Vetiver Grass in Australia and Ethiopia: Soil Organic Carbon Storage potential and Mechanisms for Carbon Sequestration

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

Soil Carbon Storage Potential of Tropical Grasses: A Review

Environmental degradation and climate change are key current threats to world agriculture and food security and human–induced changes have been significant driving forces of this global environmental change. An important component is land degradation which results in a diminished soil organic carbon (SOC) stock with concomitant loss of soil condition and function. Land management to improve soil organic matter content, condition and productivity is therefore a key strategy to safeguard agricultural production, food supply and environmental quality. Soil organic carbon sequestration through the use of plant species with high photosynthetic efficiency, deep roots and high biomass production is one important strategy to achieve this. Tropical pastures, which are adapted to a wide range of environmental conditions have particular potential in this regard and have been used extensively for land rehabilitation. Tropical pastures also have advantages over trees for biomass and carbon accumulation due to their rapid establishment, suitability for annual harvest, continual and rapid growth rates. In addition, tropical pastures have the potential for SOC storage in subsoil horizons due to their deep root systems and can be used as biomass energy crops, which could further promote their use as a climate change mitigation option. Here we aimed to review current knowledge regarding the SOC storage potential of tropical grasses worldwide and identified knowledge gaps and current research needs for the use of tropical grasses in agricultural production system

Potential for soil organic carbon sequestration in grasslands in East African countries: A review

Grasslands occupy almost half of the world's land area. Soil organic carbon (SOC) is a key indicator of soil fertility and grassland productivity. Increasing SOC stocks (so‐called SOC sequestration) improves soil fertility and contributes to climate change mitigation by binding atmospheric carbon dioxide (CO2). Grasslands constitute about 70% of all agricultural land, but their potential for SOC sequestration is largely unknown. This review paper quantitatively summarizes observation‐based studies on the SOC sequestration potential of grasslands in six East African countries (Burundi, Ethiopia, Kenya, Rwanda, Tanzania and Uganda) and seeks to identify knowledge gaps related to SOC sequestration potential in the region. In the studies reviewed, SOC stocks in grasslands range from 3 to 93 Mg C/ha in the upper 0.3 m of the soil profile, while SOC sequestration rate ranges from 0.1 to 3.1 Mg C ha‐1 year‐1 under different management strategies. Grazing management is reported to have a considerable impact on SOC sequestration rates, and grassland regeneration and protection are recommended as options to stimulate SOC sequestration. However, a very limited number of relevant studies are available (n = 23) and there is a need for fundamental information on SOC sequestration potential in the region. The effectiveness of potential incentive mechanisms, such as payments for environmental services, to foster uptake of SOC‐enhancing practices should also be assessed

Soil organic carbon in agricultural systems of six countries in East Africa – a literature review of status and carbon sequestration potential

A systematic literature review of existing evidence on soil organic carbon (SOC) responses to agronomic best management practices (BMPs) in cultivated soils of East Africa, focusing on Ethiopia, Kenya, Rwanda, Tanzania, Uganda, and Burundi. Examining current evidence on the extent to which BMPs can increase SOC stocks and whether net SOC sequestration is attainable in this region. The study also sought to identify knowledge gaps and make recommendations for future research. Independent variables: • Annual rainfall (mm year-1), as semi-arid (1500) • Temperature • Location, • Altitude - lowland (25 years). • Soil depth: 0-30 cm, 0-50 cm, and 0-100 cm. Dependent variables • Soil organic carbon stock (t C ha 1) • Soil organic carbon sequestration (t C ha 1 year-1) • Soil organic carbon loss (t C ha 1 year-1) (2020-02-20

Functional Links between Biomass Production and Decomposition of Vetiver (Chrysopogon zizanioides) Grass in Three Australian Soils

Plant roots are primary factors to contribute to surface and deep soil carbon sequestration (SCS). Perennial grasses like vetiver produce large and deep root system and are likely to contribute significantly to soil carbon. However, we have limited knowledge on how root and shoot decomposition differ and their contribution to SCS. This study examined biomass production and relative decomposition of vetiver which was grown under glasshouse conditions. Subsequently the biomass incubated for 206 days, and the gas analysed using ANCA-GSL. The results confirmed large shoot and root production potential of 161 and 107 Mg ha−1 (fresh) and 67.7 and 52.5 Mg ha−1 (dry) biomass, respectively with 1:1.43 (fresh) and 1:1.25 (dry) production ratio. Vetiver roots decomposed more rapidly in the clay soil (p < 0.001) compared with the shoots, which could be attributed to the lower C:N ratio of roots than the shoots. The large root biomass produced does indeed contribute more to the soil carbon accumulation and the faster root decomposition is crucial in releasing the carbon in the root exudates and would also speed up its contribution to stable SOM. Hence, planting vetiver and similar tropical perennial grasses on degraded and less fertile soils could be a good strategy to rehabilitate degraded soils and for SCS

Soil organic carbon in agricultural systems of six countries in East Africa – a literature review of status and carbon sequestration potential

A systematic literature review of existing evidence on soil organic carbon (SOC) responses to agronomic best management practices (BMPs) in cultivated soils of East Africa, focusing on Ethiopia, Kenya, Rwanda, Tanzania, Uganda, and Burundi. Examining current evidence on the extent to which BMPs can increase SOC stocks and whether net SOC sequestration is attainable in this region. The study also sought to identify knowledge gaps and make recommendations for future research. Independent variables: • Annual rainfall (mm year-1), as semi-arid (1500) • Temperature • Location, • Altitude - lowland (25 years). • Soil depth: 0-30 cm, 0-50 cm, and 0-100 cm. Dependent variables • Soil organic carbon stock (t C ha 1) • Soil organic carbon sequestration (t C ha 1 year-1) • Soil organic carbon loss (t C ha 1 year-1) (2020-02-20