Dissimilatory nitrate reduction to ammonium and responsible microorganisms in two Chinese and Australian paddy soils

Dissimilatory nitrate reduction to ammonium (DNRA) and the responsible microflora were studied in two typical Chinese and Australian paddy soils. The DNRA accounted for 14.9% of total reduction of (15)N-labeled nitrate added to the soil from Griffith (NSW, Australia) under anaerobic incubation without any exogenous carbon source addition, but only 5% for the soil from Yangzhou (China). Addition of reducing agents (sodium thioglycollate and L-cysteine) resulted in lower redox potentials and enhanced the DNRA process, with the majority of the product of reduction being ammonium. However, redox potential alone could not explain the difference of DNRA potentials between the two soils. Additions of glucose also resulted in substantial increases in DNRA, especially for Griffith soil, with the majority of the product being organic-N. The significantly higher DNRA in Griffith soil compared to Yangzhou soil is consistent with the higher DNRA microorganism population isolated from Griffith soil than that in Yangzhou soil, which is also consistent with higher fraction of labile carbon in Griffith soil than that in Yangzhou soil. In contrast, the denitrifiers population in the Griffith soil was about 10-fold smaller than that in the Yangzhou soil. The results demonstrate that the soil indigenous labile carbon was the key factor influencing the partitioning of nitrate reduction between denitrification and DNRA. Most DNRA bacteria and denitrifiers isolated were spore-forming bacteria. (C) 2002 Elsevier Science Ltd. All rights reserved.