Soil microbes: Precursors of stabilized soil carbon

Background/Question/Methods

In terrestrial ecosystems, microbial processes are central to the stabilization of soil organic matter (SOM). Microbial bodies themselves likely serve as precursors of SOM. We investigated the stability and fate of microbial cell materials by following added microbial cell C into three operationally-defined stabilized organic matter products in two different soil ecosystems. The influences of climate and edaphic ecosystem properties on SOM distribution were also examined. Soils were collected from both a Californian mixed conifer forest and a tropical Puerto Rican forest; ¹³C-labeled microbial dead bodies were introduced to the soils, and soils were incubated for 520 days under 3 different climate regimes (Mediterranean mixed conifer forest, redwood forest, and tropical forest). Both climate and soil type exerted significant influence on the total amount of ¹³C recovered in the incubated soils as well as the amount recovered in each of the three operationally defined stabilized carbon pools: free light fraction (FLF), occluded light fraction (OLF), and heavy fraction (HF). 

Results/Conclusions

The recovery of ¹³C was highest in the HF; 3.0-11.9% of the ¹³C added was recovered in the HF versus 0.4-2.2% in the OLF and 0.3-5.4% in the FLF. The high recovery of ¹³C in the HF is consistent with the stabilization of microbial C through interactions with soil mineral surfaces. There was clear influence of climate on ¹³C-OLF recovered from Puerto Rico soils as more ¹³C was stabilized under temperate climates compared to the tropical climate, likely due to increased aggregation resulting from wet-dry and/or freeze-thaw cycles. Puerto Rico soils retained less ¹³C-FLF than their mixed conifer soil counterparts, likely as a result of larger, more active microbial communities. The recovery of ¹³C-FLF in the mixed conifer soils under the tropical climate was minimal when compared to their counterparts incubated under  temperate climates, likely as a result of the warm moist tropical conditions. Mapping of SOM association with surface minerals using nanoscale secondary ion mass spectrometry (Nano-SIMS) showed areas of isotopic enrichment in the HF of Puerto Rican soils. We saw preferential association of ¹³C with certain mineralogies, particularly with iron-rich minerals. The magnitude and distribution of mineral-associated SOM varied between climate treatments.