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Harvard Forest Symposium Abstract 2013

• Title: Soil Biogeochemical and Microbial Response to Simultaneous Warming and Nitrogen Additions
• Primary Author: Melissa Knorr (University of New Hampshire - Main Campus)
• Additional Authors: Serita Frey (University of New Hampshire - Main Campus); Linda van Diepen (University of New Hampshire - Main Campus)
• Abstract:

  Climate warming and nitrogen (N) deposition are occurring on a global scale with unknown long-term effects on soil microbial communities and the biogeochemical processes they perform. Few studies have examined the interactive effects of elevated temperatures and N additions on soil microbial community structure and function. The overall objective of this study is to investigate whether warming and N additions interact to restructure microbial communities and their function, and/or alter the response of soil C pools to these two stressors. This research is being carried out at the Soil Warming × Nitrogen Addition Study at the Harvard Forest which includes four treatments with six replicate plots per treatment: control, warming (heating to 5°C above ambient), warming × N, and N additions only (addition of 50 kg N ha-1 yr-1). Static chamber soil respiration measurements have been made monthly during the growing season since the beginning of the experiment in 2006. From 2011 on however, instantaneous field IRGA assessments have replaced the static sampling method during the months of April through October. Soils were sampled in October 2011 from all treatment plots. Organic horizon subsamples were collected and then mineral cores were taken at incremental 10 cm depths to a maximum of 50 cm. Samples were analyzed for total C&N, labile and long-term mineralized C, soil organic matter chemistry, and microbial biomass and community structure. In addition, four replicates of all four treatments (organic and mineral soil horizon) sampled in October 2011 were analyzed for microbial (eukaryotes) gene expression by Illumina paired-end (100bp) sequencing of (poly-A tailed) mRNA libraries generated from total RNA extracts.
  
  
  
  Field season measurements of soil respiration indicate that both warming and N additions continue to stimulate CO2 flux, with warming treatments having a stronger effect on respiration than N additions alone. However, mineralized C amounts over a 180-day laboratory incubation showed no differences between warmed and ambient plots. Forest floor mass showed a 30% decline in total organic C under warmed conditions. Furthermore, soil organic matter chemistry within the organic horizon as determined by pyrolysis gc/ms, has shown a 36% reduction in the relative abundance of lignin with a simultaneous 46% increase in the relative abundance of lipids and phenols with warming. There were no discernable shifts in chemical composition under warmed conditions within the mineral soil horizons. Microbial biomass estimates continue to show declines in both bacterial and fungal biomass with warming, supporting similar findings from the same assessment (PLFA) in 2008 and 2010.
  
  
  
  Assembly of all the sequencing reads of the mRNA libraries revealed a total of ~ 2,000,000 genes expressed at the time of sampling. The N treatment had the least number of expressed genes, and the control treatment the highest, with a general pattern of less expressed genes in the mineral horizon compared to the organic horizon. Annotation of the expressed genes is currently being completed with a focus on 1) differentially expressed genes between treatments and horizons, and 2) genes involved in the decomposition process such as cellobiohydrolase (breakdown of cellulose) and peroxidases (breakdown of lignin). Around one thousand of the expressed genes were annotated as cellobiohydrolase I or II, ~400 as Class II peroxidases, and ~1000 were multicopper oxidases.
  
  In addition to gene annotation, the community profile of the expressed genes was analyzed. Of the ~2000 differentially expressed genes in the organic horizon between the control and N treatment, ~40% had no hits in the NCBI database, and of the annotated genes ~75% were expressed by Eukaryotes (of which fungi represented ~30%). In the mineral horizon more genes (~4000) were differentially expressed between control and N treatment, with similar percentages represented by Eukaryotes, fungi and “no hits” as in the organic horizon. Most of the differentially expressed genes were found within primary metabolic and cellular processes. With recent funding from JGI’s community sequencing program, we will also be sequencing DNA and RNA extracted from the organic and mineral horizon of the control and warmed plots to analyze the (active) bacterial and fungal community composition as well as the complete soil microbial metagenome (total DNA) and metatranscriptime (total mRNA).
  
  Overall, our recent results suggest that anthropogenic stressors and seasonal changes continue to interact to affect soil microbial communities, its functioning and the related biogeochemical cycles.
  
  

• Research Category: Soil Carbon and Nitrogen Dynamics