, much greater net C release was measured in the degraded site during drought periods. We tested differences in all monitored meteorological and soil variables and found it most relevant that CO2 at 1.50 m belowground was around 1000 ppm higher in the degraded site. Thus, we believe that subterranean ventilation of this vadose zone CO2, previously observed at both sites, largely drives the differences in C dynamics between them, especially during the dry season maybe due to enhanced subsoil-atmosphere interconnectivity in the degraded site. Overall, the 12 site-years of data allow direct exploration of the roles of climate and land degradation in the biological and non-biological processes that ultimately control the C sequestration capacity of semiarid ecosystems.