Dynamics of soil amino sugar pools during decomposition processes of corn residues as affected by inorganic N addition

Purpose Identifying the impact of inorganic-nitrogen (N) availability on soil amino sugar dynamics during corn (Zea mays L.) residue decomposition may advance our knowledge of microbial carbon (C) and N transformations and the factors controlling these processes in soils. Amino sugars are routinely used as microbial biomarkers to investigate C and N sequestration in microbial residues, and they are also involved in microbial-mediated soil organic matter (SOM) turnover. We conducted a 38-week incubation study using a Mollisol which was amended with corn residues and four levels of inorganic N (i.e., 0, 60.3, 167.2, and 701.9 mg N kg⁻¹ soil). The objective of this study was to examine the effects of inorganic-N availability on fungal and bacterial formation and stabilization of heterogeneous amino sugars during the corn residue decomposition in soil. Materials and methods The surface soil (0-20 cm) used for the experiment was taken from a site in Gongzhuling, Jilin Province of China (43°30′N, 124°48′E). The sampled soils were air-dried, mixed, and passed through a 2-mm sieve. The incubation experiments were carried out at 25 °C, and soil moisture was maintained at 20% gravimetric moisture content by regularly weighing the plastic containers and adding water to the plastic containers during the incubation if necessary. The incubation was destructively sampled at weeks 1, 2, 4, 8, 12, 18, 28, and 38 of the incubation. The whole samples were air-dried, ground (<0.25 mm), and analyzed for amino sugar composition and concentration. Amino sugars (d-glucosamine, d-galactosamine, d-muramic acid, and d-mannosamine) were determined by capillary gas chromatography (GC) after their conversion to aldonitrile acetates (Zhang and Amelung 1996). Results and discussion Soil amino sugar contents and their contribution to SOM accumulation were significantly enhanced by the incorporation of corn residues, and amino sugar concentration maxima were observed during the 38-week-incubation course. Inorganic nitrogen significantly affected the accumulation of amino sugars during the decomposition of corn residues which had a high C/N ratio. It was found that more amino sugars were accumulated with higher inorganic N addition rates. However, when very high N application rates were evaluated, excess inorganic N in soil could not be taken up by microorganisms and some remained as inorganic N in the soil at the end of the incubation. Moreover, the dynamics of the individual amino sugars were different in response to the level of applied inorganic N. Muramic acid, a constituent of bacterial cell walls, was more sensitive to inorganic N supply and showed a faster turnover rate, while glucosamine, a constituent of fungal cell walls, was less sensitive to inorganic N supply. Glucosamine accumulated during the decay process of corn residues, and it was eventually stabilized in the soil. Conclusions When corn residues were decomposing, individual amino sugar pools were affected by inorganic N addition. Bacterial cell-wall residues played an important role in soil N transformation in the early stages of corn residue decomposition, however, bacterial residues were rapidly remineralized. In the later stages of residue decomposition, amino sugars were assimilated into fungal cell walls which accumulated in soil and contributed to the long-term maintenance of SOM. Therefore, when plant residues with a high C/N ratio are incorporated into the soil, sufficient inorganic N is required for microbial growth. The addition of inorganic N to soils which have received plant residues with C/N ratio ≥30:1 is required to sustain microbial rapid growth; however, oversupply of inorganic N may increase the risk of inorganic N losses from agricultural fields.