Comparative assessment of urea briquette applicators on greenhouse gas emission, nitrogen loss and soil enzymatic activities in tropical lowland rice

Suitable method of application of urea briquette in flooded rice (Oryza sativa L.) not only increases nitrogen use efficiency, but at the same time sustain yield and reduce greenhouse gas emission in general and nitrous oxide emission in particular. Keeping this hypothesis, seven different treatments including manual and mechanical mode of application with two different type of applicators were tested in field in consecutive wet (2015) and dry (2016) seasons. Three row briquette applicator (TRBA) and top dressing applicator (TDA) were used in combination for basal and top dressing application along with prilled urea broadcasting and without application as well. It was observed the use of TRBA at basal with first top dressing by TDA and manual placement of urea briquette at second top dressing (TRBA + TDA) yielded highest @ 6.71 t ha(-1) and 5.47 t ha(-1) in the wet and dry seasons, respectively, with the highest agronomic use efficiency (36.3 kg kg(-1)). Average emission of methane from all treatments were higher (19.2-27.1%) during wet season than the dry season and vice versa in case of nitrous oxide (3.6-11.8%). Among the treated plots, TRBA + TDA recorded the lowest emission of nitrous oxide. The largest amount of ammonia was lost through volatilization after basal application of urea, followed by top dressing at the end of maximum tillering stage and panicle initiation stage. As a whole, volatilization loss of N was lowest in control (3.3-3.4 mg NH3-N m(-2) d(-1)), followed by mechanical (27.6-33.9 mg NH3-N m(-2) d(-1)) and manual (49.1-55.3 mg NH3-N m(-2) d(-1)) method of urea briquette application. The population of nitrite oxidizers and heterotrophs were highest in prilled urea broadcasting. Soil dehydrogenase activity was the highest in urea briquette manual placement + top dressing applicator (UBMP + TDA, 148.0 pg-160.0 TPF g(-1) soil d(-1)) treatment, while the urease activity was less was in urea applicator treated plots. Subsurface application of larger size urea (urea briquette) and applicator based precise placement of urea reduced the lossess of nitrogen through ammonia volatilization and nitrous oxide emission, thus enhanced nitrogen use efficiency. Apart from that, issue of adequate skilled labour for precise depth of application could be tackled by effectively adopting mechanical placement of urea briquette. The mechanical placement of briquette (relatively larger size particle) in tropical flooded rice is more precise and less labour intensive, hence this is an efficient environment friendly approach of N management.