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Ecological Investigation, Protection, and Restoration
Published in Benjamin Alter, Environmental Consulting Fundamentals, 2019
Most surface soils have aerobic conditions due to the ready flow of air through the unsaturated pore spaces of the soil. However, when soils become saturated with water due to rainfall and flooding, the water impedes the flow of gases, including oxygen, through the soil. Microbial activities also act to deplete the soils of oxygen, creating hydric conditions that lead to the creation of hydric soil. Hydric soil is “a soil that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part” (Federal Register, July 13, 1994). The National Technical Committee for Hydric Soils (NTCHS) defines and develops or accepts criteria for hydric soils.
Root Zone Moisture Gradients Adjacent to a Cedar Swamp in Southern Ontario
Published in George Mulamoottil, Barry G. Warner, Edward A. McBean, Wetlands, 2017
Although hydric soil is not defined explicitly in the OMNR system, hydric soil is soil that is generally saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper portion and is developed under conditions sufficiently wet to support the growth and regeneration of hydrophytic vegetation (Tiner, 1991). Hydrophytic vegetation includes plants able to grow in water or in substrates that are periodically deficient in oxygen (anaerobic) during a growing season as a result of excessive water content (Tiner, 1991).
Nutrient processes and modeling in urban stormwater ponds and constructed wetlands
Published in Canadian Water Resources Journal / Revue canadienne des ressources hydriques, 2019
Brendan Troitsky, David Z. Zhu, Mark Loewen, Bert van Duin, Khizar Mahmood
Macrophytes and phytoplankton are able to assimilate nitrogen in both NH4+ and NO3- forms, with different preference across various species; species traditionally found in wetlands with limited nitrification often prefer NH4+, whereas high NH4+ inhibits growth in many other species (Lee et al. 2009). The anaerobic conditions in hydric soil limit the oxygen intensive nitrification process, resulting in many naturally occurring wetland species displaying a preference to NH4+; this is supported by a correlation of decreased ammonia removal with greater pond depth, as traditional wetland macrophytes will have limited available habitat in these cases (Lee et al. 2009; Koch et al. 2014). Wetland macrophyte species are often adapted to intermittent flooding and anoxic conditions (Marsalek et al. 2008). This means that when selecting macrophytes to populate a stormwater pond, nitrogen type preference should be considered alongside traditional factors such as invasiveness, yield, and growing patterns (Uusi-Kämppä et al. 1996; Lee et al. 2009). The nitrogen uptake of plants is also affected by pH, temperature, sunlight, water levels, the availability of other nutrients and trace minerals, and soil moisture for shoreline macrophytes, among other factors (Glass and Siddiqi 1995; von Wirén et al. 1997; Crawford and Glass 1998). Rapid growth, high nutrient storage, high standing structure, and dense growth patterns are all ideal characteristics for maximizing nitrogen assimilation (Reddy and D'angelo 1997). Macrophytes are also seasonally active, assimilating nutrients for growth during the spring and summer, but, if they are not harvested or consumed before fall, much of the nutrients will be returned to the system as litter (Uusi-Kämppä et al. 1996; Fisher and Acreman 2004; Jefferson et al. 2017). Potential nitrogen removal rates can be effectively measured by standing stock, which relates to the amount of the plant available for harvest without removing the entire plant (Vymazal 1995). Typical aboveground N standing stock values range greatly, but have been reported to range from 0.6–72 g N m−2 (Johnston 1991), 2–64 g N m−2 (Vymazal 1999), 22–88 g N m−2 (Vymazal 1995), 2–29 g N m−2 (Mitsch and Gosselink 2000). It is important to note that root uptake for nutrients typically consists of dissolved phosphorus and nitrogen, so some sedimentated forms will not be immediately available for uptake (Carignan 1982; Vymazal et al. 1998; Cedergreen and Madsen 2002). Plant assimilation may only account for around 5% of nitrogen removal under typical conditions, however, with ideal growing conditions, low total loading, and regular harvesting, this can increase significantly (Hammer 1992).