Abstract
Intra- and interannual variability of precipitation can lead to major modifications of grassland production and carbon
storage capacity. Greater understanding of how climatic variability affects net CO2 exchange [i.e. net ecosystem
exchange (NEE)] of grazed grasslands is important to adapt grassland management and reduce risks of carbon losses.
Since 2002, we continuously measured NEE (i.e. eddy covariance technique) on an upland grassland site (7 ha),
divided in two paddocks grazed by heifers (intensive: 1 LSU ha
1 yr
1, 213 kg N ha
1 yr
1 and extensive:
0.5 LSU ha
1 yr
1, no fertilization). For years with dry and warm growing seasons (i.e. 2003, 2005 and 2008), absolute
annual NEE was higher in the intensive paddock compared with the extensive paddock. The opposite was observed
during years of ample seasonal rainfall and soil moisture (i.e. 2004, 2006 and 2007). Contrasted management led to
two distinct plant communities being different in leaf area index (LAI), soil bulk density and soil water holding
capacity. Differences in annual NEEs could thus be assigned to interactions between in carbon and water fluxes during
dry and wet growth periods. Dry growth periods led to a reduction in weekly gross primary productivity (GPP)
in the extensively managed paddock, whereas the GPP was maintained in the intensive paddock. In turn, during wet
growth periods, GPP was similar in both paddocks, whereas N amendment and frequent defoliation significantly
increased ecosystem respiration in the intensive paddock, presumably through a higher heterotrophic respiration
following on a better C substrate quality and availability (rhizodeposition and senescent fine roots). In the extensive
paddock, where plant cover was denser (reducing soil temperature) and less decomposable, C losses through heterotrophic
respiration were comparatively smaller under wet conditions. Our results demonstrate that grassland subjected
to a moderately intensive management could be more resilient in terms of carbon storage during drought and heat waves, presumably because of a trade-off between heterotrophic and autotrophic respiration.
