ABB LGR | Application of ABB LGR CCIA

Application of ABB LGR CCIA on tracking the spatio-temporal fate of ¹³C-labelled assimilates in the soil of an old-growth pine forest

Water is essential for all organisms and thus, the increasing intensity and frequency of drought in large areas of the Northern hemisphere is impacting the diversity and functioning of forest ecosystems. Impacts of drought on ecosystems range from immediate reductions of the metabolic activity of plants and soil organisms and the acclimation of physiological processes to species shifts under prolonged or reoccurring drought. The interrelation of different ecosystem processes and in particular the coupling of the above-and belowground system is one of the critical unknowns in predicting drought effect on ecosystems. In mature forests, this interlinkage and how it is impacted by drought is still poorly understood.

Based on this, to fill the research gap, in this article “Drought alters the carbon footprint of trees in soils—tracking the spatio-temporal fate of ¹³C-labelled assimilates in the soil of an old-growth pine forest”, researchers from Swiss Federal Research Institute WSL, Northeast Normal University and ETH Zurich conducted related research in the Rhone Valley near Leuk, Canton Valais, Switzerland (46°18′N, 7°37′E, 615 m a.s.l.), aiming at assessing the impact of reoccurring natural drought on the transfer of recent assimilates from canopies of mature trees to their rhizosphere at various timescales.

In summer 2003, the authors conducted the ¹³C pulse-labelling experiment within a long-term irrigation experiment. In late summer 2017, they conducted a whole crown ¹³C pulse-labelling experiment with 10 trees, five of the dry control plots and five of the irrigated plots. The whole tree canopy of each tree was wrapped in a large transparent plastic bag and highly ¹³C -enriched CO₂ (¹³C excess atom fraction 0.99); ¹³C (Cambridge Isotopes) was released to each tree for 3.5 h (Fig 1). The concentration of ¹³CO₂ and ¹²CO₂ in the sealed plastic bag was monitored with an isotope laser spectrometer (LGR, CCIA 46d) and kept manually at about 1500 ppm, which exceeded the saturation point of plant CO₂ uptake. After labelling, the plastic bags were removed and strong industrial blowers were used to flush the labelled ¹³CO₂ gas away and to prevent the labelled ¹³C-CO₂ gas to sink to the ground and diffuse into the soils as well to be taken up by the understory vegetation. The pulse labelling was conducted pairwise and every day one tree from a dry control and one from an irrigated plots were labelled with ¹³C-enriched CO₂. Canopy sizes and tree heights of pulse-labelled trees were measured with a laser-based ruler.

At the same time, the authors measured CO₂ flux rates from soil and stem, the δ¹³C values of soil-respired CO₂ and the δ¹³CO₂ values of stem-respired CO₂. They also sampled soils on the same day when CO₂ effllux rates were measured for determining the soil gravimetric water content, soil microbial biomass carbon (MBC) and soil extractable organic C (EOC). Finally, the authors calculated the soil CO2 effluxes and ¹³C mass balance.

Fig.1 Experimental set-up of the ¹³CO₂ pulse labelling of mature pine trees and the spatio-temporal ¹³C tracking in stem and soil-respired CO² using soil collars placed at several distances in three directions from each tree stem.

Fig.2 Temporal dynamics of the Δδ¹³C values within 3-m distance from tree stems (upper two panels) and ¹³C excess in soil CO₂ efflux of the entire area around the pulse-labelled trees (lower two panels) in the irrigated and the dry control treatments over more than a year. Three trees per treatment have been labelled in the time block before the rainfall event ((a) and (b)) and two trees per treatment have been labelled after the rainfall event at higher soil water contents ((c) and (d))

Fig.3 The Δδ¹³C and ¹³C excess in stem-respired CO₂ and soil-respired CO₂ 10 months after pulse labelling in July 2018.

Fig.4 Spatio-temporal dynamics of ¹³C excess in soil-respired CO₂ under the canopy of the mature pine trees in the dry control and irrigation treatment in the time block before (two upper rows) and after (two lower rows) the intermittent rainfall event. The tree stems are in the middle of each circle and tree canopies have an average radius of 1.29 ± 0.17 m and 1.52 ± 0.21 in the dry control and irrigated treatment respectively

Fig.5 Cumulative flux of soil-respired ¹³CO₂ (¹³C excess) during the first 10 days following pulse labelling, and  ¹³C excess in microbial biomass carbon (MBC) at 0–2, 2–5 and 5–10 cm depth 10 days after pulse labelling. Means and standard errors of three mature pine trees per treatment in the time block before the rainfall event and two trees per treatment in the time block after the rainfall event

[Conclusion] The whole tree pulse-labelling experiment in a mature forest documents that the drought regime affects the coupling of the above-and belowground system and the feedback between plant and soil processes at various timescales. The spatio-temporal ¹³C tracing in the soil gives evidence for an allocation of recent assimilates from tree canopies to the rhizosphere and associated microbial communities within days, but also for a retarded C transfer for more than a year through intermediate C storage within trees. Rhizosphere respiration responded highly sensitive to the 15-year long-term irrigation and the intermittent rainfall event. The authors suggest that the modulation of rhizosphere respiration by soil moisture plays a decisive role in C allocation in trees under moderate drought. The reduced C sink activity of roots and associated mycorrhizae and microbial communities below critical moisture contents appear to reduce the velocity and the amount of assimilates allocated to the belowground. This effect is amplified by the long-term acclimation of trees to repeated summer droughts, with a reduced growth and thus spatial extension of the rhizosphere system. Potentially, the reduced sink strength in the belowground in dry soils feeds back on tree's CO₂ uptake and the allocation and processing of assimilates in trees. In the rhizosphere, the decreased C supply from trees under drought has consequences not only for the microbial activity in the short term, but also for soil microbial community and soil C storage in the longer term. Predictions on the interplay of these interlinked above and belowground processes are hampered by non-linear responses of these processes to soil moisture, frequently unknown depth patterns of fine roots and their associated rhizobiome and the great temporal variability of soil moisture in the three-dimensional soil space.

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