Carbon sequestration potential of selected grasses an assessment using Carbon dioxide controlled systems

Abstract

The primary aim of the study is to assess the changes in microclimate brought about by the growth and metabolism of particular grass species in environments with elevated carbon dioxide levels. Two chambers, each with a size 6.32 m3 were installed with Polyvinyl chloride (PVC) pipes of 40mm diameter as frames and 1mm thick PVC sheet as sidewall material. The facilities associated with the chambers were a CO2 cylinder for the supply of air mixed with elevated concentrations of CO2, an air compressor, and a nebulizer (a mixing chamber where the concentrated CO2 gas from the cylinder was mixed with ambient air from the air pump). An exhaust with a control facility was also attached to the top of the chamber to adjust the outflow of gases if required. A water supply facility was attached to both chambers to facilitate the irrigation of plants during experimentation. Monitoring of carbon dioxide concentration within the chambers was made through an automated CO2 analyzer. For regular monitoring of temperature and humidity, both chambers were fitted with a Hygro- thermometer. Among two chambers one is supplied with a CO2-air mixture (Treatment chamber, TC), and the other one is supplied with ambient air (Control chamber, CC). Six grass species such as Megathyrsus maximus (Jacq.) B.K. Simon & S.W.L. Jacobs, Saccharum arundinaceum Retz., Cymbopogon flexuosus (Nees ex Steud.) W. Watson, Chrysopogon zizanioides (L.) Roberty, Arundo donax L. and Pennisetum pedicellatum Trin. were selected, multiplied, and grown for 6-7 months to attain sizable biomass for experimentation. For the experiment with each species, two sets of 3 plants each were selected and maintained in CC and TC respectively. At the beginning of the experiment, the chambers were properly sealed. Afterward, the TC was supplied with CO2+ ambient air mixture at 9.00 a.m. A concentration of 900-1000 ppm CO2 was ensured inside the chamber by monitoring through a CO2 gas analyzer. This range of CO2 is attained in about 15 minutes. Similarly, ambient air was supplied to CC for 15 minutes in the morning (9 a.m.). After the supply of air/CO2+air, the levels of CO2 (ppm) in CC and TC respectively were monitored. Subsequent levels of temperature (°C) and humidity (%) were also noted. Monitoring of CO2 concentration, temperature, and humidity was repeated at 6 p.m. Day flux of CO2, and the amount of CO2 assimilated by the plants in the chamber was then calculated. Night CO2 flux and respiratory contribution of grass species were also calculated. Growth attributes including morphological parameters such as plant height and tiller height, the number of tillers, number of leaves, leaf length, leaf breadth, leaf area, culm xxdiameter, and plant biomass were estimated. The biochemical parameters analyzed include pigments (chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids), metabolites (carbohydrate, protein, and phenol), and plant nutrients (carbon, nitrogen, calcium, magnesium, sodium, potassium). The soil characteristics analyzed include moisture, pH, total organic carbon (TOC), and nitrogen. Standardization studies were undertaken in empty chambers for 15 days to assess the flux of CO2 associated with the chambers as a result of retention or dissipation of CO2 at ideal conditions of chambers in the absence of plants. All procedures done in the experiment with grass species were repeated and day and night fluxes associated with ideal conditions of the chambers were calculated. The data obtained in the standardization study determines the pattern of CO2 flux, temperature, and humidity with rising CO2 concentrations in the chamber at ideal conditions without plants (chamber effect). The elimination of this chamber effect regarding CO2 concentrations while analyzing the CO2 flux or CO2 intake potential of individual grass species gives the actual flux/actual intake of CO2 by the respective grass species. All the grass species exhibited higher and statistically significant day flux (day variations of CO2) in TC compared to CC and standardization experiments. Morphological and biochemical results were statistically validated with Hedge’s g (effect size) values. Hedges g is an effect size measure that quantifies the difference between two groups in terms of their means, standardized by their pooled standard deviation. It enables for the comparison of effect sizes between samples. Higher effect sizes were noticed to be associated with TC. The best grass species for mitigating atmospheric CO2 were identified by analyzing a range of attributes and from these aspects, four matrices were considered including net day flux (DF (N)), net CO2 exchange, overall morphology, and CO2 uptake per plant biomass. An equation was derived for estimating CO2 uptake per plant biomass. By analyzing all four matrices it is revealed that S. arundinaceum and M. maximus could be strongly suggested for CO2 mitigation programs since the plants were superior in CO2 uptake and growth attributes. Better net CO2 exchange and more CO2 uptake per plant biomass were observed in A. donax. While the plant exhibited a negative response regarding overall morphology, due to the poor nitrogen cycling of the plant. Adequate supply of nitrogen fertiliser may benefit the plant to grow well in high CO2 environments and their high capability for CO2 uptake makes them more suitable for carbon mitigation projects.