Soil Nitrogen Distribution and Plant Nitrogen Utilization in Direct-Seeded Rice in Response to Deep Placement of Basal Fertilizer-Nitrogen

Deep placement of controlled-release fertilizer increases nitrogen (N) use efficiency in rice planting but is expensive. Few studies on direct-seeded rice have examined the effects of deep placement of conventional fertilizer. With prilled urea serving as N fertilizer, a two-year field experiment with two N rates (120 and 195 kg/hm2) and four basal N application treatments (B50, all fertilizer was broadcast with 50% as basal N; D50, D70 and D100 corresponded to 50%, 70% and 100% of N deeply placed as basal N, respectively) were conducted in direct-seeded rice in 2013 and 2014. Soil N distribution and plant N uptake were analyzed. The results showed that deep placement of basal N significantly increased total N concentrations in soil. Significantly greater soil N concentrations were observed in D100 compared with B50 at 0, 6 and 12 cm (lateral distance) from the fertilizer application point both at mid-tillering and heading stages. D100 presented the highest values of dry matter and N accumulation from seeding to mid-tillering stages, but it presented the lowest values from heading to maturity stages and the lowest grain yield for no sufficient N supply at the reproductive stage. The grain yield of D50 was the highest, however, no significant difference was observed in grain yield, N agronomic efficiency or N recovery efficiency between D70 and D50, or between D70 and B50, while D70 was more labor saving than D50 for only one topdressing was applied in D70 compared with twice in other treatments. The above results indicated that 70% of fertilizer-N deeply placed as a basal fertilizer and 30% of fertilizer-N topdressed as a panicle fertilizer constituted an ideal approach for direct-seeded rice. This recommendation was further verified through on-farm demonstration experiments in 2015, in which D70 produced in similar grain yields as B50 did.