Biological Degradation of Black Carbon in Temperate Forest Soils: Effects of clay mineralogy and N availability.

A critical knowledge gap in soil organic carbon (SOC) cycling concerns the SOC portion collectively known as pyrogenic C or black carbon (BC), which is a chemically heterogeneous class of highly reduced compounds produced by incomplete combustion.  While the stocks of BC are significant in surface soils worldwide, this SOC pool is considered to be relatively inert with negligible biologically mediated degradation of BC occurring. We will present findings from a laboratory incubation of dual-labeled (¹³C/¹⁵N) BC and its precursor wood (Pinus ponderosa) in two temperate soils (Haploxeralfs) that differ in their clay mineralogy (granite GR, versus andesitic parent material) and organic C content.  In addition, we used N additions in the GR soil to investigate the effects of N availability on soil and substrate C and N cycling. Sterile controls were used to demonstrate that the BC turnover observed was biotic. The laboratory incubations were carried out at 25°C and at 55% of soil water holding capacity. We measured the flux of mineralized ¹³C in respired CO₂, dissolved organic C, soil microbial biomass, specific microbial groups (¹³C-phospholipid fatty acids) and density-defined soil organic matter fractions. The overall flux of ¹⁵N was observed in the microbial biomass, soluble organic and inorganic pools, and organic matter fractions.  We will present rates of biologically-mediated decomposition of BC and its wood material, as well as the effects of soil mineralogy and N availability on these rates and on products of decomposition. We will also present decomposition rates of native SOM in incubations with and without substrate to investigate C priming.