| 摘要: |
Developing an understanding of the transformation kinetics of amino acids in soil is of fundamental importance to probe into the stabilization and decomposition processes central to soil organic matter (SOM) cycling. Considering the transitional function of amino acids in SOM turnover, substrate availability controls the formation and decomposition of soil amino acids critically. However, compared to broad knowledge on free amino acid turnover, how the changing bio-availability of carbon (C) and nitrogen (N) influences the accumulation and turnover of proteinaceous amino acids in soil is not well understood. Therefore, a laboratory experiment was conducted in which soil samples were incubated and supplemented weekly with glucose and N-15-labeled inorganic N as either ammonium (NH4+) or nitrate (NO3-). The concentrations of soil amino acids, differentiated into the newly synthesized (N-15-labeled) and endogenous (unlabeled) portion, were temporally quantified by an isotope-based high-performance liquid chromatography/mass spectrometric technique.
During the incubation, extraneous substrates were rapidly immobilized into soil amino acids as important metabolic constituents. However, the dynamics and maintenance of soil amino acids were determined by changing C and N availability during microbial proliferation rather than static stoichiometry of available substrates. The significantly greater amount of de-novo synthesized amino acids in treatments with NH4+ addition than NO3- addition confirmed that microorganisms preferred the reduced N than the oxidized form due to significantly lower energy and C requirements. The decomposition of endogenous amino acids during the later stages of the incubation indicated that amino acids could partly meet microbial C and energy demand, but the capacity to get decomposed was independent on N species. Therefore, higher rate of extraneous N immobilization in glucose plus NH4+ treatment was closely associated with the net accumulation of amino acids; whereas the formation of new proteinaceous amino acids after weekly additions of glucose plus NO3- was eventually offset by the loss of the endogenous portion, resulting in apparently unchanged amounts of amino acids in the microcosms.
Under the two incubation conditions with available C and N addition, both the intrinsic C percentage in molecules and the biosynthetic pathways have minor influence on the turnover pattern of HCl hydrolyzed amino acids. However, the transformation of individual amino acids was significantly correlated throughout the entire incubation. These findings suggested that the turnover of soil amino acids in proteinaceous form was critically interrelated instead of being controlled by the transformation pathways of different free amino acids. Compared to the NH4+ supply, the addition of NO3- enhanced the decomposition of glycogenic amino acid while reduced the degradation of most of ketogenic amino acids, possibly suggesting the important regulating strategy of energy yield on microbial speciation of the N forms. |
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