Summer drought influence on CO2 and water fluxes of extensively managed grassland in Germany

Net ecosystem CO2 exchange (NEE) and water fluxes of extensively managed grassland in Germany were measured in a dry year, 2003 and a normal year, 2004 using eddy covariance. Empirical descriptions of canopy capacities (light utilization efficiency and canopy capacity for maximum carboxylation) and inherent water use efficiency (IWUE*) were computed to indicate the grassland's response to environmental and management conditions. Compared with 2004, 2003 was characterized by a 2.5 °C rise in summer mean temperature and an annual precipitation deficit of 286 mm, which was almost one-third below the average precipitation of the normal year 2004 and the long-term annual average (1959–2005). Annual net water balance (precipitation minus evapo-transpiration) in 2003 and 2004 were 85 mm and 361 mm, respectively. Slightly higher IWUE* in 2003 indicates the grassland's sensitivity to drought. The grassland provided an annual gross primary production (GPP) and ecosystem respiration (Reco) of 1022 and 728 g C m−2 respectively in 2003, as compared to their corresponding values of 1233 and 973 g C m−2 in 2004. Drought-induced biomass senescence caused GPP reduction in 2003, which in turn affected the ecosystem respiration. Light explained more than 80% of NEE variability during canopy development and as a result, annual NEE in 2003 and 2004 were −295 and −260 g C m−2, respectively. Taking into account the exported carbon by biomass harvest, we calculated net biome productivity (NBP) of −113 g C m−2 yr−1. We conclude that this managed grassland acted as a moderate carbon sink, despite the difference in environmental and management conditions.

Research highlights
► Drought year recorded significant reduction in GPP and Reco.
► Drought -induced biomass senescence explained the reduction in GPP and Reco.
► Light accounted for more than 80% variation in NEE during growing period.
► Biomass harvest caused short-term negative effects on NEE.
► NBP estimates suggest that this grassland acted as moderate carbon sink.