Epsilon Open Archive

Abstract

Nitrogen (N) is recognised as a major constraint to crop production worldwide. This thesis evaluated the effects of organic and mineral fertilisers on N use in the field through studying the main N fluxes in soil and plants on a 30-day basis over four consecutive growing seasons in a maize-bean rotation in southern Nicaragua. The soil net N mineralisation response to the application of organic (chicken and cow manure applied at recommended dose and double dose) or mineral N sources was evaluated in an in situ experiment. To determine the N available for plant use, total plant N uptake was estimated by calculating the mass balance of soil mineral N pool. The measured aboveground N was related to the total plant N uptake within and between the growing seasons. Net mineralisation was found to increase significantly at the higher N rate of chicken manure compared with the other treatments. The soil mineral N increased significantly in the middle of the season in both maize and beans. Total N uptake was mainly determined by mineralisation rate and less by changes in the soil mineral N pool and very little by N leaching. For beans, the estimated partitioning of mineral N to aboveground parts was practically linearly related to the estimated total mineral N uptake. For maize, the fraction of N allocated to above ground decreased during the middle of the growing season. Grain yield increased at the higher N rates of chicken manure and mineral fertiliser. The grain yield response to added fertiliser N was similar to the corresponding response of net N mineralisation. This was explained by the aboveground N being almost proportional to the N mineralisation, the aboveground biomass being related to above ground N, and that grain yield was basically proportional to the aboveground biomass. It was concluded that the effect of fertilisation type on crop yield was mainly through its effect on net mineralisation, and that crop growth was always N-limited. Hence,the study suggests increased recommended fertiliser dose to increase yields in Nicaragua, and that quantification of soil N mineralisation is a main factor to control the effects of manure applications.