Biochar amendment and phosphorus fertilization altered forest soil microbial community and native soil organic matter molecular composition

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Fertilizer application to nutrient-deficient forest soils may alleviate soil nutrient limitations, but long-term application may be cost-prohibitive and logistically challenging to apply to large areas. Biochar has been proposed as a soil amendment to increase soil carbon storage and may emulate fertilizer application by increasing soil nutrient availability. However, biochar may also stimulate microbial activity, potentially accelerating native soil organic matter (OM) decomposition and altering soil OM molecular composition. Here we compare changes in soil microbial activity and native OM composition in a P-limited temperate hardwood forest in Ontario, Canada 3 years after the amendment of biochar and P fertilizer using a factorial design experiment. Phospholipid fatty acid analysis indicated that fungal activity was stimulated by biochar and biochar + P addition but not P amendment alone, while bacterial activity increased with all treatments. Concentrations of solvent-extractable acyclic and cyclic lipids, base-hydrolyzable cutin and suberin components, and lignin-derived phenols increased with biochar and biochar + P amendment and to a lesser extent with P fertilization. Biomarker ratios indicated soil OM compositional shifts toward greater proportions of cyclic versus acyclic aliphatic lipids and lignin-derived phenol monomers versus dimers with biochar and biochar + P amendment, but not with P fertilization. Solution-state nuclear magnetic resonance analysis of base-extractable soil OM showed increased proportions of aliphatic lipids and lignin and fewer carbohydrates with all treatments. The results suggest that biochar amendment may alleviate nutrient deficiencies in P-limited forest ecosystems. However, biochar altered the soil microbial community structure and shifted the native soil OM composition toward a greater proportion of recalcitrant OM, which may alter soil OM turnover and nutrient cycling rates with long-term biochar amendment.