Nitrogen dynamics in a shallow lake

This model describes nitrogen cycling in a dune-lake system in the Northland region of New Zealand. It is based on stock and flow diagrams where each orange oval represents an input, while each blue box represents a stock. Each arrow represents a flow. Flows reduce the stock at which they start and add to the stock at which they end.

This model describes nitrogen cycling in a dune-lake system in the Northland region of New Zealand. It is based on stock and flow diagrams where each orange oval represents an input, while each blue box represents a stock. Each arrow represents a flow. Flows reduce the stock at which they start and add to the stock at which they end.

Common nitrogen inputs to land are through nitrogen fertiliser and nitrogen fixed by legumes (such as white clover).
Nitrogen can be lost as a gas through volatilisation from the soil surface, especially if urine, dairy shed effluent, and/or fertilisers are applied on hot, windy days to alkali soils. 
Nitrogen inputs (chiefly fertilisers) leads to the entry of nitrogen to the soil water (entry).
Microbes (bugs) take up nitrogen from the soil water (much like people absorb nutrients from food). This is called immobilisation. Through this process nitrogen is locked up in the tissue of bugs as part of the soil organic matter. Soil organic matter consists of soil organisms (both alive and dead), plant material in various stages of decomposition, and dung.

Microbes (bugs) convert nitrogen locked up in soil organic matter (made up of living/dead soil organisms, decaying plants, and dung) into a form that plants can use to fuel growth. This process of conversion is called mineralisation and produces  a form of nitrogen known as ammonium. Ammonium is quickly converted by bacteria to other forms of nitrogen, such as nitrate, that can also be taken up by plants, but are also more easily lost from the soil through leaching.
Green plants take up nitrogen (uptake) to use during the process of photosynthesis - the process of generating energy from carbon dioxide and water. 
When plants die, they are broken down by fungi and bacteria and add to the soil's organic matter (plant death).
Grazing animals eat plants containing nitrogen. Some of the nitrogen that enters the animals is used to form their body tissue.
Nitrogen within supplementary feed is consumed by grazing animals (ingestion). It contributes to the nitrogen that is used to form body tissue. 
The harvest of product from grazed animals leads to a loss of nutrients from the farming system (export).
Effluent application to land on dairy farms can provide an important additional source of nitrogen input to pastures.
Animals deposit urine and faeces, which enter the reserves of nitrogen held in the soil organic matter. Urine contains a high amount of nitrogen while  inputs from faecal deposition are relatively low. Urine deposition is a major source of nitrogen in many catchments dominated by dairy farming in New Zealand. Leaching losses from urine patches are high in autumn/winter when plant uptake is low and drainage from the soil is high.
Nitrogen can be converted to a gas when soil bacteria use a form of nitrogen (nitrate) as a source of oxygen. This process of denitrification occurs most often when the soil is saturated (full with water). So, this may not be a major process around some Northland dune lakes, where soils are very free-draining.
The majority of urine deposited by animals enters water held in the soil (entry to soil water)
Nitrogen in the soil water can move to groundwater beyond the rooting zone of plants. Nitrogen in the groundwater feeding a lake may come from nitrogen inputs in the catchments that are linked to it through groundwater (flow to groundwater). Such linkages are typically difficult to identify and quantify.

Nutrients are lost to the surrounding  groundwater when deep drainage occurs (loss to deep drainage). Deep drainage is where water flows past the point where nutrients can be taken up by plant roots. Substantial amounts of nitrogen can be lost from the soil in this way. These nutrients may become inputs to other lakes, if the catchments are connected through groundwater.
Groundwater can move into the interface between land and the lake (the riparian area). This can be a dryland area or a wetland area. A dryland riparian area is not subject to frequent waterlogging, while by comparison a wetland area is saturated with water for a significant proportion of the year. Nitrogen in the groundwater flowing through the riparian zone can leave the system as a gas when soils are saturated. The key process involved (denitrification) relies on soil bacteria using a form of nitrogen (nitrate) as a source of oxygen.
Denitrification is more common in the wetland riparian zone, relative to the dryland riparian zone, because it occurs primarily under saturated soil conditions. Denitrification also requires organic matter for energy, and so is helped by the large amount of dead and decaying plant material generally present in wetland areas.
Waterfowl (especially geese and paradise ducks) graze plants, particularly in the riparian area of the catchment (waterfowl grazing).

All of the above processes are detailed in the boxed figure that depicts all processes occurring on land in the catchment of a dune lake. 
Groundwater can move from the riparian area to the littoral zone of a lake (flow into littoral zone). The littoral zone is the near-shore area where plants are present that are rooted in the lake bed but most have leaves that either just reach the lake surface or grow above it.
Sometimes stock directly deposit faeces and urine containing nitrogen into the littoral zone of a lake. Trampling of plants by stock in the littoral zone can also reduce their ability to absorb nitrogen.

Denitrification by plants in the littoral zone is a key source of nitrogen loss from a shallow-lake system. Denitrification requires organic matter for energy, which is generally provided by dead and decaying plants present in the littoral zone of the lake.
The flow of water from the littoral zone to the water column can transport nitrogen with it (flow).
Nitrogen can be deposited directly into the water column of a lake, bypassing the littoral zone. This happens predominantly through stream flows that enter a lake, especially during floods (direct input).
Nitrogen can leave a lake in the water that is flowing out from it (lake outflow). There is little loss of nitrogen in outflows from the Northland dune lakes, thus limiting the use of flushing to reduce nutrient loads. This is because there is often no defined exit point in these lakes.
Plants rooted on the lake bed (macrophytes or "major plants") absorb nitrogen mainly through their leaves from the water column but also through their roots in the lake sediments.
Plants die and become decomposing material in the stock of organic material on the lake bed (plant death).
Algae present in the water column take up nitrogen from the water (algae uptake). High algal populations in the water can lead to decreased plant populations on the lake bed because of shading.
Some species of algae can fix nitrogen from the atmosphere, like white clover and other legumes do on land (fixation).
Algae die and enter the pool of decaying organic material present on the lake bed (algae death).
Fish consume algae. Nitrogen is excreted into the water column by the fish (manure deposition), while dead fish that settle on the lake bed enter the pool of organic matter (fish death).
Dead organisms and faeces on the lake bed are subject to decomposition by microbes (bugs). The nutrients produced from this process then enter back into the water column. This process is broadly known as mineralisation. Most nitrogen in the sediment found on a lake bed is in organic form. Decomposition by bugs converts this organic nitrogen to ammonia, which can be used by plants and algae in the water column.

Denitrification is typically the major process by which nitrogen is lost from a shallow-lake ecosystem.
Sediments containing nitrogen (chiefly organic material) can accumulate on the lake bed. Burial by other sediments leads to the effective removal of these nutrients from the lake system.
This part of the system is made up of the processes occurring within the dune lake.
Together, the land and lake elements constitute the heart of a dune-lake ecosystem.

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