drivers of soil respiration of root and microbial ... - Unitus DSpace

3.4. Discussion
3.4.1. Speed of C cycling
82
New C cycled quickly within the ecosystem, with a great proportion of recently assimilated
C respired within the first day (Fig.7). The first peak in evolution of 13 CO2 occurred less than one
hour after the opening of the LC, both in above and belowground respiration, followed by the other
peaks in the course of the day (Fig.5). These initially high values in the 13 C signature of respired
CO2 is possibly attributed to the residuals of the label which may have diffused to the soil pores
during the pulse labeling procedure and consequently is not of a plant-origin. In fact some studies,
confirm this phenomenon for the in situ pulse labeling experiments (Ostle et al., 2003, Leake et al.,
2006). Ostle et al. (2003) by increasing the number of the replicates (from 3 to 6) and the sampling
frequency improved the results reported by Staddon et al. (2003). The greater replication gave more
consistent results with peak 13 C enrichment of soil respiration occurring 1 day after the labeling,
rather than 2 hours, reported by Staddon et al. (2003). In fact, laboratory conditions permit to isolate
the soil compartment during the labeling procedure, avoiding the uncontrollable penetration of the
label into the soil, while in situ its quite impossible. The 13 C signature of the aboveground
respiration experienced a small decrease during the first hour after the pulse labeling, but the first
peak originating from plant respiration occurred between 2 and 4 hour of the chase period (Fig.3b).
This time lag is similar to the one reported by Carbone and Trumbore (2007) for the grasslands and
shrublands, where the 13 C in aboveground respiration peaked in the first 4 hours after the pulse
labeling.
The 13 C fixed by shoots may be respired, used in production of new shoot growth,
temporarily stored (e.g. in starch), or allocated belowground into roots and from them to soil
organisms and belowground respiration (Leake et al., 2006). In belowground respiration there was
observed again initially high value of the 13 C content. However, we argue that the translocation of C
from leaves to roots and into soil respiration occurred later, in the time the next peak in
belowground 13 CO2 respiration was registrated (Fig. 3c). This peak occurred from 16 to 24 hours
after the pulse labeling. The same peak was observed also in aboveground respiration, which
indicates that we were not able to separate completely above and belowground respiration by
subtracting from total respiration the CO2 evolved from the bare plots. Another possible
explanation could be the re-fixation in shoots of the 13 C respired from the soil or back flow of the
pulse C from belowground, as was observed by Johnson et al. (2002), e.g. from 13 C sugar that
passed to the roots being combined with ammonium, taken previously up from the soil, to form 13 C-
enriched amino acids that are then transferred back to the shoots.
Carbone and Trumbore (2007) for a grassland site reported the time lag of less than 4 hours
for the belowground grassland respiration, but the number of sampling times in their study was less