Use-Efficiency of Nitrogen(15N) Sources and Rates in Bermudagrass in a Tropical Ecosystem.

Fertilizer source and application level can have major impacts on crop response and nitrogen (N) utilization systems. The objective of this study was to evaluate the impacts of two N sources [urea and ammonium nitrate (AN)] on Tifton 85 (Cynodon spp.) managed for hay production and N use-efficiency. The experiment was conducted in Sao Paulo State, Brazil and treatments consisted of either urea of AN surface applied at 0, 60, 120, 180, and 240 kg N ha^-1 after each harvest. Forage was harvested at 30-day intervals (December to March) for two years. Response variables included forage yield, nitrogen use-efficiency, photosynthetically active radiation (PAR), and leaf area index (LAI). Results indicate that both fertilizer sources increased dry matter (DM) yields at N levels up to the 180 kg N ha^-1 per harvest (625 kg DM ha^-1 per harvest for control vs. 4513 kg DM ha^-1 for treatments receiving 180 kg N ha^-1 per harvest). Ammonium nitrate resulted in greater DM yield and PAR during the dry months (February of 2014) compared to urea. Similarly, treatments receiving urea showed smaller LAI (2.0) than AN (2.9) when rainfall was limited. No effect of N source on yield was observed when rainfall was adequate. Despite the lack of response on cumulative annual yield, AN resulted in greater N use-efficiency (average of 36%) than urea (average of 20%). Approximately 63% of the applied N via AN remained in the plant-root-litter-soil system; while ~ 42% was retained when urea was applied. Results demonstrated that AN may be a more effective N source, particularly when rainfall is scarce, resulting in greater forage yields and N use efficiency