Effects of repeated burning on nitrogen budgets and cheatgrass (Bromus tectorum) biomass and reproduction

Background/Question/Methods

Restoration of cheatgrass dominated rangelands depends on controlling cheatgrass while simultaneously providing conditions necessary for native species establishment. Growth and reproduction of cheatgrass is highly responsive to available soil nitrogen (N), and decreasing soil N levels may decrease the competitive ability of cheatgrass. Burning volatilizes some N, but can result in an immediate increase in NH₄ and longer-term increase in NO₃. Higher N availability increases cheatgrass growth and N content and, consequently, N loss when cheatgrass is repeatedly burned. We asked if repeated burning of a cheatgrass dominated ecosystem would deplete litter and soil N and consequently, cheatgrass N content, density and biomass. We used a factorial, blocked experiment in two cheatgrass dominated sites in north-central Nevada.  Factors included two litter treatments (litter removed and litter intact) and four burn treatments (unburned, burned only, burned and seeded with cheatgrass, and burned and seeded with annual wheat). Burns were conducted in fall 2008-2011 and seeded immediately afterwards. The litter treatment was conducted in the first year only. Soils (0-5 cm) and vegetation were sampled in 2008-2012 during peak biomass and before and after burns. Analyses were conducted with mixed model ANOVAs.

Results/Conclusions

Soil mineral N increased from 2008-2010 but decreased from 2011-2012 (p<0.0001) possibly reflecting the predicted trend of decreased available resources over time following burning. Litter biomass and N content decreased from 2008-2012 (p<0.0001) and were lower in burned than unburned plots (p=0.05). Cheatgrass N increased from 2008-2009 but decreased from 2010-2012 (p<0.0001) and was lowest in plots that were burned and seeded with wheat (p=0.05). Cheatgrass N was higher in litter intact than litter removed plots (p=0.05). Cheatgrass density increased from 2008-2011 and decreased in 2012 (p<0.0001), except in wheat plots where densities did not vary significantly across time but were consistently lower than those of other treatments (p=0.05). Cheatgrass density was higher in litter intact than litter removed plots (p=0.05). Cheatgrass biomass and reproduction increased from 2008-2010 then decreased from 2011-2012 (p<0.0001) and were lowest in plots that were burned and seeded with wheat (p=0.05). Cheatgrass biomass and reproduction were higher in litter intact than litter removed plots (p=0.05). Our research indicates that repeated burning may be a useful tool in restoration of cheatgrass-dominated sites, especially if accompanied by treatments that reduce available resources such as litter reductions or seeding with a strong competitor like annual wheat.