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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 (δ¹³C_SR).

A combined shading and pulse‐labelling experiment was carried out in a mountain grassland. δ¹³C_SR was monitored at a high time resolution with a tunable diode laser absorption spectrometer.

In unlabelled plots a diurnal pattern of δ¹³C_SR 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 δ¹³C_SR 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 …