Submitted to: Kluwer Academic Publishers Netherlands
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/10/2003
Publication Date: N/A
Citation: Interpretive Summary: We investigated soil impacts associated with the introduced leguminous shrub, Cytisus scoparius (Scotch broom) in glacial outwash prairies in western Washington. We measured several properties of the surface soil (0-10 cm depth) across advancing fronts of Cytisus, including total carbon and nitrogen, the isotopic signature of the soil, and plant-available nitrogen. Total carbon and nitrogen are a measure of soil fertility; we found that carbon and nitrogen both increased slightly in soils as broom density increased. The changes, however, were very small (1.8% and 6.5% of the total C and N pools, respectively). We also measured the delta 15-N of the soil in order to assess whether any evidence of nitrogen-fixation existed in broom-invaded soils compared to non-invaded soils. If nitrogen-fixation is occurring, delta 15-N signatures should be lower (closer to zero) in those soils than their non-invaded counterpart. We found that there was no significant difference in delta 15-N signatures between broom-invaded and non-invaded soils. However, when we measured plant-available nitrogen using two different methods, we found large effects of broom. First, we observed a 3-fold increase in nitrogen-availability using a laboratory incubation method. Second, we found that a common herbaceous plant (Acchilea millefolium) grown in broom-invaded soils grew almost 30% smaller compared to those grown in non-invaded soils. This bioassay result suggests that defense compounds (such as quinolizidine alkaloids) may play an inhibitory role in broom-invaded soils; some species may respond more strongly to those compounds rather than the increased nitrogen, per se. Overall, we found larger increases in the labile (available) fraction of the nitrogen pool rather than increases in the total pool. Defense compounds in soil, aside from the increased nitrogen effect of broom, may be important in determining community compositional changes that occur in these prairies, both during invasion and in a restoration context.
Technical Abstract: Shrub encroachment of grassland is a global phenomenon that can cause substantial and rapid changes in soil nutrient levels and distribution. If the woody plants in question also have the capacity to fix nitrogen (N), the effects on soil nutrients may potentially be large. Cytisus scoparius L. Link (Scotch broom) is an introduced leguminous shrub from the Mediterranean region that colonizes open grasslands. We measured several properties of the surface soil (0-10 cm depth) across advancing fronts of Cytisus to assess changes in soil fertility associated with invasion in western Washington state (USA). Both total soil C and N increased (1.8% and 6.5%, respectively), resulting in a decrease in C to N ratio of 22.5 in uninvaded soils to 21.5 inside Cytisus patches. '15N signature of surface soil did not clearly demonstrate a signal of N2-fixation across the Cytisus gradient. On the other hand, seven-day laboratory aerobic incubations demonstrated 3 and 2.4 times greater mineralization and nitrification rate, respectively, in Cytisus soils compared to uninvaded soils. Despite this increase in N availability, bioassay plants (Achillea millefolium) grown in uninvaded soils in the greenhouse were ~ 30% larger than those grown in Cytisus-invaded soils, suggesting that Cytisus may have inhibitory effects on some plants growing in invaded soils. These results suggest that the impact of Cytisus invasion on grassland plant communities may be influenced or tempered by chemical or microbial effects on the soil other than simply increased labile N.