The Role of Nutrient and Energy Limitation on Microbial Decomposition of Deep Podzolized Carbon: A Priming Experiment.

Soil carbon decomposition is primarily driven by microbial activities and is regulated by factors which stimulate or impede microbial functions. Deep podzolized carbon (DPC), found in the United States Southeastern Coastal Plain, is situated well below the soil surface in horizons isolated from active plant input. This carbon is characterized by high C:N ratios (>30) which could reflect nutrient limitation of microbial decomposition. To uncover the energy or nutrient limitation on DPC degradation, a 90‐day priming experiment was performed with soils from the surface horizon and DPC horizons (i.e., Bh1 and Bh2) received the additions of 13C‐labeled alanine and glucose. This resulted in prominent priming effects: addition of alanine increased basal decomposition of soil organic carbon by 918 ± 51% and 737 ± 7% in Bh2 and Bh1, respectively. Glucose relative priming was 505 ± 28% in Bh1 and 606 ± 77% of basal respiration in Bh2. These strong responses to substrate input highlight the susceptibility of DPC to loss when microbial carbon and nutrient constraints are alleviated. After 90 days, glucose addition increased the microbial biomass in DPC horizons relative to alanine addition, with the latter showing no difference from ultrapure‐water control. The response of the microbial biomass indicates constraint by a lack of energy sources both by the paucity of labile substrates and reduced availability of organic matter as a result of podzolization. Our study has important implications for predicting the response of DPC in Coastal Plain soils in the context of land management and global change. Plain Language Summary: In the United States Southeastern Coastal Plain deep podzolized carbon (DPC) is found well below the soil surface, this organic matter has a high ratio of carbon to nitrogen (C:N) and is highly associated with aluminum. This suggests that microbes struggle to decompose it due to a shortage of compounds which provide nutrients such as nitrogen to create enzymes, and energy‐providing compounds such as glucose. To understand how DPC breakdown can be affected by energy or nutrient availability, we conducted a 90‐day incubation experiment where we added specially labeled compounds (alanine and glucose) to soils taken from the surface (A) and DPC layers (Bh1 and Bh2). We found that adding these compounds caused a significant increase in microbial activity, especially in the DPC layers. We also observed a greater increase in microbial growth when glucose was added compared to alanine, which did not show much difference from the water‐addition control. This suggests that the lack of easily digestible nutrients and the protection from metal association restricts microbial growth and activity in these DPC layers. Understanding deep carbon response to changes in elemental flows is crucial for managing land and anticipating changes in response to global environmental shifts. Key Points: Glucose and alanine addition to deep podzolized carbon (DPC) soils resulted in relative priming effects greater than 500% basal respirationHigh priming response suggests DPC is vulnerable to destabilization when exposed to labile substratesMicrobial nutrient and energy limitations are key to the persistence of DPC [ABSTRACT FROM AUTHOR]