Reduction of Nitrate to Ammonium in Selected Paddy Soils of China

Reduction of Nitrate to Ammonium in Selected Paddy Soils of China *1

ＩＮＴＲＯＤＵＣＴＩＯＮＡｌｔｈｏｕｇｈｄｉｓｉｍｉｌａｔｏｒｙｒｅｄｕｃｔｉｏｎｏｆｎｉｔｒａｔｅｔｏａｍｍｏｎｉｕｍ（ＤＲＮＡ）ｗａｓｏｂｓｅｒｖｅｄｉｎｐｕｒｅｃｕｌｔｕｒｅｓｏｆｍｉｃｒｏｏｒｇａｎｉｓｍｓｍａｎｙｙｅａｒｓａｇｏ（Ｗｏｏｄｓ...

Three paddy soils were examined for their capacities of dissimilatory reduction of nitrate to ammonium (DRNA). 15 N labelled KNO 3 was added at the rate of 100 mg N kg -1 . Either glucose or rice straw powder was incorporated at the rate of 1.0 or 2.0 mg C kg -1 respectively. Three treatments were designed to keep the soil saturated with water: A) a 2 cm water layer on soil surface (with beaker mouth open); B) a 2 cm water layer and a 1 cm liquid paraffin layer (with beaker mouth open); and C) water saturated under O 2 free Ar atmosphere. The soils were incubated at 28 oC for 5 days. There was almost no 15 N labelled NH + 4 N detected in Treatment A. However, there was 1.4 to 3.4 mg N kg -1 15 N labelled NH + 4 N in Treatment B and 2.1 to 13.8 mg N kg -1 in Treatment C. Glucose was more effective than straw powder in ammonium production. Because there was sufficient amount of non labelled NH + 4 N in the original soils, 15 N labelled NH + 4 N produced as such should be the result of dissimilatory reduction. Studies on microbial population showed that there were plenty of bacteria responsible for DRNA process (DRNA bacteria) in the soils examined, indicating that number of DRNA bacteria was not a limiting factor for ammonium production. However, DRNA bacteria were inferior in number to denitrifiers. DRNA process in soil suspension seemed to start after 5 days of incubation. Glycerol and sodium succinate, though both are readily available carbon sources to organisms,did not facilitate DRNA process. DRNA occurred only when glucose was available and at the C/NO 3 - N ratio of over 12. It seemed that both availability and quality of the carbon sources affected DRNA.