COMPARATIVE STUDY ON RESIDUAL EFFECTS OF ORGANIC AND INORGANIC FERTILIZER NITROGEN IN WATERLOGGED SOIL AND DRYLAND SOIL

Abstract

The balance of residual fertilizer nitrogen was studied with the application of crotalariaand ammonium sulfate in two opposite cropping sequences —rice-wheat-rice-wheat and wheatrice-wheat-rice. Air-dried plant of crotalaria tagged with(~(15)NH_4)_2SO_4 and containing 6.5% of ~(15)N in total nitrogen was used as the sources of ~(15)N labelled organic manure. Ammonium sulfate tagged with 10.6% ~(15)N was applied as inorganic fertilizer. The recovery rate of residual fertilizer N was very low, averaging only 2.5%+2% on the basis of applied fertilizer N, in the case of rice as well as wheat in each successive cropping season. After the harvest of the first crop, more fertilizer nitrogen was immobilized as residual form in dryland soil than in waterlogged soil. The percentage of cumulative uptake of fertilizer N in rice-wheat-rice-wheat cropping sequence was much greater than that in the wheat-rice-wheat-rice sequence. It might be more economical to apply this kind of manure as basal dressing first to rice. No pronounced differences in the percentage of residual N were found in the third or fourth cropping season either in waterlogged or dryland soil, indicating the transformation of residual N into stable organic nitrogen forms after one year or so. During the whole four-cropping period, the availability of residual N is 1.9+0.7 times that of indigenous soil N. All the N availability ratios of residual ammonium sulfate N were significantly greater thau the corresponding value of crotalaria N at each cropping season. Organic N-forms of residual crotalaria N and ammonium sulfate N were characterized by chemical fractionation. A major portion of residual ammonium sulfate N was amino acid N and acid soluble unidentified N whereas the residual crotalaria N relatively contained a considerable portion of acid insoluble humin N.