Biochar Affects Nutrient Leaching from Container-grown Woody Plants with Different Water Requirements

A carbon rich by-product of pyrolysis known as biochar has been described as a means to enhance soil nutrient retention. Moreover, it can cause an increase in soil fertility in the long-term by increasing the cation exchange capacity and surface area and also increase water retention, which can reduce nutrient leaching. The objective of this research was to determine the effect of switchgrass biochar on nutrient release and retention when used in a container substrate. Buxus sempervirens ×B. microphylla (‘Green Velvet’ boxwood) and Hydrangea paniculata (Pinky Winky® hardy hydrangea) were potted into 1 gallon (3.7 liter) containers filled with pine bark and amended with either 10% or 25% (v/v) biochar. Plants were irrigated with a moisture sensor-based irrigation system that provided a prescribed volume of water to plants based on the physical properties of the substrate to determine the impact of biochar on nutrient leaching. Leachate analysis over the course of the 8-week experiment showed that the average mass of phosphate, potassium and total carbon was greater in the leachate from containers that received 25% biochar compared to those receiving 0 or 10% biochar for both plant species. For hydrangea, mass of total nitrogen (TN), nitrate (NO₃), and ammonium (NH₄) in leachate was not affected by substrate. However, for boxwood, the mass of NO₃ and TN were greater for the 25% biochar treatments, while mass of NH₄ was not affected by substrate. In hydrangea, the total amount of water leached and nutrients lost from the containers was lower in biochar-amended containers (both 10 and 25% biochar) due to receiving a lower total volume of water. Amendment with biochar was also shown to affect concentration of phosphorus and potassium, increasing concentration with greater biochar amendment, in both leaf tissue and substrate.