Response of water balance components to climate change in
permanent grassland soil ecosystems

Abstract. Hydrological processes are affected by changing climatic conditions. In grassland areas, changes in the ecosystem water balance components will alter aboveground biomass production (AGB), which in turn is of great importance for ecological and economic benefits of grassland. However, the effects of climate change on the ecosystem productivity and water fluxes are often derived from climate change experiments. It is still largely unknown whether and how the experimental approach itself affects the results of such studies. The aim of this investigation was to identify the effects of climate change on the water balance and the productivity of grassland ecosystems by comparing results of two contrasting approaches of climate change experiments. The first (manipulative) climate change approach uses increased atmospheric CO2 concentrations and surface temperatures. The second (observational) approach uses data from a space-for-time substitution approach along a gradient in climatic conditions. The climate change effects on the ecosystem’s water balance was determined by using high-precision weighable monolithically lysimeters at each site over a period of four years, including the exceptionally dry year 2018. The aridity index, defined as the grass-reference evapotranspiration (ET0) to precipitation (P), was used to characterize the hydrological status of the regime (i.e. energy- or water limited system). The observational approach (grassland ecosystem moved to a drier and warmer site), resulted in a large decrease of precipitation (P) and non-rainfall water (NRW), an increase in actual evapotranspiration (ETa) and upward directed water fluxes from deeper soil and hence a decline of seepage water as well a decrease in AGB and water use efficiency (WUE). The manipulative approach (grassland ecosystem treated in place) resulted in decreasing P and NRW under conditions of elevated temperature but responded with increasing NRW for elevated CO2 as compared to the reference. Similarly, an elevated CO2 and heating increased the ecosystem’s water loss by ETa. However, the effect of increasing CO2 on ETa was largely compensated by the opposite effect of an elevated temperature in the combined treatment. The seepage water rate also increased with elevated CO2, whereas it clearly decreased for the heating treatment as compared to the reference. All treatments led to a reduction of the grassland productivity in terms of the AGB and to reduced WUE as compared to the grassland ecosystem under reference conditions. The consideration of changes in NRW and P by the treatments needs to be considered in climate change experiments to avoid an over- (elevated temperature) or underestimation (elevated CO2) of the effects of climate change on ecosystems response, especially for sites where water limitation plays a role. The impact of drought periods on seepage rates (potentially leading to groundwater recharge) was more pronounced for the relatively humid site with a longer ETa period without water stress than for a relatively dry site. The hydroclimatological and ecohydrological indicators were similarly affected by changes in temperature, atmospheric CO2 concentrations, and precipitation in both manipulative and observational climate change experiments except for the responses of ETa and AGB in the dry and warm year 2018. The resulting response differences between the two climate change approaches were explained by the actual soil moisture status. The results suggest that energy limited ecosystems tend to increase their ETa and AGB production (excluding effects from elevated CO2 and temperature), but water limited ecosystems respond with a decrease in ETa as a result of water stress, which leads to a clear decline of AGB. The results also suggest that climate change experiments should account for the possible change of the hydrological status of the ecosystem and impose sufficiently extreme levels of climatic conditions within their set-up to allow such changes to occur for capturing the full response of the ecosystem. The results may help to better understanding the impact of climate change on future ecosystem functioning.