The influence of Microstegium invasions on soil nitrifier communities

Background/Question/Methods

We are examining the influence of Microstegium vimineumon, on nitrogen cycling and soil nitrifier communities at two sites in the southeastern United States (Whitehall Forest in Athens, Ga and the Coweeta Long Term Ecological Research site in Otto, North Carolina).  While previous work has documented the effects of M. vimineumon on alteration in soil communities and enhanced nitrogen cycling, less attention has been focused on the role of this invader on the presence and activity of specific functional groups of soil microbes.  We focused our attention on soil nitrifiers.  Since nitrification, the two-step conversion of ammonium to nitrate by chemoautotrophs, represents the transformation from relatively immobile ammonium to highly mobile nitrate, soil nitrifiers function as regulators of nitrogen mobility.  Ammonification, the first step of nitrification, generally considered to be the rate-limiting step, is facilitated by two taxonomically distinct groups of microorganisms, ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA).  Since previous work suggests that AOB and AOBs are favored by different levels of nitrogen availability, we are investigating the relationship between the presence of M. vimineumon and resulting changes in soil nitrogen availability on the density of AOA and AOB populations by measuring the abundance of taxon specific ammonia monooxygenase subunit A (amoA) genes using quantitative polymerase chain reaction (qPCR).

Results/Conclusions

Our results illustrate clear differences in the abundance of AOA and AOB in M. vimineumon invaded and un-invaded (control) plots.  Quantitative-PCR documented gene copy numbers, which ranged from below detection to 4.6 x 10⁶ copies/g dw of soil for AOA amoA and 2.5 x 10⁴ copies/g dw of soil to 1.4 x 10⁷ copies/g dw of soil for AOB amoA.  Densities of AOA in M. vimineumon invaded plots were ~3X that of populations of AOA in control plots (p=0.019, two tailed paired t-test).  Similarly, AOB densities in M. vimineumon invaded plots were ~2X those of AOB populations in control plots, though this difference was only marginally significant (p=0.07, two-tailed paired t-test).  These results are consistent with previous findings that nitrogen cycling in general, and nitrification in particular are accelerated in soils invaded by M. vimineumon.  However, it is surprising that the response of AOA to these invasions was greater than AOBs, considering that AOA are generally considered to be more competitive under lower nitrogen availability.  In ongoing work we are examining the functional significance of these differences in soil nitifier communities on nitrogen cycling and mobility.