*3.1.2.2 Erosion and runoff*

Soil erosion and its impact on surface water is a complex process that depends on various factors such as soil characteristics, hydrological, and climatic conditions. Farm sediment erosion, which occurs in furrows and inter-furrows, is influenced by raindrop impact and surface currents. Raindrops typically cause soil particles to detach from each other, while surface currents cause both particle detachment and transport down the slope. The effectiveness of these processes is greatly affected by surface soil type, rainfall duration, and the amount of plant cover and residues. Soil surface coverage and erosion have an inverse relationship, so implementing soil conservation practices can reduce erosion [34, 35]. Runoff usually occurs in fields through two types of flow: surface and subsurface flow. Surface flow can sometimes penetrate into the soil along the slope and move laterally below the surface before reappearing on the soil surface. Losses of phosphorus in agricultural land occur through surface runoff, which can be in the form of dissolved or sediment-bound phosphorus. Phosphorus bound to sediments includes phosphorus that is bound with soil mineral particles and organic matter. During surface runoff in cultivated fields, 80% of phosphorus losses occur in the form of sediment-bound phosphorus [36]. In contrast, runoff from grasslands, meadows, uncultivated soils, and forests transports fewer sediments, so most of the phosphorus that is transported downstream is in the form of dissolved phosphorus in water [24, 37]. During runoff movement along sloping surfaces, phosphorus from the soil and plant residues can dissolve and enter the runoff (**Figure 3**). In summary, [39] have concluded in their research that the total phosphorus loss can be determined based on the sources of phosphorus, the chemical and biological changes and transformations of phosphorus, and the mobility of phosphorus in the soil.

**Figure 3.** *Phosphorus movement paths in the field (adapted from [38]).*

The interaction between the surface soil (1–2 inches) and water (rain or irrigation water) creates the possibility of releasing phosphorus in a soluble form from the soil and plant residues. In most watersheds, the amount of phosphorus transferred through surface runoff is greater than subsurface runoff, as phosphorus is typically low in deep soil layers. When runoff penetrates into the soil, water-soluble phosphorus is absorbed into the subsurface soil, reducing the concentration of phosphorus in the subsurface runoff compared to surface runoff. However, sandy soils, peaty soils, and soils with preferential flow can exclude this case [40].

The subsurface transfer of phosphorus has been studied very little, and it was previously assumed that the amount of transfer through subsurface runoff is very small. However, some researchers believe that a significant portion of phosphorus (more than 0.1 mg/L) is transported in the form of orthophosphate in subsurface runoff [41]. Although other forms of phosphorus may also be found in subsurface runoff [33], studies have reported a correlation between the amount of dissolved phosphorus in the runoff and the amount of phosphorus in the soil. If the amount of phosphorus in the soil exceeds 60 mg/kg, the concentration of phosphorus in the runoff will significantly increase [32]. The type of vegetation and management practices can also have a significant impact on this phenomenon.

When the concentration of phosphorus exceeds 60 mg/kg, the concentration of total phosphorus in the runoff increases sharply. The reason for this is that when phosphorus exceeds this limit, it is absorbed into sites with less absorption and storage energy and is easily released into water [42]. However, using arsenic phosphorus to predict the concentration of dissolved phosphorus in runoff has many problems, including the depth of sampling to determine arsenic phosphorus, which is usually taken from the depth of the plow. The depth of the interaction between the runoff and the soil is estimated to be between 1 and 2 inches [24]. To overcome such problems, two Dutch scientists, [43], used the soil phosphorus saturation percentage to assess the risk of phosphorus leaching into underground water. This percentage is calculated by dividing the amount of phosphorus available to the plant by the maximum amount of phosphorus fixation in the soil. During their research, a critical limit of 25% for phosphorus saturation was established (for soils in the Netherlands). If the phosphorus saturation exceeds this amount, phosphorus will be released into the water. Every soil has the potential to absorb and store phosphorus, and this potential depends on the physical-chemical characteristics of the soil. The importance of iron and aluminum oxides and other soil minerals in controlling the solubility and absorption of phosphate in the soil has been identified [2].

The rate of weathering and release of phosphorus from phosphate rocks is very low and is estimated to be about 0.015 km/ha.year [44]. The rate of weathering and release of phosphorus from phosphate rocks depends on effective factors such as the type of phosphorus rock, their size, temperature, and water quality. A positive correlation exists between the amount of chemical weathering and runoff generated in a watershed. When the amount of runoff is high, the amount of weathering is almost 10 times more than when the amount of runoff is low [45].

#### *3.1.3 Sediment*

Sediment moves downstream along slopes through waterways. Larger particles are deposited first due to the reduction in runoff transmission power, while finer particles, such as organic materials and clays, along with silt-sized particles, enter water ecosystems along with water-soluble phosphorus. Eventually, these sediments reach

rivers and water reservoirs, such as natural lakes, ponds, and reservoirs behind dams, and settle. Some of the eroded materials are so small that they settle much more slowly and can be resuspended with even slight turbulence in the water. Dynamic water conditions or changes in the redox potential can resuspend solid particles and release their adsorbed elements, such as phosphorus and nitrogen, into the water [46–48]. Therefore, the relationship between suspended particles and dissolved phosphorus in lake water is of great chemical importance.

Four main mechanisms have been identified for the release of phosphorus from sediments [49–51]. These mechanisms can cause changes in the mineral structure of sediments, which can, in turn, lead to the release of phosphorus into the water column above the sediment [46].

The four mechanisms are briefly discussed hereunder.

Absorption: Phosphorus desorption can occur in the following three different situations:


Dissolution: The dissolution of minerals containing phosphorus may cause the release of this element under the following conditions:


Ligand exchange: As a result of increasing pH, the concentration of OH increases and ligand exchange takes place with PO4 <sup>3</sup> located on iron hydroxides [57].

Enzymatic hydrolyses: Enzymatic hydrolyses in aquatic ecosystems are critical processes driven by microbial activities.

#### *3.1.3.1 Microbial activities*

Enzymes present in the structure of microbes cause mineralization of organic phosphorus and accelerate its release [14, 58]. An increase in temperature due to the activity of microbes intensifies microbial activities [53].
