Cogongrass Invasion In a Longleaf Pine Ecosystem: Pre- and Post-Eradication Impacts On Soil Nutrient Cycling Processes.

In forest ecosystems, exotic plant invasions have been shown to dramatically impact soil nutrient cycling processes.  In addition, differences in residue quality may continue to affect soil chemistry and nutrient availability following eradication.  In this study, we looked at pre- and post-eradication nitrogen and phosphorus cycling in a nutrient-poor longleaf pine (Pinus palustris Mill.) sandhill ecosystem severely impacted by cogongrass (Imperata cylindrica (L.) Beauv.).  Across a 7-year post-eradication “recovery” chronosequence, which also included untreated cogongrass monocultures and native reference plots, we used mixed bed resin bags, incubated in situ, to assess the availability of soil nitrogen and phosphorus.  We also sampled and analyzed soils in the different treatments to attain estimates of total phosphorus, total Kjeldahl nitrogen (TKN), pH and organic matter content, and we monitored the decomposition of treated cogongrass tissues using litterbags.  Compared to a native reference community, our resin bag analyses suggest that cogongrass does not affect soil nitrate availability, although there was a nitrate spike after eradication that persisted for 5 years.  No such trends were observed for ammonium.  Resin-extractable phosphorus was lowest, and pH highest, 5 years following eradication. Mehlich-1 phosphorus and TKN levels were similar in invaded and reference plots, but were lower following eradication.  Soil organic matter content was highest in invaded plots.  Our litterbag study indicates that cogongrass tissues – particularly shoots – decompose slowly, with little mineralization occurring in the first 18 months after chemical treatment.  These factors indicate that, while cogongrass invasion may affect soil nutrient cycling processes, the most significant impacts occur once the invader is gone.  We suggest that these alterations help explain why desirable native species are slow to recolonize formerly invaded sites.