Response of soil alkaline phosphatase to biochar amendments: Changes in kinetic and thermodynamic characteristics

Biochar addition to soil often increases the activity of alkaline phosphatase (ALP) involved in phosphorus (P) cycling, but the underlying mechanisms of its effect is poorly understood. This study investigated the response of kinetic parameters including maximal velocity (Vmax) and Michaelis-Menten constant (Km), and thermodynamic parameters including activation energy (Ea), enthalpy (ΔHa) and temperature coefficient (Q10) of ALP to addition of two maize biochars (400 and 600 °C) in two calcareous (Typic Haplocalcid) soils with clayey and sandy loam texture. The biochars were added to the soils at 1% (w/w) and the mixtures were incubated for 90 days under laboratory conditions (25 ± 1 °C and 70% of water holding capacity). Soils with addition of raw residue were used as positive controls and soils without biochar and raw residue were included as negative controls. The potential activity of ALP was assayed at the end of incubation period. The kinetic parameters of ALP were estimated using non-linear regression techniques and the thermodynamic characteristics were determined at different incubation temperatures (17, 27, 37, 47 and 57 °C) using the Arrhenius equation. Compared with the negative control, the addition of raw residue and biochars increased ALP activity (3.1- to 4.4-fold) after the 90-day incubation, depending upon the pyrolysis temperature and soil texture. The positive effect of biochar addition on soil ALP was greater with low than high temperature biochars and in sandy loam than clayey soils. Biochar addition increased the Km and Vmax values of ALP in the clayey soil but decreased these parameters in the sandy loam soil compared with the corresponding negative controls. Generally, application of maize raw residue and biochars increased the Ea, ΔHa and Q10 values of ALP compared with the negative controls, and the increases were similar for the two pyrolysis temperatures. Soil ALP can be strongly adsorbed by biochar particles; increasing its thermal stability and decreasing its sensitivity to elevated temperatures. In conclusion, application of maize biochar to arid-soils has a high potential to improve the ALP activity, with implications for organic P mineralization and availability. Biochar would change ALP kinetic and thermodynamic characteristics differently, depending mainly on soil texture, through the surface adsorption of this enzyme on biochar particles. These changes will be useful for modeling P mineralization and biochemical processes in biochar-amended calcareous soils.