N & P Cycle

Biogeochemistry of Lake Impurities: N vs. P Transport

1. Nitrogen (The Leacher & The Soil Binder) 💧

• Primary Transport Form: Nitrate ($NO_3^-$). It is highly soluble, not retained by soil, and primarily moves via leaching into groundwater.

• The Ammonia Link: Ammonium ($NH_4^+$) is positively charged and binds to soil but is quickly converted by bacteria into Nitrate via nitrification, freeing it to leach.

2. Phosphorus (The Surface Water Pollutant) 🏞️

• Primary Transport Form: Orthophosphate ($\text{PO}_4^{3-}$). It is highly reactive and strongly binds to soil particles.

• Binding Agents: The primary binding agents are Iron (Fe) and Aluminum (Al) compounds. Iron is generally more abundant in lake sediment, but Aluminum forms a permanent, redox-insensitive bond with phosphate. This stability is why Al salts are used to permanently lock P in lake restoration.

• Transport: The strong binding prevents leaching, so P moves mostly in surface runoff with eroded sediment.

3. Phosphorus Release (The Internal Threat) 🦠

• Phosphorus is initially locked in lake sediment by Iron(III).

• Under anoxic conditions (Dissolved Oxygen concentrations below $1 \text{ mg/L}$ at the water-lakebed surface), Dissimilatory Iron-Reducing Bacteria (DIRB) use the $\text{Fe}^{3+}$ as an electron acceptor.

• This microbial respiration reduces the insoluble $\text{Fe}^{3+}$ to soluble $\text{Fe}^{2+}$, which releases the P back into the water as bioavailable orthophosphate. This is the process of internal loading.

4. The Biological Cycle (Recycling Rates) ♻️

When dissolved phosphorus is taken up by organisms, the recycling rate depends on the resulting Organic Phosphorus (POP/DOP):

• Easily Recyclable (Fast Cycle): Organic matter like algae and bacteria is easily digestible. They decompose and release the bioavailable $\text{PO}_4^{3-}$ back into the water column quickly (hours to days), accelerating short-term algae growth.

• Difficult to Recycle (Slow Cycle): Organic matter like tree leaves and other woody debris is complex and slow to decompose. They represent a more long-term storage of phosphorus that is released back into the water much slower (months to years).