Nutrient Cycling Between Litters and Soil after Fire in Native Woodland and Pinus radiata Plantations

Abstract

Fire can change both the quantity and nature of soil organic matter during the event and can affect organic matter inputs after the event. These changes may affect the microbial biomass, nutrient availability and nutrient cycling, and may have flow on effects on soil carbon stocks and cycling. Post-fire effects may also affect success of established (re-sprouting) and recently recruited plants. In particular, changed organic matter inputs, such as the formation of a post-fire litter layer, represent a significant unknown in soil and fire ecology. The aims of the work presented in this thesis focus on the effects of fire on microbial nutrient limitation, microbial decomposition of pre- and post-fire litters, and the effects of thermal alteration of pine needles on soil nitrogen cycling and the soil microbial biomass. The materials for this work were generated from forestry reserves that had recently been exposed to an uncontrolled fire. These reserves included Eucalyptus woodlands (‘native’) and Pinus radiata plantations (‘pine’); sampling in all of these reserves initially occurred four months after the fire with further samples collected as described in Chapters 2, 3, and 6. To address the knowledge gaps, a series of field activities and experiments were conducted including: several soil surveys to determine temporal change in the soils after the fire; a study to determine microbial nutrient limitation in burnt and not burnt soils; a study to determine decomposition dynamics of native and pine litters; a study to determine decomposition of pine litters and char during a 3 month exposure; a study using thermally altered fresh pine needles to explore nitrogen cycling; and a soil microbiome study (Next Gen sequencing) using four amendments from the thermal alteration study to determine microbial responses to the post fire litter layer. The results of these field activities and laboratory experiments indicated that the forestry reserve soils were not strongly affected by fire when nutrient availability and microbial nutrient limitation are considered. The post-fire pine litter, however, caused significant disturbances to nitrogen cycling when soils were incubated with pine litter collected from the field and when incubated with thermally altered needles generated under laboratory conditions. Post-fire pine litters and pine needles heated experimentally to temperatures 􀀁 200ºC absorbed mineral nitrogen, preventing its extraction from soil and litter mixes. This ability was lost in needles heated experimentally to >200ºC. This temperature was associated with the degradation of polysaccharides and represented a step change in microbial activity and, potentially, in microbial accessibility to the added organic matter. The major conclusions from this work are that soil nutrient content and availability were not strongly affected by the fire event in native and pine forest reserves, but that fire affected pine litter has strong nitrogen absorption properties that are likely to affect the mineral N pools available for the regeneration of forest growth. It is also clear that a post-fire litter layer in pine forests can have distinctly different effects on the soil environment depending on canopy temperature conditions during the fire: post-fire litters composed of low temperature needles absorb most mineral nitrogen that they contact while high temperature needles appear relatively inert. This relationship was affected by a thermal tipping point at approximately 200ºC (detected using a combination of soil respiration and solid state 13C CP-MAS NMR spectroscopy). This research outlines an important knowledge gap in short term forest nutrient cycling and microbial responses to fire that may affect forest fire emissions estimates. degree or diploma in my name, in any university or other tertiary institution and, to the best of my knowledge and belief, contains no material previously published or written by another person, except where due reference has been made in the text. In addition, I certify that no part of this work will, in the future, be used in a submission in my name, for any other degree or diploma in any university or other tertiary institution without the prior approval of the University of Adelaide and where applicable, any partner institution responsible for the joint-award of this degree. I acknowledge that copyright of published works contained within this thesis resides with the copyright holder(s) of those works. I also give permission for the digital version of my thesis to be made available on the web, via the University’s digital research repository, the Library Search and also through web search engines, unless permission has been granted by the University to restrict access for a period of time. I acknowledge the support I have received for my research through the provision of an Australian Government Research Training Program Scholarship.