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ABB LGR | Application of ABB LGR CCIA

Date: 2021-08-02
浏览次数: 30

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.

ABB LGR | Application of ABB LGR CCIA  

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.

ABB LGR | Application of ABB LGR CCIA

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))

ABB LGR | Application of ABB LGR CCIA  

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

ABB LGR | Application of ABB LGR CCIA

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

ABB LGR | Application of ABB LGR CCIA

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.

ABB LGR | Application of ABB LGR CCIA 6376351873074688334225580.pdf

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