Effects of nitrogen fertilizer and pesticide management on floodwater ecology in a wetland ricefield

Abstract

This paper is the first in a series reporting a study on the effects of N fertilizer, green manure, and pesticide applications on the floodwater ecology of a tropical ricefield. Algal, zooplankton, and mollusc popu- lations were monitored during a crop cycle in plots sub- jected to various methods of N management and differ- ent pesticide regimes. This paper presents the experimen- tal design, general features of the statistical methods for data analysis, and the dynamics of algal growth in the floodwater. Transient green algal blooms and increases in floodwater dissolved O2 concentrations were observed after the broadcast application of N fertilizer 7 days after transplanting. These effects were less pronounced after the second N application 55 days after transplanting. Photosynthetic activity in the floodwater of planted plots decreased as the crop season progressed. Mucilaginous colonial blue-green algae proliferated in the zero-N con- trol and unplanted plots, but were inhibited by the broad- cast N fertilizer and green manure incorporation. Deep placement of urea supergranules reduced the negative im- pact of N fertilizer on blue-green algal populations. There was limited evidence that the application of pesticides promoted the development of blue-green algal popula- tions in the absence of readily available N.