Influence of urea fertilization methods on rice yield, nitrogen use efficiency and soil nitrogen loss

Objectives We studied the effects of urea fertilization methods on rice yield, nitrogen utilization efficiency, and the fate of nitrogen fertilizer in soil to propose economically and environmentally friendly N management measures.

Methods A paddy field experiment was conducted in the Taihu Lake region in 2019 and 2020, and a ¹⁵N-urea micro-plot experiment was installed in the same field plot in 2020. Urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT) and nitrification inhibitor methyl 3-(4-hydroxyphenyl) propionate (MHPP) were added at 1% of urea-N. We included six fertilizer treatments: no N application (CK); broadcasted urea N at 300 kg/hm² (conventional fertilization, CN); broadcasted urea N at 225 kg/hm² (RNB); 50% in deep and 50% in broadcasting of urea N at 225 kg/hm² (RND); broadcasted urea N at 225 kg/hm²+NBPT+MHPP (RNB+DI); 50% in deep and 50% in broadcasting of urea N at 225 kg/hm²+NBPT+MHPP (RND+DI). Broadcasted urea N was applied in three splits (4 : 3 : 3) as basal, at tillering and panicle initiation stages. Deep-placed nitrogen was applied in two splits (7 : 3) as basal, and at panicle initiation stage. Rice yield, N uptake, N utilization, absorptive capacity, soil residual and total ¹⁵N loss were quantified.

Results Compared with CN, RNB, RND, RNB+DI and RND+DI treatments recorded similar grain yield and straw biomass, except the lower straw biomass in RNB in 2020; However, all the four treatments achieved higher N use efficiency, agronomic efficiency and partial factor productivity of nitrogen fertilizer, and ¹⁵N tracing result also proved the significant reduction in ¹⁵N uptake and total N loss under the four treatments. Compared with RNB, RND enhanced apparent N recovery efficiency (NRE) and ¹⁵N use efficiency by 20.4%–27.3% and 28.2%, and reduced ¹⁵N loss by 34.6% (P<0.05). RNB+DI had no impact on NRE, but (P<0.05) increased ¹⁵N use efficiency by 11.6% and reduced ¹⁵N loss rate by 13.1%. RND+DI increased rice yield by 2.6%–4.3%, NRE by 23.4%, ¹⁵N use efficiency by 36.9%, and reduced ¹⁵N loss by 45.0%. Compared with RND, RNB+DI had no impact on rice yield and NRE but reduced the ¹⁵N loss rate.

Conclusions In the rice production of Taihu lake area, the measurement of reducing urea-N rate from 300 kg/hm² to 225 kg/hm², but increasing the basal fertilizer ratio from 40% to 70%, and changing total broadcasting to 50% by deep placement, will not decrease rice yield, but significantly increase nitrogen fertilizer use efficiency, decrease total N loss. Adding NBPT and MHPP in urea will further decrease the total N loss. We recommend that a higher proportion of urea should be allocated to the deep basal placement and a lower proportion to broadcasting, rather than adding N inhibitors, to increase rice yield and N use efficiency.