POPULATION-DYNAMICS OF BURIED WEED SEEDS AS INFLUENCED BY SHIFTING AND TERRACE CULTIVATION IN THE HUMID SUBTROPICS OF INDIA

Change in the viable seed populations of three weed species, Galinsoga ciliata (Rafin) Blake, Polygonum alatum Buch. Ham. and Spergula arven sis L. in soil was studied in potato, radish, cabbage and cabbage+radi sh (mixed together) fields under shifting agriculture (locally called jhum) and terrace cultivation in northeast India from October 1988 to September 1989. The effect of weeding frequency on the viable weed see d populations of these weeds was also investigated. In most fields see d populations were smaller during the rainy season than during other s easons. Initially, the viable seed populations of all three weed speci es showed a significant (P<0.05) decrease with increasing soil depth i n all crop fields under jhum cultivation. However, there was no clear trend in the terraced fields. At the end of the study period the trend was not clear in all crop fields under either jhum or terrace cultiva tions. In the 'normally weeded plots' the viable seed populations of S . arvensis and P. alatum recorded about a 2% increase in jhum fields, while in the terraced fields they showed a loss of 4% over the 1-year period. The 'frequently weeded plots', however, showed a sharp decline in viable seed populations in all crop fields.