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

Abstract

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 CO(2) and its C isotope composition (delta(13)C(SR)). A combined shading and pulse-labelling experiment was carried out in a mountain grassland. delta(13)C(SR) was monitored at a high time resolution with a tunable diode laser absorption spectrometer. In unlabelled plots a diurnal pattern of delta(13)C(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 delta(13)C(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 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 CO(2) fluxes.

Fig. 1

Averaged diurnal courses of (a, b) δ¹³C of soil-respired CO₂ (δ¹³C_SR) in (a) unshaded and (b) shaded plots in a mountain grassland, (c) soil respiration in unshaded (open circles) and shaded (closed circles) plots, (d) photosynthetically active radiation (PAR; black line) incident on unshaded plots and vapour pressure deficit (VPD; dashed grey line) and (e) soil temperatures (T_s) (T_s unshaded, open circles; T_s shaded, closed circles) and soil water content (SWC; dashed line) at 5 cm depth. Averages were calculated by aggregating hourly means (based on 20-min measurement intervals) obtained in three plots for each treatment during three separate field campaigns between 15 July and 4 August 2007 (for details see the Materials and Methods). Error bars indicate standard errors; n = 13–16 and 10–12 for three unshaded and three shaded plots, respectively. Horizontal dotted lines in (a) and (b) represent the daily mean of δ¹³C_SR.

Fig. 2

Time course of (a, b) δ¹³C of soil-respired CO₂ (δ¹³C_SR) after pulse labelling in (a) an unshaded and (b) a shaded plot in a mountain grassland, (c) soil respiration in shaded (closed circles) and unshaded (open circles) plots (mean values of the respective control and the labelled plot, normalized for each plot to average pretreatment values), (d) photosynthetically active radiation (PAR; black line) incident on unshaded plots and vapour pressure deficit (VPD; dashed grey line) and (e) soil temperatures (T_s) (T_s unshaded, dashed grey line; T_s shaded, black line) and soil water content (SWC; dashed line) at 5 cm depth. Insets in (a) and (b) show the period immediately after labelling on an enlarged scale.

Fig. 3

Diurnal variation of δ¹³C of soil-respired CO₂ (δ¹³C_SR) in labelled (a) unshaded and (b) shaded plots in the course of the first experiment, expressed as residuals of the respective decay curves shown in Table 1 in relation to observed values (Fig. 2a,b).

Fig. 4

Relationship between hourly means of δ¹³C of soil-respired CO₂ of unlabelled plots (δ¹³C_SR unlabelled; x) and the residuals of the decay curve of labelled plots (residual δ¹³C_SR labelled; y) for the unshaded treatments of the first experiment (y = 96.075x + 2578.2; R² = 0.85, P < 0.001). Each hourly mean is based on an average of three hourly measurements for days with complete data sets (n = 7).