Source: Geeks and Geeks website

Excessive reactive nitrogen (Nr) has broad ecological consequences. It results in a loss of plant diversity, shifts in plant dominance, increased invasive species, loss of ecosystem services, loss of biodiversity, alters food webs, oversimplification of the ecosystem, and reduction of the ecosystem’s ability to stabilize itself with predator/prey and host plant/insect interactions (Pardo et al. 2011, Bobbink and Hicks 2014, Baron et al. 2014, de Vries et al. 2014, Williams 2014, Hicks et al. 2014, Midolo et al. 2019, Fields 2004).

The mechanisms by which N force these changes are not yet understood (Midolo et al. 2019, Fenn et al. 1998). Major and important knowledge gaps remain. We lack data on how much Nr transfers to soil, groundwater, and vegetation and do we know how much N is lost to denitrification (Jeffries and Maron 1997). The other unknown is the critical load — the amount of Nr an ecosystem can tolerate without damage — making this number a “moving target’ (Jeffries and Maron 1997). These considerations are highly variable and depend on site-specific conditions, which makes setting universal guidelines a challenge.

Some of our N-sensitive plants are legumes and non-vascular plants such as lichens, mosses, hornworts, liverworts, insectivorous plants, and some grasses. Effects of excess N on woody species and ferns are varied and not very significant (Midolo et al. 2018) except for blackberries (Rhus sp); they love high N soil. The more disturbance from aboveground biomass being removed or damaged, the more N becomes available, and the blackberries thrive (Braun 2020). This makes it essential to use disturbance carefully in restoration, particularly when trying to control nitrogen-sensitive species while avoiding stimulating unwanted resprouting woody species.

Much of the literature describes nitrophilous plants in terms of nutrient amounts in their tissues or nutrient processing abilities (Moreau et al 2014, Bharucha & Dubach 1950, Braun 2020). Landowners have no easy and convenient way to measure that. Using descriptors such as fast growing, dropping their leaves when shaded, or casting their seeds far and wide can easily be misinterpreted because these are basic strategies plants use to compete successfully in their environment and may or may not have anything to do with excessive nitrogen in the soil. Curtis (1959) lists a few but certainly it’s not as complete as Britain’s Ellenberg Nitrogen Index. Developing a regional index for the Eastern US could be invaluable to landowners and restoration ecologists alike, particularly if it includes information on nitrogen cycling traits, responses to nitrogen enrichment, and interaction with native biodiversity.

Nature’s complexity insists we look at the whole system of interactions. The uncertainty in how to best approach “fixing” ecological imbalances—especially when it comes to invasive species management and nitrogen reduction—highlights the need for adaptive management and ongoing research. Every ecological intervention (aka management technique) has a trade out and carries the risk of unintended consequences and/or collateral damage. Rather than leave those consequences to chance and a hope-it-works management practice, it’s critical to explore the current knowledge base.

How much carbon addition is needed varies with the plant species, the application method, and the environmental conditions. This “it depends” response is a recurring theme when working with the dynamics of nature and the uniqueness of our lands. Adding carbon works by promoting nitrogen immobilization, slowing down plant growth, and ideally shifting the competitive balance to favor native plants (Kardol and Wardle 2010). However, it’s only successful when 1) weeds are suppressing native species, 2) weeds are nitrophilous relative to native species, and 3) the amount of carbon addition is sufficient to alter the competition between the weeds and the native species. This latter suggestion is most effective when perennial natives are not established, such as newly planted prairies (Blumenthal et al. 2003).

Removing topsoil is effective but can be costly and destructive. This solution would only be practical for initial restoration projects.

Should we be “fixing” a problem that we don’t know for certain we have? Do we understand how one factor alters another? Are there cascading effects; if so, are those positive or negative? Are they long term or short term? With issues like elevated CO2 levels altering the nitrogen cycle, predicting the outcome and long-term implications of fire and carbon addition become more challenging. Ultimately, these complexities affirm that ecological restoration efforts must proceed cautiously, often on a trial-and-error basis, and require long-term monitoring.

Annual and frequent prescribed burns are promoted to reduce excess nitrogen in native habitats, yet, prescribed fire responses aren’t homogenous. While fire removes the N from the aboveground biomass, the story doesn’t stop with this single-factor account. There are 18 variables operating at different time frames and the volatility of nitrogen makes it predictably unpredictable (Wells et al. 1979, Vasquez 2021). We might think fire is creating an N deficiency, but microbes may be offsetting that volatilization (Wells et al. 1979). This underscores how difficult it is topredict outcomes of volatile nutrients such as N in any given landscape. Additionally, our native habitats can add sufficient N to make up for any N losses due to fire (Vitousek et al. 2002, Knoepp et al. 2005). And areas where N deposition and agricultural runoff is high will compound any mitigation efforts (Perry et al. 2010).

It is well established that annual and frequent fires decrease biodiversity and lead to homogenization (Coolon et al. 2013, Reich et al. 2001 and these). This begs the question of why we would choose burning annually or frequently to allegedly decrease the nitrogen overload that is decreasing the biodiversity by increasing the dominance of tall sod-forming grasses (e.g. Indian grass, big bluestem, switchgrass). That’s a mouth full! Doesn’t this mimic the negative effects of nitrogen overloading where one group of plants becomes overly dominant? Doesn’t this just trade out one issue for another?

To learn more about how fire and grass interact to create and maintain grass dominance, click the button below.

Fire as a method to alter soil N pools is controversial. Not only are studies inconclusive, but few researchers discuss fire as a feasible management tool for intentionally reducing N. Fenn and others (2010) call the practice “unorthodox” and caution against relying on fire as a primary tool for reducing nitrogen, likely due to its inconsistent outcomes and the potential for unintended consequences (e.g. reduced biodiversity) (Kalisz and Powell 2000, Neumann and Tolhurts 1991, Collett et al. 1993, Greenslade 1993). These are wise words when research is controversial.

Fire’s unpredictability on N is highlighted by numerous studies that find fire increases, decreases, or has no change on soil N (Wan et al. 2001, Wells et al. 1979, Dijkstra & Adams 2015, Fonturbel et al. 2021, Wells 1971, Hough 1981, DeBano et al. 1998)). Here are a few examples.

• More than half of the studies reviewed show no change in nitrogen after prescribed burns (Fonturbel et al. 2021).
• One showed an N loss of 100lb/acre but when evaluated over 4 years found N losses were undetectable (Wells 1971).
• Another shows N increases of 500-900lbs/acre over 10 years because of increased microbial activity (Wells 1971).
• Other research found burning soil high in mineral elements would not significantly change the soil if it were already rich in that element (Wells et al. 1979).
• And another effect is increased mineralization which offsets any loss of N to fire (Dell et al. 2005, Coolon et al. 2013, Turner et al. 1997, Wells et al. 1979).

Fire also affects soil ecosystems. Our soil organisms drive the entire ecosystem and they depend on organic matter and can be highly sensitive to fire. It will come as no surprise that fire effects on soil microorganisms vary and there is concern some effects may be long lived (Knicker 2007, Wells et al. 1979, Dooley and Treseder 2012). Microorganisms can be affected directly or indirectly (Pelligrini et al. 2022). The indirect effects cause the highest mortality: the loss of litter oversimplifies their habitat; toxic compounds are redistributed causing death; and there are changes to soil aeration, pH, water, temperature, and food (Knicker 2007). This is not intended to be a full exploration of how fire and soil microorganisms interact.

Source: biotas.org

These declines speak to the importance of planned, intentional refugia. Planning refugia aligns with the goal of sustaining biodiversity not only in flora and fauna but in the less obvious, yet ecologically essential, microbial communities. Refugia provides the vital source for recolonization, allowing ecosystems to recover more efficiently after disturbances such as prescribed fire. Additionally, planning intentional refugia is the simplest way to “keep all the parts,” while simultaneously restoring quality habitat and experimenting with new management techniques.

Reactive Nitrogen (Nr) has increased but research on fire as a management practice to decrease it is inconclusive. Studies show fire increases, decreases, or doesn’t change soil N. The lack of quantitative, consistent data complicates its use as a reliable tool for nitrogen mitigation. It’s a bigger issue than the postage stamp (even if that is hundreds of acres) of native habitat that most of us manage.

Using fire as a nitrogen management tool is not straightforward. If native plant access is limited to other sources (e.g. light, moisture) and not just N, it changes the dynamics. The lack of long-term data and the nuanced interactions between soil, plants, and microbes suggest that pushing for frequent fire as a nitrogen management tool seems risky because of the unpredictability in both nitrogen cycling and plant responses.

There is an “urgent need” for guidelines that make fire management compatible with environmental sustainability (Fonturbel et al. 2021). Inconsistencies in ecological research of fire effects suggest that multiple factors drive fire outcomes and the complex relationship between these complicate not only the development of reliable guidelines but confidence in an outcome that relies on a single cause, a single “solution,” and a single effect (Brose and Van Lear 1998).

The broader solutions are to reduce atmospheric N depositions and are linked to fossil fuel burning and our dependence on “Big Ag.” Addressing these at the root is to become a conscientious consumer. Stop buying CAFO-raised meats and conventionally grown vegetables and grains that contribute to nitrogen pollution. Encourage restaurants to cease their support of Big Ag over the local producers. Buy local, know your farmer, and how they raise your food.

I’ve listed the references for this reserach on a separate page as there are many.