Dicksissel

Driftless Prairies: Native Ecosystems

18 Variables to Consider

  1. Fire intensity
    1. Inconsistent fire intensity (which is expected with every burn unit) may result in N radiating downward into the humus layer, thereby increasing in the soil. When this happens NH4+ is unchanged (Wells 1971, Braun 2020, DeBano 1991, DeBano et al. 1998).
  3. Fire frequency
    1. Intermittent fires, grazing, drought increase aboveground productivity and are more historically accurate; this intermittentness is critical to the dynamics of tallgrass prairies. Knowing some history is important because responses to first time disturbance vs intervals of disturbance vary soil and plant responses (Blair 1997).
  4. Fuel consumption amount
    1. Studies show there is no correlation between fuel consumption and reduction in N (Wan et al. 2001).
  5. Seasonality
    1. Dormant season fires are expected to have no effect on N availability because the N would not be sufficiently volatilized before soil microbes became active with warmer weather (Braun 2020). We could expect those highly mobile N units to leach away, polluting our ground water.
    2. Available N varies seasonally (Wan et al. 2001).
    3. Spring fires damage spring ephemerals as they are high-N plants (Braun 2020).
    4. Few studies on fire seasonality exist (Braun 2020)
    5. Season is shown to change N response (Wells et al. 1979).
    6. Microbes immobilize N in fall after plants senesce, retain it throughout the winter until spring when it is release for plant uptake (van der Heijden et al. 2008).
  6. Types of N
    1. The ones especially harmful are NO, NO2, NH3, NH4+ (Dise et al. 2011).
  7. Severity of N deposition
    1. There are a “complex number” of factors that affect severity and its impact (Bobbink and Hicks 2014).
  8. N response to these variables
    1. Six N responses were discovered from meta-analysis and within these there are 7 variables affecting these responses (Wan et al. 2001).
  9. Vegetation type –
    1. Plant responses vary depending on a variety of factors and environmental context (Midolo et al. 2018) including stand condition and successional stage (Fenn et al. 1998).
    2. N additions decreased early successional species common to N-poor soil and increased late successional species that prefer N-rich soils (Tilman 1987).
    3. Fire reduces N but to what degree is dependent upon the vegetation type (Wan et al. 2001).
    4. Changes in the plant community due to burning are more important than lowering N in explaining fire effects on N cycling in some ecosystems (Perry et al. 2010). Knowing how atmospheric nitrogen affects the biotic and abiotic soil components in response to fire is not fully grasped. We know bits and pieces but the interactions among the myriad factors are unknown. Additionally, plant responses are variable (Blair 1997). What changes when one aspect is altered?
  10. Vegetation recovery
    1. Increased N can increase plant sensitivity to disturbances and stress; this is something to consider as we face climate change (Dise et al. 2011).
    2. Fire-caused changes in N and P can worsen or improve plant growth limitations (Dijsktra and Adams 2015).
  11. Plant uptake
    1. Without prompt plant regrowth, N loss will affect plant succession, productivity, and litter production (Knicker 2007).
    2. Removal of aboveground biomass depletes Phosphorus quicker than it can be replaced (Knicker 2007). Phosphorus is key for photosynthesis.
  12. Biological factors (N transformations and N immobilizations)
    1. Cation Exchange Capacity (CEC) – low CEC exchange rates will result in species loss (Midolo et al. 2019, Dise et al. 2011). There are tests that can be done for this.
    2. Mineralization increases with precipitation (Midolo et al. 2019).
    3. Toxic compounds are redistributed in the soil after fire, killing microbes (Knicker 2007, Kim et al. 2003).
  13. Biological recovery after fire
    1. Recovery of microbes and fuius can take years and varies from study to study (Knicker 2007, Neary et al. 1999).
  14. Soil organic matter (SOM)
    1. How thick and how moist the SOM is changes the dynamics. Thin and dry heats the soil to higher temperatures; thick and moist transports the heat faster and deeper (Dijkstra and Adams 2015).
    2. Annual burning maintains a thin SOM and less N is volatilized (DeBano 1991).
    3. Fire changes to the SOM can be contradictory (Knicker 2007, Pelligrini et al. 2022).
  15. Soil type and moisture
    1. Frequent fire reduces the clay content of soil (Fonturbel et al. 2021). The clay is important because it holds and protects the organic matter.
  16. Soil pH – not consistent findings across studies
    1. pH had no effect on the relationship between plant biodiversity and N addition (Midolo et al. 2019).
    2. Increased acidity due to low pH decreases ecosystem’s ability to remove N (Dise et al. 2011).
    3. Acidity reduces the decomposition rate of OM, leaving litter to accumulate (Dise et al. 2011).
    4. Nutrient-rich ash remaining after a burn increases the pH, which positively impacts the recovery of microbes (Knicker 2007).
    5. Plant composition changes pH causing litter to decompose faster (Fields 2004).
    6. Long term frequent and annual fires increase soil pH (Fonturbel et al. 2021).
  17. Leaching and erosion
    1. Fire-release N may be nitrified (NO3-) which is easily leached (Fenn et al. 1998).
    2. Fire creates a layer of water repellency (Knicker 2007, DeBano 2000).
    3. Hydrophobic organic compounds are increased with fire (Knicker 2007).
  18. Long term vs short term
    1. Long term has adverse effects on substrate quality, loss of soil total N, significant reductions in soil bacteria, slow recovery of nitrifying bacteria (Fonturbel et al. 2021).
    2. Hard to ascertain since there are few studies and few long-term studies addressing frequent, repeated fire (Fonturbel et al. 2021).
    3. Contradictory results suggest we may be overlooking short- and medium-term changes that may lead to cumulative long term effects (Fonturbel et al. 2021).
    4. Long term effects on N, species composition, and primary production haven’t been evaluated (Wan et al. 2001).
    5. There are few long-term studies and researchers have found the repeated use of prescribed fires and its effect on soil has been scarcely investigated. Over 75% of the studies are on forest ecosystems (Fonturbel et al. 2021). Extrapolating from one ecosystem to another is questionable because individual plant species vary considerably in their physiological functioning (Wan et al. 2001, Bobbink and Hicks 2014).
  19. Ecosystem physical factors (climate, geology, topography, soils)
    1. Higher temps and precipitation increase plant growth in response to N additions (Midolo et al. 2019).
    2. There are topographical considerations, which are not well studied. When viewed at a landscape scale, nitrogen losses in uplands are greater than in lowlands (Turner et al. 1997, Christensen 1976), but the opposite happened in site specific studies (Briggs and Knapp 1995).
    3. Weather patterns must be considered. Droughts and abundant rainfall and snow “significantly alter nutrient dynamics…especially when coupled with disturbances such as fire” (Turner et al. 1997). Do we know what these alterations are?
    4. N limitations are most obvious in lowlands that are annually burned (Knapp et al. 1998).