Nitrogen acquisition strategies of soil microorganisms as affected by carbon and nitrogen availability.

摘要

The mineralization of nitrogen (N) is widely used to determine the amount of N made available in soil. In a microcosm study however, we found that soil microorganisms took up N from residue in both mineral and organic form with the direct uptake of organic molecules being dominant. When not all the organic molecules released from residue or soil N pass through the soil mineral N pool, gross N mineralization underestimates the amount of N made available. In this case, extracellular enzymes, released by microorganisms to break down insoluble organic molecules into smaller units for uptake, may better represent the bioavailability of organic N and carbon (C). In soil samples amended with cellulose and protein, protease and cellulase activity were indeed closely related to substrate degradation. An increase in activity of both enzymes resulted in a proportional increase in carbon dioxide (CO2) evolution. However, enzyme activity decreased several days after CO2 evolution started to decrease. This may be due to the fact that clay minerals and organic matter can stabilize extracellular enzymes, protecting them from degradation. The activity of protease not only depended on the amount and kind of protein added, but also on C availability. Similarly, cellulase activity was positively correlated with the amount of cellulose added, as well as with the availability of N. Close links between substrate availability and soil enzyme activity were also found in a field study. Unlike the microbial biomass, enzyme activities did not decrease in the seasonally dry top soil. In a corroborating microcosm study, enzyme activities also increased at a low soil moisture potential (-1500 kPa) in the presence of residue. However, respiration was decreased in this treatment. High solute concentrations and interactions with soil particles may have decreased the reaction rate of enzymes activity under dry conditions, resulting in large differences between the in-situ enzyme activity and the measured potential activity. These results highlight the close links between soil enzyme activities and the cycles of C and N in soil. However, more research is needed to quantify the effects of environmental factors on soil enzyme activity.