Performing Department
Natural Resources & Environmental Sciences
Non Technical Summary
Restoration of cheatgrass dominated rangelands depends on controlling cheatgrass while simultaneously providing the conditions necessary for native species establishment. The expansion and eventual dominance of exotic annual grasses and other invaders in semi-arid shrublands often has been attributed to fire and the increase in resource availability resulting from the death of fire intolerant shrubs. Soil nutrients are inherently low in these systems, but can increase dramatically following fire, especially available N (NO3- and NH4+). Cheatgrass can take advantage of the high N availability and produce significantly more shoot mass by maintaining higher growth rates than perennial grasses. Recent field studies have shown the importance of available inorganic nitrogen in controlling cheatgrass establishment and growth. Although cheatgrass tends to thrive in a high nitrogen environment, it is inhibited in a low one. A novel method to tie up mineral N might be to reduce total N supplies and, therefore, mineral N supplies by repeated burning. It is well documented that nearly all N contained in organic material that is burned is volatilized and lost from the system, potentially causing long-term declines in ecosystem N capital unless the N is replaced by atmospheric deposition, N-fixation, or fertilization. On the other hand, burning commonly causes short-term increases in soil ammonium levels, which is often followed by a pulse of nitrate and nitrate leaching once nitrifying bacteria occupy the site again. The short-term pulse of ammonium after fire is thought to be one factor favoring nitrophilic cheatgrass after rangeland fire. Over the long-term, however, one would expect that repeated burning without replacement of lost N could cause reductions in soil mineral N levels, at least after the initial post-fire pulse has passed. This project is a 5-year study established north of Winnemucca, Nevada in 2008, on cheatgrass dominated rangeland, designed to examine the effects of repeated burning and surface litter on soil nutrient dynamics, cheatgrass biomass and reproduction, and establishment of native species. We hypothesized that repeated burning of cheatgrass dominated areas will result in significant reductions in soil total and mineral N levels over time, including the magnitudes of the post-fire pulses of ammonium, significant increases in the C:N ratio of litter, which will further contribute to reductions in soil available N, and finally, significant reductions in cheatgrass biomass and seed production, increasing the potential for successful restoration of native species.
Animal Health Component
(N/A)
Research Effort Categories
Basic
50%
Applied
50%
Developmental
(N/A)
Goals / Objectives
Restoration of cheatgrass dominated rangelands depends on controlling cheatgrass while simultaneously providing the conditions necessary for native species establishment. The expansion and eventual dominance of exotic annual grasses and other invaders in semi-arid shrublands often has been attributed to fire and the increase in resource availability resulting from the death of fire intolerant shrubs (Young and Evans 1978, West and York 2002, Evangelista et al. 2004). Soil nutrients are inherently low in these systems, but can increase dramatically following fire, especially available N (NO3- and NH4+) (Stubbs and Pyke 2005) which can increase up to 12-fold (Blank et al. 1994, 1996). Cheatgrass can take advantage of the high N availability and produce significantly more shoot mass by maintaining higher growth rates than perennial grasses (Monaco et al. 2003). Recent field studies have shown the importance of available inorganic nitrogen in controlling cheatgrass establishment and growth (McLendon and Redente, 1991; Young et al., 1999). Although cheatgrass tends to thrive in a high nitrogen environment, it is inhibited in a low one (McLendon & Redente 1991; Redente et al. 1992; Young & Allen 1997; Young et al. 1999). A novel method to tie up mineral N might be to reduce total N supplies and, therefore, mineral N supplies by repeated burning. It is well documented that nearly all N contained in organic material that is burned is volatilized and lost from the system, potentially causing long-term declines in ecosystem N capital unless the N is replaced by atmospheric deposition, N-fixation, or fertilization (Blair, 1997; Neary et al., 1999; Raison et al., 1985). On the other hand, burning commonly causes short-term increases in soil ammonium levels because of the heat-induced denaturing of soil organic N (Neary et al., 1999; other refs). The pulse of ammonium is often followed by a pulse of nitrate and nitrate leaching once nitrifying bacteria occupy the site again. The short-term pulse of ammonium after fire is thought to be one factor favoring nitrophilic cheatgrass after rangeland fire (Monaco et al. 2003). Over the long-term, however, one would expect that repeated burning without replacement of lost N could cause reductions in soil mineral N levels, at least after the initial post-fire pulse has passed. In 2008, a 5-year study was established north of Winnemucca, Nevada on cheatgrass dominated rangeland to examine the effects of repeated burning and surface litter on soil nutrient dynamics, cheatgrass biomass and reproduction, and establishment of native species. We hypothesized that repeated burning of cheatgrass dominated areas will cause: 1. Significant reductions in soil total and mineral N levels over time, including the magnitudes of the post-fire pulses of ammonium. 2. Significant increases in the C:N ratio of litter, which will further contribute to reductions in soil available N. 3. Significant reductions in cheatgrass biomass and seed production, increasing the potential for successful restoration of native species.
Project Methods
The initial burns were conducted in 2008 and 2009 at two sites in North Central Nevada: Eden Valley and Orovad. Due to the five year duration of the study, additional funds and personnel are required to complete the study. This agreement provides funds to support a PhD student and soil analyses during the final years of the study. Burns and data collection will continue at the two study sites in 2010, 2011 and 2012. One quarter of each 3.5 m plot was reserved for non-destructive sampling. Two permanent quadrats (0.1 m2) were established in this area to monitor changes in plant species composition, ground cover, and soil nutrients from PRS Probes during the 5-year study. Destructive sampling was conducted in the remainder of the plot. At the time of B. tectorum seed maturation and peak production (early June), prior to the burn treatment (early September), and immediately after the burn treatment (mid-September), two new quadrats (0.1 m2) were placed in a location that had not been sampled previously. Destructive sampling was then conducted to evaluate changes in: (1) B. tectorum density, biomass, seed production and nutrient content; (2) total plant biomass and nutrient content; (3) litter biomass and nutrient content; and (4) soil nutrients from extracted soil samples. The cover and composition of the vegetation, litter, bare soil, gravel, and rock in the plots was evaluated during peak production (early June). Percent aerial and basal cover of each species was estimated ocularly for the two permanent quadrats (0.1 m2) located in the nondestructive portions of the plots. All cheatgrass plants located in the two quadrats (0.1 m2) within the destructive portions of the plots were counted and harvested in early June. These plants were then harvested by clipping at the soil surface and sorted into seed, non-seed reproductive biomass and vegetative biomass. The seed from each plot was counted and viability was assessed. Reproductive biomass and vegetative biomass was then oven dried at 60 C and weighed. A subsample of each tissue type was analyzed for carbon and nitrogen content (see analytical methods below). Species other than cheatgrass also were harvested during peak production (early June) from the same two quadrats (0.1 m2) within the destructive portions of the plots, dried and weighed, and a subsample was analyzed for carbon and nitrogen content. Litter was collected from the same two quadrats (0.1 m2) used to sample the biomass of cheatgrass and other plant species during peak production (early June) and prior to burning (early September). In addition, residual litter and ash were collected from two additional quadrats (0.1 m2) immediately after the burn treatment (mid- September). Samples were oven dried at 60 C and weighed. A subsample of each of these samples was analyzed for carbon and nitrogen content. Peak production and pre-burn samples were collected the year prior to the first burn and each subsequent year after the last burn (2008-2013). Post-burn samples were collected each year after the burn (2008-2012).