Story of nitrogen dynamics in a shallow lake

This story presents a conceptual model of nitrogen cycling in a dune-lake system in the Northland region of New Zealand. It is based on the concept of a stock and flow diagram. Each orange ellipse represents an input, while each blue box represents a stock. Each arrow represents a flow. A flow involves a loss from the stock at which it starts and an addition to the stock at which it ends.

This story presents a conceptual model of nitrogen cycling in a dune-lake system in the Northland region of New Zealand. It is based on the concept of a stock and flow diagram. Each orange ellipse represents an input, while each blue box represents a stock. Each arrow represents a flow. A flow involves a loss from the stock at which it starts and an addition to the stock at which it ends.

Common nitrogen inputs to land are the application of nitrogen fertiliser and the fixation of nitrogen 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. 
This step involves the breakdown of nitrogen inputs (chiefly fertilisers) and the subsequent entry of nitrogen to the soil water.
Immobilisation is the process by which microbes (bugs) take up nitrogen from the soil water, much like a human absorbs nutrients from food. This process leads to nitrogen being 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.

Mineralisation is the process by which 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 form of nitrogen is known as ammonium. In a different biological process, 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 through leaching.
Green plants take up nitrogen to drive the process of photosynthesis--the process of generating energy from carbon dioxide and water. 
When plants die, they become an addition to soil organic matter.
Grazing animals eat plants containing nitrogen. A proportion of the nitrogen that enters the animals is used to form their bodily tissue. 
Nitrogen within supplementary feed is consumed by grazing animals. It contributes to the nitrogen that is used to form bodily tissue. 
The harvest of product from grazed animals leads to a loss of nutrients from the farming system.
Effluent disposal to land on dairy farms can provide an important additional source of nitrogen input to pastures.
Animals deposit faeces, which enter the reserves of nitrogen held in the soil organic matter. Most nitrogen is present in the urine, thus inputs from faeces are relatively low.
Animals deposit urine that contains a high amount of nitrogen. This 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. In comparison, phosphorus is not lost to the atmosphere, so is much harder to manage.
The majority of urine deposited from animals enters water held in the soil. Generally, more than half of the nitrogen ingested by an animal exits as urine.
Nutrients in the soil water can move to groundwater beyond the rooting zone of plants typically found within a given land use. Nitrogen may arise from nitrogen inputs on land outside of that directly surrounding a lake. This involves movement through groundwater.
Nutrients are lost from the groundwater when deep drainage occurs. 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 area in the interface between land and the lake (the riparian area). This can consist of a dryland riparian area, a wetland riparian area, or a mixture of them both. The dryland riparian area is an area of riparian area that is not subject to frequent waterlogging. In comparison, the wetland riparian area is an area of land that is saturated with water for a significant proportion of the year. Nitrogen in the groundwater flowing through the riparian zone is susceptible to loss from the system in a gaseous form. The key process involved (denitrification) relies on soil bacteria using a form of nitrogen (nitrate) as a source of oxygen, especially when soils are saturated.
Waterfowl (especially geese and paradise ducks) graze plants, particularly in the riparian area of the catchment.

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.
The first part of this system is made up of the processes occurring on the land. This is commonly termed the catchment of a dune lake.
Groundwater can move from the riparian area to the littoral zone of a lake. The littoral zone is the near-shore area where plants are present that are rooted in the lake bed, but most have leaves that reach the lake surface or above it.
Stock can 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 satisfied by a significant amount of dead and decaying plants being present in the littoral zone of a lake. 
The flow of water from the littoral zone to the water column can transport nitrogen with it.
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.
Nitrogen can leave a lake in the water that is flowing out from it. There is little loss of nitrogen in outflows from the Northland dune lakes, thus limiting the use of flushing to manipulate 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") can absorb nitrogen from both the water column and lake sediments. (The chief source for nitrogen is through their leaves, however.)
Plants die and become decomposing material in the stock of organic material on the lake bed.
Algae present in the water column take up nitrogen from the water. High algal populations in the water can lead to decreased plant populations on the lake bed because of shading. This reduces the amount of algae that grow on these plants directly.
Some species of algae can fix nitrogen from the atmosphere, like white clover and other legumes do on land.
Algae die and enter the pool of decaying organic material present on the lake bed.
Fish consume algae. Nitrogen is excreted into the water column by the fish, while dead fish that settle on the lake bed enter the pool of organic matter.
Dead organisms and faeces on the lake bed are subject to decomposition by microbes (bugs). 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.
The part of this 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.

View the model in Insight Maker