Nitrogen transformations in tropical soils under conventional and sustainable farming systems

Samples of alluvial soil from mixed sandstone shale and slate and of Taiwan clay were collected from two sites, both managed under a similar crop rotation scheme. The fields were further divided into sections which were managed under either conventional farming or sustainable farming practices. When the soil samples were collected in April 1989, after 1 year of operation under conventional or sustainable practices, the nitrification activities of both soils managed under sustainable practices had improved dramatically compared to the conventional practices. The nitrifying activities in Taiwan clay samples collected in April 1993 which had been managed with chemical or with organic fertilizer were not significantly different. However, nitrifying activity in the alluvial soil was higher under sustainable than under conventional practices. Numbers of NH4-oxidizing bacteria were not significantly different in any of the soil samples irrespective of the different management practices. In contrast, higher numbers of NO2-oxidizing bacteria were detected in both soils managed sustainably. The results also indicated that the composition of NH4-oxidizing bacteria differed in the alluvial soil when managed with different kinds of fertilizer.