The persistence of soil microbes: Active community composition and capability to respond to litter addition after 10-years of no-inputs

Background/Question/Methods

It has been speculated that the high microbial diversity found in soils may be due in part to a large inactive-biomass that is able to persist in soils for long periods of time. This persistent microbial fraction may help to buffer the functionality of the soil community during times of low nutrients by providing a reservoir of specialized functions that can be re-activated when conditions improve.  A study was designed to test the hypothesis: Following 10-years of no above- or below- ground inputs from plants, new inputs will stimulate the growth and activity of a persistent microbial community able to break down litter and having a community composition similar to control soils. Soils were collected from the HJ Andrews Experimental Forest’s Detrital Input and Removal Treatments (DIRT) from the No-Input plots where all above- and below-ground litter had been excluded for 10 years.  These soils and Control plot soils that had received normal rates of litter additions were used in a microcosm experiment where Douglas-fir (the site’s dominant vegetation) needles were added to soils.  Following additions, microcosms were destructively sampled at Day 3 and Day 151. Soil microbial ribosomal DNA and RNA was extracted and characterized using Q-PCR and 454-pyrosequencing.

Results/Conclusions

Bacterial 16S rDNA gene copy numbers increased from Day 3 to Day 151 in all treatments, but no significant differences were found between soils or treatments.  At Day 3 sampling, 16S rRNA gene copy numbers were significantly higher in Douglas-fir amended soils compared to unamended, but the opposite trend was observed at Day 151. These results suggest a change in the bacterial community during the incubation and are substantiated by pyrosequences of the 16S rRNA.  Analysis of ~108,000 bacterial sequences showed a significant change in the active (RNA-based) community between Day 3 and Day 151.  Interestingly, at Day 3 all soils and treatments were similar, but at Day 151 Control soil with Douglas-fir addition was significantly different from No-Input soil with no amendment, with the other microcosms intermediate. Fungal 28S rDNA was higher in Day 3 compared to Day 151, but 28S rRNA did not vary between times, treatments, or soils. In summary these results show that even after 10 years of exclusion of plant above- and below-ground inputs, active microbial communities remained similar and that the legacy of community composition was well buffered against a dramatic disturbance.