High-alkaline coconut shell biochar modifies nutrient availability-retention and enzymatic activity in calcareous soil: A rapid incubation soil test

Biochar (BC) is a solid by-product of the pyrolysis of organic matter and has several potential benefits. However, its high pH may also allow its use in calcareous soils. This study aimed to verify the impact of four doses (0, 10, 30 and 50 g·kg-1) of coconut shell BC (pyrolyzed to 500 °C, pH 10.3) on the nutrient availability, retention and enzymatic activity of a calcareous soil with low fertility. The selected response variables were pH, electrical conductivity (EC), oxidation-reduction potential (Eh) of leachate and soil, phosphatase (PHO), arylsulfatase (ARY), fluorescein diacetate (FDA) activity and nutrient content in the soil and leachates. The experiment was carried out for 377 days under laboratory conditions in a pot test using calcareous soil (pH 8.03). In the leachates, EC decreased with the addition of 50 g·kg-1 BC. In the soil, an increase in Olsen-P (120 %), NO3- (42.5 %), K+ (96.3 %) and Cu2+ (304.2 %) content was observed with 50 g·kg-1 BC. The 30 g·kg-1 BC treatment increased 47.5 % NO3-, 62.5 % K+ and 137.5 % Cu2+ contents. BC reduced the leaching of NO3- (82 %), NH4+ (14.3 %), Olsen-P (21.8 %), Mg2+ (24.1 %), SO42- (44.2 %), Ca2+ (18 %), Fe (33 %) and B (93 %). Finally, BC at 10 g·kg-1 increased ARY and FDA activities. The use of commercially manufactured coconut shell BC (30 and 50 g·kg-1) is a viable alternative to improve nutrient availability in calcareous soils without significantly altering the naturally high pH of calcareous soils.