科研之友
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摘要
Soil microorganisms release extracellular enzymes into the soil matrix to access carbon (C) and nitrogen (N) from soil organic matter (SOM). Temperature and pH are major factors governing the rates at which these enzymes decay SOM, hence influencing the availability of C and N for microbial assimilation. As temperature increases, the rate of decomposition is also expected to increase. Recent advances provide estimates of intrinsic temperature sensitivities of key decay reactions at one, circum-neutral pH, but how temperature sensitivity of enzymatic SOM degradation is influenced by pH remains unclear. Here we expand on recent work by determining specific activities of C-acquiring (beta-glucosidase; BGase) and C- and N-acquiring (N-Acetyl-Glucosaminidase; NAGase) enzymes with purified, fluorescently labeled organic substrate at temperatures from 5 to 25 degrees C (5 degrees C steps) and at pH values from 3.5 to 8.5 (1 pH unit steps). Using specific activity data, we quantified temperature sensitivities of the reactions with estimates of activation energy (E-a) at each pH value. We then used E-a estimates to compute temperature-induced changes in the C:N flow ratio, which is defined as the ratio of enzymatic liberation rates of C to N from the substrates. Across all temperatures, BGase activity was generally high in the pH range of 5.5-8.5, while NAGase exhibited a relatively narrow optimum between pH 5.5-6.5. Temperature sensitivity of BGase differed significantly among pH values; the strongest temperature responses were observed at pH 4.5. NAGase, in contrast, did not exhibit any significant pH-dependent changes in temperature sensitivity. The temperature increase from 5 to 25 degrees C induced changes in the C:N flow ratio, with direction and magnitude strongly dependent on the pH. We observed a large, temperature-induced increase in C:N. flow ratio at pH 4.5 and decreases in C:N flow ratio at pH > 5.5 that were most pronounced at pH 7.5. Our data show that pH can induce differential effects on reaction rates and temperature sensitivity of organic C and N liberation, with consequences for changes in the relative availabilities of C and N for microbial assimilation.
- 出版日期2014-9
