Regulation effects of irrigation and nitrogen fertilization on nutrient absorption, water/nitrogen use efficiency and yield of rape

Excessive fertilizer and irrigation are often used in greenhouse cultivation in order to ensure adequate and profitable yield of vegetables. This has led to the pollution of agricultural environment that in turn hindered sustainable agricultural development. Reasonable application of irrigation and nitrogen is vital for enhancing nutrient use efficiency and controlling non-point source pollution. Using a stable isotope of ¹⁵N-traced urea, a pot experiment was conducted to evaluate the effects of different irrigation and nitrogen applications on nutrient absorption, water/nitrogen utilization and yield of rape. The study included three irrigation levelsW₁, 60%θ_f (field capacity); W₂, 75%θ_f; and W₃, 90%θ_f and four nitrogen rates (N₀, 0 g·kg^-1; N₁, 0.1 g·kg^-1; N₂, 0.2 g·kg^-1; and N₃, 0.3 g·kg^-1). The contents and absorptions of nitrogen, phosphorus and potassium by different parts of rape plant were determined along with water and nitrogen use efficiencies. The results suggested that both irrigation level and nitrogen application rate had significant effects on contents of N, P, K in the leaves and roots of rape plants. Besides, leaf P content was significantly affected by the interaction of irrigation and nitrogen. The contents of N and K in the leaves were much higher than those in the roots. While the content of P in the whole rape plant and N in the roots increased, the content of K in the whole plant and N in the leaf decreased with increasing irrigation. N application increased plant N and K contents, but reduced P content. There were significant effects of irrigation level and nitrogen rate on N, P and K uptake and the interaction effects were also prominent for P and K absorption. Medium irrigation level and low nitrogen rate (W₂N₁) had the highest nutrient uptake. The yield of rape was significantly affected by irrigation level and nitrogen rate. Yield increased with increasing irrigation level and a significant parabolic relationship was observed between nitrogen rate and yield. The application rate of nitrogen and the interaction of irrigation and nitrogen had a significant effect on irrigation water use efficiency (IWUE). Also the response of yield in terms of IWUE increased with increasing nitrogen application. Both irrigation and nitrogen fertilizer had significant effects on the utilization of fertilizer ¹⁵N. Besides, a statistically significant interaction was noted between water and nitrogen in terms of nitrogen recovery rate. Fertilizer ¹⁵N recovery rate increased with increasing irrigation level. The rates of ¹⁵N residue and ¹⁵N loss were respectively lowest and highest under medium irrigation level. With increasing nitrogen does, fertilizer recovery rate gradually decreased and the loss rate increased. It was concluded that for comprehensive nutrient uptake, yield output, and water and nitrogen utilization, W₃N₁ and W₂N₁ treatments were the best.