Fresh carbon and nitrogen inputs alter organic carbon mineralization and microbial community in forest deep soil layers

In terrestrial ecosystems, deep soils are a major reservoir of organic carbon (C). Improving knowledge of how deep soil organic carbon (SOC) mineralization responds to fresh C supply and nitrogen (N) availability is essential to better understand whether this C pool will react to climate change. However, little is known about the effects of C and N inputs on SOC mineralization and microbial communities in forest deep soils. To quantify the effects of C and N inputs on SOC mineralization, we apply two species of 13C-labeled leaf litters and ammonium chloride solution while incubating soils collected from 60 cm to 70 cm depth in a coniferous forest in subtropical China. The soil phospholipid fatty acid (PLFA) profiles are also determined to establish the effects of C and N supply on microbial community structure, and the δ13C in PLFAs is used to establish pathways of leaf litter-derived C flux among microbial communities. The addition of leaf litters stimulates deep SOC mineralization, indicating that the stability of deep SOC is attributed to a lack of fresh C input, but the addition of Michelia macclurei litter with higher C:P ratio has a greater positive priming effect than adding Pinus massoniana litter. N addition reduces the magnitude of positive priming and alters the direction of priming in soils with P. massoniana litter addition, suggesting that N deposition may suppress deep SOC mineralization and favor the maintenance of SOC storage. Leaf litter addition enhances the biomass of individual PLFA and increases the fungi:bacteria ratio, suggesting that microbes are limited by energy and that soil microbial community composition is modified by C inputs. N addition decreases the fungi:bacteria ratio, but increases the Gram-positive:Gram-negative bacteria ratio. The highest 13C-enrichment and distribution of litter-derived C are found in 16:0 and 18:1ω9c PLFAs, but litter species and N addition do not affect total PLFA-C and litter-derived PLFA-C. These results support the views that a lack of fresh C supply and N deposition may prevent the mineralization of SOC pool in deep layers and that the utilization of labile substrate by 16:0 and 18:1ω9c populations promotes positive SOC priming.