Effects of repeated burning of cheatgrass (Bromus tectorum) dominated ecosystems on litter, soil and plant nitrogen: Implications for restoration

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, 2009, and 2010 and seeded immediately afterwards. The litter treatment was conducted in the first year only. Soils (0-5 cm) and vegetation were sampled in 2008, 2009, 2010, and 2011 during peak biomass and before and after burns. Analyses were conducted with mixed model ANOVAs.

Results/Conclusions

In 2008, litter comprised the largest percentage of total quadrat N, but litter N decreased in each subsequent treatment year (p < 0.0001) and was lower in plots burned and seeded with wheat than in other treatments (p = 0.035). Litter N was higher in litter intact than litter removed plots (p < 0.0001). Soil mineral N increased from 2008-2010 but decreased in 2011 (p < 0.0001) possibly reflecting the predicted trend. However, no differences existed in burn or litter treatments, and generally high precipitation from 2008-2011 may have masked treatment differences. Cheatgrass biomass and N content increased from 2008-2009 but decreased in 2010-2011 (p = 0.0003, p < 0.0001), and were lower in plots burned and seeded with wheat than in other treatments (p = 0.0005, p = 0.001). Cheatgrass biomass and N were higher in litter intact than litter removed plots (p < 0.0001, p < 0.0001). The trends in litter, soil, and vegetation were reflected in total quadrat N which increased from 2008-2010 and decreased in 2011 (p < 0.0001).  Our results indicate that repeated burning can potentially decrease ecosystem N and cheatgrass competition, and that both litter and seeded species like wheat may play important roles.