Traditional lowland rice agriculture in Sri Lanka : Nitrogen cycling and options for biofertilizers

• The aim of this thesis has been: to study the dynamics of soil organic-matter, with regard to nitrogen in a traditional lowland-rice agricultural system; to consider options for and nitrogen cycling of biofertilizers, especially the green-manure legume Sesbania sesban; to discuss fertilizer sources and management constraints as parts of a sustainable agricultural development; and to discnss the methodological problems of controlled experiments vs. investigations in farmer's fields.The traditional lowland rice (paddy rice, Oryza saliva) system was low-intensive and little mechanization and fertilizers were used. Water and nutrients were considered the largest constraints to crop yield, and their spatial variability within a paddy tract was studied. Mineralization of soil organic matter is the main nitrogen source for the rice crop. Since the studied rice fields are cultivated for extensive periods, irrigation water transports clay particles to low-lying areas (a decrease in clay content from 45"/o to 15%, over a distance of about 300 m), resulting in accumulation of soil organic-matter in areas with higher clay content. Both the recalcitrant as well as the more labile fraction (N0) of soil organic nitrogen were higher in low-lying areas (7% N0 oftot-N with a clay content of6% and 15% N0 with a clay content between27 and 42%). Rice yieldduring the minor cultivation season 1985 varied considerably (from 720 to 4 180 kg ha-1), but was not related to soil organic-matter content. However, differences in water coverage duration of the fields partly explained yield variations.The legume Sesbania sesban was evaluated as a green manure crop. The nitrogen rich leaves (N-content 3.8%), which contained 88% of the nitrogen in the above-ground parts, decomposed and released nitrogen much more rapidly than the stems (N-content 0.41%) and roots (N-content 0.73%). After four days the leaves had released 53 kg N ha·1• The total N-input from S. sesban was 92 kg ha·1, which gave a rice yield of 3 960 kg ha·1 (corresponding yield with 96 kg N ha~I industrial fertilizer was 4 420 kg ha·1).For a sustainable development of this agricultural system a strategy based on active adaptation to local resources is discussed. This strategy is also in line with the view of the farmer. Concerning N-fertilizers, the use of green-manure legumes in combil)ation with industrial fertilizers shows the greatest potential.

Ämnesord

SAMHÄLLSVETENSKAP  -- Annan samhällsvetenskap -- Tvärvetenskapliga studier inom samhällsvetenskap (hsv//swe)

SOCIAL SCIENCES  -- Other Social Sciences -- Social Sciences Interdisciplinary (hsv//eng)

Nyckelord

INTERDISCIPLINARY RESEARCH AREAS

TVÄRVETENSKAPLIGA FORSKNINGSOMRÅDEN

Publikations- och innehållstyp

vet (ämneskategori)

dok (ämneskategori)