Divergent Impacts of Straw and Biochar on Organic Carbon Fraction Allocation and CO2 Emissions in Coastal Saline Soil

How homologous straw and biochar alter soil carbon fractions and associated microbial processes in coastal saline soils remains unclear. A 120-day microcosm experiment compared reed (Phragmites australis) straw and its biochar at equal mass rates of 1%, 5%, and 9% of dry soil mass. Both amendments increased the soil organic carbon (SOC) and reduced electrical conductivity. At 9%, straw increased the particulate organic carbon share of SOC to 78.2%, decreased the bacterial oligotroph/copiotroph ratio from 1.82 to 0.86, and raised potential hydrolytic enzyme activities to 4.3–9.1 times the control levels. Across straw treatments, cumulative CO2 emissions were 375.0–1645.9% higher than in the unamended control. Biochar maintained operationally defined mineral-associated organic carbon dominance (62.1–68.0% of SOC), with weaker hydrolytic responses and cumulative CO2 emissions 58.0–102.2% above the control levels. Exploratory structural equation models showed that lower bacterial oligotroph/copiotroph ratios were associated with higher hydrolytic enzyme activity under straw. Under biochar, higher fungal Basidiomycota/Ascomycota ratios were associated with higher polyphenol oxidase activity. Both enzyme indicators were positively associated with cumulative CO2 emissions in their respective models. These findings demonstrate contrasting carbon fraction distributions and respiratory responses to homologous amendments, with implications for carbon management in coastal saline soils.