The effect of physical back-diffusion of 13CO2 tracer on the coupling between photosynthesis and soil CO2 efflux in grassland

Pulse labelling experiments provide a common tool to study short-term processes in the plant–soil system and investigate below-ground carbon allocation as well as the coupling of soil CO₂ efflux to photosynthesis. During the first hours after pulse labelling, the measured isotopic signal of soil CO₂ efflux is a combination of both physical tracer diffusion into and out of the soil as well as biological tracer release via root and microbial respiration. Neglecting physical back-diffusion can lead to misinterpretation regarding time lags between photosynthesis and soil CO₂ efflux in grassland or any ecosystem type where the above-ground plant parts cannot be labelled in gas-tight chambers separated from the soil. We studied the effects of physical ¹³CO₂ tracer back-diffusion in pulse labelling experiments in grassland, focusing on the isotopic signature of soil CO₂ efflux. Having accounted for back-diffusion, the estimated time lag for first tracer appearance in soil CO₂ efflux changed from 0 to 1.81±0.56 h (mean±SD) and the time lag for maximum tracer appearance from 2.67±0.39 to 9.63±3.32 h (mean±SD). Thus, time lags were considerably longer when physical tracer diffusion was considered. Using these time lags after accounting for physical back-diffusion, high nocturnal soil CO₂ efflux rates could be related to daytime rates of gross primary productivity (R²=0.84). Moreover, pronounced diurnal patterns in the δ¹³C of soil CO₂ efflux were found during the decline of the tracer over 3 weeks. Possible mechanisms include diurnal changes in the relative contributions of autotrophic and heterotrophic soil respiration as well as their respective δ¹³C values. Thus, after accounting for physical back-diffusion, we were able to quantify biological time lags in the coupling of photosynthesis and soil CO₂ efflux in grassland at the diurnal time scale.