Soil carbon dynamics under perennial pastures in south-western Australia

Kanako Tomita

Research output: ThesisDoctoral Thesis

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This study aimed to clarify carbon dynamics under perennial pastures by investigating factors related to soil carbon in south-western Australia. The investigations targeted soil under perennial pastures in the Great Southern and southern Wheatbelt regions to address the accumulation of soil carbon in relation to spatial distribution and the duration of perennial pasture since establishment on root development. Soil bacterial community structure associated with perennial pastures was also explored. In addition, farmers’ views on carbon sequestration through perennial pastures were evaluated.

The spatial distribution of soil carbon with respect to root development was investigated in veldt grass perennial pasture (Chapter 3). Soil carbon, root mass and soil pH were assessed at three depths (0-10, 10-20 and 20-30 cm) and at two sampling locations (i.e. soil between the plants and soil under the plants). There was a significant difference in the vertical distribution, with a higher total carbon found in the surface soil (0-10 cm) compared with the deeper soil layers (10-20 cm and 20-30 cm) (P<0.001). The mean total carbon of the surface soil (0-10 cm) under the plants (25.7 Mg ha-1) and between the plants (25.7 Mg ha-1) were similar, although the mean root mass of the surface soil under the plants (2.9 mg cm-3) was significantly higher than root mass between the plants (2.0 mg cm-3) (P=0.03) in this layer. Root mass appeared to be a major factor that controlled soil carbon in the surface soil and in the soil between plants, whereas soil pH was likely to play a critical role in deeper soil and in the soil under plants. This study also showed that even bunchgrass-type perennial pasture, which does not cover the entire ground surface, has the potential to increase carbon in the soil both between and under plants.

The influence of the duration since perennial pasture establishment was assessed in two kikuyu pastures chronosequence and one tall wheat grass pasture chronosequence (Chapter 4). Total carbon at 0-30 cm depths for the kikuyu sites ranged from 32.6 to 52.0 Mg ha-1 (pasture age range: 8 to 22 years) at the first site and from 21.0 to 36.5 Mg ha-1 (pasture age range: 6 to 20 years) at the second site. Total carbon (0-30 cm) in the tall wheat grass site (pasture age range: 1 to 9 years) ranged from 36.3 to 54.5 Mg ha-1. Soil carbon in these perennial pastures tended to be lower at the early stages of perennial pasture establishment, regardless of perennial species, until approximately 10 years following establishment; higher levels of soil carbon were found in older, established pastures compared with annual pastures. Root mass, soil pH and plant species were likely to impact the process of soil carbon input and output. The relationships between root mass and total carbon and between humus and particulate carbon suggested that soil pH and the plant species present, including both C4 and C3 species of perennial plants, impact the carbon input process which convert roots to soil carbon. Conversely, the process of carbon output, decomposition from particulate carbon to humus, was most influenced by plant species.

The bacterial community in soil under perennial pastures, was investigated by assessing the resident soil bacterial diversity in relation to edaphic factors (Chapter 5). Characterisation of dominant bacterial phyla in a kikuyu pasture chronosequence with adjacent agricultural land was assessed using 16S rRNA-based sequencing. The most dominant phylum across all land uses was Firmicutes, which is in stark contrast to the reports of globally distributed surveys that report Actinobacteria and Acidobacteria to be dominant phyla. However, there was a significant correlation between Actinobacteria and both total soil carbon and total soil nitrogen, which increased with perennial pasture age. The dominance of the bacterial phylum Firmicutes was consistent with the inherent soil environment and land history, but the bacterial community’s diversity and structure were influenced by environmental change, including the duration of perennial pasture. Soil pH, electrical conductivity and the amount of extractable copper are good predictors of bacterial diversity.

As suggested by the ongoing discussion concerning soil carbon, it was assumed that farmers are becoming increasingly aware of the potential of carbon sequestration for mitigating the impact of climate change and the opportunities offered by the Australian carbon offset initiatives. Hence, their view on carbon sequestration through the establishment of perennial pastures was evaluated (Chapter 6). A questionnaire and interviews were conducted to determine what changes farmers have noticed on their land and in their circumstances since they established perennial pastures. In contrast to expectations, adoption of the concept of carbon sequestration was not observed to be influenced by economic benefit. Farming experience and farmers’ observations of their land were the major factors that influenced an increase in their interest in carbon sequestration. Farmers who noticed some changes in soil properties in perennial pastures were more likely to become aware of carbon sequestration.

Overall, this study demonstrated that perennial pastures can play a critical role in maintaining or increasing soil carbon sequestration. The accumulation of soil carbon was influenced by key factors such as root development and microbiological activities which are likely to be controlled by soil pH, resulting in vertical and horizontal variability. Plant species were revealed to be another critical factor influencing soil carbon accumulation. As the perennial pastures aged, soil carbon increased and soil bacterial structure changed. If farmers noticed an increase in soil carbon and associated changes in the soil under perennial pastures, they were more likely to become interested in soil carbon sequestration, thereby enhancing their contribution to mitigating the impact of climate change.

Original languageEnglish
QualificationDoctor of Philosophy
Publication statusUnpublished - 2015

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