We assessed pre- and posteradication nitrogen and phosphorus dynamics in longleaf pine sandhill stands severely affected by cogongrass. Across a 7-yr posteradication (glyphosate imazapyr) “recovery chronosequence,” which included untreated cogongrass, uninvaded reference, and treated plots, we analyzed soils for total N, potentially available P (Mehlich-1 [M1]), pH, and organic matter content. We also used resin bags to assess fluxes of plant available N and P in the soil solution. Additionally, we used litterbags to monitor the decomposition and nutrient mineralization patterns of dead rhizome and foliage tissue. Our results indicate similar total N and M1-P contents in both cogongrass-invaded and uninvaded reference plots, with levels of M1-P being lower than in cogongrass plots for 5 yr after eradication. Soil organic matter did not differ between treatments. Resin bag analyses suggest that cogongrass invasion did not affect soil nitrate availability, although a pulse of NO2 NO3 occurred in the first 3 yr after eradication. No such trends were observed for ammonium. Resin-adsorbed PO4 was lowest 3 yr after eradication, and pH was highest 5 yr after eradication. Our litterbag study showed that approximately 55% of foliar biomass and 23% of rhizome tissue biomass remained 18 mo after herbicide treatment. Substantial N immobilization was observed in rhizomes for the first 12 mo, with slow mineralization occurring thereafter. Rapid P mineralization occurred, with 15.4 and 20.5% of initial P remaining after 18 mo in rhizomes and foliage, respectively. Overall, our findings indicate that cogongrass invasion has little to no effect on soil nutrient cycling processes, although some significant—but ephemeral—alterations develop after eradication.
Nomenclature: Glyphosate, imazapyr, cogongrass, Imperata cylindrica (L.) Beauv. IMCY, longleaf pine, Pinus palustris Mill
Management Implications: An improved understanding of how invasive alien plant species alter soil properties and of how novel soil properties persist after eradication is essential for practitioners to develop effective restoration strategies for invaded natural communities. Toward these ends, we undertook this study to assess nutrient dynamics in forest stands affected by cogongrass—an invasive alien plant that affects tropical and subtropical ecosystems on six continents. We chose a longleaf pine sandhill ecosystem as the study site because these forests are frequently targeted in restoration efforts and are commonly invaded by cogongrass in the southeastern United States. Because nitrogen (N) and/or phosphorus (P) availability often affect plant establishment after disturbance, we focused on processes that affect N/P availability and uptake. Our results indicate that cogongrass invasion does not substantially alter soil properties. This is intriguing, given that this species is known to be a superior competitor for belowground resources and often forms dense, single-species stands in longleaf pine ecosystems. The decomposition of cogongrass biomass after herbicide treatment resulted in a rapid pulse of P into the soil and the temporary immobilization of soil N. All measured soil properties (N, P, pH, organic matter), however, returned to a preinvasion state within 5 yr. These results suggest that long-term posteradication soil “legacy” effects, which have been observed with other invasive alien plant species (e.g., alterations to the nitrogen cycle and other biogeochemical cycles), might not occur in cogongrass-affected pine ecosystems—at least those that have been invaded for a relatively short period of time. In light of these findings, we see little evidence to indicate that altered soil properties would impede groundcover restoration efforts in sites that have been invaded by cogongrass for 1–2 yr, as long as practitioners give the soi