Effect of nitrogen fertilization on the fate of rice residue-C in paddy soil depending on depth: 13C amino sugar analysis

A 100-day incubation experiment was conducted to (i) trace the fate of rice residue-derived C-13 in the amino sugar (AS) pool in 0-1-cm (oxic) and 1-5-cm (anoxic) layers of paddy soil and (ii) evaluate the effects of inorganic N ((NH4)(2)SO4) fertilization on the formation of AS at early and late incubation times (5 and 100 days, respectively). The accumulation of rice residue-derived AS occurred at 5 and 100 days in both soil layers as a result of AS stabilization. Inorganic N addition increased the contents of rice residue-derived muramic acid, glucosamine, and galactosamine in the 0-1-cm soil layer for both incubation times by average on 14.7-20.8%, 23.7-31.8%, and 11.6-23.3%, respectively. In contrast, no effects of N fertilization on AS content in the 1-5-cm soil layer were found. The amount of rice residue-derived AS was higher in the 1-5-cm than in the 0-1-cm soil layer at early incubation time, probably due to the higher contents of ammonium here compared to the upmost oxic layer where nitrate was the dominated N form. Thus, the preferential uptake of ammonium but not nitrate by microorganisms led to the higher formation of rice residue-derived AS in the anoxic soil layer. The ratio of fungal to bacterial residues (fungal glucosamine/muramic acid) ranged between 1.0 and 1.7 for rice residue-derived AS and was 12.5-14.6 for total AS, indicating that fungi and bacteria have similar contributions to the decomposition of fresh rice residue whereas native soil organic matter (SOM) is a fungi-predominated process. This study emphasized that coupling of C and N cycles in paddy soils is different in oxic and anoxic layers, resulting in variation of plant residue decomposition and formation of SOM.