Seeding, nitrogen and irrigation management optimize rice water and nitrogen use efficiency

Adoption of appropriate agronomic practices, such as optimum seeding and nitrogen (N) rates, in synchronization with proper water management practice could help improve yield, water productivity and N use efficiency (NUE) of rice (Oryza sativa L.). Field experiments were conducted at the research farm of the Asian Institute of Technology during the dry seasons (November–March) of 2017–2018 and 2018–2019 to evaluate the effects of seeding (S) (95 [S95], 125 [S125] and 155 [S155] kg ha−1) and N (0 [N0], 60 [N60] and 120 [N120] kg ha−1) rates under different water regimes (continuous flooding [CF] and safe alternate wetting and drying [AWD15]) on yield, water productivity and NUE of wet direct-seeded rice. Application of 120 kg N ha−1 (N120) resulted in the highest grain yield and water productivity (by 76% and 78%, respectively, compared with N0), which were statistically similar with N60. Less plant population in lower seeding rates (S95 and S125) provided better results in terms of vegetative and reproductive growth, grain yield and water productivity. Rice grown under AWD15 resulted in statistically similar grain yield with rice maintained under CF, but there was 40–44% more water savings depending on seeding rates and 68% higher water productivity in AWD15-treated plots. Improving sink capacity and dry matter accumulation is key to ensuring better grain yields even with reduced N and seeding rates under AWD15. A stronger relationship between grain yield and spikelet number panicle−1 as well as between shoot dry matter and spikelet number panicle−1 was observed under CF (r = 0.61 and r = 0.57, respectively) compared with AWD15 (r = 0.45 and r = 0.41, respectively). N60 had significantly higher partial factor productivity (64.6) of applied N than N120 (36.4). S95 and S125 resulted in a significant improvement in NUE compared with S155. This result suggests that desirable grain yield and water productivity as well as better NUE could be achieved by decreasing N rate from 120 to 60 kg ha−1, and thus significant reduction in fertilizer input cost could be possible along with environmental benefits. In addition, decreasing seeding rate coupled with AWD15 is recommended to reduce the input cost and total water input to make the wet direct-seeded rice cultivation system more sustainable and profitable.