Coupled effects of atmospheric CO₂ and soil nutrients on induced suction in vegetated soil

Abstract

Current atmospheric CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ in nutrient poor heavily compacted soil, Leaf area index (LAI) of plants decreased by 35% under 1000 ppm CO₂ due to soil N scarcity which impedes the plants ability to metabolize increased atmospheric CO₂ concentration. Despite having LAI of 0.5-0.7, suction decreased by 10-25 kPa during drying under 1000 ppm CO₂. This is because leaves cannot open stomata wide enough to transpire more water when they absorb more CO₂ from the atmosphere resulting in soil matric suction reduction. Although a strong positive linear correlation (R²=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 CO₂, a weak linear correlation (R²=0.6; P-value>0.1) of the root-shoot biomass ratio with suction under 1000 ppm CO₂ was observed. In contrast, while growing under 400 and 1000 ppm atmospheric CO₂ in NPK nutrient supplied heavily compacted soil, LAI of plants increased by 22% under 1000 ppm CO₂ compared to 400 ppm CO₂. 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 CO₂ 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 CO₂. LAI could be a reliable parameter to understand soil matric suction behaviour due to having strong correlation (R²=0.85-0.98; P-value<0.1) with peak matric suction even under different atmospheric CO₂. 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 CO₂ 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.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology