This model describes phosphorus 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 involve a loss from the stock at which they start and add to the stock at which they end.
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 (sorption). 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 (desorption).
This makes it once again available to plants.
Microbes (bugs) take up phosphorus from the soil water (much like people absorb nutrients from food). This is called immobilisation. In this process, phosphorus 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 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 at which this process of mineralisation takes place 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 (uptake).
When plants die, they are broken down by fungi and bacteria and add to the soil organic matter (plant death)
Grazing animals eat plants that contain
phosphorus and some of it is utilised to form body tissue.
Phosphorus within supplementary feed is consumed by grazing animals (ingestion). It contributes to the phosphorus that is used to form body tissue.
The harvest of product from grazed animals leads to a loss of phosphorus from the farming system (export).
Disposal of dairy-shed effluent onto pastures can provide another primary avenue through which
phosphorus excreted from animals returns to pasture (effluent application).
Animals deposit faeces, contributing to the
reserves of phosphorus stored in the soil organic matter (faecal deposition). 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.
Phosphorus in the soil water can move to groundwater beyond the rooting zone of plants. Phosphorus entering into a lake may come from phosphorus inputs in other parts of the catchment moving through the groundwater (flow to groundwater). 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 interface between land and the lake (the riparian area). This can be a dryland zone, a wetland zone, or a mixture of them both. A dryland zone is not subject to frequent waterlogging while, in comparison, a wetland is saturated with water for a significant proportion of the year. The riparian area
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.
Waterfowl (especially geese and paradise ducks) graze plants, particularly in the riparian area of the catchment (waterfowl grazing).This can lead to the transfer of phosphorus from the riparian zone to the lake.
All of the above processes are detailed in the boxed figure, which 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 and 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 (direct deposition). 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 (deposition in littoral zone).
The flow of water from the littoral zone to the water column can transport phosphorus with it (flow into column).
Phosphorus can be deposited directly into the water column of a lake, bypassing the littoral zone (flow). This happens predominantly through stream flows that enter a lake, especially during floods.
Algae present in the water column take up phosphorus from the water (algae uptake).
Algae die and enter the pool of decaying organic material present on the lake bed (algae death).
Fish consume algae, with phosphorus being excreted into the water column by the fish (manure deposition). 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 (fish death).
Dead organisms and faeces 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 the top layer of sediments remains oxygenated or is released if oxygen levels are low.
Sediment is the chief source of phosphorus for plants rooted to the lake bed (plant uptake).
Plants die and become decomposing material in the stock of organic material on the lake bed (plant death).
High levels of phosphorus in the water column
typically lead to high rates of phosphorus becoming locked into the lake sediments (sorption).
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 (bed deposition). 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 (lake outflow). There is little loss of phosphorus in outflows from the Northland dune lakes. 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.
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.