Do soil enzyme activities explain stimulated carbon mineralization following liming?
Sajjad Raza, Rolf Sommer, Andrew J. Margenot
Abstract
Losses of soil organic carbon (C) observed following liming have been proposed to result from stimulated C mineralization, which could occur through a variety of mechanisms including soil enzymes. Given the pH-sensitivity of enzyme activities, liming could impact catalysis of C mineralization by extracellular enzymes independently of microbial response. To this end, we evaluated the responses of C mineralization as CO2 flux, permanganate-oxidizable C (POXC), and the activities of C-hydrolytic enzymes to liming of a Haplic Ferralsol (Typic Kandiudox) in western Kenya under three differing 11-year soil fertility managements: unfertilized (UNF), mineral fertilized (MIN), and organic fertilized (ORG). Five rates of Ca(OH)2 (1.5, 3.0, 4.5, 6.0, and 7.5 t ha−1) and an unlimed control were used to establish a pH gradient of 4.6–6.4. Soil C mineralization rates assessed as CO2 production peaked within 5 d for high liming rates, but converged to a similar rate as unlimed soils after 21 d. At 28 d, activities of β-glucosidase and cellobiohydrolase decreased with increasing pH, increased for β-glucosaminidase, and were not affected for β-galactosidase. POXC generally increased with lime for fertilized soils, more so in ORG and less in MIN soils, with relative increases of 18%, 5%, and 1% for ORG, MIN and UNF soils, respectively, at 7.5 t ha−1 liming. This study identifies potentially rapid albeit short-lived stimulation of C mineralization following addition of a highly soluble liming agent, concurrent with an increase in POXC. Notably, decreases in activities of soil enzymes that catalyze the first and last step of the cellulosic C depolymerization suggests that short-term losses of soil C were not driven by enzyme activities, and may occur in spite of decreases in activities of C-hydrolytic enzymes. The enhanced C mineralization observed could reflect augmentation of labile C pools via non-extracellular enzyme means in the short-term.