Effect of nitrogen source options on grain yield, nitrogen uptake and use efficiency of rice along an inland valley in Central Uganda
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Only 29% of the Inland valleys in Uganda has been harnessed for rice production yet, little is known about N management for optimizing rice yields across hydrological positions within these inland valleys. This study was conducted to determine the effect of N source on grain yield, N uptake and use efficiency by rice across three hydrological positions: center, middle and fringe for three growing seasons of September to December, 2014, 2015 and 2016. Field experiments were conducted at the National Crop Resource Research Institute, Namulonge in Central Uganda. There were five treatments each replicated four times: control, 60 kg N ha-1 mineral N, 60 kg N organic N, 120 kg N ha-1 mineral N and 120 kg N ha-1 organic N in a randomized complete block design with hydrological position as the blocking factor. Data were collected on number of tillers, dry matter yield, number of panicles, 1000 grain weight, harvest index, grain yield, N uptake, agronomic and recovery efficiency of N. The data were subjected to ANOVA using Genstat 12th edition and means compared using Tukey’s HSD. The main effect of hydrological position was not significant on all parameters except for HI and 1000 grain weight. Interaction between hydrological position and N source was also not significant on all parameters except number of tillers. Similarly, the tow-way interactions between hydrological position and season did not differ significantly for all the parameters measured. However, the main effect of N source was significant on all the parameters as did its interaction with season except for dry matter yield, harvest index and agronomic N use efficiency. The results indicate that the growth and yield of rice were influenced solely by N source and that these responses occurred regardless of the hydrological position. Overall, the highest grain yield (6.0 ± 0.2 t ha-1) was realized with application of 120 kg N ha-1 as mineral N and even the 60 kg N ha-1 as either organic or mineral sustained rice grain yield above 3 t ha-1 over the three years of this study compared to the control treatments in which yields plummeted below the national average of 2.5 t ha-1 by the third year. About 63% of applied N was recovered (ANRE) with application of 120 kg N ha-1 from mineral source compared to 20% ANRE reported in most rice fields. This study indicates that the real benefits of mineral fertilizers are realized with the higher rate of 120 kg N ha-1 but that at the lower rate of 60 kg N ha-1, rice grain yields from the organic N source are as good as those from mineral N. A multi-locational study across different inland valleys within different agro ecological zones is needed to develop N management packages that can be scaled throughout Uganda and related agro-ecologies.