Post-fire Recovery of Carbon and Nitrogen in Sub-alpine Soils of South-eastern Australia
Creators: name: Shrestha, Hari
Publisher: name: figshare
Year: 2019
Subjects: Soil Science

description: The forests of south-eastern Australia, having evolved in one of the most fire-prone environments in the world, are characterized by many adaptations to recovery following burning. Thus forest ecosystems are characterized by rapid regenerative capacity, from either seed or re-sprouting, and mechanisms to recover nutrients volatilized, including an abundance of N<sub>2</sub> fixing plants in natural assemblages. Soil physical, chemical and biological properties are directly altered during fire due to heating and oxidation of soil organic matter, and after fire due to changes in heat, light and moisture inputs. In natural ecosystems, carbon (C) and nitrogen (N) lost from soil due to fires are recovered through photosynthesis and biological N<sub>2</sub> fixation (BNF) by regenerating vegetation and soil microbes. This study investigated post-fire recovery of soil C and N in four structurally different sub-alpine plant communities (grassland, heathland, Snowgum and Alpine ash) of south-eastern Australia which were extensively burnt by landscape-scale fires in 2003. The amount and isotopic concentration of C and N in soils to a depth of 20 cm from Alpine ash forest were assessed five years after fire in 2008 and results were integrated with measurements taken immediately prior to burning (2002) and annually afterwards. Because the historical data set, comprised of three soil samplings over the years 2002 to 2005, consisted of soil total C and N values which were determined as an adjunct to <sup>13</sup>C and <sup>15</sup>N isotopic studies, it was necessary to establish the accuracy of these IRMS-derived measurements prior to further analysis of the dataset. Two well-established and robust methods for determining soil C (total C by LECO and oxidizable C by the Walkley-Black method) were compared with the IRMS total C measurement in a one-off sampling to establish equivalence prior to assembling a time-course change in soil C from immediately pre-fire to five years post-fire. The LECO and IRMS dry combustion measurements were essentially the same (r<sup>2 </sup>&gt;0.99), while soil oxidizable C recovery by the Walkley-Black method (wet digestion) was 68% compared to the LECO/IRMS measurements of total C. Thus the total C measurement derived from the much smaller sample size (approximately 15 mg) combusted during IRMS are equivalent to LECO measurement which require about 150 mg of sample. Both total C and N in the soil of Alpine ash forests were significantly higher than soils from Snowgum, heathland and grassland communities. The ratio of soil NH<sub>4</sub><sup>+</sup> to NO<sub>3</sub><sup>-</sup> concentration was greater for Alpine ash forest and Snow gum woodland but both N-fractions were similar for heathland and grassland soils. The abundance of soil <sup>15</sup>N and <sup>13</sup>C was significantly depleted in Alpine ash but both isotopes were enriched in the heathland compared to the other ecosystems. Abundance of both <sup>15</sup>N and <sup>13</sup>C increased with soil depth. The natural abundance of <sup>15</sup>N and <sup>13</sup>C in the foliage of a subset of non-N<sub>2</sub> fixing and N<sub>2</sub> fixing plants was measured as a guide to estimate BNF inputs. Foliage N concentration was significantly greater in N<sub>2</sub> fixers than non-N<sub>2</sub> fixers while C content and <sup>13</sup>C abundance were similar in both functional groups. Abundance of <sup>15</sup>N was depleted in the N<sub>2</sub> fixing species but was not significantly different from the non-N<sub>2</sub> fixers to confidently calculate BNF inputs based on the <sup>15</sup>N abundance in the leaves. The total C pool in soil (to 20 cm depth) had not yet returned to the pre-fire levels in 2008 and it was estimated that such levels of C would be reached in another 6-7 years (about 12 years after the fire). The C and N of soil organic matter<sup> </sup>were significantly enriched in <sup>15</sup>N and <sup>13</sup>C isotopes after fire and had not returned to the pre-fire levels five years after the fire. It is concluded that the soil organic N pool can recover faster than the total C pool after the fire in the Alpine ash forests.