Relationship between carbon and nitrogen availability and extracellular enzyme activities in soil

Microorganisms rely on extracellular enzymes to break down insoluble organic polymers such as cellulose, protein, and chitin into smaller units for uptake. Our objective was to investigate the factors affecting the relationship between soil extracellular enzyme activities and C and N turnover. Several aerobic incubations were carried out with ammonium (NH₄⁺) and proteins as N sources and cellulose as the main C source. Cellulase (exocellulase and β-glucosidase) activity was positively correlated with the amount of cellulose added, as well as with the availability of N. A decrease in the C to N ratio of the amendments from 40 to 10 resulted in an increase in exocellulase and β-glucosidase activity of 18% and 10%, respectively. Similarly, the activity of protease initially depended on the amount and kind of protein added; later, however, an increase in carbon availability resulted in an elevated protease activity. Initially, protease and cellulase activity were induced by their corresponding substrates and an increase in activity of both enzymes resulted in a proportional increase in carbon dioxide (CO₂) evolution. Over time, however, the level of enzyme activity became increasingly determined by factors other than substrate availability. In addition, N turnover, while initially determined by the amount and kind of N source added, became increasingly dominated by the C to N ratio of the substrates added. Our study showed that even though enzyme activities alone may not be sufficient to describe the decomposition process, they can yield valuable information about the availability of specific organic compounds and their degradation over time.