Effects of Different Soil-water-fertilizer Management Practices on Tagged Fertilizer N Uptake by Rice in Amazon Varzea and Sacramento Clay

Abstract

Using 15N-tagged ammonium sulfate fertilizers as tracers, the uptake of fertilizer N by rice (Oryza sativa L.) as influenecd by different soil-water- fertilizer management practices in Amazon varzea soil and Sacramento clay were studied under pot culture conditions. Depending on the practices the recovery of fertilizer N by rice at flowering stage was 60 to 67% in the Amazon varzea soil, while this was 25 to 59% in the Sacramento clay. Those practices which may increase development of aerobic conditions or diffusion of the applied N to the oxidized soil layers greatly reduced the applied N uptake in the Sacramento clay. This was not the case in the Amazon varzea soil. The ammonium-N applied in the Sacramento clay was found to be quickly nitrified under aerobic conditions while this was not so in the Amazon varzea soil. The soil organisms in the two soils immoblized nearly the same amount of fertilizer N (19 to 21%) while only the former soil showed some fertilizer N fixation (2 to 5%) by clay minerals which appeared to contribute some reduction in the recovery of the fertilizer N by rice. It was concluded that marked difference in the tagged N uptake depending on various practices or conditions between the two soils is attributed mainly to the high nitrification and partly to the N fixation by the clay minerals in the Sacramentoclay. Therefore, the practices (deep placement, continous flooding, slowly releasing N fertilizers and nitrification inhibitors) which may minimize the fertilizer N loss through minimizing development of nitrification may not be particularly needed in the Amazon varzea soils having very low nitrification. 本研究使用 N15 標識硫酸銨，以盆栽試驗探討巴西亞馬遜河下游沼澤土和美國沙加緬度粘土，在不同土壤一水分一肥料管理下，所施氮肥被水稻吸收利用情形。 試驗結果顯示，亞馬遜沼澤土與沙加緬度粘土水稻在開花期之氮肥回收率分別為 60~67%及 25~59%；後者依不同的土壤管辦而有很大差異。會導致土壤好氣狀態之發展或導致所施氮肥往氧化層擴散之各種土壤管理均沙使加緬度粘土之肥料回收率降低，而亞馬遜沼澤土則無此現象；前者土壤中所施銨態氮肥在好氣狀態下迅速硝酸化，而後者則否。所施肥料氮素被土壤微生物固定之量在兩種土壤差異不大（19~21%），只是沙加緬度粘土之粘土礦物略有固氮能力（2~5%）而已。故兩種土壤隨者土壤管理差異所致氮肥收率之差異主要係因沙加緬度土壤之硝化作用較盛，易導致脫氮損失所致。 為防止水田硝化作用，吾人常利用氮肥深肥，持續淹水，施用緩效性氮肥或硝化抑制度等處理，但在本試驗之亞馬遜沼澤土，因硝化作用進行極為緩慢，故上述各處理似非必要。