Can biochar increase the bioavailability of phosphorus and why is phosphorus a limiting nutrient and where is most phosphorus found?
Biochar and Phosphorus Availability
Yes, biochar can actually increase the bioavailability of phosphorus in some situations. Here's why:
- Biochar Adsorption: Biochar has a porous structure that can adsorb or hold onto phosphorus. This can prevent it from leaching out of the soil and make it more accessible to plants when needed.
- Cation Exchange: Biochar can also act like a sponge for certain positively charged ions (cations) like those holding onto phosphorus. This can indirectly make phosphorus more available for plant uptake by swapping these cations with others that plants can use more easily.
However, it's important to note that the impact of biochar on phosphorus availability can vary depending on the type of biochar, the soil conditions, and the form of phosphorus present.
Why is Phosphorus a Limiting Nutrient?
Phosphorus is a vital nutrient for plant growth, but it often becomes a limiting factor in ecosystems for a couple of reasons:
- Low Availability: Unlike some other nutrients, readily available phosphorus is often scarce in natural environments.
- Soil Binding: Phosphorus tends to bind strongly to soil particles, making it difficult for plants to access directly.
Because of this limited availability, even a small increase in accessible phosphorus can significantly impact plant growth.
Where is Most Phosphorus Found?
Most of the earth's phosphorus is actually locked away in rocks and minerals. However, in terms of biological availability, here's a breakdown of where phosphorus is typically found:
- Rock Phosphate: This is the largest reservoir of phosphorus, but it's not directly usable by plants. It's mined and processed to create fertilizers.
- Soil: While only a small portion of soil phosphorus is readily available, it's a crucial source for plants.
- Living Organisms: Plants and animals store phosphorus, and when they die and decompose, it can become cycled back into the soil.
- Aquatic Systems: Phosphorus can be found in freshwater and saltwater, but again, the available amount can be a limiting factor for aquatic plant growth.
The application of biochar to acid (pH < 6.5) and neutral soils (pH 6.5–7.5) significantly increased plant-P availability by a factor of 5.1 and 2.4, respectively (95% confidence interval 3.5–6.7 and 1.4–3.4, respectively), while there was no significant effect in alkaline soils (pH > 7.5). Precipitation and dissolution reactions greatly influence the availability of phosphate in the soil. Phosphate minerals can dissolve over time to replenish the phosphate in the soil solution. This reaction increases the availability of phosphorus. Adding biochar to the soil changes the soil environment and soil microbial communities, and these results in a no effect to positive effect on P availability. Incorporating biochar into the soil raises the soil pH, soil water holding capacity, and soil cation exchange capacity (CEC).Phosphorus is usually considered the “limiting nutrient” in aquatic ecosystems, meaning that the available quantity of this nutrient controls the pace at which algae and aquatic plants are produced. In appropriate quantities, phosphorus can be used by vegetation and soil microbes for normal growth. This means only a fraction of what we put into the soil will ever make it into the plant. Furthermore, when plants detect high levels of phosphorus they reject their symbiotic partners, arbuscular mycorrhizal (AM) fungi. Biochars produced at temperatures higher than 600 °C had no significant effect on P availability. Low temperature biochars (450 °C) and mid temperature biochars (450–600 °C) significantly enhanced the P availability in soils amended with biochar and also significantly differed from each other. After biochar amendment, the bioavailable P content in the red soil increased significantly. Specifically, when biochar was added at concentrations of 1%, 3%, and 5%, the bioavailable P content in the red soil substantially increased to 1.32, 1.52, and 1.92 mg kg−1, respectively, which was an increase of 159–255%.One of the reasons for this is the high reactivity of phosphorus. It tends to bind to soil particles and complex with metals in the soil. This makes it unavailable to plants even if it is present in the soil.