Lepidium latifolium (perennial pepperweed) is a widespread non-native herb that has invaded many riparian corridors and wetland areas in the western United States.  Once established, Lepidium can spread rapidly both by seed and vegetatively, and can crowd out native vegetation to form a monoculture.  We were interested in determining the effects of invasion on soil chemistry in an agricultural system, and how quickly after invasion these effects could be detected.

Using maps generated from HyMap hyperspectral imagery gathered from 2004-2007, we were able to track the spread of Lepidium on Bouldin Island in the Sacramento–San Joaquin River Delta.  We used these maps to identify two major zones of Lepidium invasion.  Both zones were located between the actively cultivated land and the earthen levee that encloses the island.  Within each zone we identified uninvaded areas, as well as patches of Lepidium from one to four years in age.  Uninvaded areas were primarily covered with non-native grasses and forbs.  In June 2008 we collected samples from plots randomly chosen from each invasion stage (0-4 years).  At each plot, we collected soil from the top five centimeters of the A horizon for analysis.  

Results/Conclusions

Invaded soils showed a number of significant differences from uninvaded soils.  After four years of invasion, mean Ca⁺⁺ concentrations dropped from 2350 to 1725 mg/g, Na⁺ dropped from 195 to 62 mg/g, and total P rose from 3.5 to 36 mg/g.  Invasion also increased the mean ratio of inorganic to organic phosphate, from 1.15 to 3.11, and raised the pH from 5.59 to 6.47.  There were no significant changes to soil organic matter, nitrate or ammonium levels.

These results suggest that the alterations to the soil environment from Lepidium invasion can occur within a relatively short time frame, perhaps as little as one year. These changes to the soil chemistry may promote the invasion process, and may also affect the ability of other vegetation to successfully re-establish after the removal of Lepidium.