The effect of differences in the active microbial community on decomposition on cedar and pine litter on native sandy prairie

Background/Question/Methods
Afforestation is providing soil C sequestration of atmospheric CO₂ in both the plant biomass and soil. Its other effects such as those on the soil profile, microorganisms and nutrient cycling are not well established. The Nebraska National Forest at Halsey was sampled to explore the effects on amounts and distribution of SOM and associated microorganisms under cedars and pines planted on native sandy prairie approximately 75 years ago. Cedars contained the largest amount of soil C attributed to large input of fine roots and litter. Pine stands contained the largest amount of litter C but contained the least amount of total soil C. This is possibly due to two processes, soil C stabilization or slower litter decomposition. We set out to answer the question of how microbial populations affect decomposition. Litter samples from all three vegetation types were added to prairie soil to monitor decomposition and decomposer communities. Stable isotope probing of FAME samples was used to determine community shifts in δ¹³C values which indicated which groups of organisms are utilizing the transplanted litter. BrdU was used to corroborate this result by extracting the DNA of active microbes and QPCR on the BrdU extracted samples was used to determine differences in the microbial community. Chloroform fumigations measured changes in microbial biomass.

Results/Conclusions

Cedar had higher decomposition rates than pine which coordinated well with field observations. However, δ¹³C-CO₂ values showed that the cedar were respiring more prairie C than was expected based on the results of a previous study of soil C in the cedar stands at Halsey. Results of fieldδ ¹³C analysis indicated there was very little decomposition of prairie C under cedars pines accelerated decomposition. Key differences were seen in the poly and monosaturated 18:2 and 18:1 groups, which are fungal markers, the cyclocarbon groups, which are markers for G- bacteria, straight chain fatty acids, likely markers for eubacteria, and branched chain fatty acids, which represent either eubacteria or G+ bacteria. Our research suggests that decomposition differences may be related to the active microbial community and the new substrate available. Overall, if ecosystems are to be used as sinks for atmospheric C, then we must know the relative impacts of different management strategies on soil C pools and microbial populations to prescribe the best management options.