Globally, soil organic carbon (SOC) has declined as a result of human induced disturbance with negative effects on production and productivity. Maintaining SOC has the combined effect of contributing to climate change mitigation efforts and agro-ecosystem functioning in addition to its potential for sustaining soil health. A primary source that can contribute to soil carbon (C) sequestration is plant biomass, and an important component of this is the biomass found belowground. SOC sequestration using plant species with high photosynthetic efficiency, deep roots and high biomass production therefore has considerable potential for soil carbon storage. Perennial tropical grasses, particularly those with deep root systems, are therefore likely to contribute significantly to SOC and the introduction of perennial tropical grasses could potentially contribute large quantities of C through the soil profile and facilitate SOC sequestration. A range of tropical pasture species have been investigated for their SOC storage potential, but vetiver grass, given its extensive use globally and its large biomass production, has considerable, as yet unquantified, potential for long term C storage. The main aim of this research was to examine the SOC quantity, nature and distribution in soils under vetiver. Specifically, the work 1) examined SOC content, stock and profile distribution under vetiver; 2) determined the quantity of SOC attributable to vetiver (C₄-C) compared with soil dominated by previous C₃ carbon; 3) examined the above- and below-ground vetiver biomass production and the relative rate of decomposition, and 4) determined the allocation of soil C under vetiver to its component fractions (POC, HOC and ROC) differentiated on the basis of particle size and chemical composition.

A series of research questions were examined under this PhD research work: In chapter 3 undisturbed soil core samples were collected to 1.0 m soil depth from Gunnedah, Australia to determine the soil carbon content and depth distribution down the soil profile under vetiver compared with native and tropical pastures and cropland soil. The result showed a larger TOC stock under vetiver (123 Mg ha^-1) compared with tropical pasture (93 Mg ha^-1) and cropping soils (78 Mg ha^-1) while vetiver and native pastures (111 Mg ha^-1) showed no significant difference in TOC stocks. For all plant types, a decrease in SOC content was observed with increasing soil depth but a larger stock of C was found under vetiver at almost all depths through the soil profile compared with cropping soils, but on an annual basis, not much more than other tropical grasses. Soils under vetiver had higher (less negative) δ¹³C compared with native, tropical pastures and cropping soils. This was particularly true in the surface soil layers but persisted to some degree through the whole soil profile. Both litter and roots probably contributed to the additional C stock by vetiver (43.5%) and results indicated a significant C turnover through the whole soil profile resulting in a modest net accumulation of soil C.

In chapter 4 the impact of vetiver grass on carbon sequestration and its SOC input and the quantity of SOC attributable to vetiver (C₄ carbon) compared with soil dominated by pre-existing (C₃) Carbon determined. Undisturbed soil core samples were collected to 1.0 m soil depth from Southwest Ethiopia. The result showed a larger TOC stock under vetiver (mean 262 Mg C ha^-1) compared with coffee (mean 178 Mg C ha^-1), particularly, at the surface soil layers and decline was observed with increasing soil depth between plant types. Low δ¹³C (more negative) values were recorded at the soil surface layers increasing with increasing soil depth for both vetiver and coffee. However, the δ¹³C values were significantly higher (less negative) under vetiver in comparison with coffee, particularly at the surface soil layers which suggests a continuous new C addition and a significant C turnover in the soil system.

In chapter 5 vetiver plant material was therefore grown under a glasshouse condition for biomass production assessment and subsequently incubated to determine the relative decomposition rate between the above- and below-ground vetiver biomass in different soil types. Vetiver showed a high biomass production (268 Mg ha^-1 of fresh and 120.2 Mg ha^-1 of dry biomass) potential and the shoot to root biomass ratio was determined to be 1.49 and 1.28, for the fresh and dry biomass, respectively.

In chapter 6 the amount of allocation of soil carbon to particulate, humus and resistant fractions differentiated based on particle size and chemical composition. The stocks of soil C fractions indicated significant variations which changes from the labile POM to the HOM across site and vegetation types. Hence, the dominant C fraction was HOC (58%) for vetiver and all vegetation types. The ratio of POC to HOC stocks was also very low indicating the lesser vulnerability of C because of the high proportion of HOC component fraction given its less labile nature which could help the carbon stay in the soil for longer time and changes quite slowly.

Despite the continuous new C addition under vetiver the significant soil C turnover could be due to the more rapid decomposition of the root material than the shoot which could have been impacted by the lower C:N ratio of the root compared with the shoot. Hence, promoting the use of vetiver, particularly due to its potential to produce a large biomass, is a promising strategy to enhance soil C storage. Hence, growing vetiver has the potential for high rate of C accumulation because this grass is building up the more stable HOC fraction which is less vulnerable to change and to use this in the C accounting program can be feasible. This study investigated that vetiver due to its fast growth, large biomass production (both above- and below-ground) potential and extensive use has considerable potential for C sequestration, particularly on C depleted soils. In conclusion, in this work it has been demonstrated that vetiver grass has an important role in storing large TOC stock, has the potential to add new carbon despite high rates of turnover; produce high biomass and have high root to shoot decomposition which might be a reason for high turnover rates and larger organic carbon accumulation in the more resistant (hemic organic carbon fraction) carbon pool throughout the 1.0 m soil profile and has considerable potential for both restoration of soil health and for storing additional soil carbon to offset greenhouse gas emissions.