Abstract

Wildfires alter aggregate microhabitats that regulate microbial access to carbon in volcanic ash soils, but aggregate-scale catabolic responses across contrasting climatic windows remain poorly understood. The hypothesis was that wildfire would reduce cumulative substrate-induced respiration, with declines concentrated in native forest microaggregates and plantation macroaggregates. The MicroResp whole-soil multiple substrate-induced respiration (MSIR) assay quantified cumulative CO2-C production in topsoil (0-5 cm) macroaggregates (2000 -250 mu m) and microaggregates (250 -53 mu m) from paired burned and unburned Nothofagus forest and Pinus radiata plantation sites in La Araucan & iacute;a, south-central Chile. The same five georeferenced points per condition were sampled in February 2024 (dry/warm) and August 2024 (wet/cool). Composite dry/warm samples provided chemical, wavelength-dispersive X-ray fluorescence (WDXRF), and X-ray micro-computed tomography (micro-CT) data for analysis. Across campaigns, amino-acid cumulative CO2-C in native-forest microaggregates decreased from 475 to 181 mg kg- 1 in the dry/warm campaign and from 535 to 342 mg kg- 1 in the wet/cool campaign, whereas carbohydrate responses in plantation macroaggregates decreased from 372 to 119 mg kg- 1 and from 419 to 234 mg kg- 1, respectively. Substrate-level log-response ratios and a normalized differential aggregate fire sensitivity (DAFS) index indicated that fire-associated declines were concentrated in native forest microaggregates and plantation macroaggregates. The results indicate that wildfires reorganized microbial substrate use within aggregates in ways that depended on ecosystem/site context and aggregate fraction, while structural and chemical descriptors should be interpreted as contextual information.