Fate and stabilization of labile carbon in a sandy boreal forest soil: A question of nitrogen availability?

Labile carbon (C) fractions, such as sugars, may persist in soil due to their incorporation into microbial biomass and are ultimately stabilized as microbial necromass as part of stable soil organic matter (SOM). However, the underlying factors and mechanisms are currently highly debated. To address this knowledge gap, we conducted a 1-year greenhouse experiment including four treatments: (1) bare soil, (2) bare soil and nitrogen (N) fertilization, (3) soil planted with a tree, and (4) tree and N. The boreal forest soil was a sandy and nutrient-poor Podzol taken from 0 to 20 cm depth and trees were Pinus sylvestris. We hypothesized that: (1) originally labile C does not accumulate under N-deficient conditions, as microbial residues may be intensely recycled for N acquisition and (2) differences in N supply and demand change the functionality and composition of the microbial community, which will be reflected in the stabilization of microbial C. We added ¹³C glucose to the soil and measured ¹³C recovery to trace the fate of added C in soil, microbial biomass (MBC), dissolved organic C (DOC), phospholipid fatty acids (PLFA), and amino sugars as biomarker for microbial necromass. We also analyzed microbial community structure and enzyme activities. Around 40 % of the added C was mineralized after one day. Mineralization of the added C continued for 6 months, but stabilized thereafter. After 1 year, the treatment with both tree and N fertilization had the highest amount of added ¹³C (34 %) remaining in soil compared to the other treatments (18 %). The recovery of ¹³C in DOC was <1 % from the 3rd day onwards, but remained higher in MBC (2 %) and microbial necromass (1.5 %) after 1 year. N fertilization increased bacterial growth on ¹³C-glucose and abundance of gram-positive bacteria, while trees increased the abundance of symbiotrophic fungi. The formation of more stable C in the treatment with both tree and N indicates that under those conditions, recycling of microbial necromass for N acquisition is lower and the changed microbial composition leaves behind more stable residues.