Current atmospheric CO
2 concentration (400 ppm) might reach up to 500-1000 ppm by year 2100
due to global climate change predicted by different gas emission scenarios from International panel
of climate change (IPCC). Elevated atmospheric CO
2 has been proved to reduce plant-transpiration
in some studies while some studies did not have any differences in different soil. Since most of the
previous studies proved that elevated atmospheric CO
2 interacts with soil nutrients to enhance
plant growth, whole plant transpiration could increase through enhanced leaf area. However, plant
induced soil suction besides plant growth due to interaction of increasing atmospheric CO
2 and
soil nutrients in heavily compacted soil for the design purpose of vegetated soil structures still
remain elusi...[
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Current atmospheric CO
2 concentration (400 ppm) might reach up to 500-1000 ppm by year 2100
due to global climate change predicted by different gas emission scenarios from International panel
of climate change (IPCC). Elevated atmospheric CO
2 has been proved to reduce plant-transpiration
in some studies while some studies did not have any differences in different soil. Since most of the
previous studies proved that elevated atmospheric CO
2 interacts with soil nutrients to enhance
plant growth, whole plant transpiration could increase through enhanced leaf area. However, plant
induced soil suction besides plant growth due to interaction of increasing atmospheric CO
2 and
soil nutrients in heavily compacted soil for the design purpose of vegetated soil structures still
remain elusive. The major objectives of this study were to investigate the coupled effects of
atmospheric CO
2 and nitrogen-rich NPK (Nitrogen, Phosphorous, Potassium) nutrient on single
plant growth, induced soil matric suction, soil water retention behaviour and correlations of plant
characteristics with plant-induced peak matric suction in heavily compacted soil.
Three series of laboratory experiments were conducted under different atmospheric CO
2 in nutrient
poor and nutrient supplied soil. For all tests, completely decomposed granite soil was used at a
95% degree of compaction and 3 replicates of Schefflera heptaphylla (Ivy tree) in each test were
grown for 3 months. Different plant characteristics, soil matric suction, volumetric water content
and macro and micro nutrients of soil were quantified during and after 3 months of plants growth.
While growing under 400 and 1000 ppm atmospheric CO
2 in nutrient poor heavily compacted soil,
Leaf area index (LAI) of plants decreased by 35% under 1000 ppm CO
2 due to soil N scarcity
which impedes the plants ability to metabolize increased atmospheric CO
2 concentration. Despite
having LAI of 0.5-0.7, suction decreased by 10-25 kPa during drying under 1000 ppm CO
2. This
is because leaves cannot open stomata wide enough to transpire more water when they absorb
more CO
2 from the atmosphere resulting in soil matric suction reduction. Although a strong
positive linear correlation (R
2=0.8-0.9; P-value<0.1) of LAI and root-shoot biomass ratio with
induced peak soil suction was found under 400 ppm CO
2, a weak linear correlation (R
2=0.6; P-value>0.1) of the root-shoot biomass ratio with suction under 1000 ppm CO
2 was observed.
In contrast, while growing under 400 and 1000 ppm atmospheric CO
2 in NPK nutrient supplied
heavily compacted soil, LAI of plants increased by 22% under 1000 ppm CO
2 compared to 400
ppm CO
2. Due to adding N-rich NPK nutrient in soil, plants could uptake more nitrogen (N) to
leaves which increased the nitrogen amount in chloroplasts to produce higher amount of
chlorophyll thus increased leaf area. Because of 40% larger LAI of plants grown under 1000 ppm
CO
2 with additional soil nutrient, larger leaf area induced higher soil suction substantially since
no significant difference in suction was found between current and elevated atmospheric CO
2. LAI
could be a reliable parameter to understand soil matric suction behaviour due to having strong
correlation (R
2=0.85-0.98; P-value<0.1) with peak matric suction even under different
atmospheric CO
2. However, root-shoot biomass ratio should not be a recommended parameter to
observe soil suction behaviour due to having contradictory correlations in different studies.
This study implies that the effects of atmospheric CO
2 on plant induced soil suction must be
considered for the conservative design of vegetated soil structures. They also need proper
maintenance with additional nitrogen-rich nutrients to thrive under future atmospheric condition
and to maintain higher soil suction hence soil shear strength. Given the 3 months of plants growth
and small-scale investigations, research findings from this study should be treated with caution.
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