Investigating the effect of biochar on microbial activities and biological processes in soil
Creators: name: Abujabhah, IS
Publisher: name: University Of Tasmania
Year: 2023

description: Soil amendment with biochar has been widely described as a suitable approach to improve soil fertility, sequester carbon and reduce greenhouse gas (GHG) emissions to mitigate climate change. The purported benefits of biochar addition to soils include improved soil physical properties and nutrient retention as well as changes in microbial composition and abundance which in turn affect nutrient cycling in the biochar amended soils. However, the impacts of different application rates of biochar and its interactions with different soils have received less attention and need to be explored. The aim of this thesis was to investigate the impact of biochar application rates on microbial activity and related biological processes in a range of different topsoils. This thesis focuses on understanding the behaviour of soil microbes in relating to soil biological processes that occur following biochar application and attempts to assess the relationship between these microbes and the physico-chemical properties that are altered in soil matrices after biochar application. Field and laboratory experiments were conducted to examine the effect of biochar amended-soil on the physico-chemical and biological properties. A field experiment was conducted for 3.5 years to investigate the impact of biochar and compost amendments on soil physico-chemical properties and the total microbial community in a sandy loam apple orchard site at Mountain River in Tasmania, Australia. This was followed by a 10-month pot trial to determine the effects of biochar application rates on selected soil parameters, microbial composition and related biological processes in three topsoils. These included a reactive black clay loam (BCL), a non-reactive red loam (RL) and a brown sandy loam (BSL) topsoils. In the field experiment, soil pH decreased in both biochar and compost treatments compared to control. However, significant differences in bacterial and fungal but not archaeal or other eukaryote community components were observed in the biochar and compost treatments. The results also indicated that biochar and compost amendments can subtly affect the community structure of the orchard soils even with active application of inorganic and organic fertilizers. There were no significant differences across a panel of enzyme activities among treatments. There were slight increases in alkaline phosphatase while fluorescein diacetate activity and hydrolysis activity slightly decreased. The overall effects on fundamental activity however are largely neutral, and likely due to the enormous structural resilience and functional redundancy present. The 10 month pot trial showed that biochar additions had a significant impact on NH\\(_4\\) and NO\\(_3\\), total C and N, pH, EC and soil moisture content in both soil types and biochar loading. There was a relatively limited effect on microbial biomass in amended soils; however biochar addition reduced the potential nitrification at the higher biochar rate in the two lighter soils (RL and BSL). The addition of biochar at different loading rates was reflected in significant differences in the bacterial diversity between biochar treatments in the BSL and RL soils, while the BCL soil was more resilient to soil amendment. Complete ammonia oxidizing (Nitrospira spp.) and nitrite oxidizing bacteria (NOB) were more abundant than standard ammonia oxidizing bacteria (AOB) in all soils. Increased biochar loading raised the abundance of nitrifying bacteria in BCL soil while Nitrospira became more abundant in BSL soil. Biochar addition affected the abundance of certain N2-fixer groups in a soil dependent manner. Strong positive correlations were observed in Rhizobium (r=0.99) and Azospirillum abundance (r=0.70) with increased biochar loading rates in BCL. Greater biochar loading also significantly increased the relative abundance of methanotrophs, especially in BCL soil. The impact of biochar on community structure and nitrogen cycling bacteria depended on soil type and biochar rates which correlated to the differences in soil properties. Overall, the abundance of nitrogen cycling bacterial groups seemed to be most affected by the changes in soil conditions, including aeration, C/N ratio, nutrients and pH in relation to biochar application in different soils.