Transformation and stabilization of pyrogenic organic matter in a temperate forest field experiment

Pyrogenic organic matter (PyOM) decomposes on centennial timescale in soils, but the processes regulating its decay are poorly understood. We conducted one of the first studies of PyOM and wood decomposition in a temperate forest using isotopically labeled organic substrate, and quantified microbial incorporation and physico-chemical transformations of PyOM in situ. Stable-isotope (¹³C and ¹⁵N) enriched PyOM and its precursor wood were added to the soil at 2 cm depth at ambient (N0) and increased (N+) levels of nitrogen fertilization. The carbon (C) and nitrogen (N) of added PyOM or wood were tracked through soil to 15 cm depth, in physically separated soil density fractions and in benzene polycarboxylic acids (BPCA) molecular markers. After 10 months in situ, more PyOM-derived C (>99% of initial 13C-PyOM) and N (90% of initial ¹⁵N-PyOM) was recovered than wood derived C (48% of 13C-wood) and N(89% under N0 and 48% under N+). PyOM-C and wood-C migrated at the rate of 126 mm yr ⁻¹ with 3-4% of PyOMC and 4-8% of wood-C recovered below the application depth. Most PyOM C was recovered in the free light fraction(fLF) (74%), with 20% in aggregate-occluded and 6% in mineral associated fractions – fractions that typically have much slower turnover times. In contrast, wood C was recovered mainly in occluded (33%) or dense fraction (27%).PyOM addition induced loss of native C from soil (priming effect), particularly in fLF (13%). The total BPCA-C content did not change but after 10 months the degree of aromatic condensation of PyOM decreased, as determined by relative contribution of benzene hexa-carboxylic acid (B6CA) to the total BPCA C. Soil microbial biomass assimilated 6-10% of C from the wood, while PyOM contributions was negligible (0.14–0.18%). The addition of N had no effect on the dynamics of PyOM while limited effect on wood.