Fire is an important driver of carbon storage in Australian temperate forests. During vegetation fires, plant biomass is transformed into pyrogenic organic matter (pyrOM) and deposited in varying amounts and forms onto soil surfaces. To date, most studies related to soil carbon have investigated the impact of fire on carbon cycling and storage. A far less examined area is the role of pyrOM from prescribed fire in carbon cycling. The broad aim of this research was to examine the effects of prescribed fire on soil carbon and nutrients and, in particular, changes caused by deposition of pyrOM. The first study encompassed nine sites in mixed eucalypt forest in Victoria, Australia. A carefully designed sampling strategy allowed potentially confounding influences of season and site variation to be minimised. Following this sampling strategy, soil samples were collected before and after prescribed fire (one week, one month and one year). Robust evidence of the changes in total carbon and nitrogen after fire and the dynamic nature of pools of carbon and nitrogen (e.g. labile, microbial biomass and pyrogenic) over time since fire are presented. In further studies, two soil incubations were done to examine the influence of pyrOM on the dynamics of soil carbon and nutrient availability. The first incubation investigated changes in microbial respiration and available forms of carbon, nitrogen and phosphorus after the addition pyrOM to soil over a 72 h period. A second, longer term, incubation (96 d) investigated the influence of pyrOM on soil carbon balance via loses through microbial respiration. The results varied with different size fractions of pyrOM supplied, the amount of pyrOM added to the soil and with soil characteristics. Overall, pyrOM stimulated short-term microbial respiration and thus represents a substantial substrate for soil microbial activity. However, over the long-term, addition of pyrOM to soil represents a net gain of carbon and the capacity for carbon storage. The final component of this study quantified and characterised soil carbon held as pyrOM at different times since fire. Quantification was achieved using mid-infrared spectroscopy and the cubist model and a calibration model was built using spiked soil samples as measured using the Kurth-MacKenzie-DeLuca digestion method. Molecular characteristics of soil were identified using pyrolysis gas chromatography-mass spectrometry in the presence of tetramethylammonium hydroxide. This study contributes new knowledge to the complex patterns associated with redistribution of carbon after prescribed burning. Recommendations were made for land management agencies for development of fire management plans that consider protection of soil carbon in temperate forests in Australia.
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