Distinct fungal and bacterial δ13C signatures as potential drivers of increasing δ13C of soil organic matter with depth

Soil microbial biomass is a key source of soil organic carbon (SOC), and the increasing proportion of microbially derived SOC is thought to drive the enrichment of C-13 during SOC decomposition. Yet, little is known about how the delta C-13 of soil microbial biomass differs across space or time, or with the composition of the microbial community. Variation in soil microbial delta C-13 may occur due to variation in substrates used by soil microorganisms, and variation in how different microorganisms synthesize biomass. Understanding these variations in soil microbial delta C-13 would enable more accurate interpretation of patterns in the delta C-13 of SOC. Here, we report the variation in delta C-13 values of individual phospholipid fatty acids (PLFA) within podzolic soils from mesic boreal forests characterized by steep decreases in fungal to bacterial (F:B) ratios. By comparing trends in delta C-13 of PLFA indicative of either fungi or bacteria to those PLFA common across both microbial groups, we tested the hypothesis that the enrichment of C-13 in bacterial relative to fungal biomass represents a mechanism for the increase of bulk SOC delta C-13 with depth. We demonstrate that PLFA derived from fungi were consistently depleted in C-13 (-40.1 to -30.6 aEuro degrees) relative to those derived from bacteria (-31.1 to -24.6 aEuro degrees), but unlike bulk SOC the delta C-13 of PLFA from either group did not vary significantly with depth. In contrast, the delta C-13 of PLFA produced by both fungi and bacteria, which represent the delta C-13 of soil microbial biomass as a whole, strongly increased with depth (increase of 7.6-8.4 aEuro degrees) and was negatively correlated with the fungi/(fungi + bacteria) ratio (R-2 > 0.88). The steep increase of the delta C-13 of general PLFA with depth cannot be explained by an increase in the delta C-13 of either fungal or bacterial biomass alone since the PLFA indicative of those groups did not vary with depth. Instead, these data demonstrate that the increase in soil biomass delta C-13 with depth is driven by a change in the proportion of bacterial relative to fungal biomass. We suggest that the increased proportions of soil bacterial relative to fungal biomass with depth may represent an important mechanism contributing to increasing delta C-13(SOC) with depth via contributions from 'necromass' to SOC.