Eastern deciduous forests have been invaded by two exotic species that often dominate the understory vegetation. Berberis thunbergii, a woody shrub, forms dense thickets, while Microstegium vimineum, a C4 grass, forms continuous lawns; the two species often co‐occur. We hypothesized that a variety of characteristics of the exotic species may cause soil‐based ecosystem processes to change following invasion, and that such changes could establish positive feedback processes that enhance the spread of the exotic. We examined changes in soil pH, nitrogen cycling, and litter dynamics in stands infested with both species in three locations in northern New Jersey (USA). Soils directly beneath each of the two exotics had higher pH values and higher nitrification rates and often had higher net N mineralization rates than did soils beneath adjacent patches of the most common native understory shrub, Vaccinium pallidum. These differences could be replicated in the greenhouse by growing the exotics in previously uninvaded soils. Berberis litter was much higher in N concentration than was litter of the native species; it decomposed more rapidly than litter of native dominant tree species, with little or no immobilization of N. In contrast, Microstegium litter decomposed more slowly than the native species' litters, and it immobilized N. We suggest that the same change in soil—increased pH and nitrification—result from different combinations of mechanisms in the two exotic species. Both species evidently favor uptake of nitrate, which may elevate pH. However, Berberis combines large biomasses of N‐rich roots with N‐rich leaf litter, while Microstegium populations combines small biomasses of N‐rich roots with small biomasses of N‐poor litter that leave much of the surface soil with few roots. The ability to change soil functions may be an important and previously unrecognized characteristic that renders species capable of invading intact communities, while the likelihood that changes in soil functions can alter the growth of the exotics may be an important component of invasibility. For reprints of this Invited Feature, see footnote 1, p. 1259.

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Eastern deciduous forests have been invaded by two exotic species that often dominate the understory vegetation. Berberis thunbergii, a woody shrub, forms dense thickets, while Microstegium vimineum, a C 4 grass, forms continuous lawns; the two species often co-occur. We hypothesized that a variety of characteristics of the exotic species may cause soil-based ecosystem processes to change following invasion, and that such changes could establish positive feedback processes that enhance the spread of the exotic. We examined changes in soil pH, nitrogen cycling, and litter dynamics in stands infested with both species in three locations in northern New Jersey (USA). Soils directly beneath each of the two exotics had higher pH values and higher nitrification rates and often had higher net N mineralization rates than did soils beneath adjacent patches of the most common native understory shrub, Vaccinium pallidum. These differences could be replicated in the greenhouse by growing the exotics in previously uninvaded soils. Berberis litter was much higher in N concentration than was litter of the native species; it decomposed more rapidly than litter of native dominant tree species, with little or no immobilization of N. In contrast, Microstegium litter decomposed more slowly than the native species' litters, and it immobilized N. We suggest that the same change in soil-increased pH and nitrificationresult from different combinations of mechanisms in the two exotic species. Both species evidently favor uptake of nitrate, which may elevate pH. However, Berberis combines large biomasses of N-rich roots with N-rich leaf litter, while Microstegium populations combines small biomasses of N-rich roots with small biomasses of N-poor litter that leave much of the surface soil with few roots. The ability to change soil functions may be an important and previously unrecognized characteristic that renders species capable of invading intact communities, while the likelihood that changes in soil functions can alter the growth of the exotics may be an important component of invasibility.

Exotic plant species are increasingly becoming the focus of research and have been identified as a component of human-induced global change. Successful invaders may alter soil conditions, but the effect of exotic species on soil microbial communities has not been studied. We studied two exotic understory plant species (Japanese barberry [Berberis thunbergii] and Japanese stilt grass [Microstegium vimineum]) in hardwood forests in northern New Jersey, USA. We sampled bulk and rhizosphere soils under the two exotic species, as well as under a co-occurring native species (blueberry [Vaccinium spp.]). We indexed the structure (by measuring phospholipid fatty acid [PLFA] profiles) and function (by measuring enzyme activities and substrate-induced respiration [SIR] profiles) of microbial communities in the sampled soils. Soils under the three species differed in microbial community structure and function. These differences were observed in both the rhizosphere and bulk soil samples. Differences in the structural variables were correlated to differences in the functional variables as demonstrated by canonical correlation analysis. These results indicate that successful exotic invasive species can have profound effects on the microbial community of the soil.

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) and organic nitrogen (DON) were measured in cloud water samples collected over the northern lower peninsula of Michigan. Within a given cloud field, several altitudes were sampled to examine changes in concentration and speciation with altitude. Several samples were analyzed for bacterial content and activity. Convective cumulus (cumulus congestus) were more concentrated than fair weather cumulus (cumulus humilis) for all major ions and DON, with the cloudy air DON concentrations in convective cumulus being twice as large as for fair weather cumulus, and for all other ions, the droplets were 4-6 times more concentrated. The molar average distribution of nitrogen in the cloud water was 43 (±10, 1s)% ammonium, 39 (±7)% nitrate and 18 (±11)% DON. High concentrations of bacteria were observed in the clouds with an average concentration of 2.9 Â 10 5 (±1.0 Â 10 5 , 1s) bacteria m À3 of cloudy air but which contributed less than 1% of the nitrogen in the cloud water. In addition, nitrifying bacteria were identified, indicating bacterial processing of nitrogen in the cloud water may occur. Air mass origin and altitude influence observed cloud water concentrations, with the exception of DON. The correlation of ammonium and sulfate, and calcium and nitrate suggest that ammonium sulfate and calcium nitrate aerosol may be important sources of these ions.