Invasive scotch broom alters soil chemical properties in Douglas-fir forests of the Pacific Northwest, USA

Scotch broom is an N-fixing invasive species that has high potential to alter soil properties. We compared soil from areas of Scotch broom invasion with nearby areas that had no evidence of invasion to assess the influence of broom on soil P fractions and other chemical properties. The study was conducted at two contrasting Douglas-fir sites in Oregon (OR) and Washington (WA), USA with broom invasion for 10 years. We used the Hedley sequential fractionation procedure to assess effects of Scotch broom invasion on P pools of varying bioavailability, and also measured total C, N and extractable nutrient cations. Total soil C and N were significantly higher with broom present at the fine-textured OR site, but there was no effect at the coarse-textured WA site. There was no difference in labile-P measures between the presence and absence of Scotch broom at either site, but there were notable reductions (25-30 %) in the intermediately-available P fraction when broom was present. Extractable nutrient cations (notably K) were lower in the presence of broom at both sites, with the effects most pronounced at the fine-textured OR site. Lasting effects of Scotch broom invasion are likely to be associated with variable changes in soil C, N, and decreases in extractable nutrients and available P. These changes, and other documented effects of Scotch broom on soil, are likely to have lasting effects on Douglas-fir growth after Scotch broom removal that will vary depending soil nutrient status at a given site.