Douglas fir biochar enriched with plant nutrients as a controlled release fertilizer

Numerous agricultural soils may lack essential macronutrients crucial for plant growth. Utilizing highly water-soluble commercial fertilizers over extended periods can be both expensive and ecologically unfavorable. Herein, we developed nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg)-enriched Douglas fir biochar (BCF) as an eco-friendly and cost-effective fertilizer for plants. The BCF was synthesized by precipitating MgNH 4 PO 4 . 6H 2 O and MgKPO 4 . 6H 2 O on the biochar surface, which was characterized using XRD, XPS, SEM, and elemental analysis. BCF contained 43.76±3.26 mg g -1 of NH 4 + (3.40±0.25 as N%), 120.14±5.95 mg g -1 of PO 4 3- (8.98±0.31 as P 2 O 5 %), 3.79±0.58 mg g -1 of K + (0.41±0.01 as K 2 O%), and 34.05 mg g -1 of Mg 2+ . The releasing behaviors of NH 4 + , PO 4 3- , K + , and Mg 2+ were investigated through a series of batch leaching experiments at different pHs (1-13), different water matrices, and three different temperatures (5, 25, and 40°C), as well as fixed-bed continuous flow column experiments. Finally, greenhouse studies were carried out to study the performance of BCF fertilizer as a slow-release P fertilizer for corn growth. In comparison with conventional P fertilizer and the simple blends biochar (BC) and P fertilizer, the use of BCF yielded ∼23% higher plant heights on the 35 th day, ∼64% higher plant dry weight, and ∼111% higher P uptake compared to triple superphosphate fertilizer at 29.70 kg (P 2 O 5 ) ha -1 nutrient rate for corn growth. • Struvite and struvite-K were synthesized on biochar (BCF). • N%, P 2 O 5 %, and K 2 O% values of BCF were 3.4, 9.0, and 0.4, respectively. • BCF acts as a slow-release fertilizer. • BCF produced ∼1.23x higher corn plant heights compared to TSP fertilizer. • BCF produced ∼1.64x higher corn plant dry weights than TSP fertilizer. • BCF produced ∼2.11x higher P uptake compared to TSP fertilizer.