Does photosynthesis affect grassland soil-respired CO₂ and its carbon isotope composition on a diurnal timescale?

Soil respiration is the largest flux of carbon (C) from terrestrial ecosystems to the atmosphere. Here, we tested the hypothesis that photosynthesis affects the diurnal pattern of grassland soil-respired CO₂ and its C isotope composition (δ¹³CSR). A combined shading and pulse-labelling experiment was carried out in a mountain grassland. δ¹³CSR was monitored at a high time resolution with a tunable diode laser absorption spectrometer. In unlabelled plots a diurnal pattern of δ¹³CSR was observed, which was not explained by soil temperature, moisture or flux rates and contained a component that was also independent of assimilate supply. In labelled plots δ¹³CSR reflected a rapid transfer and respiratory use of freshly plant-assimilated C and a diurnal shift in the predominant respiratory C source from recent (i.e. at least 1 d old) to fresh (i.e. photoassimilates produced on the same day). We conclude that in grasslands the plant-derived substrates used for soil respiratory processes vary during the day, and that photosynthesis provides an important and immediate C source. These findings indicate a tight coupling in the plant-soil system and the importance of plant metabolism for soil CO₂ fluxes.