Abstract
Although microorganisms will preferentially allocate resources to synthesis of nitrogen (N)-acquiring enzymes when soil N availability is low according to the resource allocation model for extracellular enzyme synthesis, a robust link between microbial N-acquiring enzyme activity and soil N concentration has not been reported. To verify this link, we measured several indices of soil N availability and enzyme activity of four N-acquiring enzymes [N-acetyl-β-glucosaminidase (NAG), protease (PR), urease (UR), and L-asparaginase (LA)] and a carbon (C)-acquiring enzyme [β-D-glucosidase (BG)] in arable and forest soils. Although the ratios of NAG/BG and PR/BG were not significantly related with indices of soil N availability, ratios of LA/BG and UR/BG were strongly and negatively related with potentially mineralizable N estimated by aerobic incubation but not with pools of labile inorganic N and organic N. These results suggest that microorganisms might allocate their resources to LA and UR synthesis in response to N supply rate rather than the size of the easily available N pools. It was also suggested that the underlying mechanism for synthesis was different between these N-acquiring enzymes in soil microorganisms: microbial LA and UR were primarily synthesized to acquire N, whereas NAG and PR syntheses were regulated not only by N availability but also by other factors.
Highlights
Gains in ecosystem N are made through molecular N2 fixation by microorganisms and N deposition from the atmosphere, the amounts are usually low, resulting in widespread N limitation to primary production in many terrestrial and aquatic ecosystems [1]
We compared the response of synthesis of four N-acquiring enzymes to soil N availability and examined which soil N form and/or N availability index microorganisms respond to when investing their resources in N-acquiring enzyme synthesis in arable and forest soils
The LA/BG and UR/BG ratios showed the most significant negative relationships with AerNmin and to a lesser extent with Ana-Nmin and UV absorbances at 205 nm (UV-205) but not with pool sizes of labile inorganic N in the soils, suggesting that soil microorganisms might allocate their resources to LA and UR synthesis in response to N supply rate
Summary
Gains in ecosystem N are made through molecular N2 fixation by microorganisms and N deposition from the atmosphere, the amounts are usually low, resulting in widespread N limitation to primary production in many terrestrial and aquatic ecosystems [1]. Microbial decomposition of plant litter and soil organic matter plays an important role in N supply to plants and microorganisms in terrestrial ecosystems. Microorganisms acquire N through decomposing organic N to ammonium (NH4+) or assimilable small organic N Microbial resource allocation for synthesis of nitrogen-acquiring enzymes preparation of the manuscript. There was no additional external funding received for this study
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