Soil amendment application after road construction alters resource availability and can benefit native over non-native species
Road construction increases plant invasion potential through altered soil resource availability. Bare roadsides have higher resource availability due to the absence of plants and nitrogen deposition from cars. Non-native species on roadsides can serve as propagule sources, enabling invasion of native vegetation and threatening natural biodiversity. On newly constructed roadsides, we tested whether seeded native species can be favored over non-native species using soil amendments that (1) reduce available nitrogen (N), water loss, and soil temperature fluctuations (wood mulch) (2) supply organic matter and increase water holding capacity (yard waste compost), and (3) extend water and N availability (super-absorbent polymers). All possible two-way combinations of these three treatments were installed at six replicate field sites along Bear Lake Road in Rocky Mountain National Park, Colorado. Soil amendments were incorporated before and mulch was applied after seeding in fall of 2013. In September 2013, a mixture of ten native perennial grasses and forbs were hydro-seeded onto roadsides. In summer of 2014, we measured plant density, plant cover, soil moisture and plant-available N.
Density of seeded native species was significantly higher in the mulch (97± 23) and mulch + compost (90 ± 26) treatments mid-growing season compared with control (34 ± 11) and compost (27 ± 5), measured in plants/m2 (mean ± standard error). Unseeded native density was significantly higher in the polymer treatment (15 ± 3) compared to mulch (5 ± 2.5), mulch + compost (4 ± 2), and mulch + polymer (4 ± 3). We detected no difference in non-native species density among treatments. Soil nitrogen (N) was highest in compost and lowest in mulch + polymer mid-season. No difference in soil N was detected late season. One week after 14-21 mm of rainfall in July, soil moisture was highest in mulch + compost (27 ± 5%) and mulch (20 ± 4%), and lowest in control (11 ± 4%), polymer (7.8 ± 2%), compost (12.5 ± 2.4%), and compost + polymer (12.8 ± 3%). Although there was great variation in response to treatments among sites, soil amendments altered availability of N and water. In the first year of monitoring, this benefited establishment of both seeded and unseeded native species, but not non-native species.