Nitrogen transformation in tropical soils: Influence of fertilization and crop species

The experimental field plots were divided into two sections managed under either conventional or sustainable farming practices plus two different rotation schemes. The planting order for rotation System 1 was seen manure, cabbage, sweet corn, rice, vegetable soybean while under System 2, the order was green manure, sweet corn, vegetable soybean, rice, radish, vegetable soybean. In samples of sandstone shale and slate mixed alluvial soil and Taiwan clay taken from field plots operated under rotation System 1, the nitrifying activities were always higher in soils receiving organic fertilizer as compared with chemical fertilizer treatment. When comparing the nitrifying activities of soils collected from fields managed under different rotation systems, the results indicated that the activity was always higher in soil collected from fields grown with legume. In our attempts to compare the numbers of ammonium-oxidizing and nitrite-oxidizing bacteria in soils receiving different fertilizers or between rotation systems, we failed to detect any significant differences between those samples taken at the same period of time. However, when comparing the trends of nitrite and nitrate accumulation in alluvial soil and Taiwan clay incubated at 30 degrees C and 40 degrees C, changes were observed. Therefore, we believe that the differences in nitrite and nitrate accumulations in soils receiving chemical or organic fertilizer were caused by variation in the composition of nitrifying bacterial populations in these soils. (C) 1997 Elsevier Science B.V.