Fire influences needle decomposition: tipping point in Pinus radiata carbon chemistry and soil nitrogen transformations

Abstract

As climate change proceeds, a change in the frequency and intensity of fire events is expected to affect soil organic matter (SOM) transformations within forestry systems. A likely consequence is the development of post-fire litter layers composed of thermally altered non-senescent materials that have fallen during a fire event. In this study, Pinus radiata needles were thermally altered to determine the effect of changes in carbon chemistry on needle decomposition and nitrogen cycling. Live needles were collected and dried at 40 °C before being further heated for 1 h in a muffle furnace at a range of temperatures >40 °C (max. = 320 °C) to simulate a range of canopy temperatures that can occur during a fire, and then coarsely ground and screened (0.5–1.0 mm fraction retained). These needles were characterised for carbon and nitrogen content, and carbon chemistry (solid-state ¹³C NMR spectroscopy); they were also used in an incubation experiment (14 days) which was performed to assess the impact of heating on nitrogen transformations. Soil respiration and extractable nitrogen pools (mineral, potentially mineralizable, and microbial biomass) were measured throughout the incubation. During the incubation, cumulative respiration and nitrogen absorption capacity decreased with increasing thermal alteration. The results indicate a step change in the response of nitrogen pools to thermal alteration of pine needles, with a critical change occurring at or before 200 °C. This step change in nitrogen response may be due to the thermal degradation of light fraction organic matter, simple polysaccharides, or both. From this experiment it is clear that a post fire litter layer can have distinctly different effects on the soil environment depending on canopy temperature conditions during the fire with post fire litters composed of low temperature needles absorbing most mineral nitrogen that they contact. This will in turn affect post-fire plant recovery and therefore ecological succession.