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

Abstract

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 ¹³C during SOC decomposition. Yet, little is known about how the δ¹³C of soil microbial biomass differs across space or time, or with the composition of the microbial community. Variation in soil microbial δ¹³C 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 δ¹³C would enable more accurate interpretation of patterns in the δ¹³C of SOC. Here, we report the variation in δ¹³C 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 δ¹³C of PLFA indicative of either fungi or bacteria to those PLFA common across both microbial groups, we tested the hypothesis that the enrichment of ¹³C in bacterial relative to fungal biomass represents a mechanism for the increase of bulk SOC δ¹³C with depth. We demonstrate that PLFA derived from fungi were consistently depleted in ¹³C (−40.1 to −30.6 ‰) relative to those derived from bacteria (−31.1 to −24.6  ‰), but unlike bulk SOC the δ¹³C of PLFA from either group did not vary significantly with depth. In contrast, the δ¹³C of PLFA produced by both fungi and bacteria, which represent the δ¹³C of soil microbial biomass as a whole, strongly increased with depth (increase of 7.6–8.4 ‰) and was negatively correlated with the fungi/(fungi + bacteria) ratio (R² > 0.88). The steep increase of the δ¹³C of general PLFA with depth cannot be explained by an increase in the δ¹³C 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 δ¹³C 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 δ¹³C_SOC with depth via contributions from ‘necromass’ to SOC.