Altering water and nitrogen availability after roadside disturbance to favor native plant species

Abstract

This study evaluates the use of soil amendments in roadside restoration to promote native species and discourage invasive species establishment through manipulation of water and nitrogen (N) availability. Our goal was to decrease soil N availability and increase soil water content to foster growth of perennial native species on roadsides in Rocky Mountain National Park, Colorado. Roadside construction can increase soil bulk density, reduce aggregation, and cause an initial increase in resource availability, which encourages growth of early successional species. In addition, N deposition from the Front Range of Colorado is increasing nitrate and ammonium availability in this National Park. The study objective was to increase or decrease water and/or N availability with soil amendments to reduce weedy annual species establishment on roadsides. Treatments were hypothesized to 1) increase soil moisture and reduce plant-available N (synthetic polymer incorporation), 2) reduce soil surface temperatures, increase moisture and indirectly decrease N (wood mulch blanket), 3) decrease bulk density by changing soil structure and slowly increase N (yard-waste compost incorporation). These amendments were applied alone and in pairwise combinations to six southeast facing roadsides slopes concurrent with seeding in fall of 2013. Ten perennial grass and forb species were hydro-seeded with tackifier to all roadsides. Plant density, cover, mineral nutrients, soil moisture, total C:N, soil temperature, and rainfall were measured during the growing season in 2014 and 2015. A paired greenhouse study was conducted in spring 2014 with analogue native and non-native grasses. Grass root and shoot biomass, plant height, seedling density, and soil moisture were measured after 9 weeks of growth. On these roadsides, soil moisture, and density of native seeded species was significantly changed by soil treatments through time (p=0.039, p=0.040). Wood mulch alone and combined with compost or polymer increased soil moisture after rainfall in the field (p=0.0007) and after irrigation in the greenhouse (p=0.0001). In the field, seeded species density was highest in mulch/compost treatments in 2014 (p=0.029) and mulch/polymer treatments in 2015 (p=0.003). After one year of decomposition, none of the treatments significantly changed carbon to nitrogen ratios (p=0.27) which averaged 18.7:1, although mulch/compost treatment had the lowest C:N ratio of 13.5:1. Mulch or mulch/polymer treatments combined had much less nitrate than yard-waste compost incorporation (p=0.0002). Mulch blanket immobilized N and decreased non-native density in summer 2015 to 2 plants/m2. In contrast, compost/polymer treatments had 12 non-native plants/m2 (p=0.02). In the greenhouse, nitrate was more limiting than water, and mulch blanket increased native grass growth relative to non-native grasses (p=0.002). Because of the immobilization of N, mulch also decreased germination rates (p=0.001) and biomass (p=0.001) across all species. With higher soil moisture availability in the greenhouse, non-native growth was lower than natives in control (p=0.001), polymer and mulch soil treatments. In contrast, compost incorporation, which increased N availability in the field, drastically increased growth of all species (p<0.0001) in the greenhouse. These results begin to demonstrate how increased soil moisture and decreased soil N favors germination and seedling survival of desirable native perennial species, while simultaneously reducing non-native species establishment. By managing soil resource availability after disturbance, we can achieve resilient plant communities dominated by perennial native species.