The Nitrogen Cycle – everyone learns it at varying degrees of detail as they pursue their leaving certificate biology course and onward in university natural sciences but why is it important? Nitrogen has been identified as a major driver of eutrophication not only on this blog (The Hidden Depths of Kinvara: Underground Rivers; What’s eating Ria Formosa Lagoon? ) but internationally to government level with the implementation of the Nitrates Directive 1991. After more than 20 years of measures aimed at reducing nitrogen loading to fresh and coastal waters, water quality blighted by nitrogen enrichment has yet to improve (Grinsven et al. 2012; Gulati and van Donk 2002).
Figure 1 shows the Nitrogen Cycle as we know it. N2 gas, well known to be the major constituent of the air we breathe, is fixed to ammonium and converted to nitrite and further to nitrate for use by plants to grow. The process then occurs in the reverse where the nitrogen is returned to the gaseous state by denitrification. So what is the problem? Historically, the earth contained far less fixed nitrogen than was required by higher organisms for energy. As a result, the amount of nutrients available to plants was LIMITED. This is still the case in pristine waters. However, in waters where additional fixed nitrogen has been made available, either by discharge from sewage treatment or agricultural runoff, serious problems can occur with nuisance algal blooms reducing light penetration and therefore reducing biodiversity as well as the effects on DO cycling within freshwaters, particularly lakes and slow moving rivers.
Figure 1: The Nitrogen Cycle (modified from Thamdrup (2012))
While measures have been focused on reducing the available fixed nitrogen to aquatic plants – particularly phytoplankton, our understanding of the nitrogen cycle has deepened. Instead of the classical picture as shown in Figure 1, scientists are beginning to realise the increasing importance of newer pathways as shown in Figure 2. While these pathways were discovered as far back as 1999, they are yet to be fully considered when the Nitrogen Cycle is conceptualised.
Figure 2: The “full” Nitrogen Cycle (Thamdrup 2012)
Microbial communities have long been identified as the foundation of all cycles, but it is only in recent years that pathways such as Methane Denitrification and Phototrophic Nitrite Oxidation have been included in the Nitrogen Cycle. So are these important? And if so, why?
One of the research questions I am investigating in a small interdrumlin lake (Core Blimey! Using Lacustrine Records to test Eutrophication Trends) is the role of thermal stratification breakdown in nutrient loading. Initially I need an understanding of the nutrient cycling while thermal stratification is occurring and this is primarily focused on anaerobic or anoxic processes.
These new pathways, particularly the Annammox pathway, not only provide ammonia to algae such as cyanobacteria; but further complicate the use of fixed nitrogen in the hypolimnion (deep part of the lake) and exacerbate the progression through the oxidation phases (Figure 3). Importantly, once all the available oxygen and fixed nitrogen have been used by microbial communities to decompose organic matter, decomposition looks to iron to aid microbial processes.
Within this phase, iron is reduced and forms a soluble molecule with phosphate. This is the primary pathway for release of phosphorus from sediments during hypolimnetic anoxic periods during summer. As a consequence we see an increase in phosphate concentrations in the deep part of a lake during this time and occasionally these phosphorus-charged waters are made available to surface waters, if conditions prevail, providing nutrients to P-starved algae. So the Nitrogen Cycle not only influences nitrogen use but can greatly impact on phosphorus concentrations in lakes and ensuing algal blooms.
Figure 3: Degradation pathways in mud but also applicable to anoxic hypolimnia
Grinsven HJM, ten Berge HFM, Dalgaard T, Fraters B, Durand P, Hart A, Hofman G, Jacobsen BH, Lalor STJ, Lesschen JP et al. 2012. Management, regulation and environmental impacts of nitrogen fertilization in northwestern Europe under the Nitrates Directive; a benchmark study. Biogeosciences, 9(12):5143-5160.
Gulati RD, van Donk E. 2002. Lakes in the Netherlands, their origin, eutrophication and restoration: state-of-the-art review. Hydrobiologia, 478(1-3):73-106.
Thamdrup B. 2012. New pathways and processes in the Global Nitrogen Cycle. Annual Review of Ecology, Evolution and Systematics, 43:407-428.