Hydroseeding increases ecosystem nitrogen retention but inhibits natural vegetation regeneration after two years of chaparral post-fire recovery

Fire is a natural disturbance in many ecosystems such as semi-arid chaparral shrublands, but rates of vegetation regeneration may be slow after intense fires. Thus, land managers may resort to practices such as mulching or seeding in areas that are prone to soil erosion and nutrient loss. These practices, in particular seeding with annual grasses, are controversial because they may inhibit rates of natural vegetation regeneration, introduce exotic species, and be ineffective at soil and/or nutrient retention. We assessed how hydroseeding affected rates of chaparral vegetation regrowth and ecosystem nitrogen (N) storage during the first 2 years of post-fire recovery. We selected three north-facing slopes that were within 500m of each other: one slope was unburned while the other slopes burned during the “Cocos” fire in May 2014 and were either hydroseeded with a mix of native grasses and suffrutescent shrubs (seeded) or left to naturally regenerate (control). We hypothesized that seeding would reduce native woody shrub cover and production but that ecosystem N storage would be higher in the seeded stand. Seeding significantly reduced woody shrub cover, aboveground biomass, shrub and total species richness, and rates of shrub growth. However, herbaceous biomass, dominated by grasses that were absent on the naturally regenerating slope, was significantly higher in the seeded areas, causing significantly higher total vegetation cover in the seeded area. The higher herbaceous cover led to an increase in plant N storage and a reduction in soil extractable NO3. Hydroseeding also caused a reduction in potential N mineralization, and during the growing season, an increase in soil microbial biomass. Our results suggest that seeding recently burned chaparral slopes reduces nutrient loss but at the expense of native plant abundance and diversity. While the long-term effects of seeding on vegetation composition are unknown, changes in vegetation from a shrub- to a grass-dominated system may fundamentally alter future fire regimes.