This story contains a conceptual model of phosphorus 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.
Key phosphorus inputs to land are the application of manures and fertiliser (such as superphosphate).
The entry of phosphorus into the soil water involves the breakdown of inputs (especially manures and fertilisers).
Phosphorus in the soil water binds strongly to iron and aluminium oxides on soil particles when oxygen is available. This makes it unavailable to plants. Iron oxides are compounds containing iron and oxygen, whereas aluminium oxides are compounds containing aluminium and oxygen.
Phosphorus bound to iron and aluminium oxides on soil particles is released when no oxygen is present.
This makes it once again available to plants.
Immobilisation is the process by which microbes (bugs) take up phosphorus from the soil water, much like a human absorbs nutrients from food. This process leads to phosphorus being locked up as a component of 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 phosphorus 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. The rate of mineralisation varies with soil temperature, moisture, and soil oxygen content.
Plants take up phosphorus from the soil water to
promote the development of new tissue and drive complex energy transformations.
When plants die, they become an addition to the stock of organic matter present in the soil.
Grazing animals eat plants that contain
phosphorus and assimilate part of it into their bodily tissue.
Phosphorus within supplementary feed is consumed by grazing animals. It contributes to the phosphorus that is used to form bodily tissue.
The harvest of product from grazed animals leads to a loss of phosphorus from the farming system.
Animals deposit faeces, contributing to the
reserves of phosphorus stored in the soil organic matter. Most phosphorus that is emitted from grazing animals exits as faeces, rather than urine that is the primary source of nitrogen lost from these animals.
Disposal of dairy-shed effluent onto pastures can provide another primary avenue through which
phosphorus excreted from animals returns to pasture.
Phosphorus in the soil water moves to groundwater
beyond the rooting zone of plants.
Phosphorus usually only moves through groundwater when no oxygen is present or when the soil is saturated with this nutrient.
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. The riparian zone
cannot provide for the loss of phosphorus in a gaseous form, as it can for
nitrogen. Thus, its ability to take phosphorus out of groundwater is defined
by its limited capacity to absorb it.
Phosphorus bound to sediment can arise from erosion and enter either of the riparian or littoral zones. 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. Erosion in the areas of land found around Northland lakes is particularly high in summer, driven by
frequent storm activity and soils becoming repellent to water after prolonged dry spells.
Specific types of waterfowl (mainly geese and paradise ducks) graze
within the riparian zone. This can lead to the transfer of phosphorus from the riparian zone to the lake.
The first part of the dune-lake system presented here 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 containing phosphorus into the littoral zone of a lake. Trampling of plants and their roots by stock in the littoral zone can also reduce their ability to absorb phosphorus. Animals can also promote the entry of phosphorus into the lake through promoting the collapse of sediment from the lake edge through trampling.
Phosphorus bound to sediment can arise from erosion and flow to either of the riparian or littoral zones.
Phosphorus can be transported in the flow of
water from the littoral zone to the water column of the lake.
Phosphorus bound to sediment can flow through
erosion to streams that enter the lake, and therefore be deposited directly into the
water column, especially during floods.
The primary way that algae in the water take up
nutrients is through obtaining them from the water. Phosphorus is a
key nutrient required for algal growth.
Algae die and enter the pool of decaying material.
Fish consume algae, with phosphorus being excreted into the water column by the fish. Also, bottom feeders (such as koi carp and catfish) can promote the entry of phosphorus to the water column by stirring up sediment. Dead fish that settle on the lake bed enter the pool of organic matter found there.
Dead organisms on the lake bed are subject to decomposition by microbes (bugs). After phosphorus in the
organic matter is broken down to forms that are accessible to plants through the process of mineralisation, this leads to an increase
in the phosphorus stock in sediment where it will stay provided the top of the lake sediments remain
oxygenated. By remaining oxygenated, the top of the lake sediments provides a cap that prevents free phosphorus from escaping from deeper lake sediments. Much phosphorus is free (not bound to iron and aluminium oxides) in the lower layers because of low oxygen levels there.
Phosphorus is stored in the sediment. It remains bound to the sediment if sediments are oxygenated or is released if oxygen levels are low.
Sediment is the chief source of phosphorus for plants rooted to the lake bed.
Plants die and enter the pool of decaying material.
High levels of phosphorus in the water column
typically lead to high rates of phosphorus becoming locked into the lake sediments.
The top 1-5 cm of
lake sediments are well oxygenated and therefore phosphorus remains bound. This layer prevents the escape of phosphorus from lower layers, which is
not held strongly because there are low levels of oxygen there. If the top layer
becomes saturated with phosphorus or has a low oxygen content too, then greater flows of phosphorus from iron and
aluminium oxides to the water column will be seen. A major driver of low oxygen levels on the lake bed is in deep lakes where high temperatures cause the lake volume to divide into several layers. Here, the deepest layer is often unable to exchange oxygen with the atmosphere and the oxygen supply is exhausted as it is used in decomposition processes. These low oxygen levels lead to a release of phosphorus from the sediments.
Sediment can be directly deposited on the lake
bed, if it stays suspended through the littoral zone. This is particularly
common if it enters through stream flow, particularly during flood conditions.
Phosphorus can leave a lake in the water that is
flowing out from it. There is little loss of
phosphorus 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.
Sediments containing phosphorus (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 second part of the dune-lake system presented here is made up of the processes occurring in the lake.Together, the land and lake elements constitute the heart of a dune-lake ecosystem.