Black carbon decreases soil activity of biomass degrading enzymes and increases protease activity

Background/Question/Methods

Black carbon (BC), charcoal or what has recently been dubbed biochar, resides in soil for hundreds or thousands of years, creating important potential to sequester recently fixed atmospheric carbon in soil.  The long residence time of BC is due to high microbial decay resistance.  Yet microbes appear to flourish with BC in soil due to improvements in soil factors such as increased porosity, lower bulk density, and improved water and nutrient retention.  Favorable conditions created by BC amendments typically increase microbial biomass, creating potential to increase decomposition and nutrient cycling through enhanced microbial activity, including soil exoenzyme production.  Increased decomposition of native soil organic matter (SOM) would negate some or all of the sequestration potential of black carbon additions to soil.  On the other hand, greater decomposition would enhance nutrient availability, increasing nutrient cycling, and improving soil productivity potential.  The objective of this study was to evaluate the effects of BC on the activity of soil exoenzymes capable of biomass decomposition and nutrient release.  Two different soils, a forest Andisol and an agricultural Molisol, were each incubated with various amounts of BC (0, 12.5, 25, and 50% v/v).  After 121 and 226 days, we measured activities of five soil exoenzymes responsible for biomass decomposition and nutrient release (protease, beta-glucosidase, chitinase, phosphotase and peroxidase). 

Results/Conclusions

Incubation time had little or no impact on exoenzyme activity, but soils and BC amounts did.  Enzyme levels for the Andisol were higher than for the Molisol (protease, 224%; beta-glucosidase, 53%, chitinase, 57%, phosphatase, 49%). One exception was peroxidase, which did not different between soils nor did it different among BC amounts.  Increasing amounts of BC did cause a decrease in the activity of beta-glucosidase, chitinase and phosphatase, however, the decline with increasing BC amounts was greater for Andisol (-59% to -64%) than Molisol (-32% to -42%).  Beta-glucosidase and chitinase decompose fractions of SOM and their decreased activity with increasing BC indicates that decomposition of native SOM will not increase with BC additions.  The relatively large decrease in Andisol suggests that native organic matter might be protected from enzymatic decomposition to a much greater extent in forests than in agricultural or grasslands ecosystems having Molisols.  In contrast, BC caused increased protease activity (Andisol, 31%; Molisol, 164%), which implies greater peptide degradation and nitrogen availability, especially in Molisols.