How does climate change alter carbon cycling in temperate European grasslands?
Climate change has the potential to significantly alter carbon (C) and nutrient dynamics in grasslands, with projections indicating that drier summers will be more common in northern Europe. This research investigates the mechanisms by which temperate grasslands respond to summer drought events and how nutrient availability will influence these responses. Questions remain for understanding both aboveground and belowground responses and their potential feedbacks. An important parameter for plants and soil microbes is their carbon use efficiency (CUE), the proportion of C assimilated into biomass relative to C respired. CUE can shift in response to nutrient availability and climate change, although it has not been studied in both plants and the soil microbial community in combination. To investigate aboveground and belowground changes in C cycling we set up manipulation experiments in the Yorkshire Dales, UK, to test for interactive effects of drought and nutrient availability. We investigated CUE using a 13CO2 ‘pulse-labelling’ experiment in the field to measure the fate and rate of C transfer in plants and soil microbial community. We hypothesised that under drought conditions grasslands with nutrient addition would have greater declines in net ecosystem exchange, ecosystem respiration and plant and soil microbial CUE.
We found that both drought and nutrient availability altered grassland CO2 fluxes. In particular, drought reduced ecosystem respiration, with the largest reductions in nutrient addition plots which had the greatest biomass. This demonstrates that drought resistance in temperate grasslands can be biomass dependent. Drought also increased C:N in aboveground biomass for 3 plant functional groups (forbs, legumes and parasitic plants) but not for grasses. This suggests that drought stress reduced the plants’ ability to take up nitrogen (N), as soil inorganic N was increased under drought. In situ 13CO2 ‘pulse-labelling’ revealed that plants and soil microbes rapidly cycled C, however the soil respiration from nutrient addition plots had lower 13CO2 enrichment than control plots, indicating greater assimilation into microbial biomass. Drought had no effect on 13CO2 respired. In conclusion we show that the mechanisms determining grassland resistance to climate change are complex. Increased nutrient availability can increase grassland vulnerability to drought, however drought can also drive shifts in N cycling both above and belowground. Additionally nutrient addition has the ability to alter CUE which can drive positive or negative feedbacks to climate change.