THUWEBA DIWANI, MATHIAS BECKER
University of Bonn, Plant Nutrition in the Tropics and Subtropics, Germany
In the seasonally wet tropics, alternating soil drying and wetting cycles characterise the dry-to-wet season transition period (DWT) after the harvest of the dry season crop and before the transplanting of wetland rice. Large soil N losses during DWT have been shown to occur from lowland fields in Asia and Africa. Particularly under low-input conditions, this native soil fertility needs to be conserved for crop (lowland rice) production. Previous field experiments have shown that native soil N losses during DWT can be minimized through the temporary N immobilization in the microbial biomass after application of low quality crop residues (e.g., cereal straw). While soil N conservation may potentially benefit the subsequent crop of rice, too much straw may lead to N immobilization for longer than desired, resulting in yield reduction. The effect of different rates and methods of straw application during DWT on soil N dynamics has been studied under controlled conditions in a greenhouse. Wheat straw at rates of 1.5, 3.0 and 6.0 Mg ha has been either incorporated or applied as surface mulch. Soil moisture was gradually increased from 50% field capacity to full water saturation during DWTs of varying lengths. The dynamics of soil Nmin, soil microbial biomass N and in situ NO emissions were quantified at weekly intervals. The growth and N uptake by a subsequent crop of lowland rice was evaluated. Implications of various straw management options on the N nutrition of wetland rice will be validated in participatory on-farm experiments in rice-wheat rotation systems in South Asia.
Keywords: Denitrification, microbial biomass, N-immobilisation, Oryza sativa