The strategy of microbial utilization of the deposited N in a temperate forest soil

Abstract

A laboratory incubation experiment was conducted using 15NH4NO3 or NH415NO3 (0.1 mg N g−1 soil (0.1 N) and 0.5 mg N g−1 soil (0.5 N)) labeled soil, with or without litter addition. The newly synthesized amino sugars derived from exogenous nitrogen (15N) were differentiated and quantified using a gas chromatography/mass spectrometry technique. Litter addition enhanced the incorporation of deposited N (exogenous N) to microbial residues (amino sugars), while the contribution of bacterial and fungal residues to N transformation varied at different incubation stages. There was a succession from fungal to bacterial residues after the deposited N addition, while the contribution of fungal residues to the NO3− transformation increased at the later stage of litter decomposition. The contents of total 15N (15NH4+ + 15NO3−) derived microbial residues in the low N (litter + 0.1 N) treatment were similar to that in the high N (litter + 0.5 N) treatment, which indicated that microorganisms had a saturation effect on the utilization of deposited N at different concentrations. Moreover, there was a preferential uptake of NH4+ than NO3− by microorganisms, while the strategy of microorganisms to utilize different forms of N depended upon the deposited N concentration. When the deposited N concentration was low, the microorganisms would have no choice but to use NO3−, while when the deposited N concentration was high, the microorganisms would selectively utilize NH4+ and decrease the utilization of NO3−. Our study indicated that with the increase in atmospheric N deposition in the future, selective utilization of deposited NH4+ and NO3− by microorganisms may increase the potential losses of NO3− from forest soil, especially in China, where the NH4+/NO3− ratio has recently decreased in atmospheric deposition.